Synthesis and characterization of nanocrystalline UO2 ceramics

Author: Jovani Abril, Raquel

Year: 2014

Source: https://minerva.usc.es/bitstreams/97425a65-af96-4281-82cc-fba35865da9a/download

Syn hesis and cha ac e iza ion o
nanoc ys alline UO2ce amics
DOCTORAL THESIS
Disse a ion by
M.Eng. Raquel Jo ani-Ab il1,2
Di ec o s:
P o .D . A u o López Quin ela1
D . José Luis Spino2
1Uni e sidad de San iago de Compos ela (USC), Spain
2Ins i u e o T ansu anium Elemen s (ITU), Ge many
San iago de Compos ela (Spain), 2014
Memo ia de esis p esen ada po Raquel Jo ani-Ab il pa a la ob ención del í ulo de
Doc o po la Uni e sidad de San iago de Compos ela den o del P og ama de Doc o -
ado en Ciencia de los Ma e iales.
Raquel Jo ani-Ab il
i
ii
D. A u o López Quin ela, P o eso Doc o Ca ed á ico del Depa amen o de
Química-Física de la Uni e sidad de San iago de Compos ela, y D. José Luis Spino,
In es igado Doc o Senio del Ins i u e o T ansu anium Elemen s Join Resea ch
Cen e de la Comisión Eu opea,
in o man:
Que la p esen e memo ia, i ulada “Syn hesis and cha ac e iza ion o nanoc ys-
alline UO2ce amics” (“Sín esis y ca ac e ización de ce ámicas nanoc is alinas de
UO2”), que pa a op a al í ulo de Doc o po la Uni e sidad de San iago de
Compos ela den o del P og ama de Doc o ado en Ciencia de los Ma e iales
p esen a Dª. Raquel Jo ani-Ab il, ha sido ealizada en el Ins i u e o T ansu a-
nium Elemen s Join Resea ch Cen e de la Comisión Eu opea en colabo ación con el
Depa amen o de Química Física de la Uni e sidad de San iago de Compos ela bajo
nues a di ección.
Conside ando que cons i uye abajo de Tesis, au o izan su p esen ación en la
Comisión de Te ce Ciclo de la Uni e sidad de San iago de Compos ela.
Y pa a que así cons e, ﬁ mamos el p esen e in o me:
P o .D . A u o López Quin ela D . José Luis Spino
iii

i
Al ﬁn y al cabo somos lo que hacemos pa a cambia lo que somos.
Edua do Galeano
A Daniel i Jo di
i
Acknowledgemen s
The wo k p esen ed in his s udy has been ca ied ou in he amewo k o a Eu opean
hesis o ob ain he deg ee o Doc o om he Uni e si y o San iago de Compos ela
(USC) and sponso ed by he Eu opean Commission. This hesis has been enabled by
he collabo a ion and di ec ion o P o .D . A u o López Quin ela om he Depa men
o Physical-Chemis y o he USC, and D . José Luis Spino om he Nuclea Fuels
Depa men o he Ins i u e o T ansu anium Elemen s (ITU). I wish o exp ess my
g a i ude o bo h o hem o ake he challenge o he supe ision o he hesis and he
collabo a ion in he dis ance.
I would like o hank he Di ec o o ITU, P o .D . Thomas Fanghänel, o oﬀe ing
me he oppo uni y o make he esea ch in his enowned ins i u e.
I wan o g ea ly acknowledge D . Daniel Ba on, D . Joaquin Cobos Saba é,
P o .D . Joan de Pablo Ribas, P o .D . Ian Fa nan, P o .D . Haas Didie , D . Ralph
Hania, D . Rika d Malmbeck, P o .D . F ancisco Ri adulla, D . Vicenzo V. Rondinella,
D . Joseph Some s, P o .D . Ca los Vázquez Vázquez and D . Ma cus Wal e , who
accep ed wi hou complica ions o be pa o he possible elec ed membe s o he
ju y/ e ise s o my hesis de ence.
I wan now o special hank all he colleagues who made ha expe ience possible.
I do no use he e you i le, bu you name. No because I do no wan o ea you
wi h he espec you all dese e, bu because you i le does no say any hing abou
you as a pe son. I is you name, which ep esen s o me he pa ience, he lo e, he
ime and he laugh e we enjoyed oge he .
Fi s , hank o he Nuclea Fuels Depa men because, al hough I was mo ing
a ound any co ne o he ins i u e, his was he uni I was belonging o. I eally
enjoyed he ou yea s wi h you.
Thanks o Ma c Couland, He win Hein and Se ge Fou caudo . You showed me how
o mo e inside his special ins i u e in my ﬁ s ime in ITU as a ainee and la e on in
he PhD. You opened doo s o me which would emain longe closed o a new s uden .
Thanks also o slowly us ing me making you baby-expe imen s. Fo ﬁnishing he
p epa a ion o my samples o an u gen p ojec when I needed o ﬂy o Spain wi hou
expec ing i om one day o he o he . Thanks o he oﬃce-con e sa ions.
Thanks o Michael Holzhäuse , Co Bosho en, Mai ead Mu ay Fa hing, John
Mcginley, Sa ah S oh , Pa ick Laja ge, Sebas ien Ga deu , An ony Guio , Emmanuel
Ve mo el, And ea Camb iani, Alexand e Dockendo and Anne e Küs , who wo ked
in one o o he s ep o he pelle pe o mance. Thank o pu you speciﬁc expe ience
in his wo k.
ii
Es a ans o mación comienza en el bo de de la pas illa del combus ible y de
mane a cons an e p og esa hacia el in e io mien as se sucede la i adiación
[Ma zke and Spino, 1997][Spino and Papaioannou, 2000]. La mic oes uc u a o ig-
inal de mic o- g anos (o g anos-g andes) se ans o ma en una ma iz nc-po osa
[Nogi a and Une, 1994] a a és de la ees uc u ación de los de ec os de i adiación
acumulados. Se a a de un ipo de acción de “au o-cu ación”, donde el ma e ial
se cu a del daño su ido eo denándose a sí mismo [Spino e al., 2012]. La nue a
nc-es uc u a que apa ece en onces, ecibe el nomb e de es uc u a de al o g ado
de combus ión (HBS; high bu n-up s uc u e), ambién llamada es uc u a de bo de
( im-s uc u e) po que en los combus ibles de UO2és a se inicia en el bo de o en las
zonas ex e io es de las pas illas ( egión que ecibe mayo can idad de ﬁsiones).
En un p incipio se pensó que la HBS e a la causa de los allos que obse ados en el
e es imien o de las a illas debido a aumen o adicional del olumen (hinchazón) en
pila y un supues o compo amien o ágil del ma e ial así ans o mado [Ma zke, 1992].
Además se c eyó que es a es uc u a que apa ece en el bo de de la pas illa pod ía
ac ua como una nue a uen e de libe ación de gas. De hecho oda ía hay líneas de
in es igación que deﬁenden es a opinión. Sin emba go, o os es udios han demos ado
que el po cen aje de gas libe ado desde el bo de de las pas illas de combus ible, donde
apa ece la HBS, es bajo en compa ación con el gas que iene de las pa es in e nas de
la pas illa donde la es uc u a o iginal oda ía es á p esen e [Mogensen e al., 1999]. A
es a segunda ﬁloso ía le han seguido o as publicaciones que demues an un e olución
en gene al a o able de las p opiedades del combus ible con la apa ición de es a nue a
es uc u a a al os alo es de combus ión, en pa icula el aumen o en la e ención de
los gases de ﬁsión.
Asímismo, las p opiedades de la HBS indican una mayo ole ancia a la adiación
[Spino e al., 2012]. Es e es udio coincide con una e idencia impo an e e inno ado a
en la li e a u a que demues a que los ma e iales de g anos nano esis en más el daño
po adiación que los co espondien es ma e iales de g ano mic o, debido a la ecom-
binación de de ec os en los múl iples lími es de g ano [Ni a e al., 2005]. Se an icipan
ambién o as endencias en el ma e ial como la mejo a de la conduc i idad é mica
y o os e ec os de la adiación en las p opiedades del ma e ial debido a la libe ación
de es és en el pa áme o de ed después de la ec is alización [Ronchi e al., 2004], así
como el aumen o de la enacidad a la ac u a y cu ación de g ie as [Spino e al., 2003].
En lo que concie ne a la segu idad, los úl imos expe imen os ealizados en com-
bus ibles some idos a al o g ado de combus ión en eac o es nuclea es de agua lige a
(LWR; ligh wa e eac o ), no indica on aumen o en la libe ación de gas, así como
ampoco en la suscep ibilidad de allo du an e acciden es de eac i idad iniciados (RIA;
eac i i y ini ia ed acciden ) ansi o ios [Sasajima e al., 2010][Fuke a e al., 2006].
También ha sido obse ada una disminución de la elocidad de co osión acuosa bajo
condiciones ípicas de depósi o geológico simuladas en combus ibles con p esencia
de HBS [Eke o h e al., 2009][Ca bol e al., 2009]. Ambos hechos conﬁ ma on la
es anqueidad de es a es uc u a.
Así pues, se ha demos ado que la HBS iene cualidades excepcionales incluso en
compa ación con la ma iz o iginal (es uc u a de g ano-g ande), con una mejo a de
p opiedades que se ían muy en ajosas pa a un combus ible. En onces, ¿po qué no
imi a es e ma e ial ec is alizado?. ¿Po qué no imi a es a es uc u a (HBS) que
xi

apa ece en el bo de del combus ible a al os g ados de combus ión y se in oduce en es a
o ma como un combus ible nue o en el in e io del eac o ?. ¿Po qué no sin e iza
una ma iz de combus ible mime izando la HBS que debido a su apa en e esiliencia
al daño esis i ía iempos más la gos bajo i adiación?.
Aquí es cuando empieza el desa ollo de es e p oyec o con el obje i o de sin e-
iza pol o nanoc is alino (nc)-UO2pa a la ab icación de pas illas (monoli os) de
nc-combus ible. Las piezas monolí icas ab icadas a pa i de es e pol o, end ían un
olumen de ap oximadamen e 1 cm3y una ma iz uni o me de g anos de un amaño
en e 100 y 250 nm imi ando la es uc u a que apa ece a al os g ados de combus ión
(HBS; high bu n-up s uc u e). Du an e es e abajo se consiguió la c eación de es a
nue a mic oes uc u a de combus ible pasando po dis in as ases. Desde la sín esis
del nc-ma e ial, a la ab icación de la pas illa de combus ible, dis in as e apas de
es e p oceso, p e iamen e desconocido o inexplo ado, u ie on que se especialmen e
desa olladas y/u op imizadas.
Sín esis de nc-UO2y nc-ThO2
Un abajo conside able ue dedicado al desa ollo del pol o inicial pa a la p oducción
de los monoli os o pas illas de nc-UO2imi ando la HBS que apa ece en los combus ibles
de los LWR. Dos ías de sín esis química di e en es ue on es udiadas pa a ob ene
p ecipi ados deﬂoculados de nc-UO2y nc-ThO2como compensación a la al a de
disponibilidad come cial. El ThO2 iene una es uc u a simila al UO2pe o iene, a su
ez como en aja, una sola alencia (IV). Pa a consegui ob ene mayo es can idades
de nc-UO2que las publicadas en li e a u a, y p opo ciona así ma e ial suﬁcien e pa a
la ab icación de los monoli os, ambos mé odos ue on con enien emen e ajus ados,
desa ollados y escalados de acue do a las necesidades. El ma e ial así p oducido ue
obje o de es udio median e mic oscopio elec ónico de ansmisión (TEM) y di acción
de ayos X (XRD).
El p ime mé odo desa ollado ue una p ecipi ación con olada que u iliza una
disolución acuosa elec olí icamen e educida de ni a o de u anilo como p ecu so
y una solución go eada de NaOH como agen e de alcalinización pa a desencadena
la p ecipi ación del nc-ma e ial lo más p óximamen e posible a la línea de solubil-
idad del UIV . Es e mé odo ue o iginalmen e desc i o po [Rousseau e al., 2002],
[Rousseau e al., 2006]. Un es udio in ensi o de los ango de concen ación de U y
acidez en los que se p oduce la p ecipi ación de nc-UO2+x ue lle ado a cabo. Se
u iliza on pa a ello disoluciones de ni a o de u anilo elec olí icamen e educidas,
usando U-concen aciones más ele adas (10−1M) que las obse adas en li e a u a
(10−2M) [Rousseau e al., 2006], y po an o in e alos de pH de abajo más bajos,
siemp e siguiendo la línea de solubilidad del UIV . Como esul ado se ob u ie on has a
10 g de nc-UO2+xpo expe imen o, en luga de los pocos nanog amos publicados has a
aho a en li e a u a. La ase sólida así ob enida y es udiada bajo XRD, c is alizó bajo la
ípica es uc u a de ﬂuo i a UO2- cc (g upo espacial Fm-3m), con un pa áme o de ed
a=0.5417(1) nm y un amaño de c is al p omedio de 3.79 nm, ambién en conco dancia
con el amaño medio obse ado po TEM de 3.9(8) nm. El di ac og ama p edominan e
de las mues as co espondía inequí ocamen e a UO2pe o en un es ado lige amen e
oxidado. Es o úl imo se mani es ó a a és de una con acción del pa áme o de ed
de ap oximadamen e 0.9%de la ase p ecipi ada (a=0.5417(1) nm) con espec o a los
x
alo es ípicos de UO2es equiomé ico (a=0.547 nm).
El segundo mé odo es udiado ue una descomposición é mica en ase o gánica
usando ace ilace ona o de u anilo (UAA) como p ecu so . És e se añade a una
mezcla de ácido oleico y oleilamina calen ándose a con inuación has a empe a u as
po encima de los 300°C pa a induci la p ecipi ación de las nanopa ículas de UO2
po descomposición é mica del UAA. Es e p ocedimien o ue desc i o o iginalmen e
po [Wu e al., 2006] y ue aquí modiﬁcado pa a educi la can idad de agen es
ensoac i os espec o al po cen aje de me al u ilizado. AsÌ mismo, se escala on los
0.1gdenc-UO
2po expe imen o ob enidos según lo publicado po [Wu e al., 2006],
a2.3gdenc-UO
2po expe imen o en el p esen e abajo. El mismo mé odo ué
ex apolado pa a la sín esis de nc-ThO2, u ilizando ace ilace ona o de o io (ThAA)
o ace a o de o io (ThA) como p ecu so es, ob eniéndose ﬁnalmen e nano- a illas de
ThO2. La azón po la cual se ob u ie on p ecipi ados en o ma de a illa, en ez de
la o ma es é ica ob enida pa a nc-UO2, es oda ía desconocida en es e es udio. El
endimien o po expe imen o (0.3 g nano- a illas de ThO2) ue más bajo que en la
sín esis de nc-UO2po el mismo mé odo. Tan o en la sín esis de nc-UO2como en la
de ThO2bajo es e mé odo, se p oba on di e en es condiciones de abajo: elocidad
de calen amien o, iempo de en ejecimien o de la disolución ﬁnal, empe a u a de
en ejecimien o, así como dis in os p ecu so es iniciales (ThAA y ThA) en la sín esis
de nc-ThO2. No obs an e, se encon a on siemp e esul ados simila es en é minos de
es uc u a y geome ía (es é icas pa a nc-UO2y en o ma de a illa pa a nc-ThO2)de
los p ecipi ados. En el es udio bajo el XRD se de e minó una ase sólida pe ec amen e
c is alizada con la ípica es uc u a de ﬂuo i a UO2- cc (g upo espacial Fm-3m), un
amaño de c is al p omedio (diáme o de la es e a) de 5.52 nm y un pa áme o de ed
de 0.5431(0) nm, ambién en conco dancia con el amaño p omedio obse ado con el
TEM de (4.9(3) nm) y po dispe sión dinámica de luz (DLS) de (3.7(1) nm). En las
nano- a illas de ThO2, se encon ó una es uc u a ípica de ﬂuo i a (g upo espacial
Fm-3m)ThO
2- cc, con un amaño de c is ali o (diáme o a illa) de 1.42 nm y un
pa áme o de ed de 0.5579(1) nm. No se obse ó ag egación de pa ículas en las
imágenes de TEM del ma e ial sin e izado po ambos mé odos.
C is alización y c ecimien o de g ano en (T) del nc-
UO2
La composición de los p ecipi ados ob enidos po ambos mé odos a iba mencionados
y su p opensión a la expansión é mica en el es ado no consolidado, se es udia on
en el ma e ial así sin e izado y ecocido a di e en es empe a u as. Pa a ello se
u iliza on écnicas de análisis é mico y de di acción de ayos X, como el análisis
e mog a imé ico y el análisis é mico di e encial (TGA/DTA), di acción de ayos X
(XRD) y di acción de ayos X a al a empe a u a (HT-XRD), écnicas espec oscópi-
cas ales como la espec oscopía de abso ción de ayos X (XAS), espec oscopía de
esonancia magné ica nuclea (MAS-NMR), espec oscopía in a oja (IR), y écnicas
de ca ac e ización como la mic oscopía elec ónica de ansmisión (TEM).
La e olución del amaño de los c is ales, el pa áme o de ed y la ensión de ed se
de e mina on pa a el ma e ial así sin e izado y a dis in as empe a u as de ecocido
del ma e ial (bajo a mós e a ine e) has a 1200°C. Pa a el caso del ma e ial nc-UO2
x i
p ecipi ado en ase acuosa se obse ó an sólo un pequeño aumen o en el amaño de
los c is ales, pe maneciendo és os po debajo de los 7 nm has a llega a la empe a u a
de ecocido de 700°C. A pa i de es a empe a u a, el amaño de los c is ales c eció
acusadamen e y de mane a cons an e con la empe a u a, has a alcanza un alo de
73 nm a 1200°C. Po el con a io, el inc emen o mayo del pa áme o de ed se obse ó
en el in e alo más bajo de empe a u a 20°C-700°C. En el in e alo 700°C-1200°C
sólo se obse ó un pequeño aumen o en el pa áme o de ed coincidiendo con la
expansión é mica e e sible ípica del UO2. Hay que añadi que en la mediciones del
pa áme o de ed a empe a u a ambien e y después del a amien o a empe a u a
bajo a mós e a es á ica de He, se obse ó una ecupe ación de la es uc u a ípica
c is alina del UO2, pasando del alo de pa áme o de ed inicial de 0.5417 nm pa a el
nc- UO2así sin e izado, a un alo de 0.5473 nm después de la exposición a 1200°C.
La con acción del pa áme o de ed inicialmen e medida pa a el ma e ial nc-UO2así
sin e izado po debajo del alo no mal ípico pa a UO2(a=0.5470 nm) se a ibuyó
p incipalmen e a oxidación.
En el caso del nc-UO2p ecipi ado po el mé odo en ase o gánica, se obse ó un
compo amien o simila en el c ecimien o de las pa ículas con la empe a u a, sin
apenas cambio en el amaño de c is al has a una empe a u a de 700°C (debido al
p e- a amien o a empe a u a aplicado), seguido de un c ecimien o in enso del amaño
de c is al has a ob ene un amaño de 150 nm a 1100°C. Se de e minó una oxidación
inicial de las pa ículas sin e izadas bajo el mé odo en ase o gánica de i ada de los
alo es de pa áme o de ed, aunque menos acusada que en las pa ículas sin e izadas
en ase acuosa. Las pa ículas ecob a on el alo ípico de pa áme o de ed pa a UO2
a empe a u as de ecocido po encima de los 750°C.
En cuan o al pa áme o de es és de ed, un compo amien o simila ue ambién
obse ado pa a las pa ículas ob enidas po ambos mé odos. La obse ación p incipal
en ambos casos ue la disminución con inua del pa áme o de es és con la empe a u a,
has a ex ingui se p ác icamen e a la empe a u a en que los c is ales empeza on a
c ece . Es o conﬁ maba que la p esencia del pa áme o de es és de ed ac uó en
ambos casos como inhibido del c ecimien o c is alino.
Es udio de la es uc u a y es equiome ía del oxígeno
median e XRD, XANES, EXAFS, NMR AND FTIR
La es uc u a del ma e ial nc-UO2p oducido en unción de la empe a u a pe o en es e
caso bajo a mós e a educ o a, se es udió ambién bajo el XRD y se compa ó con el
ma e ial de e e encia UIV O2-g ano-g ande (g anos de amaño mic omé ico). Se midió
el pa áme o de ed del ma e ial as en ia se después de alcanza di e en es empe -
a u as máximas (600°C y 1200°C). Es o pe mi ió la sepa ación de la con ibución de
la expansión é mica en los alo es medidos a empe a u a pa a ob ene cu as más
limpias de expansión é mica en e a empe a u a, y pa áme o de ed en e a amaño
de c is al. No se encon a on g andes di e encias en el amaño de c is al, pa áme o de
ed y ensión, en e las mediciones ealizadas bajo a mós e a ine e (a iba comen ado)
y bajo a mós e a educ o a (medición ealizada as el en iamien o) pa a el ma e ial
ob enido po el mé odo acuoso. Sin emba go. se obse ó un cambio no able en el
amaño de los c is ales pa a el ma e ial ob enido po el mé odo o gánico al llega a
x ii
empe a u as de ecocido de 1100°C bajo a mós e a de He con una alla de c is al de
150 nm, y un amaño de an sólo de 12 nm bajo a mós e a de A /5%H2. Compa ando
el ma e ial nc-UO2ob enido po ambos mé odos, acuoso y o gánico, bajo a mós e a
educ o a (A /5%H2) y sin ningún o o a amien o é mico p e io del pol o, no se
de ec a on g andes cambios has a la empe a u a de ecocido de 600°C. Pe o sí se
obse a on a la empe a u a de 1200°C, ob eniendo un amaño de 82 nm pa a el
ma e ial del mé odo acuoso en e a 12 nm pa a el ma e ial del mé odo o gánico.
Es o pod ía se a ibuido a la capa o gánica p o ec o a p esen e pa a es as úl imas
nanopa ículas.
Jun o al es udio de XRD se lle ó a cabo un es udio de XANES pa a de e mina
el es ado de oxidación de los ca iones de U, las acciones mola es co espondien es y
la elación de O/U de i ada. Los espec os de XANES en el bo de U-L3mos a on
endencias simila es pa a el nc-UO2sin e izado po ambos mé odos (acuoso y
o gánico): educción de la es equiome ía (x)delUO
2+x, al aumen a la empe a u a
de a amien o. Se es udia on mues as de nc-UO2así sin e izado y después del
a amien o é mico a 600°C y 1200°C en a mós e a de A /5%H2y se compa a on
con el ma e ial de e e encia UIV O2(g ano-g ande), de ec ándose con la empe a u a
un lige o desplazamien o del pico de la WL (whi e line) hacia ene gías más bajas, así
como un aumen o de la in ensidad y de las oscilaciones en las egiones de XANES.
La ampli ud de es as oscilaciones disminuyó con el aumen o de la empe a u a de
a amien o é mico mos ando un mayo o den de la es uc u a pa a las mues as
ecocidas.
Es e e ec o pod ía debe se al pequeño amaño de los c is ales del ma e ial nc-UO2
o al simple desplazamien o de la es equiome ía del ma e ial así p oducido espec o
al ma e ial de e e encia UIV O2(g ano-g ande). Pa a cuan iﬁca la con ibución del
amaño del c is al a es e e ec o se ía necesa io un es udio con nc-UO2de dis in os
amaños de c is al y una es equiome ía de oxígeno ﬁja (a se posible la ca ac e ís ica
del UO2de e e encia). Dado que en el momen o de ealización del es udio es a sín esis
selec i a de nc-UO2no ue posible, un es udio al e na i o pa a de e mina el e ec o
de la alla del c is al en la es equiome ía ue lle ado a cabo. Pa a ello se u ilizó un
subs i u o como es el oxido de o io (ThO2) que c is aliza con la misma es uc u a de
ﬂuo i a que el UO2y que posee además un único es ado de alencia (ca ión) ThIV .
Se lle o a cabo un es udio de una mues a de nc-ThO2así p oducido (no a ado
é micamen e). En los espec os XANES co espondien es al bo de Th-L3, el pico de la
WL co espondien e al nc-ThO2así sin e izado, se encon ó en una posición y ampli ud
idén icas a las de los espec os del ThIV O2(g ano-g ande) de e e encia. Tan sólo se
de ec ó una lige a disminución en la in ensidad y núme o de oscilaciones, indicando
solo un e ec o débil en las dis ancias in e a ómicas y el o den eﬂejado en el espec o de
XANES debido a la alla del c is al del ma e ial nc-ThO2. Es e p ác icamen e idén ico
compo amien o del ma e ial de e e encia ThIV O2(g ano-g ande) y el nc-ThO2,
sugie e que los desplazamien os obse ados an e io men e pa a el ma e ial de nc-UO2
end ían su o igen, no al amaño de pa ícula sino más bien al desplazamien o de
la alencia-ca ión hacia un es ado más oxidado (UVI) del ma e ial así sin e izado.
Teniendo es o en cuen a, la de e minación de la O/M de nc-UO2apa i dela écnica
de XANES es a ía jus iﬁcada.
En los espec os de EXAFS k3-ponde ado pa a el ma e ial nc-UO2sin e izado po
x iii
el mé odo acuoso, las oscilaciones y su ampli ud aumen a on con la empe a u a de
ecocido y el c ecien e amaño de los c is ales, ap oximándose g adualmen e al espec o
ípico pa a la es uc u a del UO2( cc). El ma e ial nc-UO2así p ecipi ado de 4 nm
esul ó di ícil de ajus a a la es uc u a de ﬂuo i a pu a. Los ajus es no e an es ables y
los da os enían mucho uido. La mues a ecocida a 600°C y amaño de c is al de 9 nm
mos ó un o denamien o in e medio con oscilaciones cla amen e iden iﬁcables. Tan o
la mues a o iginal así sin e izada de 4 nm como la mues a ecocida a 600°C (9 nm)
mos a on cla amen e dis ancias de enlace U-O y O-O más co as en compa ación con
la e e encia de UIV O2-(g ano-g ande). Es o esul ó compa ible con los es udios de
XRD que mos a on una conside able con acción del pa áme o de ed pa a la mues a
así sin e izada de 4 nm, siendo és a meno con la empe a u a de ecocido. En úl ima
ins ancia, pa a la empe a u a de ecocido de 1200°C y un amaño de c is al de 82 nm,
las oscilaciones de EXAFS ue on simila es, si no coincidie on o almen e con las del
ma e ial de e e encia UIV O2, indicando misma es uc u a- cc (Fm-3m) y mismas
dis ancias in e a ómicas. Es o conco dó con la simila es uc u a mos ada en XRD
en e la mues a de nc-UO2 ecocida a 1200°C y la e e encia de UIV O2-(g ano-g ande).
También esul ó cohe en e con el es udio de XANES que no mos ó p ác icamen e
ninguna di e encia espec o a la es uc u a ípica de ﬂuo i a pa a el caso de la mues a
nc-UO2 ecocida a 1200°C. Los esul ados de los espec os de EXAFS de k3-ponde ada
pa a el ma e ial nc-UO2de o igen o gánico, ue on dis in os a los a iba comen ados
pa a el ma e ial nc-UO2de o igen acuoso, siendo no sólo el ma e ial de 5 nm nc-UO2
así p ecipi ado di ícil de ajus a a la es uc u a de ﬂuo i a pu a, sino ambién las
mues as a adas a 600°C y 1200°C. Todas las mues as p esen a on un al o g ado de
deso den y no se pudie on ajus a al ma e ial de e e encia UIV O2-(g ano-g ande), po
lo que debe ía ene se en cuen a o a ase oda ía aquí no iden iﬁcada.
Se adqui ie on espec os de NMR Hahn-echo 17O MAS de mues as a adas a
dis in as empe a u as de ecocido en a mós e a educ o a (A /5%H2). Se iden iﬁca on
es ipos di e en es de oxígeno a pa i del ajus e del desplazamien o químico de los
egis os ob enidos pa a es as mues as, es deci , del desplazamien o del pico 17O
de ecuencia de esonancia espec o al del espécimen de e e encia y exp esado en
unidades ela i as (ppm). En el p esen e es udio se omo como e e encia la esonancia
del 17O de una mues a de H2O dopada con 17O y se deﬁnió como 0 ppm. La p ime a
iden iﬁcación co espondió a especies de oxígeno con un desplazamien o químico de
casi 900 ppm pa a las mues as ecocidas has a una empe a u a 650°C. Los o os
dos ipos de especies de oxígeno iden iﬁcados apa ecie on cla amen e en el ango de
empe a u a en e 650°C y 1200°C. Es as especies mos a on una un pico agudo y
la o a un pico 17O amplio. És os pod ían a ibui se a un 17O en un en o no más
bien c is alino y en o o en o no más deso denado (debido a la ampli ud del pico),
espec i amen e. Ambos picos disminuye on ue emen e su desplazamien o químico
y su anchu a a media al u a (FWHM; Full Wid h a Hal Maximum)enelin e alo
de empe a u a de 650°C a 800°C, pa a con e ge ápidamen e a empe a u as po
encima de 800°C a alo es ce canos a los de la mues a ecocida a 1200°C con 717 ppm
(desplazamien o químico) y 5 ppm (FWHM), espec i amen e. Es e pequeño alo de
FWHM es sinónimo de un en o no bien c is alizado, aunque sigue siendo lige amen e
más ancho que los 3 ppm encon ados pa a el UIV O2-(g ano-g ande) de e e encia. A
pesa de ello, el desplazamien o químico (717 ppm) ue el mismo que el encon ado pa a
dicha e e encia UIV O2-(g ano-g ande). Puede deci se en onces que el en o no, pa a
la mues a con el c is al de mayo amaño (∼80 nm), de las posiciones del pa áme o
de ed del oxígeno, es á muy p óximo al medido pa a la mues a de e e encia
xix

UIV O2-(g ano-g ande). Basándose en la FWHM puede deci se que pa a que la señal
de UO2-c is alino sea obse able, es necesa io llega a un amaño de c is al po encima
de los 80 nm. Es o es a ía en consonancia con la obse ación hecha median e XRD
pa a es e amaño de c is al de una es uc u a UO2- cc con pa áme o de ed 0.5472 nm.
Va ias mues as de ma e ial nc-UO2sin e izadas po el mé odo acuoso y a adas
a dis in as empe a u as-cla e de ecocido, ue on analizadas bajo el espec óme o
de FTIR. Se obse a on has a 4 picos en el in e alo 400-4000 cm−1pa a la mues a
de nc-UO2así sin e izada (RT). Es os pod ían asigna se a la ib ación bending (o
de ije e eo) del H-O-H del agua coo dinada, y a un posible es ado más oxidado del
ma e ial (UO2+x). Todos los picos disminuye on en in ensidad con la empe a u a
de ecocido de las mues as. Además, pa a la mues a a ada a la empe a u a de
ecocido de 1200°C, el espec o de IR se asemejaba al espec o de la mues a de
e e encia de UIV O2. Es o concue da con los esul ados de XANES a iba comen ados
donde se obse ó una es uc u a elec ónica di e en e pa a la mues a a ada a 600°C,
mien as que la mues a a ada a una empe a u a de 1200°C p esen ó una es uc u a
simila a la de la mues a de e e encia de UIV O2. También los esul ados del EXAFS
se ca ac e iza on po un o den pob e a 600°C, pe o pa es de oscilación o almen e
ajus ados a los de la mues a de e e encia de UIV O2pa a la mues a a ada a 1200°C.
Se lle o a cabo un es udio iso é mico pa a el ma e ial nc-UO2sin e izado, del
c ecimien o de g ano du an e pe iodos de iempo la gos y bajo el HT-XRD. Pa a las
empe a u as de ecocido de 500°C, 700°C y 900°C, bajo a mós e a es á ica e ine e
de He, el c ecimien o de g ano se p odujo en las p ime as ho as de a amien o a
empe a u a cons an e alcanzando un amaño de c is al p omedio es able a dicha
empe a u a (el c ecimien o de g ano cesó a pa i de ese momen o). En el caso de la
iso e ma a 1200°C y bajo a mós e a es á ica de He, el ma e ial p esen ó un c ecimien o
con inuo sin llega a alcanza un amaño cons an e de g ano en las p ime as 50 h. Se
ob u o una ene gía de ac i ación de la di usión en e 0.93 eV a 1.25 eV. Es os pequeños
alo es de ene gía de ac i ación ob enidos, pod ían debe se p incipalmen e a los lími es
de di usión de g ano (supe ﬁcie e in e az).
Se midió un pa áme o de ed de 0.5472 nm pa a las mues as a adas du-
an e 50 h a 900°C bajo a mós e a de A /H2(y as el en iamien o del ma e ial),
ob eniendo una alla ﬁnal de c is al de unos 50 nm. Es po ello que, en p incipio
no se ía necesa io alcanza una empe a u a de 1200°C (y po an o una alla
de c is al de 80 nm) pa a consegui un ma e ial con el ípico alo de pa áme o
de ed del UO2de g anos g andes (a=0.5472 nm), como se había comen ado más a iba.
Además, un amaño medio de c is al de 322 nm ue medido a 1200°C du an e
50 h en a mós e a es á ica de He. Teniendo es o en cuen a, du an e el p oceso de
sin e ización de los monoli os se ía necesa ia una empe a u a po debajo de 1200°C
pa a e i a el c ecimien o ex emo de las pa ículas (>200 nm). Sin emba go, se
midió un amaño ﬁnal de c is al de 85 nm pa a las mues as de nc-UO2 ecocidas a
1200°C du an e 50 h bajo a mós e a educ o a de A /H2. Incluso después de 200 h
a es a empe a u a en condiciones educ o as, se midió un amaño ﬁnal de c is al de
150 nm (bas an e meno que el alo de 322 nm obse ado bajo a mós e a de He y
50 h de a amien o). Es a di e encia pod ía debe se al es ado inicial de oxidación de
las mues as así sin e izadas de nc-UO2y su e olución bajo a mós e a es á ica e ine e
de He. Un UO2sob ees equiomé ico p esen a ía un aumen o mayo de los coeﬁcien es
xx
de au o-di usión y del ﬂujo de masa, inc emen ando así el mo imien o en el lími e de
g ano (o c is al) y c ecimien o del g ano. De hecho las di e encias en e el coeﬁcien e
de di usión del UO2de g anos g andes y el nc-UO2, son compa ibles con una mejo a
de los p ocesos di usión, ya sea po e ec o del amaño de g ano o po el a io O/U>2.
Consolidación y ca ac e ización de monoli os de nc-
UO2
Se p oba on di e en es u as al e na i as pa a la consolidación de los monoli os (ej.
p ensado con encional uniaxial, ﬂoa packing, e c). Las pas illas así p ensadas, ue on
sin e izadas a empe a u as en e 900°C y 1200°C bajo a mós e a de A /H2. Las
condiciones óp imas de sin e ización se deduje on a pa i del es udio de c ecimien o
de c is al iso e mo du an e la gos pe iodos de iempo bajo a mós e as de He y de
A /H2. Es o e i ó el iesgo de un c ecimien o de g ano desp opo cionado incluso a
la empe a u a más al a es udiada de 1200°C. En algunos casos se p ac icó ambién
un p e- a amien o é mico del pol o de nc-UO2pa a e i a la o mación de g ie as
du an e la e apa de sin e ización debido a la p esencia de agua o compues os o gánicos
en el ma e ial dependiendo de la sín esis u ilizada. Las pas illas sin e izadas p e-
sen a on una apa iencia ue e aunque se podían obse a g ie as ﬁnas en algunas
de ellas. Se ob u ie on densidades de sin e ización en e 75.5-90.5%de la densidad
eó ica (TDUO2=10.96 g/cm3). Se ob u o un amaño de g ano p omedio de ∼200 nm,
eplicando la es uc u a que apa ece a al os g ados de combus ión (HBS; high bu n-up
s uc u e) pa a los di e en es ipos de monoli os de nc-UO2sin e izados.
También se lle a on a cabo expe imen os de dila ome ía donde se compa ó la
con acción en e la pas illa ab icada con ma e ial de nc-UO2, con la pas illa ab icada
con el ípico UO2-(g ano-g ande) y a pa i del p oceso es ánda de ab icación.
Se obse a on mejo es ac i idades de sin e ización a empe a u as in e io es pa a
el ma e ial nanoc is alino en compa ación con las medidas pa a las pas illas de
UO2-(g ano-g ande). El ango de empe a u a desde el inicio has a la comple a
densiﬁcación, ocu ió a empe a u as mucho más bajas pa a las pas illas de nc-UO2
(200-955°C, con un a io máximo de sin e ización a 740°C), en compa ación con las
pas illas de UO2-(g ano-g ande) [Lahi i e al., 2006] (900-1540°C, con un máximo
de sin e ización a 1200°C). Es o pod ía debe se a la mayo supe ﬁcie p esen e en el
ma e ial de nc-UO2compa ado con el ípico UO2-(g ano-g ande), lo que lle a ía a una
sin e ización más e ec i a (a empe a u as más bajas). Se encon ó una ene gía de
ac i ación de la sin e ización de Q= 171 ±7kJ/mol asumiendo di usión de supe ﬁcie,
yQ= 114 ±5kJ/mol asumiendo di usión de olumen pa a el monoli o de nc-UO2.
La ene gía de ac i ación de e minada pa a un monoli o de UO2de g ano g ande es de
Q= 287 kJ/mol según [Lahi i e al., 2006]. Ambos mecanismos de di usión mos a on
pues alo es bajos pa a las ene gías de ac i ación de sin e ización como es ípico pa a
los nano-ma e iales. Es o se aduce en una cla a en aja ecnológica en la ab icación
de monoli os de nc-UO2debido a su al a capacidad de densiﬁcación a empe a u as
bajas. El man enimien o de un ango de empe a u as acep ablemen e bajo du an e el
p oceso de sin e ización disminui á cos os y simpliﬁca á la ecnología de ab icación.
Los monoli os de nc-UO2o ecen ambién la posibilidad de ajus a el amaño de g ano
a olun ad median e la a iación de las empe a u as y iempos de sin e ización.
xxi
Las mac oes uc u as de los dis in os monoli os de nc-UO2se ca ac e iza on po
mic oscopía óp ica (OM). Las mic oes uc u as se ca ac e iza on po obse ación de
la ac u a- esca a dis in as ampliaciones bajo el SEM. En las imágenes de SEM de
la ac u a- esca de las pas illas sin e izadas a la empe a u a más baja de 900°C,
se obse a on g anos no del odo deﬁnidos. La sin e ización de los c is ales pa ecía
oda ía en ase de desa ollo, po lo que a pa i de es e momen o se u iliza on
empe a u as de sin e ización de 1200°C. Se obse a on mac o-ﬁsu as en algunas de las
pas illas, pe o no pa a los monoli os ab icados con nc-UO2sin e izado po el mé odo
o gánico con pol o con p e- a amien o é mico, y ampoco pa a la mues a del mé odo
de consolidación ﬂoa -packing y pol o sin e izado po el mé odo acuoso. Todas las
mac oes uc u as, con excepción de es a úl ima, mos a on densiﬁcación no homogénea
(po osidad esidual en e las zonas densiﬁcadas). También la mues a con p ensado
con encional de pol o nc-UO2p e-deshid a ado sin e izado po el mé odo acuoso,
mos ó una buena calidad en compa ación con el es o, desde el pun o de is a de la
densiﬁcación. Sin emba go, es necesa io la in oducción de mejo as en la o mación de
los monoli os pa a e i a el p oblema de las g ie as du an e el sin e izado (y po lo
an o disminución de las p opiedades del ma e ial). En cuan o a la mic oes uc u a
de la supe ﬁcie de ac u a- esca, la pas illa de nc-UO2sin e izado po el mé odo
acuoso y ab icada po consolidación ﬂoa -packing y sin e ización a 1200°C, ue la
ap oximación más ce cana al ma e ial HBS ob enido has a aho a. El amaño medio de
g ano pa a los di e en es monoli os es u o en e 170 nm y 250 nm. Es o ue un de los
g andes log os de es e abajo.
P opiedades mecánicas como la du eza Vicke s (HV), du eza Knoop (HK) y módulo
de Young (E) se de e mina on pa a las pas illas de nc-UO2sin e izadas. Un aumen o
en la du eza (HV) y alo es bajos pa a módulo-E(de has a un 30%) ue on en gene al
obse ados pa a los di e en es monoli os de nc-UO2en compa ación con aquellos
de UO2-(g ano-g ande). También se u ilizó mic oscopía de ba ido acús ico (SAM)
pa a la es imación y la compa ación del módulo de Young ob enido po iden ación.
Los esul ados ob enidos po SAM (E=155 GPa) coincidie on con los de i ados po
mic o-inden ación (E=155 GPa). La di e encia obse ada con espec o a pas illas
de g ano-g ande de UO2(220 GPa), pod ía es a inﬂuenciada po las impe ecciones
de la mic oes uc u a (nano-ca idades en las in e secciones de es g anos, po os,
g ie as, e c.). Sin emba go, es a caída del módulo es oda ía demasiado g ande
como pa a se o almen e a ibuida a la p esencia de ca idades. El mismo ipo de
endencia obse ado en las mues as de nc-UO2,esdeci ,aumen odelos alo esde
HVy disminución de los alo es del módulo-E, ya se había de e minado an es en
el combus ible es ánda de UO2 as habe alcanzado alo es ele ados de BU. En
es e caso la disminución del módulo-E ampoco pudo se o almen e a ibuída a un
aumen o de la po osidad, y con adijo el e ec o de la disolución de los p oduc os de
ﬁsión que p o oca en ealidad un aumen o de la igidez del ma e ial. Dado que los
combus ibles nuclea es i adiados se ans o man en una es uc u a nano- ec is alizada
con el aumen o de BUs [Spino e al., 2012], los e ec os (pa cial) del aumen o de HV
(la disolución de los p oduc os de ﬁsión p o ocan ambién endu ecimien o) y además
la disminución del módulo-E(sumado al causado po la po osidad), al igual que los
e ec os obse ados en el p esen e abajo, pod ían a ibui se a la nano-es uc u a de
los combus ibles some idos a ele ados BUs.
Se conﬁ mó con éxi o la dependencia con el amaño del c is al, de las p opiedades
ísico-químicas del nc-UO2. Así, se comp obó que la comp esibilidad del nc-UO2e a
xxii
de hecho mayo que la del es ánda -UO2de amaño g ande. Se conﬁ mó ambién
una dependencia de las p opiedades de expansión é mica con el amaño del c is al
pa a el ma e ial de nc-UO2. La expansión é mica aumen ó con la disminución del
amaño de c is al, al mismo iempo que el módulo de comp esibilidad disminuyó. Es o
es compa ible con la elación G üneisen que p esen a un p oduc o cons an e en e la
conduc i idad é mica y el módulo de comp esibilidad. Sin emba go sigue pendien e la
e iﬁcación de es a endencia sob e el calo especíﬁco (Cp), necesa ia pa a comple a
el análisis de la elación de G üneisen.
En cuan o a la comp esibilidad del ma e ial bajo di acción de ayos X in si u
de al a p esión (HP-XRD), se ealizó un es udio de la dependencia del módulo de
comp esibilidad con el amaño de c is al pa a el ma e ial nc-UO2. Se es udia on es
amaños de nc-UO2di e en es (4 nm, 6 nm y 34 nm) has a una p esión de 27 GPa,
y se de e mina on las cons an es de comp esibilidad co espondien es B0yB
0.El
módulo de comp esibilidad del UO2su ió una disminución ex ema pa a las pa ículas
de amaño den o del ango nanomé ico. Pa a las pa ículas de nc-UO2de 4 nm se
obse ó un módulo de comp esibilidad (B0)en o noaun40%meno que el medido
UO2-g ano-g ande (g anos de amaño mic omé ico) [Pujol e al., 2004]. Es o conﬁ mó
la dependencia del módulo de comp esibilidad con el amaño de las pa ículas. Sin
emba go, un es udio con pa ículas de amaños mayo es que los aquí conside a-
dos (>34 nm) se ía necesa io pa a ga an iza que la endencia obse ada en es os
monoli os (disminución del módulo de elas icidad) se debe al amaño de los g anos,
y no sólo debido a las impe ecciones y po osidad posiblemen e p esen e en las mues as.
Los esul ados de las p uebas de di usi idad é mica pa a el ma e ial de nc-UO2
compac ado mos a on un compo amien o simila al del ma e ial UO2-es ánda
(mic o-g ano). La di usi idad é mica pa a las pas illas sin e izadas de nc-UO2
(∼200 nm, 90%densidad), se de e minó en el ango de empe a u a 254°C a 1165°C.
Se hizo una ex apolación de los esul ados ob enidos has a una densidad de 95%yse
encon ó la misma di usi idad é mica que en las pas illas ab icadas con es ánda -UO2
(g ano-g ande) y densidad del 95%[Fink, 2000]. Respec o al emido empeo amien o
de la conduc i idad é mica del ma e ial en la HBS debido al e ec o del amaño de
g ano ( esis encia Kapi za), quedó aquí demos ado el no-de e io o de las p opiedades
é micas pa a las pas illas de UO2con un amaño de g ano de 200 nm imi ando la HBS.
Se lle o a cabo la de e minación del pun o de usión po calen amien o-láse y
de ección de la empe a u a pi omé ica pa a nc-UO2-compac ado de dos amaños
di e en es de nano-g ano (ap oximadamen e 10 nm y 200 nm), e aluándose su a iación
con espec o al UO2-es ánda de g ano g ande. Se encon ó una disminución del pun o
de usión pa a el compac o con ma e ial nc-UO2-(10 nm), de ap oximadamen e 150°K
con espec o al alo ípico pa a UO2-es ánda . Es a educción se ía a p io i debido
al amaño nano de los g anos. Sin emba go, el pa áme o de ed medido pa a dicha
mues a an es de aplica la usión (a=0.5438 nm) esul ó in e io al alo ípico de la
e e encia UO2-es ánda (a=0.547 nm), indicando po an o la p esencia de un óxido
sob e-es equiomé ico el cual ambién pod ía se causan e de es a disminución del
pun o de usión. Pa a co obo a la endencia medida con la educción de amaño
de g ano, se ía necesa ia una mues a de nc-UO2es ic amen e es equiomé ica. Sin
emba go pa a el compac o con ma e ial nc-UO2-(200 nm) se encon ó un pun o de
usión igual al de la e e encia UO2-es ánda . Una es equiome ía de O/M=2.00 del
pa áme o de ed ue medida pa a es a mues a an es de p o oca la usión. És e es
xxiii
diame e ) o 1.42 nm and a la ice pa ame e o 0.5579(1) nm, was ound. In bo h cases,
no agg ega ion o he p ecipi a ed nanopa icles has been obse ed on he TEM images.
C ys alliza ion and g ain g ow h in (T) o nc-UO2
To s udy he composi ion o he p ecipi a es ob ained by bo h me hods abo e
men ioned and hei p opensi y o he mal g ow h in he unconsolida ed s a e, u he
analysis o he p ecipi a ed ma e ial annealed a diﬀe en empe a u es was pe o med
by applying he he mal analy ical and X- ay diﬀ ac ion echniques like he mog a i-
me ic analysis and diﬀe en ial he mal analysis (TGA/DTA), X- ay diﬀ ac ion (XRD)
and high empe a u e X- ay diﬀ ac ion (HT-XRD), and spec oscopic echniques such
as X- ay abso p ion spec oscopy (XAS), magic angle spinning nuclea magne ic eso-
nance spec oscopy (MAS-NMR) and in a ed spec oscopy (IR) and cha ac e iza ion
echniques like ansmission elec on mic oscopy (TEM).
The e olu ion o he c ys alli e size, he la ice pa ame e and he la ice s ain
we e de e mined om ambien empe a u e up o 1200°C unde ine a mosphe e.
Fo he aqueous p ecipi a ed nc-UO2, only a weak eﬀec o empe a u e on he
c ys alli e size occu ed below 700°C, emaining his below 7 nm in his empe a u e
ange. On exceeding 700°C, he c ys al size g ew, howe e , s eadily wi h empe a u e,
o each he alue o 73 nm a 1200°C. Opposi e, he s onges la ice pa ame e
inc ease was measu ed in he lowes empe a u e ange 20°C-700°C, whe eas in he
empe a u e ange 700°C-1200°C only a weak la ice expansion was obse ed, which
almos coincided wi h he e e sible he mal expansion o UO2. Thus, on he base
o measu emen s done a e cooling, a eco e y o he UO2 ypical c ys al s uc u e
was achie ed du ing his annealing unde s a ic He a mosphe e, passing om he
ini ial la ice pa ame e alue o 0.5417 nm o he as-p oduced nc-UO2, o he alue
o 0.5473 nm a e exposu e o 1200°C. The e iﬁed ini ial la ice con ac ion o he
as-p oduced nc-UO2below he no mal alue o bulk s oichiome ic UO2(a=0.5470 nm)
is a ibu ed mainly o oxida ion.
Fo he o ganic p ecipi a ed nc, a simila pa icle-g ow h beha iou wi h empe a-
u e was obse ed, wi h almos no c ys al-dimension changes up o 700°C (because o
he p e- he mal ea men pe o med), ollowed by an in ense c ys al-g ow h be ween
his empe a u e h eshold and he ﬁnal annealing empe a u e o 1100°C, ob aining
a ﬁnal c ys al size o 150 nm. As o he de i ed oxygen s oichiome y om he
la ice pa ame e alues, also an ini ial oxida ion o he nc-pa icles p oduced by he
o ganic me hod was conﬁ med, al hough in lowe ex en as o he case o he pa icles
p oduced by he aqueous me hod. The pa icles eco e ed he no mal la ice dimension
o bulk s oichiome ic UO2 o annealing empe a u es abo e 750°C.
As o he de e mined la ice s ain, also a simila beha iou was obse ed o pa i-
cles ob ained om bo h p epa a ion me hods. The main obse a ion in bo h cases was
ha he la ice s ain dec eased con inuously wi h empe a u e, un il being p ac ically
ex inguished a he empe a u e a which he boos ed c ys al g ow h s a ed. This
conﬁ med he la ice s ain o ha ing ac ed in bo h cases as c ys al-g ow h inhibi o .
xxx

S uc u e and oxygen-s oichiome y s udies by XRD,
XANES, EXAFS, NMR AND FTIR
The s uc u e o he p oduced nc-UO2ma e ial as a unc ion o empe a u e and, in
his case unde educing a mosphe e, was also s udied by XRD and compa ed o he
e e ence bulk-UIV O2. The la ice cons an o he ma e ial in he cooled s a e a e
eaching diﬀe en maximum empe a u es (600°C and 1200°C) was measu ed. This
allowed he sepa a ion o he he mal expansion con ibu ion in he high- empe a u e
alues o ob ain cleane cu es o he mal expansion s. empe a u e and la ice
dimension s. c ys al size. No big diﬀe ences in c ys al size, la ice and s ain, we e
obse ed be ween measu emen s made unde ine (abo e commen ed) and educing
a mosphe es (measu emen a e cooling) o he ma e ial ob ained by he aqueous
me hod. Howe e , a no able change in he c ys alli e size was obse ed o he ma e ial
ob ained wi h he o ganic me hod a 1100°C, which showed a size o 150 nm unde
He and a size o only 12 nm unde A /5%H2. Compa ing he aqueous and o ganic
p oduced ma e ial unde educing a mosphe e (A /5%H2) and wi hou p e- he mal
ea men , no big change was obse ed un il 600°C anneal, bu a 1200°C. A he
las empe a u e, a size o 82 nm was measu ed o he aqueous me hod ma e ial
compa ed o he 12 nm ob ained o he pa icles om he o ganic me hod a he same
empe a u e unde educing a mosphe e (A /5%H2), we e measu ed. Tha could be
asc ibed o he su ace laye p o ec ing he o ganic p ecipi a ed nanopa icles.
In addi ion o he XRD s udies, XANES was used o de e mine he oxida ion s a e
o he U ca ions and he co esponding mola ac ions and he de i ed O/U a ios.
The XANES spec a a he U-L3edge o he aqueous me hod ma e ial and o he
o ganic me hod ma e ial, showed simila imp o ing ends wi h inc easing empe a u e
and as he s oichiome y shi (x) dec eased (UO2+x). The samples s udied we e
nc-UO2as p oduced and a e he mal ea men a 600°C and 1200°C unde A /5%H2.
Compa ed o he e e ence sample o bulk (la ge g ain) UIV O2ma e ial, he peak o
he WL shi ed sligh ly o lowe ene gies and inc eased in in ensi y, and he oscilla ions
wi hin he XANES egions inc eased. The ampli ude o hese oscilla ions dec eased
wi h he inc easing empe a u e o he mal ea men showing a highe s uc u al
o de o hese annealed samples.
This eﬀec could be ei he due o he small c ys al size o nc-UO2samples o o
he s oichiome y shi o he syn hesised ma e ial. To quan i y hese con ibu ions
a dedica ed s udy wi h nc-UO2wi h ﬁxed oxygen s oichiome y and diﬀe en c ys al
sizes would be needed. Since a his momen his kind o selec i e syn hesis o nc-UO2
was no possible, an al e na i e sepa a e s udy o he size eﬀec in he s oichiome ic
nano-oxide-ma e ial was a emp ed using he subs i u e ho ium dioxide (ThO2),
known o c ys allize wi h he same ﬂuo i e s uc u e as UO2and o main ain a unique
ca ion- alence s a e ThIV .
In his wo k, a s udy o as-p oduced nc-ThO2(no he mally ea ed) was done.
In he co esponding XANES spec a a he Th-L3edge, he peak o he WL co e-
sponding o nc-ThO2a RT (as-p oduced) had an iden ical posi ion and ampli ude
as he one o he e e ence spec a o la ge-g ain bulk ThIV O2. Only a sligh peak
in ensi y dec ease and somewha ewe oscilla ions we e de ec ed, which indica ed only
a weak eﬀec o he c ys al size on he in e a omic dis ances and o de ing eﬂec ed
in he XANES spec a. This iden ical beha iou o he la ge g ain ThIV O2and he
xxxi
nc-ThO2sugges ed ha he displacemen s obse ed o me ly o nc-UO2would ha e
been no due o he pa icle size, bu a he o he shi o he ca ion- alence owa ds
he oxidised s a e (UVI). Ha ing ha in o accoun , de e mining he O/M o nc-UO2
om he XANES shi seems o be jus iﬁed.
In he k3-weigh ed EXAFS spec a o nc-UO2pa icles om he aqueous me hod
he oscilla ions and hei ampli ude inc eased wi h he annealing empe a u e and he
esul ing g owing c ys al size, app oaching g adually hose ypical o he UO2( cc)-
s uc u e. The as-p ecipi a ed 4 nm as-p ecipi a ed sample was e y diﬃcul o ﬁ
wi h a pu e ﬂuo i e s uc u e, as he ﬁ s we e non s able and he da a noisy. The
600°C annealed 9 nm sample showed an in e media e o de ing wi h clea ly iden iﬁed
oscilla ions. Bo h he o iginal 4 nm-sample and he 600°C-annealed 9 nm-sample
showed clea ly sho e U-O and O-O bond-dis ances compa ed o he e e ence
bulk-UIV O2sample. This was compa ible wi h he XRD s udies showing conside able
la ice con ac ion o he as- ecei ed sample and in lowe ex en , wi h in ensi y
dec easing wi h empe a u e, o he annealed samples below 1200°C. Ul ima ely, o
pa icles annealed a 1200°C and wi h a c ys al size o 82 nm, he EXAFS oscilla ions
we e simila , i no en i ely ma ching, o hose o he bulk-UIV O2, indica ing he
same cc-s uc u e (Fm-3m) and same in e a omic dis ances and subs an ial c ys al
pe ec ion. Tha was in ag eemen wi h he s uc u e-simila i y shown in he XRD
analysis be ween he nc-UO2sample annealed a 1200°C and he e e ence la ge-g ain
bulk-UIV O2sample; and was also consis en wi h he XANES s udies, showing no
depa u e om he ﬂuo i e s uc u e o he ully annealed UO2nanopa icles. In he
k3-weigh ed EXAFS spec a o he UO2nanopa icles om he o ganic o igin, he
esul s we e diﬀe en as abo e, being no only he as-p ecipi a ed 5 nm sample e y
diﬃcul o ﬁ wi h a pu e ﬂuo i e s uc u e, bu he samples ea ed a 600°C and
1200°C, oo. All samples p esen ed a high deg ee o diso de and could no ma ch a
all he e e ence signa u e o bulk-UIV O2, wi h he meaning ha ano he uniden iﬁed
phase mus be aken in o accoun in his case.
NMR Hahn-echo 17O MAS spec a could be acqui ed o samples p epa ed
by he aqueous me hod a e annealing a diﬀe en empe a u es unde educing
a mosphe e (A /5%H2). Th ee diﬀe en oxygen en i onmen s could be iden iﬁed
om he ﬁ ing o he chemical-shi signa u es o hese samples, i.e., he eco ds o
he 17O- esonance- equency peak displacemen wi h espec o ha o a e e ence
specimen, exp essed in ela i e uni s (ppm). In he p esen case, he 17O- esonance
o a 17O-dopped H2O sample was aken as e e ence, and deﬁned as 0 ppm. The ﬁ s
iden iﬁca ion co esponded o oxygen species ha ing a chemical shi o nea ly 900 ppm
and was ound o samples annealed up o 650°C. The wo o he ypes o oxygen
species iden iﬁed appea ed clea ly in he empe a u e ange 650°C-1200°C. These new
species, i.e., one showing a sha p and he o he a b oad 17O-peak, could be espec i ely
a ibu ed o 17O in a well c ys alline en i onmen and in a mo e diso de ed one; he
las due o he la ge peak b oadening. Bo h peaks diminished s ongly hei chemical
shi s and hal -maximum wid hs in he empe a u e ange 650°C-800°C, o con e ge
apidly a empe a u es abo e 800°C o alues nea hose o he sample annealed a
1200°C, i.e., espec i ely, 717 ppm (chemical shi ) and 5 ppm (FWHM), which due o
e y small peak b oadening (FWHM) indica ed a e y well c ys allized en i onmen .
This las was s ill sligh ly bigge han he 3 ppm ound o UIV O2-bulk. Despi e his,
he chemical shi (717 ppm) was he same as ha ound o UIV O2-bulk. Hence,
one can say ha he en i onmen a ound he oxygen la ice posi ions in he case
xxxii
o he sample wi h he bigges c ys alli e size (∼80 nm) was e y close o ha o
UIV O2-bulk. Based on he FWHM, one can say ha o obse e he signal o c ys alline
UO2a c ys alli e size abo e 80 nm should be eached. This is in line wi h he ob-
se a ion by XRD o an UO2- cc s uc u e wi h la ice pa ame e 0.5472 nm in his case.
Se e al samples om he aqueous me hod a key annealing empe a u es we e also
analysed unde he FTIR spec ome e . In he case o nc-UO2in he as-p oduced
condi ion (RT), ou peaks in he ange 400-4000 cm−1could be obse ed. They could
be assigned o he bending ib a ion o H-O-H o he coo dina ed wa e , and o a
possible mo e oxidised s a e (UO2+x). All hese peaks diminished in in ensi y wi h
he annealing empe a u e. Hence, a 1200°C he IR spec a looked like he one o
he UIV O2 e e ence sample. Tha was also in ag eemen wi h he abo e commen ed
XANES esul s whe e a diﬀe en elec onic s uc u e was seen a 600°C, while a
1200°C a simila s uc u e o bulk-UIV O2was ound. Also EXAFS was cha ac e ized
by poo o de ing a 600°C bu en i ely ma ching wi h he bulk-UIV O2oscilla ion pai s
a 1200°C.
Iso he mal g ain-g ow h s udy o he syn hesized nc-UO2was hen pe o med by
XRD and HT-XRD. Fo he annealing empe a u es o 500°C, 700°C and 900°C and
a s a ic and ine a mosphe e o He, he g ain g ow h ook place in he ﬁ s hou s
o iso he mal hold un il a s able a e age c ys al size was es ablished a he applied
empe a u e, a which ime g ain g ow h ceased. Fo he iso he m a 1200°C and a
s a ic a mosphe e o He, he ma e ial had a con inuous g ow h no eaching a cons an
g ain alue in he ﬁ s 50 h. An ac i a ion ene gy o diﬀusion o 0.93 eV o 1.25 eV
was ob ained. The low ac i a ion ene gies ob ained could be ela ed p edominan ly o
g ain bounda y (su ace and in e ace) diﬀusion.
A la ice pa ame e o abou 0.5472 nm was al eady ound o he samples ea ed
a 900°C a e 50 h dwell ime unde A /H2ob aining a ﬁnal size abou 50 nm.
The e o e a empe a u e o 1200°C (and in consequence a ﬁnal c ys alli e size o
80 nm) would be, in p inciple no necessa y o each he ypical la ice pa ame e o
he e e ence la ge-g ained UO2(a=0.5472 nm), as abo e commen ed.
An a e age c ys al size o 322 nm was measu ed a e cooling o he hea ea men
a he highes empe a u e o 1200°C a e 50 h dwell ime unde He. Taking ha
in o accoun , i appea s ha a empe a u e below 1200°C would be necessa y in he
sin e ing p ocess o he monoli hs o a oid ex eme g ow h o he pa icles (>200 nm).
Ne e heless o he nc-UO2samples annealed a 1200°C du ing 50 h unde A /H2
dynamic a mosphe e, a ﬁnal c ys al size o 85 nm was measu ed a e cooling. E en
a e 200 h dwell ime a his empe a u e unde educing a mosphe e, a ﬁnal c ys al
size o 150 nm was seen (qui e a om he 322 nm obse ed unde He a mosphe e
a e 50 h). This diﬀe ence could be due o he ini ial oxida ion s a e o he nc-UO2
samples and hei e olu ion unde a s a ic He a mosphe e. An hype s oichiome ic
UO2would p esen a s onge inc ease o he sel -diﬀusion coeﬃcien s and in he same
way aise he mass-ﬂow, o which enhanced g ain-bounda y mo ion and g ain (o
c ys al) g ow h will occu . In ac he diﬀe ences in he diﬀusion coeﬃcien be ween
bulk-la ge-g ain-UO2and nc-UO2a e compa ible wi h an enhancemen o he diﬀusion
p ocesses ei he by a diminishing o he g ain size o by O/U>2 eﬀec s.
xxxiii
nc-UO2monoli h consolida ion and cha ac e iza ion
Diﬀe en al e na i e ou es o consolida ion in o g een bodies (e.g. con en ional
uniaxial p essing, ﬂoa packing, e c.) ha e been es ed. A e wa ds he g een bodies
we e sin e ed a empe a u es be ween 900°C and 1200°C unde A /H2a mosphe e.
The op imum sin e ing condi ions we e deduced om he long-iso he mal c ys alli e
g ow h s udies unde He and A /H2a mosphe e. This ensu ed lack o disp opo iona e
g ain g ow h isks e en a he highes empe a u e used o 1200°C. Also he mal
p e-condi ioning o he powde be o e p essing was in some cases done o a oid c acks
du ing he sin e ing s ep due o he p esence o wa e o o ganics (depending on he
case) in he ma e ial. The pelle s sin e ed p esen ed a s ong appea ance al hough ﬁne
c acks we e isually obse able in some cases. Sin e ing densi ies be ween 75.5-90.5%o
he heo e ical densi y (TDUO2=10.96 g/cm3), we e ob ained. An a e age g ain size o
∼200 nm, eplica ing he HBS, was ob ained o all he diﬀe en sin e ed nc-UO2pelle s.
Addi ional dila ome y expe imen s we e pe o med o compa e he sh inkage
o he ab ica ed nc-UO2pelle wi h ha o bulk-UO2(la ge g ain) p oduced by
a s anda d ab ica ion p ocess. Enhanced sin e ac i i ies o he nanoc ys alline
ma e ials compa ed o mic oc ys alline UO2we e ound a lowe empe a u es. The
empe a u e ange om onse o comple ion o he densiﬁca ion occu ed a much
mo e lowe empe a u es o he nc-UO2(200-955°C, wi h a maximum sin e ing a e a
740°C), compa ed o he bulk-UO2[Lahi i e al., 2006] (900-1540°C, wi h a maximum
sin e ing a 1200°C). The eason o ha migh be he highe su ace p esen in he
nc-UO2compa ed wi h he bulk-UO2ma e ial, ende ing he sin e ing o become mo e
eﬀec i e (a lowe empe a u es). The sin e ing ac i a ion ene gy was de e mined as
Q= 171 ±7kJ/mol assuming su ace diﬀusion and Q= 114 ±5kJ/mol assuming
olume diﬀusion o he nc-UO2monoli h, compa ed o Q= 287 kJ/mol de e mined
o bulk-UO2in he li e a u e [Lahi i e al., 2006]). Bo h diﬀusion mechanisms showed
low alues o he sin e ing ac i a ion ene gies as ypical o nanopowde s. Tha means
a clea echnological ad an age in he ab ica ion o nc-UO2monoli hs due o i s
high densiﬁca ion capaci y a low empe a u es. Fu he mo e, he nc-UO2oﬀe ed he
possibili y o adjus ing he g ain size a will by a ying sin e ing empe a u es and
imes. Main aining an accep ably low empe a u e ange in he sin e ing p ocess, i
will diminish he cos s and simpli y he manu ac u ing echnology.
Cha ac e iza ion o mac os uc u es by op ical mic oscopy (OM), and mi-
c os uc u es by esh- ac u e obse a ion by SEM, o diﬀe en samples a diﬀe en
magniﬁca ions, was pe o med. No well deﬁned g ains we e obse able in he
esh- ac u e SEM images o he pelle s sin e ed a low empe a u e o 900°C. The
sin e o he c ys als was s ill unde de elopmen , he e o e sin e ing empe a u es o
1200°C we e used a e wa ds. Mac oc acks ac oss diﬀe en samples we e obse ed,
bu no o he monoli hs om nc-UO2syn hesized by he o ganic- ou e wi h powde
he mal p e- ea men , and no o he monoli hs om he ﬂoa -packing consolida ion
me hod and powde o he aqueous-syn hesis. All mac os uc u es, wi h excep ion
o he las one, showed non-homogeneous densiﬁca ion ( esidual po osi y be ween
densiﬁed a eas). Also he con en ional p essed sample o p e-dehyd a ed powde
om he aqueous ou e showed a good quali y in compa ison o he es , om he
poin o iew o he densiﬁca ion. Howe e , imp o emen s in he pe o mance o he
monoli hs would be necessa y o a oid he p oblem o c acks in he sin e ed pelle s
(and he e o e he diminishing ma e ial’s p ope ies). Looking a he esh ac u e
su aces, he mic os uc u e o he aqueous- ou e-powde pelle p oduced by ﬂoa
xxxi
packing consolida ion and sin e ing a 1200°C, was he closes app oxima ion o he
HBS ma e ial ob ained un il now. The a e age g ain size o he diﬀe en mono-
li hs was in he 170 nm o 250 nm ange. He e a majo success o his wo k was achie ed.
Mechanical p ope ies as Vicke s Ha dness (HV), Knoop Ha dness (HK)and
Young’s modulus (E) we e de e mined o sin e ed nc-UO2pelle s. An inc ease in
ha dness (HV) and low alues o E-modulus (up o 30%) we e in gene al seen o
he diﬀe en nc-UO2monoli hs in compa ison wi h bulk-UO2. Also scanning acous ic
mic oscopy (SAM) was used o he es ima ion and compa ison o he Young’s E-
modulus ob ained by inden a ion. The esul s by SAM (E=150 GPa) ma ched he ones
de i ed om mic oinden a ion (E=155 GPa). This diﬀe ence obse ed wi h espec
o bulk-UO2pelle s (220 GPa), could be inﬂuenced by mic os uc u e impe ec ions
(nanoca i ies a iple-g ain junc ions, po es, c acks, e c.). Howe e , he d op was s ill
oo la ge o be a ibu ed only o he p esence o ca i ies. The same ype o endency
obse ed in he nc-UO2specimens, i.e. wi h inc ease o HV alues and dec ease o
he E-modulus alues, has been ound be o e in i adia ed s anda d-UO2 uel a high
BUs. In his case also he E-modulus dec ease could no be ully a ibu ed o a
po osi y inc ease and was o con adic he eﬀec o he ﬁssion p oduc s dissolu ion,
which causes in eali y an inc ease o he ma e ial’s s iﬀness. Since wi h he inc ease
o BU he i adia ed nuclea uels ans o m in o a nano- ec ys allized s uc u e
[Spino e al., 2012], he eﬀec s o (pa ial) HV-inc ease (ﬁssion p oduc s dissolu ion
causes as well ha dening) and addi ional E-modulus dec ease (beside ha caused by
po osi y) like he eﬀec s obse ed in he p esen wo k could be a ibu ed in high BU
uels due o he nanos uc u e.
The conﬁ ma ion o he size-dependen physical-chemical p ope ies o nc-UO2has
been success ully accomplished. So he comp essibili y o nc-UO2wasp o edin ac o
be la ge han ha o s anda d-UO2. A size-dependence o he he mal expansion p op-
e ies o nc-UO2was also conﬁ med. The he mal expansion was shown o inc ease wi h
he size-dec ease, a he ime ha he bulk modulus dec eased. This is compa ible wi h
he G üneisen ela ionship showing a cons an p oduc be ween he he mal conduc i -
i y and he bulk modulus. Howe e , e iﬁca ion o he end in he speciﬁc hea (Cp)
is s ill lacking, which is indeed neeeded o comple e he G üneisen- ela ionship analysis.
Rega ding he ma e ial’s comp essibili y, in-si u high p essu e X- ay diﬀ ac ion
(HP-XRD) has been pe o med o he s udy o he bulk modulus dependence on he
c ys al size in nc-UO2. Th ee diﬀe en nc-UO2sizes (4 nm, 6 nm and 34 nm) we e
s udied up o a p essu e o 27 GPa and he co esponding comp essibili y cons an s
B0and B
0de e mined. The bulk modulus o UO2suﬀe ed an ex eme dec ease in
he nano-size pa icle ange. Fo he 4 nm-size nc-UO2-pa icles, a bulk modulus
(B0) a ound 40%lowe han he one measu ed o bulk-UO2(mic on-size g ains)
[Pujol e al., 2004], has been obse ed. This conﬁ med he dependence o he bulk
modulus wi h he c ys alli e size. Howe e , s udies wi h bigge pa icle sizes as he
ones he e s udied (>34 nm) would be necessa y o gua an ee ha he endency
obse ed in he monoli hs (dec ease o E-modulus), is due o he size o he g ains and
no jus because o impe ec ions and po osi y possibly p esen in he samples.
The esul s o he mal diﬀusi i y es s o he compac ed nc-UO2-ma e ial showed
simila beha iou as ha o s anda d, nuclea g ade UO2(bulk). The he mal
diﬀusi i y o sin e ed nc-UO2(∼200 nm, 90%densi y), was de e mined be ween
xxx

254°C o 1165°C. Ex apola ion o 95%densi y was done and same he mal diﬀusi i y
as s anda d bulk-UO2pelle [Fink, 2000]wi h95%densi y was ound. Rega ding
he ea ed wo sening o he he mal conduc i i y o he HBS ma e ial due o g ain-
size eﬀec (Kapi za esis ance), i has been he e shown ha no he mal p ope ies
de e io a ion has o be expec ed o he 200 nm-UO2pelle ma e ial mimicking he HBS.
De e mina ion o he mel ing poin by lase -hea ing and py ome ic empe a u e
de ec ion has been pe o med o compac ed nc-UO2wi h wo diﬀe en nano-g ain sizes
(abou 10 nm and 200 nm) and hei a ia ion wi h espec o bulk-UO2(la ge-g ain),
assessed. A mel ing poin dep ession o abou 150°K wi h espec o he no mal alue
o bulk-UO2was ound o he 10 nm-size nc-UO2sample. This educ ion would be
a p io i due o he nano-size g ains. Howe e , he measu ed la ice cons an o he
sample be o e mel ing (a=0.5438 nm) was below he alue o bulk-UO2(a=0.547 nm)
and indica ed in eali y a hype s oichiome ic oxide, which would also cause a mel ing
poin dec ease. To co obo a e he measu ed endency wi h he g ain-size educ ion, a
s ic ly s oichiome ic nc-UO2sample would be needed. Howe e , an iden ical mel ing
poin as o bulk-UO2, was ound o he 200 nm-sample o which a s oichiome y o
O/M=2.00 was conﬁ med om he la ice cons an measu emen be o e mel ing. This
is an impo an echnological esul o he use o nc-UO2ce amics as nuclea uel.
Indeed, a lowe mel ing poin would pose a p oblem o he licensing o he monoli hs
as a uel o he eac o . Fo una ely he possibili y o a lowe mel ing poin disappea s
o he 200 nm-UO2samples, as i would occu o he HBS ma e ial in he eac o , oo.
So, pos ula ed nano-eﬀec s such as diminu ion o he he mal conduc i i y and he
mel ing poin could be he e excluded as weak poin s o he use o nc-UO2as a nuclea
uel. These eﬀec s migh be ele an o e y low c ys al/g ain sizes (∼10 nm) bu hey
disappea o g ain sizes o ∼200 nm, whe e, con enien ly, he sough ad an ageous
p ope ies o he nano-s uc u e (supe -plas ici y, low swelling unde Xe-bomba dmen
[Spino e al., 2012], sel -limi ing g ain g ow h, e c.), s ill emain. This an icipa es he
lack o p ope y loss o he de eloped nc-UO2monoli hs o echnical applica ions in
his size ange.
Fu u e Recommenda ions
The licensing o nuclea uel is made on basis o i s sa e y pe o mance no jus only
unde no mal ope a ion condi ions, bu also when a empe a u e ise occu s in he
uel. This could be caused in a Loss o Coolan Acciden (LOCA) o in a Reac i i y
Ini ia ed Acciden (RIA). Unde such ex eme condi ions uel agmen a ion could
occu . Du ing his hesis, one a emp was made o mimic such an acciden in an ou
o pile expe imen using nc-Y-Z O2as a sample ins ead o nc-UO2. This es was made
in a acili y a ITU (Ins i u e o T ansu anium Elemen s) known as POLARIS, which
pe mi s e y apid lase hea ing o he sample.
The ini ial ma e ial es ed in POLARIS was po e ee and i s su ace was ﬂa . The
lase ea men showed ha a local swelling occu ed h ough o ma ion o po osi y.
This expe imen was no pe ec ly well con olled, bu i is likely ha he obse ed
swelling was due o CO o CO2gas gene a ed when he ca bon impu i y in he
ma e ial eac ed wi h oxygen om he a mosphe e, which caused po e o ma ion, in
a p ocess simila o he p oduc ion o oamed glass. A pa icula ly in e es ing esul
xxx i
o he es is ha he o med po es we e closed and as onishingly simila o hose o
he HBS-zone in high bu n-up uels. Chances appea he e o e ha du ing such kind
o pos ula ed uel mel ing acciden , a leas pa o he ﬁssion gas could be apped
in po en ially o ming closed po es, as i occu s in he HBS ma e ial a low empe -
a u es. Al hough hese expe imen s a e p elimina y hey sugges a p omising no el
me hod o es he gas e en ion capabili y o he nc-UO2 uel unde acciden condi ions.
Finally, ano he impo an me hod o unde s and he esis ance o nc-ma e ials o
i adia ion can be p o ided by ion beam i adia ion es s. This can be done a acili ies
like he ANL (A gonne Na ional Labo a o y) IVEM-Tandem acili y in Chicago, whe e
i adia ion wi h ine gas ions (He o Xe) wi h on-line TEM obse a ion p o ides a
e y use ul way o implan he gas a oms and o e alua e how hey beha e in he
ma ix, e.g. dissolu ion he ein, o ma ion o bubbles, anspo o bubbles along g ain
bounda ies, e c.
Concluding ema ks
Success ul consolida ion o he syn hesized nanoc ys alline UO2nanopowde s in o
dense pelle s mimicking o he High Bu n-up S uc u e (HBS) as ideal sys em has
been achie ed. F om he diﬀe en syn hesized nc-UO2powde s (4-5 nm size) o he
nc-UO2compac ed monoli hs wi h 200 nm a e age g ain size and abou 90%densi y
we e achie ed. S abili y o he s uc u e a e ageing and sel limi ing g ain g ow h
kine ics up o empe a u es o 1200°C, we e shown. The ou -o -pile mechanical
p ope ies o sin e ed pelle s (in e ms o ha dness and elas ic modulus) we e conﬁ med
o closely esemble hose o he HBS-ma e ial in-pile. Beneﬁcial p ope ies ound,
like s abili y o he s uc u e, enhanced mechanical p ope ies and sel -limi ing g ain
g ow h, s ongly encou age he pe o mance o i adia ion es s o e i y he in- eac o
beha iou . As de e mined p e iously in ou -o -pile es s o monoli hs o he b o he
sys em o nanoc ys alline nc-Y-Z O2[Spino e al., 2012], a s ong educ ion o he
gas bubble swelling, long e m he mal s abili y o he po e-g ain conﬁgu a ion, and
s iking supe plas ic beha iou and accele a ed c eep, would be expec ed as well o he
de eloped nc-UO2. Conﬁ ma ion o anomalies in he physical p ope ies o he ma e ial
o g ain sizes in he absolu e nano ange (<30 nm), consis en wi h obse a ions in
o he nc-sys ems was also achie ed. These pe nicious nano-eﬀec s, as diminu ion o
he he mal conduc i i y and he mel ing poin , which could be a weak poin o he
use o nc-UO2as a uel, we e ound, howe e , o become ele an only a e y low
c ys al/g ain sizes (<30 nm) and o disappea o g ain sizes o ∼200 nm, whe e,
sui ably, he o he sea ched beneﬁcial p ope ies o his nanos uc u e supe -plas ici y,
low gas-bubble swelling, sel -limi ing g ain g ow h, e c., emain. This an icipa es he
lack o p ope y loss o he de eloped nc-UO2monoli hs o echnical applica ions in
his size ange. This has been a e y ewa ding wo k, wi h a numbe o b eak h oughs
achie ed. Much has been lea ned, bu mo e needs o be done o de e mine he ue
po en ial o his in iguing ma e ial.
xxx ii
xxx iii
Con en s
Abs ac xii
Resumen xx i
Summa y xxx iii
Lis o Symbols and Abb e ia ions xliii
1 In oduc ion 1
1.1 Backg ound and s a e o he a . ..................... 1
1.2 Goal o he hesis. .............................. 10
2 Analy ical and cha ac e iza ion echniques 13
2.1 Elec ochemical analysis .......................... 13
2.1.1 Cyclic Vol amme y (CV) ..................... 13
2.1.2 Elec olysis ............................. 14
2.2 Spec oscopy ................................ 15
2.2.1 Ul a iole - isible spec oscopy (UV-Vis) ............. 15
2.2.2 Dynamic Ligh Sca e ing (DLS) ................. 15
2.2.3 X- ay Abso p ion Nea Edge S uc u e (XANES) and Ex ended
X- ay Abso p ion Fine S uc u e (EXAFS) ............ 16
2.2.4 Nuclea Magne ic Resonance spec oscopy (NMR) ........ 17
2.2.5 In a ed spec oscopy (IR) ..................... 17
2.3 Elec on mic oscopy ............................ 18
2.3.1 Scanning Elec on Mic oscopy (SEM) ............... 18
2.3.2 T ansmission Elec on Mic oscopy (TEM) ............ 18
2.4 X- ay sca e ing ............................... 18
2.4.1 Room Tempe a u e X-Ray Diﬀ ac ion (RT-XRD) ........ 19
2.4.2 High Tempe au e X-Ray Diﬀ ac ion (HT-XRD) ......... 19
2.5 The mog a ime y/Diﬀe en ial The mal Analysis (TGA/DTA) ..... 19
2.6 Dila ome y ................................. 20
2.7 Mechanical Cha ac e iza ion ........................ 20
2.7.1 Mic oinden a ion .......................... 20
2.7.2 High P essu e X-Ray Diﬀ ac ion (HP-XRD) ........... 20
2.7.3 Scanning Acous ic Mic oscopy (SAM) ............... 21
2.8 The mophysical cha ac e iza ion ...................... 21
2.8.1 The mal Diﬀusi i y ......................... 21
2.8.2 Mel ing Poin ............................ 22
xxxix
Chap e 1. In oduc ion
3




3
Figu e 1.1: Ligh Wa e Reac o [The ene gy ne , 2012][U.S.NRC, 2012].
This hea is ans e ed o he coolan (wa e ), which passes by he clad, gene a ing
wa e apou . This s eam eeds con en ional gene a o s (s eam-d i en u bines) in he
p ima y ( o BWR) o seconda y ( o PWR) loop which p oduce he elec ici y.
The uel pelle inside he nuclea eac o is a ma e ial subjec ed o ex eme
condi ions which change i s p ope ies wi h ime and i adia ion dose. Each a om in
he uel is displaced se e al imes du ing i s i adia ion his o y bu many e u n o
equi alen c ys allog aphic posi ions. Damage and local de ec s like in e s i ials, loops
and acancies a e c ea ed. UO2is a poo hea conduc o , he e o e he hea ans e
om he cen e o he su ace o he pelle is slow. A ypical empe a u e p oﬁle o
a LWR pelle is shown in Fig. 1.2 [Konings e al., 2011]. Fu he mo e, accumula ion
o solid ﬁssion p oduc s in he la ice and o ma ion o gas bubbles make he pelle
he mal conduc i i y o dec ease. The ﬁssion gases p ecipi a e in bubbles and lead
e en ually o composi ional and mic os uc u al changes, swelling o he uel, as
well as o emb i lemen and ha dening o he cladding. O he eﬀec s occu in he
cen al sec ions o he uel pelle (a highe empe a u e) and include g ain g ow h,
po osi y build-up and an augmen ed gas elease [Kleykamp, 1979][S ehle e al., 1975].
Fig. 1.3-le shows a uel deco a ed wi h c acks a e ou eac o cycles (app oxima ely
ou yea s). On he igh side o he ﬁgu e, he e olu ion o he geome y o he pelle s
inside a od can be obse ed. Fuel c acks appea om he beginning o he i adia ion
due o he mal s esses. The uel pelle s swell owing o he accumula ion o ﬁssion
gas bubbles in he ma ix and he seg ega ion o low densi y ﬁssion-p oduc s phases
(me allic and ce amic p ecipi a es). As a esul o he swelling, he uel app oaches
he clad. Physical and/o chemical in e ac ion can occu upon con ac , which can
induce clad de e io a ion and up u e [Ga za olli e al., 1979]. These ype o changes
can aﬀec also he empe a u e p oﬁle o he uel pelle (Fig. 1.2) by modiﬁca ion o
2

1.1. Backg ound and s a e o he a .
he gap he mal ans e condi ions, limi ing he li e ime o he uel (and BUs) inside
he eac o , i p ema u e od up u e occu s.
Figu e 1.2: A ypical empe a u e p oﬁle o a LWR uel as a unc ion o he uel pin adius
[Konings e al., 2011].
A as amoun o wo k has been made in he las decades o cha ac e ise he
beha iou o high BU uels and o deepen he knowledge o he unde lying phenomena,
wi h he aim o inc ease he usage ime o he uel in he eac o [Wa eau e al., 2001].
Besides ha , in he las 20 yea s also a la ge inc ease o he esea ch ac i i ies in
nanoc ys alline (nc)-ma e ials o diﬀe en aims and applica ions has been os ensible
[Kulisch e al., 2009][Ma hu and Singh, 2009]. The ques ion a ises whe he hese
wo appa en ly disconnec ed esea ch a eas would o e lap and whe he a link be ween
nc-ma e ials and high BU nuclea uel ma e ials would exis . The answe o his
ques ion is a emp ed below.

 




Figu e 1.3: Mac og aph o a uel pelle a e i adia ion showing he ypical adial c acks
(le ). Pelle inside he pin illus a ing he swelling wi h he i adia ion ime ( igh ).
[Bailly e al., 1996].
Indeed, nuclea uels app oxima ely a he end o he hi d i adia ion cycle
3
Chap e 1. In oduc ion
(abou 40 GWd/ M) unde go a s uc u e ans o ma ion which begins a he edge
o he uel pelle and s eadily p og esses o i s cen e as he i adia ion p oceeds
[Ma zke and Spino, 1997][Spino and Papaioannou, 2000]. The o iginal mic os uc u e
ans o ms in o a nc-po ous ma ix [Nogi a and Une, 1994] h ough es uc u ing o
he accumula ed i adia ion de ec s. This is a so o “sel -healing” ac ion whe e he
ma e ial ge s cu ed om damage by eo de ing i sel [Spino e al., 2012]. Fig. 1.4
shows wo ce amog aphs o diﬀe en uel zones a inc easing local BU and whe e he
men ioned change in he s uc u e is clea ly app ecia ed. The new nc-s uc u e appea -
ing (Fig. 1.4- igh -mic og aph) is called high bu n-up s uc u e (HBS). I is called also
im-s uc u e because in UO2 uels i ini ia es a he im o ou e zones ( /R>0.98) o
he pelle s (colde pe iphe y o he pelle ; Fig. 1.4- igh ). This happens because he im
is he egion which ecei es he highes dose ( he mos ﬁssions) and he e o e is exposed
o highes local inc ease o BU (∼70 GWd/ M in he hi d cycle, a empe a u es
a ely exceeding 800°C [Sonoda e al., 2002]), and so he highes adia ion damage, oo.
!
"!$" $

~P=15-20% (GAS TIGHT); CLOSED POROSITY
1 µm
/ o=0.98
~160 GWd/ M
" $# $
P<4% (GAS PERMEABLE )
1 µm
/ o=0.80
67 GWd/ M




Figu e 1.4: Mic og aphs a diﬀe en pelle adius a eas [Spino and Papaioannou, 2008]. High
bu n-up s uc u e (HBS o im-s uc u e) ans o ma ion [The ene gy ne , 2012].
Mos o he p ope ies obse ed in he HBS, esemble hose seen in he nanoma-
e ial’s s uc u es. Likewise hea ily cold wo ked me als which a e se e e plas ic
de o ma ion show g ains in he nm-size ange [Villegas and Shaw, 2009], he hea ily
damaged high-BU uel egion a he im o he pelle s display a p o ound modiﬁca ion
o he mic os uc u e on exceeding a c i ical dose. A e his h eshold, he o iginal
nuclea uel wi h la ge-g ains (10-20 μm) suﬀe s p og essi ely g ain subdi ision
(low-angle sub-g ain o ma ion o low coo dina ion numbe ) and ec ys alliza ion
(high-angle sub-g ain o ma ion o high coo dina ion numbe ) changes (Fig. 1.5)a he
end o which a new s uc u e ( he HBS) wi h uni o mly nm-sized g ains (100-250 nm)
appea s [Spino e al., 2012]. This ans o ma ion o a s uc u e wi h nm-g ains a he
edge ( im) o he pelle esul s in la ice con ac ion [Spino and Papaioannou, 2000].
Fu he mo e, o ma ion o new 1 μm-sized po es embedded in he ma ix (Fig. 1.5)
occu s, which en aps mos o he c ea ed ﬁssion gas [Spino e al., 1996]. This po osi y
can each alues abo e 20%.
A he beginning i was hough ha he HBS could be esponsible o cladding
ailu es due o addi ional in-pile olume inc ease (swelling), which, in addi ion o
4
1.1. Backg ound and s a e o he a .




 

100 nm
52 GWd/ M – 500 °C
Xe

$ #
 #
%
Figu e 1.5: Coo dina ion s a e change in he ans o ma ion o he HBS (o im-s uc u e).
he supposed b i le beha iou o he ans o med ma e ial [Ma zke, 1992], lead o
p oposals o pelle -design changes o coun e ac he ans o ma ion [Swam, 1997]
[Tulenko and Wang, 2008]. Also, i was hough ha he im-s uc u e could ac
as a new sou ce o gas elease. In ac , one could hink ha he la ge he g ains
(e.g. 10-20 μm, as in he o iginal uel o he uel wi h lowe high-BUs), he longe
he pa hways o he ﬁssion gases owa ds he g ain bounda ies and he ex e io ,
and consequen ly, he mo e gene ally imp o ed ﬁssion gas e en ion beha iou in
compa ison wi h he small g ains o he HBS (0.2 μm g ains). Indeed, he e a e s ill
in es iga ion lines which de end his opinion.
Howe e , o he s udies ound ha he pe cen age o gas libe a ed om he im
o he uel pelle s, whe e he HBS appea ed, was low in compa ison wi h he gas
coming om he inne pa s o he pelle whe e he o iginal la ge -g ain s uc u e
is s ill p esen [Mogensen e al., 1999]. This philosophy has been ollowed by o he
publica ions, which demons a es a gene ally beneﬁcial e olu ion o he uel p ope ies,
as in pa icula he e en ion o he ﬁssion gases, a e he s uc u e ans o ma ion
[Rondinella and Wiss, 2010], [Spino e al., 2012]. Speciﬁcally, i has been obse ed
ha he HBS does no de elop an open po e s uc u e wi h in e connec ed channels,
e en a highe po osi ies [Noi o e al., 2008].
In ac , i has been seen ha s uc u es wi h la ge g ains (10-20 μm) and wi h
1μm po es and po osi ies below 4%(in he o iginal uel and he uels wi h low
BUs; Fig. 1.4-le ), a e pe meable o he ﬂow o gases. In con as , he im-s uc u e
wi h small g ains (100-250 nm) and wi h 1 μm po es and 20%po osi y (Fig. 1.4-
igh ), was sugges ed o emain gas igh ly because o he o med closed po osi y
[Spino e al., 2004][Hie nau e al., 2008]. Mo eo e he la ged-g ained s uc u es a e
ound o e ain la ge amoun o disloca ion loops and gas bubbles inside he g ains,
which diminishes he mechanical p ope ies (c eep s ain), inc easing he isk o
5
Chap e 1. In oduc ion
Figu e 1.6: SEM mic og aphs o a uel pelle a high-BU om he ou e adius o im (HBS
in he ﬁ s mic og aph) o inne adial posi ions om [Manzel and Walke , 2002].
PCMI. In con as , he small-g ained s uc u es wi h hei high-angle sub-g ain (high
coo dina ion s a e; Fig. 1.5) acili a ing he GBs sliding de o ma ion mechanisms,
show imp o emen o he plas ici y and c eep s ain, diminishing he PCMI- ailu e
isks [Chung and Da ies, 1979][Spino e al., 2008].
Besides ha , he p ope ies o he HBS indica e an enhanced adia ion ole ance as
epo ed by [Spino e al., 2012]. This s udy coincided wi h impo an e idence in he
li e a u e ha nano-g ained ma e ials a e mo e esilien o adia ion damage han he
co esponding la ge-g ained ma e ials due o de ec ecombina ion a hei mul iple
g ain bounda ies has been epo ed by [Ni a e al., 2005]. Also imp o emen o he
he mal conduc i i y and o he adia ion-de ec s depending p ope ies was ound due
o la ice-s ain elease a e ec ys alliza ion [Ronchi e al., 2004], as well as ac u e
oughness inc ease and c ack-healing endency we e an icipa ed [Spino e al., 2003].
Mo eo e , in ela ion o sa e y issues, he la es expe imen s on he high BU LWR
uels indica ed no inc ease in he gas elease and in he ailu e suscep ibili y du ing e-
ac i i y ini ia ed acciden (RIA) ansien s [Sasajima e al., 2010][Fuke a e al., 2006].
Also diminu ion o he aqueous co osion a e unde simula ed geologic eposi o y condi-
ions o uels con aining HBS was ound ou [Eke o h e al., 2009][Ca bol e al., 2009].
Bo h ac s conﬁ med he igh ness o he s uc u e.
So i has been demons a ed ha he HBS has excep ional quali ies e en in
compa ison wi h he o iginal ma ix (la ge-g ain s uc u e), wi h a numbe o imp o ed
p ope ies ha a e eally ad an ageous o a uel. Then, why no imi a e his ec ys-
6
1.1. Backg ound and s a e o he a .
allized ma e ial?. And why no imi a e he s uc u e appea ing in he im (HBS) and
in oduce i in his o m as a esh uel inside he eac o ?. Why no syn hesise a uel
ma ix like his HBS, which due o i s appa en damage- esilience would wi hs and
longe imes unde i adia ion?.
This is a adical s ep in nuclea uel concep ion, hi he o no conside ed, since, as
indica ed be o e, he gene al hinking o he indus y ( uel p o ide s and u ili ies) was
un il now jus he con a y one, i.e. ying o make la ge and la ge g ains, unde
he p emise o imp o e only one aspec o he uel pe o mance, namely he ﬁssion-gas
elease unde s eady s a e condi ions, dis ega ding he implied wo sening o he uel
plas ici y due o g ain-size inc ease, and also i s poo e beha iou unde powe - amps.
7
"$%
"$%

&(*
 &(''#)+'

Figu e 1.7: A no el uel mic os uc u e: nc-UO2[The ene gy ne , 2012].
On he con a y, apa om i s s ill unp o en imp o ed esis ance agains adia ion-
damage (e.g. educed swelling), he p incipal i ues o nc- uels in-pile compa ed o
con en ional uels will be he as e elaxa ion o PCI s esses h ough he highe
plas ici y induced by g ain- eﬁnemen , plus he possibili y o e en ion o mos ﬁssion
gases in o med closed po es. As a po en ial echnological applica ion, and as inspi ed
in he beha iou o HBS-ma e ial in pile, he nc- uel could e ain ﬁssion gases inside
he po es up o e y high-BUs (>300 GWd/ M). Up o hese BUs alues, he po osi y
could inc ease up o 30%, un il incipien po e in e connec ion would ﬁ s begin
[Konings e al., 2011].
The main aim he e is hence o syn hesise nc-UO2powde s o he manu ac u e
o bulk nc- uel compounds o he cha ac e iza ion o hei ou -o -pile mechanical
p ope ies and i adia ion beha iou . The p oduced monoli hic pieces would ha e a
olume o app oxima ely 1 cm3, wi h a uni o m g ain size be ween 100 and 250 nm o
mimic he im-s uc u e.
The ﬁ s ques ion which appea s is how o ob ain enough amoun o nc-UO2powde
o ab ica ion ials o bulk-pieces, when he known me hods o nanopa icle syn hesis
a e gene ally uned o yield jus small amoun s o ma e ial (mg ange), p ima ily
7

Chap e 1. In oduc ion
dedica ed o analy ical o esea ch uses [Rousseau e al., 2002][Menneca e al., 2004]
[Wu e al., 2006][Rousseau e al., 2006][Opel e al., 2007][Rousseau e al., 2009].
The second impo an in e oga e ha a ises hen is how o ob ain he eo dense
la ge monoli hs wi h uni o m g ain size a ound 200 nm, when he achie emen o bulk
nc-bodies is oday one o he mos demanding challenges in nano echnology.
The adi ional me hods o he p oduc ion o UO2a e based on he p ecipi a ion
o UVI sal om liquid solu ions. Then he p ecipi a ed ma e ial is oxidised o U3O8
by oxida i e he mal ea men and a e wa ds he con e sion o UO2(UIV )is eached
by a second hea ea men unde educing condi ions (800°C unde A /H2). All hese
p ocesses oge he p o ide a powde ma e ial which is u he condi ioned by diﬀe en
physical-chemical/mechanical me hods o gi e a composi ionally s able (in ai ) and
ee-ﬂowing agglome a e, sui able o be compac ed by s anda d powde me allu gy
echniques (basically uniaxial-bidi ec ional p essing). The s anda d cha ac e is ics
o comme cial “ eady- o-p ess“ UO2-powde s ha e a UO2.1composi ion, agglome a e
pa icle size 20-40 μm and c ys alli e size 200-500 nm. A e p essing o desi ed
geome y, ﬁnal sin e ing o he compac (1600°C du ing 16 h) is necessa y o ge
he desi ed densi y, o 95 o 98%o he heo e ical densi y (TDUO2=10.96 g/cm3).
Howe e , he ﬁnal g ain size ob ained by his con en ional ma e ial syn hesis ou e is
in he ange o 5-10 μm, which is a abo e he goal he e (100-250 nm).
1.2 Goal o he hesis.
The main goal in he p esen wo k was o de elop an accessible ou e o p oduce
de ec - ee nc-UO2-based monoli hic ce amic specimens wi h ailo ed g ain/po e
mic os uc u e. The a ge conﬁgu a ion consis ed o a dense, uni o m ma ix wi h
100-250 nm sized g ains wi h po osi y le els o 10 o 20% o ep oduce ou -o -pile he
p ope ies o he HBS ma e ial, using simila me hods as u ilized in he p e iously
s udied case o nc-Y-Z O2[San a-C uz, 2009]. Once his goal was accomplished, a
s udy o de e mine hei physical-chemical p ope ies and hei damage esis ance in
compa ison wi h mic on-g ained ma e ials was pe o med.
The c ea ion o he abo e no el uel mic os uc u e has been achie ed in his wo k
by passing h ough e y diﬀe en s eps. F om he ma e ial syn hesis o he uel pelle
manu ac u e, many indi idual p ocess s ages, p e iously unknown o unexplo ed, had
o be speciﬁcally de eloped and/o op imized. Hence, ega ding he ini ial powde ,
conside able wo k was de o ed o he de elopmen o wo diﬀe en chemical syn hesis
ou es leading o deﬂoccula ed nc-UO2and nc-ThO2p ecipi a es. ThO2is simila in
s uc u e o UO2bu has he ad an age o a single alency (i.e. IV).
Al hough much in o ma ion can be ound abou nano-chemis y and ac inides-
chemis y, no e y much in o ma ion on he syn hesis o nanopa icles wi h ac inides
compounds is a ailable. The objec i e o he ew epo ed wo ks is, in he majo i y
o cases, conce ned wi h he issue o adionuclide elease con ol du ing spen uel
geological disposal, namely he ul ima e dissolu ion/ e-p ecipi a ion o ac inides in a
in a ac u ed geological eposi o y, by non-excludable con ac o damaged spen uels
wi h wa e [Rousseau e al., 2002], [O’Loughlin e al., 2003]. The a ge in hese cases
is o examine he adiological haza ds which could eme ge om hese si ua ions and
o quan i y he eﬀec o adioac i i y elease (and po en ial en i onmen con amina-
8
1.2. Goal o he hesis.
ion) on possible long- e m pe manence o spen uels in con ac wi h g oundwa e
[Menneca e al., 2004], [Rousseau e al., 2006]. Also he s udy o he elemen a y
oxida ion mechanisms occu ing on an a omic scale on uel/wa e con ac du ing
spen uel s o age has been pe o med wi h ac inide oxide colloids [Opel e al., 2007],
[Rousseau e al., 2009]. O he nanos uc u es based on u anium oxides ha e been
used o ca aly ic pu poses [Wang e al., 2008]. In o he cases, he possible s udy
o size-dependen physical and chemical p ope ies has mo i a ed he syn hesis o
high-quali y colloidial o UO2[Wu e al., 2006].
Despi e ha , no publica ions o he abo e desc ibe he p oduc ion o nc-ac inides
o he han as in he e y small quan i ies needed o esea ch o analy ical s udies.
Al hough enough o hese pu poses, no monoli h ce amic as desi ed in his wo k could
be achie ed wi h such (small) amoun s o ma e ial. Fo he mimicking o he HBS
nc-mic os uc u e in whole UO2-pelle be ween 0.5 and 1.0 g o nc-UO2powde a e
needed. The e o e, a me hod wi h a deﬁni ely highe ma e ial’s yield mus be de eloped.
In he p esen wo k, wo o he abo e epo ed me hods o he syn hesis he
nc-UO2we e chosen and de eloped as a sou ce o he nc-powde needed o pe o m
he desc ibed monoli hs. In Chap. 3 he ﬁ s one is desc ibed. This is a con olled
p ecipi a ion me hod ha uses an elec oly ically educed aqueous solu ion o u anyl
ni a e as p ecu so and d opped NaOH-solu ion as alkalinisa ion agen o igge
he p ecipi a ion o he nc-ma e ial in he icini y o he U4+ solubili y line, which
was o iginally desc ibed in [Rousseau e al., 2002], [Rousseau e al., 2006]. The second
me hod, desc ibed in Chap. 4, is a he mal decomposi ion o an o ganic phase
con aining u anyl ace ylace ona e (UAA) as p ecu so , which is added o a mix u e o
oleic acid and oleylamine which is hen hea ed as a whole up o empe a u es a ound
300°C o induce he p ecipi a ion o UO2nanopa icles by he mal decomposi ion o
he UAA. This o iginal p ocedu e was desc ibed in [Wu e al., 2006]. To ob ain la ge
amoun s o nc-UO2as equi ed, bo h me hods we e con enien ly adjus ed, de eloped
and scaled-up acco ding o he aim needs. The ma e ial in he as-p oduced condi ion
was s udied by TEM and XRD. C ys alliza ion and g ain-g ow h kine ics o he
syn hesized powde s as a unc ion o he empe a u e and ime, as well as s uc u e
cha ac e iza ion a he diﬀe en empe a u es, ollow in Chap. 5and Chap. 6, o he
aqueous con olled p ecipi a ion and he o ganic p ecipi a ion, espec i ely. Tools such
as TGA/DTA, XRD, HT-XRD, TEM, NMR, IR and XANES/EXAFS we e used o
his pu pose. The esul s on he mally ea ed powde s p o ided use ul in o ma ion o
he selec ion o ideal sin e ing condi ions o he pos e io syn hesis nc-monoli hs. In
Chap. 7 he consolida ion o he g een monoli hs, as well as diﬀe en he mal ou ines
o he d ying and sin e ing s eps, which ep esen a challenge o he achie emen o
c ack- ee dense specimens, we e explo ed. Op ical, mechanical and he mophysical
cha ac e iza ion o he sin e ed bodies ollowed o e i y hei ap i ude o mee nuclea
eac o uel speciﬁca ions. Cha ac e iza ion echniques as SEM, inden a ion, HP-XRD,
SAM, he mal diﬀusi i y and mel ing poin we e used. Chap. 8summa izes he
discussions o he diﬀe en chap e s in a ﬁnal conclusion o he o e all esul s. Finally
Chap. 9concen a es diﬀe en u u e ecommenda ions o he p ojec . Some o hem
ha e been al eady ini ia ed du ing he hesis and p esen ed he e, e.g., al e na i es o
monoli hs compac ion, s udy o nc-UO2magne ic p ope ies and ou -o -pile simula ion
expe imen s.
9
Chap e 1. In oduc ion
10
Chap e 2
Analy ical and cha ac e iza ion
echniques
2.1 Elec ochemical analysis
The ins umen used was a SP-50 ol amme e om BioLogic Science Ins umen s
(wo king anges 2.8-10 V and 10 μA - 400 mA).
2.1.1 Cyclic Vol amme y (CV)
A h ee elec ode sys em conﬁgu a ion was u ilized. A wo king elec ode (~1 mm hick
od in spi al o m, composi ion P /I 90/10%me al om He aeus and Fishe ype
wi h in oduced a ea=1.885 cm2Fig. 2.1a), an auxilia y o coun ing elec ode (ne ,
composi ion P /I 90/10%me al om He aeus and Fishe ype wi h 0.12 mm ne -wi e
hickness, 38 mm in diame e and 50 mm in heigh Fig. 2.1a), and a e e ence elec ode
(Ag/AgCl InLab® Re e ence sa u a ed om Me le Toledo). A 150 mL glass- eac o
buil o hese expe imen s was also used (Fig. 2.1b).
(a) Ne , spi al, i (b) Reac o
Figu e 2.1: Cyclic ol amme y expe imen al a angemen .
In a cyclic ol amme y (CV) analysis, he cu en densi y i(mA/cm2) is plo ed
e sus he applied ol age o po en ial E(V). Whe e iis he in ensi y o cu en a he
11
Chap e 2. Analy ical and cha ac e iza ion echniques
a Cu X- ay ube (40 kV, 40 mA), and a Posi ion Sensi i e de ec o B aun co e -
ing an angula ange o 6°(2θ) and an An on Paa HTK2000 hea ing chambe . The
alignmen o he machine is done wi h e e ence ma e ial MgO a diﬀe en empe a u es.
2.5 The mog a ime y/Diﬀe en ial The mal Anal-
ysis (TGA/DTA)
The mog a ime ic analysis o he de e mina ion o mass changes and decomposi ion
empe a u es we e ca ied ou wi h a simul aneous he mog a ime y diﬀe en ial
he mal analysis (TGA-DTA) sys em (NETZSCH Simul aneous Analyze STA 449
Jupi e ). Fo he es s Al2O3c ucibles we e employed. Fo he co ec ion o he
gas buoyancy eﬀec , base lines we e measu ed wi h emp y c ucibles using he same
expe imen al condi ions as o he in es iga ed samples.
2.6 Dila ome y
The sin e ing beha iou o g een monoli hs was de e mined by diﬀe en ial dila ome y
wi h a Bäh The moanalyse DIL-802 S. The leng h change o he monoli h was
measu ed as a unc ion o empe a u e ela i e o he leng h changes o a pa allel
e e ence sample (polyc ys alline Al2O3) wi h simila dimensions.
2.7 Mechanical Cha ac e iza ion
2.7.1 Mic oinden a ion
The mic oinden a ions we e pe o med wi h a F ank-Fino es ha dness- es e acco ding
o he s anda d me hods o ad anced ce amics, ASTM C1327 o Vicke s inden a ion
ha dness and ASTM C1326 o Knoop inden a ion ha dness. The o m o he hese
ypical inden e s is shown in Fig. 7.24. Loads o 1.96 N, 4.90 N and 9.80 N we e
applied o 15 seconds. Fo compa ison an ins umen ed mic oinden e de eloped in
ITU, was also used. In his case, loads o 0.49 N, 0.98 N, 1.96 N, 4.90 N and 9.80 N
we e applied also o 15 seconds. The acquisi ion o es da a poin s was a oom
empe a u e. A leas 3 o 5 diﬀe en inden a ions (a diﬀe en loca ions h oughou
he sample) we e pe o med o each load applied and he a e age alue aken o he
calcula ions. Minimum dis ances be ween inden a ions we e espec ed ollowing he
espec i e ASTM me hods. Random ep esen a i e a eas o he ma e ial we e always
aken o he es s.
The samples we e embedded in ha d epoxy- esin, ollowed by g inding and polishing
o he es su ace. Special holde s wi h wo-sides plane-pa allel geome y we e used
o ensu e he pe pendicula i y o he inden e o he sample and, su ace and in
consequence, symme ical inden a ions. G inding o he samples was ca ied ou wi h
SiC pape (600 o 1200 mesh), as well as a ﬁnal polishing wi h diamond suspensions
was done, educing p og essi ely he pa icle size om 15 μm o1μm. A ﬁne polishing
was needed o achie e a pe ec ﬂa mi o su ace and o a oid he addi ion o e o s in
18

2.8. The mophysical cha ac e iza ion
he de e mina ion o he inden a ion leng hs due o su ace oughness and impe ec ions.
2.7.2 High P essu e X-Ray Diﬀ ac ion (HP-XRD)
The comp essibili y s udy o he diﬀe en nano-sized UO2was pe o med by means o
in-si u X- ay diﬀ ac ion (XRD). The samples we e loaded in a Diacell- ype memb ane
diamond an il cell wi h 500 μm cule size using p e-inden ed Re gaske s wi h 200 μm
diame e holes. P essu e was de e mined using he uby scale [Pie ma ini e al., 1975]
and Cu equa ion o s a e (Cu-EOS). Silicone oil was used as p essu e ansmi ing
medium. High p essu e X- ay diﬀ ac ion (HP-XRD) was pe o med using a modiﬁed
B uke D8 x- ay diﬀ ac ome e wi h ocusing mi o op ics ins alled on a molybdenum
o a ing anode sou ce (Mo Kα1=0.70926 Å), coupled wi h a B uke SMART Apex
II Cha ged-Coupled De ice (CCD). The eco ded diﬀ ac ion images we e in eg a ed
using he ESRF FIT2D so wa e [Rod íguez-Ca ajal, 1993].
2.7.3 Scanning Acous ic Mic oscopy (SAM)
The scanning acous ic mic oscopy we e pe o med in a collabo a ion wi h he g oup
o P o . Laux as pa o a collabo a ion wi h IES (Ins i u d’Elec onique du Sud) a
Uni e si y o Mon pellie wi h an acous ic mic oscope in ITU de eloped (Fig. 2.2).
The de ice includes a ansla ion s age, mic ome ic mo o s, echog aphic bench, and
acous ic ocused senso s wi h sphe ical lens wi h an ape u e angle o 50°. The samples
should ha e a hickness o abou 1 mm. A e wa ds he samples a e embedded in
a esin and polished o ob ain a smoo h su ace. The sample is in oduced in an
aluminium basked and me hanol coupling liquid is pou ed un il he embedded sample
is comple ely subme ged. A e ho izon al alignmen wi h wo adjus ing sc ews he
acous ic senso is lowe ed a oiding o ap any ai bubble which could lead o alse
eadings. The senso is u he lowe ed un il a ew μm dis ance om he sample and
de ocusing is s a ed in o de o ge an acous ic image. The signal om he senso is
con e ed o an op ical signal o be displayed on a compu e sc een.
2.8 The mophysical cha ac e iza ion
2.8.1 The mal Diﬀusi i y
The measu emen s o he he mal diﬀusi i y a e pe o med in a lase -ﬂash de ice
(LAF I) [Ronchi e al., 1999] inside a lead-shielded glo e box wi h emo e manipula o s.
The sample is hea ed up (A a mosphe e o 10−2mba ) in a high equency u nace
o he measu emen empe a u e. A lase pulse is applied o one o he su aces o
he sample. A he opposi e su ace , he ou - empe a u e pe u ba ion is eco ded by
a pho o-diode py ome e (0.05°K sensi i i y) wi h an in ITU de eloped log-ampliﬁe
wi h a ise- ime o he o de o 50 μs. The expe imen al se -up and he measu emen
echnique a e explained in de ail by [S aicu, 2007].
The he mog am is eco ded by a 14 bi digi alize (T=T( ) consis ing o se e al
housands o poin s) and is analysed by a ealis ic and accu a e ma hema ical model
o he pulse p opaga ion in he sample. The he mal diﬀusi i y and hea losses a e
19
Chap e 2. Analy ical and cha ac e iza ion echniques
(a) (b)
Figu e 2.2: O e iew o he acous ic mic oscope (in ITU de eloped) de ice showing acous ic
senso s, coupling liquid holde , sample pla o m and ansla ion s ages.
calcula ed by a nume ical ﬁ ing me hod. Co ec measu emen s o he mal diﬀusi i y
can be ob ained e en wi h samples o small sizes and i egula con ou s, due o he
highly homogeneous p obe lase -beam. The p ecision o he indi idual measu emen s
is be e han 1%. Ne e heless, he accu acy o he measu ed he mal diﬀusi i y
is lowe han he p ecision o he me hod, being p incipally de e mined by sample
hickness a ia ions.
2.8.2 Mel ing Poin
Same condi ions as desc ibed in [Cappia e al., 2013] we e used. A schema ic o he
lase hea ing expe imen al se -up can be also he e seen and he e ep oduced (Fig. 2.3).
The sample is in oduced in an au ocla e unde con olled a mosphe e and hea ed by
a 4.5 kW cw Nd:YAG TRUMPFlase . The powe in unc ion o he ime p oﬁle
is p og ammable wi h a esolu ion o 1 ms. The onse o mel ing is de ec ed by he
appea ance o ib a ions in he signal o a p obe A +lase eﬂec ed by he sample
su ace ( eﬂec ed ligh signal echnique) [Mana a e al., 2008]. The cooling o he
sample occu s when he lase beam is swi ched oﬀ. The mal a es s co esponding o
solidiﬁca ion can hen be obse ed on he he mog ams eco ded by as py ome e s.
These ope a e in he isible-nea in a ed ange be ween 488 nm and 900 nm. The
e e ence py ome e wa eleng h is he e 655 nm. This was calib a ed acco ding o he
p ocedu e epo ed in [Mana a e al., 2008][Böhle e al., 2012]. A dense sample o
a leas se e al mic ons in hickness a e needed in o de ha he measu emen is no
inﬂuenced by he sample suppo . The no mal spec al emissi i y o u ania has been
assumed o be equal o 0.83 [Mana a e al., 2005][Cappia e al., 2013]. In Fig. 2.4 a
pic u e o he sample mel ing poin se -up, is shown.
20
2.8. The mophysical cha ac e iza ion
Figu e 2.3: Lase hea ing expe imen al se -up [Cappia e al., 2013].
Figu e 2.4: Sample mel ing poin se up. In he yellow a ea, he nc-UO2pelle hold wi h h ee
sc ews is obse able.
21
Chap e 2. Analy ical and cha ac e iza ion echniques
22
Chap e 3
Syn hesis o nc-UO2by con olled
massi e p ecipi a ion in Aqueous
phase
3.1 In oduc ion and p inciples.
3.1.1 U-s abili y: en i onmen al s udies.
The con ol o used and p esen u anium mines, spen nuclea uels and was e
eposi o ies ins alla ions is an ongoing subjec . Rela ed o he su eillance o hese
si es, he unde s anding o he dissolu ion/ e-p ecipi a ion and anspo beha iou o
diﬀe en adionuclides a diﬀe en pH in aqueous media is essen ial o he a oidance o
he con amina ion o he g oundwa e [Ryan and Rai, 1983]. The beha iou in wa e
o U in i s a ious o ms, pa icula ly as UO2in c ys alline and amo phous s a es,
needs o be unde s ood. In he case o educing condi ions he disposal is acili a ed
by immobiliza ion o he soluble UVI species by i s educ ion and p ecipi a ion as
insoluble UIV in o m o UO2, and pos e io emo al a e localiza ion om he aqueous
media [Lo ley and Phillips, 1992].
The inc ease in he solubili y and mobili y o UIV o UVI species due o
complexa ion wi h chlo ide [Hennig e al., 2005], ca bona e [Suzuki e al., 2006]o
sulpha e [Hennig e al., 2007] anions, cons i u es also a subjec o a en ion o
adiological issues. The possible o oxida i e emobiliza ion o he nanome e -sized
p ecipi a es [Suzuki e al., 2002][Ling e al., 2008] by acciden al con ac wi h a -
mosphe ic O2[Zhong e al., 2005] o by adioly ical induced oxida ion in wa e
[Menneca e al., 2004], is also ma e o s udy.
A a ie y o pa ame e s inﬂuence he sys ems in ol ing UO2colloids. In pa -
icula , he p ecipi a ion/solubili y o U colloids in aqueous media is aﬀec ed by
wo issues. The end o oxida ion om UIV o UVI, as well as he hyd olysis o
UIV a e y low pH (pH∼1) by complexa ion o U4+ in o Um(OH)(4m−n)
nin solu ion
(mU4+ +nH2O←→ Um(OH)(4m−n)
n+nH+)[Neck and Kim, 2001]. The p esence o
he colloids in solu ion is also unc ion o he deg ee o acidi y o he media. Finally,
he colloids agg ega ion s a e (c ys alline, amo phous), also plays a ole in hei
p ecipi a ion/solu ion ends [Opel e al., 2007][Rai e al., 2003].
23

Chap e 3. Syn hesis o nc-UO2by con olled massi e p ecipi a ion in Aqueous phase
3.1.2 U- edox chemis y pe inen o nc-UO2syn hesis.
The p ecipi a ion in aqueous media o u anium colloids species a diﬀe en pHs has
been s udied o gain a be e unde s anding o hese p ocesses, wi h he objec i e
o examine he adiological haza ds implied in he deep disposal o adioac i e
was e [Rousseau e al., 2002]. The elemen a y mechanisms occu ing on an a omic
scale du ing uel oxida ion s o age ha e been also s udied wi h ac inide colloids
[Opel e al., 2007], [Rousseau e al., 2009].
The calcula ed equilib ium solubili y lines o UIV and UVI species in aque-
ous solu ion as a unc ion o pH ha e been epo ed by [Neck and Kim, 2001]
[Fanghänel, Th. and Neck, 2002]. A plo o hese esul s as compiled by
[Gil e al., 2010] is shown in (Fig. 3.1). The e a e se e al o de s o magni ude
diﬀe ence be ween he solubili y o UIV in p esence o i s c ys alline dioxide phase,
UO2(c), o in p esence o he amo phous o m o his phase (hyd a ed u anini e,
UO2·xH2O(am)). Also se e al o de s o magni ude sepa a e he wo las s wi h he
solubili y line o UVI in he p esence o he c ys alline hyd oxide phase schoepi e,
UO2(OH)2(s) (Fig. 3.1)[Gil e al., 2010]. Also aluable expe imen al da a o he
oxida ion s a e o U in he p ecipi a es and o he size o he co esponding colloids
and hei agglome a es, along he solubili y line ha e been epo ed a diﬀe en pH
and low U-concen a ions [Opel e al., 2007][Rai e al., 2003][Fujiwa a e al., 2003]
[Fujiwa a e al., 2005]. The da a show p ecipi a es sizes o UO2 anging om 8-13 nm
a a ound pH=1 in he c ys alline s a e, o 80-150 nm a pH>2.5 in he amo phous
s a e (Fig. 3.1). One can conclude ha by p o oking he p ecipi a ion close o
he UIV solubili y line a lowes pH, he smalles and mo e c ys alline o m, o he
p ecipi a es can be ob ained, a oiding possibly he p ecipi a ion o any u anium
hyd oxide compound.
Acco ding o he hi he o expe ience, he syn hesis o nc-ma e ial using his
me hod esul s, howe e , in small quan i ies o ma e ial p oduced and la ge
quan i ies o was e. The concen a ions o U pe ba ch we e o he o de o
10−3 o 10−3M[Rousseau e al., 2002][Rousseau e al., 2006][Opel e al., 2007]
[Rousseau e al., 2009]. A challenge o ackle in his wo k will be hence o s udy he
easibili y o his syn hesis me hod o nc-UO2species, bu ex apola ed o quan i ies
o p oduc as high as 1 g pe ba ch o highe so as o be able o p oduce dense
monoli hs wi h such ma e ial.
In he publica ions o [Rousseau e al., 2002][Rousseau e al., 2006]
[Rousseau e al., 2009], wo me hodologies o ob ain nano-p ecipi a es we e used.
Two pH anges we e s udied, ≤4and≥4, wi h u anyl ni a e (UO2(NO3)2) solu ions
dissol ed in chlo ide media wi h U-concen a ions o 0.005 M and 0.03 M, espec-
i ely. In bo h in e als, UO2+xp ecipi a es wi h elemen a y size 20 nm (wi hou
excluding agglome a ion) and O/U a io ∼2.19 we e ob ained [Suzuki e al., 2006]
[Rousseau e al., 2002]. In he ﬁ s me hod, an aliquo o UVI solu ion was added
o he p ecipi a ion eac o whe e educing condi ions we e applied by a gal anos a
(cons an in ensi y) a a ﬁxed pH. The d opped UVI was educed elec ochemically
o UIV a a ﬁxed pH and UO2+xwas so p ecipi a ed. In he second me hod used
by hese au ho s, he UVI-solu ion was ﬁ s educed elec oly ically o UIV ( o a oid
p ecipi a ion o UVI compounds which begins a pH≥4) and he ea e i was slowly
d opped in o he p ecipi a ion essel unde educing condi ions, which we e kep
by applica ion o cons an po en ial. Simul aneously, he pH was held cons an by
24
3.1. In oduc ion and p inciples.
Nanoscale u ani ni e p ecipi a es
Nanoscale u anini e p ecipi a es (sulpha e media)
[Gil e al., 2010], me hod 3, (pH≈4.5-5)
Nanoscale u anini e p ecipi a es (sulpha e media)
[Opel e al., 2007], LIBD, UO2 (c)
[Opel e al., 2007], LIBD, UO2·xH2O (am)
[Rousseau e al., 2002], me hod I, UO2.19
[Rousseau e al., 2002], me hod II, UO2.19
The modynamic p edic ions (solubili y limi s)
[Fanghänel, Th. and Neck, 2002] I=0.5 M
[Fanghänel, Th. and Neck, 2002] I=1 M
[Fanghänel, Th. and Neck, 2002] I=1 M
[Neck and Kim, 2001] I=0.5 M
Figu e 3.1: Theo e ical solubili y limi s o UIV and UVI species in aqueous solu-
ions [Fanghänel, Th. and Neck, 2002][Neck and Kim, 2001] and expe imen al de e mi-
na ions o U-sulpha e [Gil e al., 2010] and U-chlo ide solu ions [Rousseau e al., 2002]
[Opel e al., 2007]. Compounds shown beside each equilib ium line show he p ecipi a ed
solid phase when hese condi ions a e exceeded.
balancing he acidi y o he U solu ion wi h con olled NaOH addi ions. P ecipi a ion
occu ed hence ins an aneously.
An in ensi e s udy o he ange o U-concen a ion and acidi y o nc-UO2+x
p ecipi a ion om elec oly ically educed u anyl ni a e (UO2(NO3)2) solu ions is
hence endea ou ed in he p esen wo k, using highe concen a ions anges, and
he e o e lowe pH anges, ollowing he solubili y line o UIV . The use o highe
concen a ions and hei co espondingly lowe p ecipi a ion pH ange, was al eady
sugges ed bu no es ed by [Opel e al., 2007]. In hei pe chlo a e sys em, hese
au ho s p oposed mo ing upwa ds he UIV -p ecipi a ion line owa ds lowe pHs as a
means o ob ain nano-UO2p ecipi a es in i s c ys alline o m bu upon diminishing
he size o he agglome a es. In he p esen chap e , he same kind o concep will be
applied.
3.1.3 nc-UO2syn hesis p inciples.
Fo he nc-UO2syn hesis by con olled p ecipi a ion in aqueous media, he s a ing
UO2(NO3)2solu ion was educed om UVI o UIV , be o e being p ecipi a ed by adding
NaOH o he sys em. An ini ial s udy o he elec ochemical educ ion by cyclic ol am-
me y (CV) and an op imiza ion o he diﬀe en pa ame e s ha e been also pe o med.
A ypical se -up used o he educ ion and con olled p ecipi a ion, is shown in Fig. 3.2.
25
Chap e 3. Syn hesis o nc-UO2by con olled massi e p ecipi a ion in Aqueous phase
I consis ed o a glo e-box o house he en i e conﬁgu a ion unde ine a mosphe e
and as he ﬁ s adiop o ec ion ba ie o he handling oxic ma e ials. A se en-
necked eac o was used whe e pH-elec ode, e e ence elec ode, coun e elec ode,
wo king elec ode, A -ﬂow inle ube, NaOH dosing ube and ou -gas neck, we e placed.
0.1/02
%'$$$
#$,")!'!&$"
&,"$!&$"
%%&)*&$&
$&
&"$
$


0-

#
$!&$"
#&$"
"% &$ 0
Figu e 3.2: Gene al se -up o educ ion and con olled p ecipi a ion in a aqueous media
me hod.
A ﬂow cha showing he diﬀe en s eps pe o med in he p ocess a e shown in
Fig. 3.3. The solu ion con aining he U-sal was p epa ed and he pH was adjus ed
by HCl addi ion. The pH was kep below 1 o a oid hyd olysis o he U4+ ca ion (see
Sec. 3.1.1) once he educ ion o he UVI-solu ion began.
In a second s ep, he ini ial UVI-solu ion was elec ochemically educed a cons an
po en ial o UIV , keeping he acidi y o he solu ion below he p ecipi a ion pH o he
gi en ini ial U-concen a ion in he solu ion (UIV -solubili y line Fig. 3.1). The s a e o
elec olysis o he solu ion was con olled by con inuous CV-checks, a he ime ha
UV- is abso p ion es s we e pe o med.
A e educ ion o he solu ion, p ecipi a ion p oceeded. Aliquo s o NaOH we e
added in o he solu ion whe e a se ies o s epwise p ecipi a ions we e ca ied ou . In he
me hod epo ed by [Rousseau e al., 2009] he p ocedu e was he o he way a ound.
In ha case [Rousseau e al., 2009] aliquo s o U-solu ion ( educed o un educed
depending on he me hod used) we e added in a solu ion wi h highe pH whe e he
p ecipi a ion was immedia ely aking place. The quan i ies o ma e ial pe day and
ba ch ob ained we e e y small. By changing he p ocedu e, and by inc easing a
he same ime he U-concen a ions in he solu ion (i.e. wi h espec o he ones
26
3.2. Mo he solu ion p epa a ion.
used by [Rousseau e al., 2009] and [Gil e al., 2010]), an inc ease o he yield o he
p ecipi a es was sea ched.
Black nc-UO2-p ecipi a es appea ed so con inuously un il no mo e UIV -ca ion was
p esen in he solu ion. All educ ion and p ecipi a ion expe imen s we e conduc ed
unde anoxic condi ions in a glo e-box unde N2a mosphe e (oxygen < 0.5%).
Cen i uga ion o he blackened solu ion was hen pe o med o collec he p e-
cipi a es. Remo al o he Na+and Cl−species s ill p esen on he su ace o he
we nc-UO2-p ecipi a es was achie ed by epea ed washing wi h deionised wa e
and soniﬁca ion. Cen i uga ion a e each washing s ep was needed o sepa a e he
p ecipi a es om he wa e con aining he dissol ed Na+and Cl−species washed.
In he cou se o he abo e expe imen s, he u anyl con aining solu ions changed
i s colou om yellow-g een, cha ac e is ic o he u anium ni a e hexahyd a e sal , o
g een da k colou , once he specie UVI was educed o UIV . In bo h cases (oxidised
and educed), he solu ion had in ense colou bu no u bidi y was obse ed. Once he
ﬁ s aliquo s o NaOH we e d opped in o he eac o , he g een colou ed anspa en
UIV -solu ion began o acqui e u bidi y because o he incipien UO2c ys als p e-
cipi a ed. This g een- u bid colou changed p og essi ely o black wi h he ollowing
p ecipi a ion (see Fig. 3.3).
The mo phology and s uc u e o he ob ained nc-UO2we e cha ac e ized by means
o T ansmission Elec on Mic oscope (TEM) and conﬁ med by X-Ray Diﬀ ac ion
(RT-XRD).
# #
 
  
&$'%&
$%
→
$% 


 

##&

&
!
#
Figu e 3.3: Con olled massi e p ecipi a ion in aqueous phase s eps.
3.2 Mo he solu ion p epa a ion.
The mo he solu ion was p epa ed by dissolu ion o UO2(NO3)2powde (CAS:
10102-06-4; yellow g een c ys als; Fig. 3.4) in deionised wa e by con inuous s i ing a
80°C o each a solu ion concen a ion o 500 gU/L. This mix u e a oom empe a u e
(RT) was dilu ed in a NaCl 1 M solu ion o ob ain ﬁnal u anium concen a ions in he
27
Chap e 3. Syn hesis o nc-UO2by con olled massi e p ecipi a ion in Aqueous phase
3.3.2 Elec ochemical educ ion o he mo he solu ion.
3.3.2.1 Expe imen al a angemen .
The expe imen al a angemen used o he elec ochemical educ ion was in essence
ha shown in he gene al illus a ion o he p ecipi a ion me hod in Fig. 3.2.The
gene al p inciples o he elec olysis a e desc ibed in Sec. 2.1.2. Th ee elec odes
we e so imme sed in he p epa ed solu ion in a 150 mL glass eac o buil o hese
expe imen s. The solu ion con aining he U-analy e (UO2(NO3)2) was p epa ed a
diﬀe en concen a ions be ween 0.02 un il 0.5 M in U (depending on he expe imen ).
And an excess o he suppo ing elec oly e (1 M NaCl o 1 M HCl) was added he e,
oo. The expe imen al se -up o he educ ion cell used is shown in Fig. 3.10,whe e
some modiﬁca ion can be app ecia ed wi h espec o one used o he CV- es s (see
Fig. 3.5). In he CV expe imen s, he ne elec ode was used as coun e elec ode,
bu o he p esen case ( educ ion o he solu ion), he ne was used as a wo king
elec ode (ca hode). Du ing he educ ion o he pai UO2+
2/U+4, a la ge wo king
elec ode a ea is desi ed o achie e highe educ ion a es, o which he ne elec ode
(wi h la ge su ace ha o he spi al elec ode) was employed o ha pu pose. The
spi al was used as a coun e elec ode (anode).
















Figu e 3.10: Elec ochemical educ ion se -up.
To p e en he e-oxida ion o he ob ained UIV o UVI because he anodic (coun -
ing elec ode o spi al) p oduces oxygen, he anode was in oduced in a sepa a ed glass
ube o 12 mm in diame e , wi h a glass- i (40 μm as de e mined in he CV s udy)
on i s bo om. This is seen as a undamen al modiﬁca ion o he app oach adop ed by
[Rousseau e al., 2009] and [Gil e al., 2010] whe e ca hode and anode we e imme sed
in he same ba h. The i allows he passage o he elec oly e bu pa ially a oids
he passage o he oxygen o he solu ion, so ha he en i e p ocess is mo e eﬀec i e
34

3.3. UIV /UVI cyclic ol amme ic and spec opho ome ic s udy.
speeding he educ ion p ocess.
Con inuous bubbling o he solu ion wi h ine gas (A ) was applied o a oid any
aces o O2which could pass h ough he i . A humidiﬁca ion bo le was ins alled
be ween he a gon supply and he eac o essel, so ha he A was p e-bubbled
and sa u a ed wi h wa e be o e en e ing in he eac o . This ensu es d y A gas is
sa u a ed wi h wa e and he losses o liquid du ing he expe imen a e minimised.
The bubbling oge he wi h he s i ing ein o ced he homogeniza ion o he mix u e
(s ock solu ion) du ing he whole expe imen . Same as in he CV- es s, all educ ion
and p ecipi a ion expe imen s we e conduc ed unde anoxic condi ions in a glo e-box
unde N2a mosphe e (O2<0.5%).
In a ypical expe imen , an aliquo o he u anyl (UVI) ni a e s ock-solu ion was
dilu ed in a 1 M NaCl solu ion o yield a ﬁnal U-concen a ion o 0.1 M in a 75 mL
ba ch. Be o e beginning any elec ochemical educ ion s ep, he pH was measu ed
wi h a pH glass elec ode (iEco ode Plus Me ohm 3 M KCl) in combina ion wi h a
Ti ando 906 measu ing ins umen om Me ohm. Adjus men wi h HCl 1 M was
pu sued o main ain he solu ion wi h a pH<1 o a oid he hyd olysis (see Sec. 3.1.1)
o he al eady educed U4+, be o e he onse o he p ecipi a ion was induced by
con olled NaOH-alkalinisa ion.
The educ ion o UVI-solu ion o UIV was pe o med unde a cons an po en ial
(-0.3 V s. Ag/AgCl as de e mined in he CV s udy) and he cu en a ied be ween
|-50| o |-5|A. The e o e he calcula ion o he needed ime o he elec olysis was
app oxima ed. The educ ion was apid a ﬁ s bu slowed as he ion concen a ion
dec eased as would be expec ed om Ohm’s Law (V=I·R), i.e. a cons an ol age,
he cu en is in e sely p opo ional o he esis ance.
The UVI-solu ion was p o ed o be elec ochemically educed o UIV almos
en i ely. Con inuous checks o he educ ion s a us we e ca ied ou by CV- es s du ing
he elec olysis o he solu ion o e i y he ex en o educ ion o he UVI ca ions
amoun (on obse ing he in ensi y diminu ion o he UO2+
2/U+4 ca hodic peak). Thus,
he educ ion was empo a ily hal ed and he ypical expe imen al CV a angemen
was adop ed in each check (Fig. 3.5). The spi al was disposed ou side he glass- i and
pu inside he eac o ba h in di ec con ac wi h he educing solu ion. A ﬂow and
s i e agi a ion we e s opped o a oid dis u bances. Scan speed as high as 0.1 V/s
was used as de e mined in he CV-s udy.
Du ing he educ ion, he eac o was showing inc easingly e idence o an al eady
educed g een UIV -solu ion, while inside he glass ube wi h he i a yellow UVI-
solu ion was s ill obse ed, due o he con inuous O2p oduc ion on he anodic side
(spi al).
3.3.2.2 Resul s and discussion.
Fig. 3.11 shows diﬀe en CV plo s eco ded a diﬀe en imes du ing he educ ion
s ep. The plo s show he dec ease o he ca hodic peak UO2+
2/U+4 wi h inc easing
elec ochemical educ ion ime, in e ms o he cu en o densi y (A/cm2) passing
be ween he wo ine P elec odes a he co esponding po en ial. The elec ochemical
esis ance o he media inc eased wi h ime because he ions concen a ion o he
35
Chap e 3. Syn hesis o nc-UO2by con olled massi e p ecipi a ion in Aqueous phase
species o educe diminished. The diﬀusion o he ca hode o he ions, which a e s ill
no educed, became hence mo e diﬃcul , explaining so he dec eased ca hodic cu en .
Figu e 3.11: Cyclic ol ammog am 0.1 M U and pH<1. UO2(NO3)2solu ion in HCl 0.33 M
scanned be ween -0.4 and +1.4 V s. Ag/AgCl (sa u a ed) a a scan a e o 0.1 V/s. The
heo e ical educ ion a e o he ion UO2+
2a each momen was calcula ed wi h Eq. 2.7.
The pH ose om 0.2 o 0.6 a he end o he elec ochemical educ ion. Tha
could be due o he acid consump ion du ing he elec oly ic educ ion o he u anyl
ion (ca hodic eac ion; see Eq. 2.1). Tha could be ele an i a pH>1 was in he end
ob ained because hyd olysis o UIV is expec ed.
3.3.3 UV-Vis spec opho ome y o he solu ion.
3.3.3.1 Expe imen al.
Diﬀe en Aliquo samples we e aken along he elec olysis o ul a iole - isible
spec oscopy (UV-Vis) analysis, which we e pe o med o moni o he alence s a e o
U be o e and du ing he con olled po en ial educ ion o UO2(NO3)2in HCl solu ion
(Fig. 3.12). The UV-Vis/NIR spec opho ome e used o eco d he UV-Vis abso p ion
spec es is desc ibed in Sec. 2.2.1. The in o he solu ion was changing along he
educ ion om an ini ial clea -yellow colou ypical o UO2+
2ion, o a da k-g een colou
ypical o U4+ a he end o he p ocess (see Fig. 3.13 showing he p og essi e colou
change o he U-solu ion upon i s educ ion).
3.3.3.2 Resul s and discussion.
The abso p ion bands in he ange om 375 o 500 nm, which a e cha ac e is ic o
he abso p ion o u anyl (UVI) ions, g adually dec eased and ﬁnally disappea ed wi h
36
3.3. UIV /UVI cyclic ol amme ic and spec opho ome ic s udy.
Figu e 3.12: Change in isible abso p ion spec a o he educ ion o 0.1 M U and ini ial
pH<1. UO2(NO3)2solu ion in HCl 0.33 M. The heo e ical educ ion a e o he ion UO2+
2
a each momen was calcula ed wi h Eq. 2.7.
inc easing he elec olysis educ ion ime. A he same ime, he abso p ion bands
om 400 o 700 nm, which a e consis en wi h abso p ion ypical peaks o UIV (426,
492, 548 and 646 nm), appea ed wi h s onge in ensi y as he ime o elec ochemical
educ ion o he solu ion inc eased. Tha was in ag eemen wi h he in ensi y dec ease
o he UO2+
2/U+4 ca hodic peak obse ed in he CV es s o he solu ion (Fig. 3.11)
wi h he ime o educ ion. This esul s ongly suppo s he ac ha UO2+
2in he
acidic solu ion was almos ully educed o UIV a he P elec ode. The p og ess o
he elec ochemical educ ion, as analysed by spec opho ome y, is shown in Fig. 3.12.
In Fig. 3.14, he in ensi y peak dec ease by bo h cyclic ol amme y (e.g. UO2+
2/U+4
ca hodic peak, Fig. 3.11) and UV-spec opho ome y (e.g. UO2+
2abso p ion peak



Figu e 3.13: U-solu ion a diﬀe en s eps du ing elec ochemical educ ion
37
Chap e 3. Syn hesis o nc-UO2by con olled massi e p ecipi a ion in Aqueous phase
a 412.43 nm, Fig. 3.12), a e plo ed as a unc ion o he pe cen age o UO2(NO3)2
elec ochemical con e sion (analysis pe o med on he same aliquo s o solu ion). Co -
espondence be ween bo h me hods in he de e mina ion o he UO2+
2/U+4 con e sion,
is e y sa is ac o y.

















          



Figu e 3.14: In ensi y peak dec ease by bo h CV (e.g. UO2+
2/U+4 ca hodic peak, Fig. 3.11)
and UV-spec opho ome y (e.g. UO2+
2abso p ion peak a 412.43 nm, Fig. 3.12) as a unc ion
o he pe cen age o UO2(NO3)2elec ochemical con e sion.
3.4 P ecipi a ion and sepa a ion o he UO2-
nanoc ys als.
3.4.1 In oduc ion ema ks.
The p ecipi a ion om he elec ochemically educed UIV -solu ion (0.5 M U o he
highes concen a ion) was achie ed by g adual alkalinisa ion o he solu ion ollowing
as close as possible he heo e ical solubili y limi line o UIV species in aqueous
media Fig. 3.1. This was pu sued o a oid signiﬁcan agglome a ion o he p ecipi a es
(UO2(c)o UO
2·xH2O(am)), which was ai ed o occu i he sys em en e ed in
he egion o ank supe sa u a ion, as i could be induced by uncon olled b usque
alkalinisa ion (pH-inc ease much beyond he equilib ium line).
Indeed, he heo y o p ecipi a ion om supe sa u a ed solu ions and a numbe
o dedica ed expe imen al wo ks indica e ha he size o he nuclei-p ecipi a es is
in e sely p opo ional o he supe sa u a ion deg ee, while he numbe o nuclei is
di ec ly p opo ional o i (homogeneous p ecipi a ion) [Li shi z and Slyozo , 1961]
[B is ow e al., 2001][Wu e al., 2008][Maeda e al., 2009]. Howe e , unde conside -
a ion o kine ic aspec s (e.g. coagula ion a es), also conside able numbe o s udies
exis indica ing ha he supe sa u a ion deg ee is a key ac o igge ing he agglom-
e a ion o p ecipi a es, wi h he e idence ound ha he la ge he supe sa u a ion
deg ee he la ge he size o he agglome a es o med (including bo h homogeneous
and he e ogeneous p ecipi a ion) [Yu e al., 2007][Claassen and Sandenbe gh, 2006]
38
3.4. P ecipi a ion and sepa a ion o he UO2-nanoc ys als.
[Zums ein and Rousseau, 1989][Pack e , 1958][Sa ig e al., 1978].
The quan i y o NaOH heo e ically needed o each a gi en pH can be calcula ed
as he sum o [OH−]molsA+[OH−]molsB, whe e he ﬁ s e m Aindica es he
numbe o OH mols needed o inc ease he ini ial pH o he wished ﬁnal pH, as gi en by:
[OH−]molsA=[H+]molsIni ial −[H+]molsEnd (3.1)
whe e
[H+]molsIni ial =10
−pHIni ial
and
[H+]molsEnd =10
−pHEnd
and he second e m Bindica es he numbe o OH mols needed o p ecipi a e he
species U+4 as UO2, as gi en by he eac ion:
U4+ +4OH−↔UO2+2H2O(3.2)
and whe e
[OH−]molsB=4·U4+mols (3.3)
This heo e ically amoun o NaOH needed o se a gi en pH was hence es ima ed
and was aken in o accoun as indica i e alue o he subsequen p ecipi a ion s ep.
3.4.2 Expe imen al s eps.
The p ecipi a ion was pe o med in he same eac o used o he elec olysis expe -
imen s. The a angemen used o his s ep is shown in Fig. 3.15. No po en ial was
applied du ing he p ecipi a ion bu he a mosphe e condi ions we e main ained in
he same way as in he elec ochemical educ ion s ep: i.e. he a angemen was
kep inside a glo e-box unde anoxic condi ions (N2a mosphe e wi h O2<0.5%)and
dynamic A gas ﬂow (passed h ough a humidiﬁca ion bo le o sa u a e i wi h wa e
be o e en e ing in he eac o o a oid he d yness o he solu ion), was applied.
The pH a he beginning o he p ecipi a ion, was below 0.5. A se ies o s epwise
p ecipi a ions be ween pH<0.5 and 3 we e hen ca ied ou by addi ion o 3 M NaOH
solu ion a a a e o 10-20 μl/min, as con olled by a dosage ins umen (Ti ando 906
om Me ohm). The slow alkalinisa ion was au oma ically s opped when he solu ion
eached he pH desi ed, and was e-s a ed when he pH e ol ed backwa ds ( h ough
p ecipi a ion, Eq. 3.3) and was again below he se pH.
Black nc-UO2-p ecipi a es appea ed a ound pH∼1 and he solu ion began o look
u bid om his poin onwa ds. The pH a ia ion du ing he p ecipi a ion s ep was
39

Chap e 3. Syn hesis o nc-UO2by con olled massi e p ecipi a ion in Aqueous phase











Figu e 3.15: P ecipi a ion o nc-UO2se -up.
moni o ed con inuously. An example o his a ia ion wi h ime o he case o he
highes U-concen a ion used in he expe imen s (0.5 M) is shown in (Fig. 3.16).
In he displayed case, a cons an dosage a e o he alkalinising NaOH-solu ion o
app oxima ely 1.2 ml/h was used. Small pH-d i s occu ed because o he o ma ion
o nc-UO2c ys als. In addi ion o his, on eaching he pH he alue o 1, a main jump
o ab up slope inc ease was obse ed in he pH s. cu e (Fig. 3.16), which e ealed
he main p ecipi a ion o UO2c ys als in he solu ion has al eady occu ed, when he
pH su passed his p ecipi a ion h eshold.
The quan i y o NaOH heo e ically needed o each he se pH was calcula ed as
p e iously indica ed as he sum o [OH−]molsA+[OH−]molsB. The NaOH-solu ion
was slowly added a cons an a e and he pH was moni o ed (Fig. 3.16). The NaOH
addi ions s opped au oma ically a e app oxima ely 30 h o p ecipi a ion on eaching
he pH he alue 3. A e his poin , no u he essen ial modiﬁca ions o he pH
we e egis e ed du ing a leas 3 days mo e (>100 h a e ini ia ion o he p ocess),
indica ing ha he p ecipi a ion had been al eady almos comple ed when he pH
achie ed he alue 3.
Cen i uga ion (3500 pm and 30 min) o he blackened solu ion was pe o med
o ga he he p ecipi a es. Na+and Cl−species we e p esen in he ﬁnal solu ion
because o he ini ial addi ions o HCl o acidi y solu ion and he pos e io addi ions o
NaOH used o p ecipi a e he UIV -species as oxide in he solu ion. The we nc-UO2-
p ecipi a es hold hese impu i y species on i s su ace, which will p ecipi a e as small
NaCl c ys als once he ﬁnal p oduc d ies. The e o e, he we nc-UO2-p ecipi a es
we e e-dispe sed in deionised wa e and soniﬁca ed o dilu ing and washing ou he
possible Na+and Cl−species concen a ion p esen . Subsequen cen i uga ion, o
40
3.4. P ecipi a ion and sepa a ion o he UO2-nanoc ys als.
Figu e 3.16: Con olled nc-UO2p ecipi a ion om a elec ochemically educed UIV -solu ion
0.5 M U in HCl 1M. pH moni o ing s. NaOH addi ion and ime.
sepa a e he nc-UO2-p ecipi a es om he wa e con aining he dissol ed Na+and
Cl−species, was done. This washing-cen i uga ion s ep was pe o med 5 imes using
a olume o 40 ml o deionised wa e p o washing ope a ion and a pa ial cha ge
o 1.25 g nc-UO2each ube (4 ubes we e cen i uged in pa allel). Fo he highes
U-concen a ions o (0.5-0.6 M), a ound 10 g o nc-UO2p ecipi a es we e ob ained in
o al om each p ecipi a ion ba ch.
3.4.3 Spec opho ome y o he solu ion.
The UV-spec opho ome e was used du ing he p ecipi a ion p ocess o iden i y he
dec ease in U concen a ion in he solu ion (see Fig. 3.17 whe e he abso p ion band a
647.04 nm ypical om U+4, has been moni o ed). Wi h inc easing o he pH (addi ion
o NaOH o he solu ion), inc easing o nc-UO2p ecipi a es occu ed and educ ion o
he UIV in solu ion diminished. The change in he isible abso p ion spec a could no
be obse ed eﬀec i ely ill he end o he p ecipi a ion because o he u bidi y o he
solu ion. A pic u e o he educed g een UIV -solu ion be o e o begin he p ecipi a ion
and he black ﬁnal solu ion con aining he nc-UO2-p ecipi a es, is shown in Fig. 3.18.
Wi h he solubili y cons an s o UO2(c) and UO2(am), and he s abili y cons an s
o UIV hyd olysis epo ed by [Guillaumon e al., 2003], a g aph wi h he specia ion
o all u anium en i ies p esen a diﬀe en pH in he solu ion as well as he solubili y
lines o UO2(c) and UO2·xH2O (am) can be ep esen ed, as i was unde aken in
Fig. 3.19. The specia ion cu es add knowledge o wha is happening in he solu ion
as he pH is inc eased in he p ecipi a ion s ep. The yellow line ep esen s he piece o
solubili y/p ecipi a ion line ollowed du ing p ecipi a ion. The yellow ci cles ep esen
he expe imen al p ecipi a ion poin s obse ed in he UV-abso p ion spec a (Fig. 3.17)
ha ing in accoun he diﬀe en in ensi ies ob ained. These expe imen al p ecipi a ion
poin all all on he heo e ical p ecipi a ion line.
41
Chap e 3. Syn hesis o nc-UO2by con olled massi e p ecipi a ion in Aqueous phase
Figu e 3.17: Change in isible abso p ion spec a o he ypical UIV band be ween 630 and
665 nm, o a con olled nc-UO2p ecipi a ion om a elec ochemically educed UIV -solu ion
0.1 M U in HCl 1 M and pH<1 (le ). Diminishing o in ensi y wi h inc easing o he pH
because he p ecipi a ion o he U+4 in solu ion as nc-UO2( igh ).
(a) UIV -elec ochemically educed solu-
ion 0.5 M U in HCl 1M and pH<0.5.
(b) ∼10 g o black nc-UO2p ecipi a es
Figu e 3.18: U-solu ion be o e and a e he p ecipi a ion.
-16
-14
-12
-10
-8
-6
-4
-2
0
0.00
0.20
0.40
0.60
0.80
1.00
0 1 2 3 4 5 6
log[(U(IV)]
ac ion
pH
U4+
U(OH)22+
U(OH)3+
U(OH)31+
U(OH)4
Figu e 3.19: U anium Specia ion a diﬀe en acidic media and solubili y lines o UO2(c)
and UO2·xH2O (am) ep esen ed wi h he cons an s da a by [Guillaumon e al., 2003].
Yellow ci cles ep esen expe imen al poin s. Yellow line ep esen s he piece o solubili y
line ollowed du ing p ecipi a ion.
42
3.5. Cha ac e isa ion o he as-p oduced nanoc ys als.
3.5 Cha ac e isa ion o he as-p oduced nanoc ys-
als.
The mo phology and s uc u e o he ob ained nc-UO2pa icles we e cha ac e ized
by means o ansmission elec on mic oscope (TEM) and he s uc u e conﬁ med by
X- ay diﬀ ac ion (XRD).
3.5.1 P ecipi a es mo phology and composi ion.
Quali a i e composi ion analysis o he p ecipi a es ob ained was ca ied ou by TEM.
The cha ac e is ics o he ins umen used, as well as he p epa a ion o he analysed
specimens a e desc ibed in Sec. 2.3.2.
Fig. 3.21 shows he TEM image o nc-UO2syn hesised by he p esen ly desc ibed
me hod. The a e age p ecipi a e size was o 3.9(8) nm, as ob ained om he size
dis ibu ion (Fig. 3.20). The pa icles showed agglome a es o 50 nm (Fig. 3.21a).
Howe e , e y o en sepa a ed nanopa icles exis in suspension in he o iginal solu ion,
bu one sees agglome a es on he TEM g id, due o he p epa a ion o he TEM sample.
Figu e 3.20: Size dis ibu ion his og am om TEM measu emen s o nc-UO2syn hesized by
p ecipi a ion in aqueous media. Diame e a e age size o 3.9(8) nm diame e a e age size.
The collec ed black p ecipi a es p esen ed he ypical cc ﬂuo i e s uc u e o UO2.
The selec ed a ea elec on diﬀ ac ion (SAED) pa e n (inse o Fig. 3.21c) e ealed
he polyc is allini y o he ma e ial wi h he cc s uc u e. The calcula ed in e e ence
inge spacing in he HRTEM image (Fig. 3.21c) was abou 0.315 nm, which was in
ag eemen wi h he in e plana dis ance o he [111] plane in he cc ﬂuo i e s uc u e
(0.3153 nm o he UO2s anda d 00-041-1422-ICCD).
3.5.2 P ecipi a es c ys al s uc u e.
The c ys al s uc u es and c ys al size o he as syn hesised nc-UO2obse ed in
he TEM analysis we e conﬁ med by XRD. The cha ac e is ics o he ins umen
used a e desc ibed in Sec. 2.4.1. The c ys al s uc u e o he nano-p ecipi a es was
43
Chap e 4. Syn hesis o nc-UO2and nc-ThO2by a p ecu so he mal decomposi ion in O ganic phase
Su ac an s inside he eac o bind on he syn hesized c ys als o ming an o ganic
capping laye ha p o ec s and s abilizes he c ys als agains ﬂoccula ion o excessi e
g ow h. La ge molecules su ac an s which p o ide g ea e s e ic hind ance, as well
as su ac an s ha cap s onge o he su ace o he c ys als, diminish he eagen s
inco po a ion a e in he nuclei and as a consequence hei ﬁnal size. Pa icles g ow h
can be s opped (be o e he ﬁnishing o he eagen s) by as cooling o he eac o
[Mu ay e al., 2000][Wang e al., 2003][Mu ay e al., 1993][Peng and Peng, 2001]
[Qu e al., 2001][Bu da e al., 2005].
To collec he pa icles a p ecipi a ion is induced by addi ion o a non-sol en
which is cha ac e ized o be pa ially miscible wi h he media o he eac ion bu has
no in e ac ion wi h he capping agen s. The dispe sion is des abilized and he solu ion
becomes u bid due o ﬂoccula ion o he c ys als. Cen i uga ion accele a es he
p ocess and c ys als a e collec ed on he bo om o he cen i uge ubes. A e wa ds
he liquid is decan ed and he c ys als collec ed. The ma e ial ob ained is o med
by he nc-ma e ial and he o ganic capping laye which pe mi s i s e-dispe sion
in an o ganic sol en [Mu ay e al., 2000][Wang e al., 2003][Mu ay e al., 1993]
[Peng and Peng, 2001][Qu e al., 2001][Bu da e al., 2005].
The o ganic solu ion phase decomposi ion ou e has been widely used o he
syn hesis o me al-oxide nanoc ys als. In his p ojec he syn hesis o nc-UO2in
o ganic media was de eloped ollowing he second app oach abo e desc ibed and he
me hod epo ed by [Wu e al., 2006] whe e u anyl ace ylace ona e (UAA Fig. 4.1a)is
used as p ecu so in a mix u e o long chain sol en s as oleic acid (OA), oleylamine
(OAM) and oc adecene (ODE) which a e s able a high empe a u es whe e his kind
o me al-oxide nanoc ys als a e no mally o med [Willis e al., 2007].
(a) UAA (b) ThAA (c) ThAc
Figu e 4.1: P ecu so molecula s uc u es used in he o ganic syn hesis.
The decomposi ion o he UAA p ecu so and he amine (OAM) is ollowed by a
educ ion o UVI (UAA) o UIV (UO2) a highe empe a u es. OAM migh ac as
educing agen [Wu, 2008]. A condensa ion eac ion be ween OA and OAM occu s
ending in he o ma ion o N-(cis-9-oc adecenyl)oleamide (OOA Fig. 4.2-le )and
wa e , and is almos e mina ed be o e he beginning o he nuclea ion, as epo ed by
[Wu e al., 2006]. OOA is desc ibed he e as une o he eac ion in e media e s eps
o o m he UO2nanopa icles, whe e ee UO2uni s and clus e s ha e in e ac ion
wi h he amide (R-NH+
3) and he ca boxylic (R-COO−) g oups (see Fig. 4.2- igh )
[Wu, 2008].
50

4.1. In oduc ion and p inciples.
29




2
9
Figu e 4.2: Oleic Acid (OA), Oleylamine (OAM) and N-(cis-9-oc adecenyl)oleamide (OOA)
ob ained a e he condensa ion eac ion oge he wi h wa e [Wu e al., 2006](le ).In e -
media e s eps o he nc-UO2syn hesis whe e ee UO2uni s and clus e s in e ac wi h he
o med OOA ( igh ) [Wu, 2008].
Howe e , no OOA bu jus an olea e was ound bonded h ough chela ing biden a e
in e ac ion on he su ace o he nc-UO2[Wu e al., 2006]. A s ong a ac ion appea s
be ween he olea e and he me al g oup making a e y compac monolaye a ound he
nanoc ys als inhibi ing hei g ow h and p o ec ing hem agains agglome a ion (see
Fig. 4.3).


Figu e 4.3: Olea e as capping ligand bonded h ough chela ing biden a e in e ac ion on he
su aceo henc-UO
2.
51
Chap e 4. Syn hesis o nc-UO2and nc-ThO2by a p ecu so he mal decomposi ion in O ganic phase
4.2 Expe imen al.
In he p esen wo k he p ecu so u anyl (UVI)-ace ylace ona e (UAA) o UO2
nanoc ys als and ho ium (ThIV )-ace ylace ona e (ThAA) o ho ium (ThIV )-ace a e
(ThA) o ThO2nanoc ys als (see Fig. 4.1), we e dissol ed in a solu ion o oleic
acid (OA Fig. 4.2) and 1-oc adecene (ODE) a 110°C unde con inuously s i ing.
Oleylamine (OAM Fig. 4.2) was added o he mix u e and was hea ed a an a e age
a e o 20°C/min un il 310 o 350°C was eached (depending on he expe imen ).
A e wa ds he g ow h solu ion was aged o 1-6 h (depending on he expe imen ) and
cooled o oom empe a u e.
The expe imen al se -up in ol ed a hea e , a small- ound essel, a coole and a
con inuous A ﬂow. A scheme as ep esen a ion o he se -up is shown in Fig. 4.4.
The solu ion p epa a ion as well as he eac ion s eps we e conduc ed unde anoxic
condi ions in a glo e-box unde N2a mosphe e (O2<0.5%). Pos e io washing o he
oxide-c ys als, as well as cen i uging was pe o med. As a ﬁnal p oduc , nc-p ecipi a es
o UO2and ThO2 edispe sible in o ganic sol en s we e ob ained. A pic u e o he
diﬀe en s eps is shown in Fig. 4.5.
111
2
3
4567
8
9
11
2
3
4567
8
9
11
The mocouple
A
S i e
Hea ing man le
S i ing pla e
N2Glo e-Box
Wa e cooling
nc-UO2Syn hesis Reac o
Figu e 4.4: A angemen o he o ganic he mal decomposi ion me hod.
Reduc ion o su ac an quan i ies wi h espec o he me al con en in compa ison
wi h [Wu e al., 2006] was made. Pos e io scale-up o he me hod om ini ial
quan i ies o abou 0.1 g o nc-UO2as epo ed by [Wu e al., 2006], o quan i ies o
abou 2 g o nc-UO2achie ed he e, was done. The same me hod was also ex apola ed
o he syn hesis o nc-ThO2.
52
4.2. Expe imen al.





#&"!


'
%""!$&! 
'


#""!
Figu e 4.5: S eps in he he mal Decomposi ion o UAA in o ganic media.
4.2.1 Solu ion p epa a ion.
4.2.1.1 UO2nanoc ys als p epa a ion.
In his me hod UAA (UO2(CH3COCHCOCH3)2CAS: 18039-69-5 yellow o ange
c ys als om Ibilabs based on deple ed u anium 235U 0.3-0.4%-238U 99.6%Fig. 4.1a)
was used as a p ecu so . Indeed, 4 g o UAA we e dissol ed in a solu ion o 15 mL
OA (9-Oc adecenoic acid CAS: 112-80-1 99% om Sigma Ald ich) and 27 mL ODE
(Oc adecene-1 CAS: 112-88-9 90% om Sigma Ald ich) inside a 100 mL h ee-necked
glass eac o unde con inuous s i ing. The h ee necks o he essel we e occupied
by an A ﬂow, a condense and a empe a u e senso connec ed o a empe a u e
con olle . The eac o was placed on a hea ing man le and he solu ion was slowly
hea ed up o 150°C and main ained a his empe a u e du ing 20 min. A e wa ds he
solu ion was le o cool o oom empe a u e and 21 mL OAM (1-Amino-9-oc adecene,
CAS: 112-90-3, 70%, Sigma Ald ich) we e added o he mix u e. The solu ion was hen
slowly a hea ed 100°C unde con inuous s i ing du ing 15 min. Du ing his hea ing
he solu ion passed om an ini ial u bid yellow s a e o a good mixed, anspa en
o ange s a e (see Fig. 4.6).
(a) (b)
Figu e 4.6: (a) UAA + ODE + OA a RT (be o e applying any empe a u e). Tu bid yellow
solu ion. (b) UAA + ODE + OA + OAM a RT (a e s i ing a 100°C). T anspa en o ange
solu ion.
53
Chap e 4. Syn hesis o nc-UO2and nc-ThO2by a p ecu so he mal decomposi ion in O ganic phase
As sol en s ODE, OA and OAM we e in oduced in he eac o . The long chained
alcohol (OA) was used as su ac an , and as educing agen a long chained amine
(OAM). The boiling poin s o he OA, OAM and ODE a e 360, 364 and 315°C
espec i ely.
4.2.1.2 ThO2nanoc ys als p epa a ion.
In his case ThAA (Th(CH3COCHCOCH3)4CAS: 17499-48-8 om Ibilabs colo less
c ys als Fig. 4.1b) o ThA (Th(C2H3O2)4CAS: 13075-28-0 om Ibilabs colo less
c ys als Fig. 4.1c) we e used as p ecu so s. The s eps ollowed o he p epa a ion o
nc-ThO2we e simila o hose used o he syn hesis o nc-UO2. In his case 0.60 g
o ho ium ace ylace ona e (ThAA) o ho ium ace a e (ThA) we e dissol ed in a
solu ion o 3 mL OA and 4 mL ODE inside a 50 mL h ee-necked glass eac o unde
con inuous s i ing. The h ee necks o he essel we e as be o e occupied by an A
ﬂow, a condense and a empe a u e senso connec ed o a empe a u e-con olle . The
eac o was placed on a hea ing man le and he solu ion was slowly hea ed up o 150°C
and main ained a his empe a u e du ing 20 min. A e wa ds he solu ion was le
o cool o oom empe a u e and 4 mL OAM we e added o he mix u e. The solu ion
was hen slowly hea ed a 100°C ( empe a u e high enough o allow a good mixing bu
no high enough o p o oke he eac ion) unde con inuous s i ing du ing 15 min. The
solu ion passed om an ini ial u bid-whi e o a good mixed anspa en whi e solu ion.
4.2.2 Reac ion s ep.
4.2.2.1 UO2nanoc ys als p oduc ion.
Once he OAM was also mixed in he solu ion, e e y hing was hea ed wi h an
a e age a e o 25°C/min un il 300°C. The mix u e was so p og essi ely changing i s
colou om anspa en o ange-b own a 100°C o u bid da k b own a 190°C, o
b own-black a 200°C, un il ﬁnally u ning o comple ely black a 250°C (see Fig. 4.7).
Small explosions inside he eac ion essel occu ed a ha empe a u e. Once he
empe a u e pla eau a 300°C was eached, he solu ion was aged a his empe a u e
o 60 min. Du ing his ageing ime, small explosions con inued inside he essel.
The solu ion p epa a ion as well as he eac ion s eps, we e conduc ed unde
anoxic condi ions in a glo e-box unde N2a mosphe e (O2<0.5%). Fu he mo e,
a con inuous A gas ﬂow was supplied inside he eac o essel in o de o keep
an ine a mosphe e. E en so, a lo o wa e and oxygen molecules a e al eady
inside he expe imen al sys em (UAA con ains wo wa e molecules and wo oxy-
gen a oms). The small explosions obse ed up o 250°C could be due o an eﬀec o
oxygen eac ing wi h he sol en s du ing he mal decomposi ion, a hese empe a u es.
The UAA and he OAM we e he mally decomposed and subsequen educ ion
o he UVI (UAA) o UIV (UO2) occu ed using OAM as a educing agen , as
sugges ed by [Wu, 2008]. As a ﬁnal ma e ial, nc-UO2we e ob ained. The su ac an
bonded o he nanoc ys als o ming a lipophylic su ace on hem. This capping would
a oid he agglome a ion o he nanoc ys als and would allow he pos -collec ion o he
c ys als in an o ganic sol en . Finally he solu ion was le o cool o oom empe a u e.
54
4.3. P ecipi a ion and sepa a ion o he nanoc ys als.
Figu e 4.7: U-solu ion du ing he eac ion s ep a diﬀe en empe a u es (160, 190 and 250°C).
4.2.2.2 ThO2nanoc ys als p oduc ion.
Once he OAM was added o he mix u e, e e y hing was hea ed a 25°C/min up o
300°C. In he expe imen s whe e highe empe a u es we e achie ed (330°C), ODE
was subs i u ed because i s boiling poin a 315°C, and jus OA and OAM we e used
as sol en s o he eac ion. The mix u e p og essi ely changed i s colou om u bid
whi e a 100°C o yellow anspa en a 200°C, and o o ange anspa en du ing he
ageing ime a 300°C and o pale yellow anspa en a he end o he ageing (see
Fig. 4.7). No explosions occu ed du ing any o he ThO2-expe imen s. Diﬀe en
ageing imes be ween 1 o 5 h we e es ed.
Figu e 4.8: Th-solu ion du ing he eac ion s ep a diﬀe en empe a u es.
4.3 P ecipi a ion and sepa a ion o he nanoc ys-
als.
4.3.1 UO2nanoc ys als eco e y.
The pa icles coa ed and s abilized wi h a non-pola laye , we e he e o e soluble
in highly non-pola sol en s such as ODE o oluene. The p ecipi a ion o he
nanoc ys als was induced by adding a non-sol en , a mix u e o hexane/ace one (1/4)
o he aged solu ion. The mix u e was suﬃcien ly apola as o selec i ely p ecipi a e
he ela i ely non-pola pa icles wi hou p ecipi a ing ou he ODE and excess OAM
and OA (which a e aken by he hexane ac ion). Pu e ace one would be oo pola
o be di ec ly miscible wi h ODE. Ins ead, he use o a mix u e wi h a mode a ely
non-pola sol en such as hexane pe mi s hei addi ion being miscible in he solu ion
wi hou o ming a seconda y immiscible liquid laye .
55

Chap e 4. Syn hesis o nc-UO2and nc-ThO2by a p ecu so he mal decomposi ion in O ganic phase
Figu e 4.9: Typical ﬁnal UO2solu ion a e he eac ion s ep and be o e p ecipi a ion.
Once he mix u e o hexane/ace one was in oduced, he dispe sion was des abilized
and he solu ion became u bid due o he ﬂoccula ion o he black nc-UO2p ecipi a es.
The sepa a ion was o ced by cen i uging he new mix u e a 3500 pm (EBA 20
cen i uge om He ich) o 10 min. The pa icles accumula ed a he bo om o he
cen i uging ubes and he supe na an liquid con aining he emaining o ganics (ODE,
OA and OAM) was aken away. New hexane/ace one was in oduced in he ubes
and he cen i uging s ep epea ed. This p ocedu e was ei e a ed un il he ex ac ing
liquid was clea (see Fig. 4.10). A e cleaning he nanoc ys als se e al imes wi h
he mix u e hexane/ace one, he black nc-UO2we e e-dispe sed in an o ganic sol en
such as hexane o oluene.
2
Figu e 4.10: P ecipi a ion, cleaning and ecollec ion in an o ganic sol en (hexane) o he
nanoc ys als o UO2.
4.3.2 ThO2nanoc ys als eco e y.
The ThO2nanoc ys als whe e ob ained in he same way, bu mo e epea ed washing
and cen i uging s eps we e needed o ob ain whi e ThO2p ecipi a es a he end o he
sepa a ion whi e ThO2p ecipi a es. The hexane/ace one (1/4) mix u e was in oduced
in he pale yellow eac ion solu ion, which became whi e u bid. Then he c ys als began
o ﬂoccula e (see Fig. 4.11) and a e cen i uging a pale-o ange gel appea ed on he
bo om on he ubes. A e se e al washing s eps, howe e , whi e ThO2c ys als we e
ﬁnally ob ained. The ThO2whe e edispe sed in an o ganic sol en (hexane o oluene).
56
4.4. Cha ac e isa ion o he as p oduced nanoc ys als.
Figu e 4.11: P ecipi a ion, cleaning and ecollec ion in an o ganic sol en ( oluene) o he
nanoc ys als o ThO2.
4.4 Cha ac e isa ion o he as p oduced nanoc ys-
als.
The mo phology and s uc u e o he ob ained nc-UO2pa icles we e cha ac e ized by
means o ansmission elec on mic oscope (TEM), dynamic ligh sca e ing (DLS),
and he s uc u e conﬁ med by X- ay diﬀ ac ion (XRD).
4.4.1 P ecipi a es mo phology and composi ion.
The mo phologies and dimensions o he samples we e e ealed by TEM. The cha ac-
e is ics o he ins umen used, as well as he p epa a ion o he analysed specimens
a e desc ibed in Sec. 2.3.2. The o ganic ou e led o high-quali y monodispe sed UO2
nanoc ys als and ThO2 od-shaped nanoc ys als.
4.4.1.1 UO2nanoc ys als mo phology.
In Fig. 4.13, he TEM image o syn hesised nc-UO2by he mal decomposi ion in
o ganic phase is shown. The a e age p ecipi a e size was o 4.9(3) nm, as ob ained
om he size dis ibu ion (Fig. 4.12).
The collec ed black p ecipi a es p esen ed he ypical cc ﬂuo i e s uc u e o UO2
and we e polyc ys alline, as shown by he ings o he SAED in Fig. 4.13c.The
selec ed a ea elec on diﬀ ac ion (SAED) pa e n (inse o Fig. 4.13c) e ealed he
polyc is allini y o he ma e ial wi h he cc s uc u e. The calcula ed in e e ence
inge spacing in he HRTEM image (Fig. 4.14) was abou 0.315 nm, which was in
ag eemen wi h he in e plana dis ance o he [111] plane in he cc ﬂuo i e s uc u e
(0.3153 nm o he UO2s anda d 00-041-1422-ICCD).
Dynamic ligh sca e ing (DLS) measu emen s in hexane medium du ing 80 s o
he dispe sed sample o nc-UO2ob ained by he mal decomposi ion in o ganic phase,
yielded an hyd odynamic a e age size o 3.7(1) nm wi h a polydispe si y index (PI) o
0.139. Cha ac e is ics o he ins umen a e shown in Sec. 2.2.2. The size dis ibu ion
his og am is shown in Fig. 4.15). The DLS size alues co espond well wi h he alues
obse ed by TEM.
57
Chap e 4. Syn hesis o nc-UO2and nc-ThO2by a p ecu so he mal decomposi ion in O ganic phase
Figu e 4.12: Size dis ibu ion his og am om TEM measu emen s o UO2nanopa icles
syn hesized by he mal decomposi ion o UAA in o ganic media. Diame e a e age size o
4.9(3) nm.
(a) The scale ba is 50 nm. (b) The scale ba is 20 nm.

(c) The scale ba is 5 nm. The in-
se o he ﬁgu e shows selec ed a ea
elec on diﬀ ac ion (SAED).
Figu e 4.13: TEM mic og aphs o UO2a low esolu ion, showing an assembly o nanoc ys als,
and a high esolu ion, e ealing la ice imaging o he nanoc ys als.
4.4.1.2 ThO2nanoc ys als mo phology.
In Fig. 4.13 he TEM image o syn hesised nc-ThO2by he mal decomposi ion in
o ganic phase is shown. Single c ys alline 1±0.5 nm in diame e ThO2-nano ods
esul ed upon p ecipi a ion. The e is s ill no ull unde s anding o he eason o he
ob ained shape. The p ecipi a e ma e ial ob ained was qui e diﬀe en in geome y
om he one ob ained om he UAA p ecu so , which was ins ead almos pe ec ly
sphe ical in shape. Diﬀe en ageing empe a u es (290-330°C) and imes (1-6 hou s),
as well as diﬀe en p ecu so s (ThAA and ThA) we e es ed, bu he esul s gained
we e always ThO2-nano ods.
The collec ed whi e p ecipi a es p esen ed he ypical cc ﬂuo i e s uc u e o ThO2
and we e polyc ys alline, as shown by he ings o he selec ed a ea elec on diﬀ ac ion
(SAED) pa e n (inse Fig. 4.16c) and he HRTEM image (inse Fig. 4.17) showing
58
4.4. Cha ac e isa ion o he as p oduced nanoc ys als.



Figu e 4.14: TEM mic og aph o UO2a high esolu ion, e ealing la ice imaging o he
nanoc ys als and in e plana dis ances.
Figu e 4.15: Size dis ibu ion his og am om DLS es o UO2nanopa icles syn hesized by
he mal decomposi ion o UAA in o ganic media. Hyd odynamic a e age size o 3.7(1) nm.
la ice spacings o an indi idual nanoc ys al. The calcula ed in e e ence inge spacing
in he HRTEM image (inse Fig. 4.17) was abou 0.322 nm, which was in ag eemen
wi h he in e plana dis ance o he [111] plane in he cc ﬂuo i e s uc u e (0.3232 nm
o he ThO2s anda d 00-042-1462-ICCD).
(a) HAADF STEM mic og aph.
The scale ba is 20 nm.
(b) The scale ba is 10 nm.





(c) The scale ba is 20 nm. The
inse o he ﬁgu e shows selec ed
a ea elec on diﬀ ac ion (SAED).
Figu e 4.16: STEM and TEM mic og aphs o ThO2nano ods.
59
Chap e 5. C ys alliza ion and G ain G ow h in (T) o nc-UO2by Aqueous ou e
DTA signal and is shown in Fig. 5.1. Upon hea ing, an almos in isible endo he m
wi h a maxima a ound 155°C occu s. This is accompanied by a sligh weigh loss
o abou 2.3 w %un il 207°C, which a e mos likely a ibu ed o wa e deso p ion.
Un o una ely, we we e no in a posi ion o iden i y he deso p ion p oduc s, as
he in eg a ion o a mass spec ome e in a glo e-box en i onmen o his pu pose
p esen ed oo many echnical diﬃcul ies (pa icula ly subsequen main enance) o
be o e come. The mass loss con inues wi h a u he 3%un il 600°C, which can be
ela ed o c ys alliza ion (pe ec ionism o he UO2 cc-s uc u e). This coincides
wi h he exo he mic peak in he DTA a 280°C which eﬂec s his hea ealise due o
he c ys alliza ion, on u he eac ion o deso b wa e . Howe e , he e is no eason
o belie e ha he weigh loss did no include as well some loss o oxygen due o
ma e ial’s educ ion. A deepe analysis on he la ice pa ame e and c ys al g ow h
unde ine and educing a mosphe e has been in he ollowing sec ions pe o med o
conﬁ m he la es .
Figu e 5.1: TGA and DTA signal o nc-UO2un il 1200°C unde A /5%H2.
5.3 La ice pa ame e and c ys al g ow h in neu al
a mosphe e.
The c ys al g ow h, la ice pa ame e o he nc-UO2, has been in es iga ed unde ine
condi ions (s a ic He a mosphe e) using in si u HT-XRD. The eﬀec o empe a u e
on he c ys alli e size, which is a undamen al pa ame e in he sin e ing p ocess
has been analysed. The in si u HT-XRD pa e ns we e acqui ed wi h an ins umen
desc ibed in 2.4.2. The empe a u e ange explo ed was 30 o 1200°C a a hea ing a e
o 5°C/min unde a s a ic He a mosphe e.
The e olu ion wi h empe a u e o he nc-UO2XRD pa e n is shown in Fig. 5.2
( esul s p esen ed in [Jo ani-Ab il e al., 2011]). The obse ed eﬂec ions a e assigned
o UO2- cc phase s uc u e and o P phase co esponding o he hea e pla e, plus an
impu i y peak a a ound 2θ= 26°. The pa e n is simila o he one epo ed a oom
66

5.3. La ice pa ame e and c ys al g ow h in neu al a mosphe e.
empe a u e (RT) by [Rousseau e al., 2009]. They epo ed he impu i y peak as Na
polyu ana e, coming om p ecipi a ion o U(VI) wi h NaOH. The eﬀec o empe a u e
on he peaks can be obse ed mo e clea ly in Fig. 5.2- igh , which displays he e olu ion
o wo main peaks (111) and (200) o he UO2s uc u e. A shi in he peak posi ion o
lowe angles is he e obse ed, possibly ela ed o a he mal la ice pa ame e expansion.
Figu e 5.2: In si u HT-XRD pa e ns o nc-UO2unde He (le ). The ypical UO2and P
( om he hea ing pla e) B agg peak posi ions a e also ma ked. The a ow on down- igh
side o he g aph shows a esidual impu i y which disappea s wi h empe a u e. E olu-
ion o (111) and (200) peaks o UO2cubic s uc u e as a unc ion o empe a u e ( igh )
[Jo ani-Ab il e al., 2011].
An eﬀec o he empe a u e is seen in he wid h o he peaks which dec eases wi h
inc easing empe a u e while he in ensi y o he peaks inc eases. Since he con ibu ion
o ins umen al b oadening is independen o he empe a u e, he b oadening a lowe
empe a u es is mainly ela ed o he c ys alli e size and s ain p esen in he ma e ial,
as well as inc ease o he s uc u al o de . Bo h con ibu ions, c ys al size (p opo ional
o cos−1θEq. 5.1) and s ain (p opo ional o anθ Eq. 5.2), ha e diﬀe en angula
dependences, and a e so sepa able. A s udy o hose inﬂuences has been in he ollowing
pe o med.
D=Kλ
βcosθ (5.1)
e=β
4· anθ (5.2)
whe e Dis he a e age c ys alli e size, Kis a cons an (0.87-1) ha depends
upon he pa icle shape and he Mille -indexes (hkl), λis he wa eleng h o he adi-
a ion, βis he ull peak wid h a hal maximum, θis he B agg angle and eis he s ain.
67
Chap e 5. C ys alliza ion and G ain G ow h in (T) o nc-UO2by Aqueous ou e
5.3.1 G ain g ow h as a unc ion o empe a u e unde neu al
a mosphe e.
The eﬀec o empe a u e on he c ys alli e size, which is a undamen al pa ame e in
he sin e ing p ocess has been he e analysed. The c ys alli e size o he nc-UO2has
been de e mined by he XRD Rie eld eﬁnemen (see Sec. 2.4) o he B agg peaks,
and used also o cha ac e ize he mic os uc u e o he ma e ial.
F om hese esul s i is possible o gene a e a uni e sal ep esen a ion o he
c ys alli e size as a unc ion o empe a u e (XRD measu emen s done a empe a u e
wi h he HT-XRD ins umen unde s a ic He a mosphe e) and epo ed in Fig. 5.3.
E en hough, his in o ma ion is aken as uni e sal, sligh de ia ions om i may
occu , in pa icula due o dwell imes and empe a u e amps, bu mo e impo an ly
due o he a mosphe e o s a ic He du ing he mal ea men .
Figu e 5.3: E olu ion o he nc-UO2c ys alli e size in unc ion o he empe a u e
[Jo ani-Ab il e al., 2011].
A oom empe a u e, he size o he c ys alli e was abou 4 nm a e p ecipi a ion
Sec. 3.5.1, which is in ag eemen wi h p e ious s udy by [Rousseau e al., 2009]. The
c ys alli e size change wi h empe a u e shows wo domains sepa a ed a 700°C (see
Fig. 5.3). Below ha empe a u e, he e was a weak inﬂuence on he c ys alli e size
which e ol ed om 2 o 7 nm (measu ed in si u a empe a u e). Abo e 700°C, he size
o he c ys alli e inc eased quasi linea ly bu d as ically wi h empe a u e, eaching a
size abou 73 nm a 1200°C.
5.3.2 La ice pa ame e and linea he mal expansion coeﬃ-
cien as a unc ion o empe a u e.
The c ys al g ow h o he sample unde ine condi ions (s a ic He a mosphe e) using
in si u HT-XRD, ha e been al eady desc ibed. In addi ion, he a ia ion o he la ice
pa ame e e sus c ys al size and empe a u e, as well as da a on he linea he mal
expansion, ha e been epo ed and compa ed o bulk ma e ial UO2. The c ys al
s uc u e o he p ecipi a es ha e been, as he c ys alli e size, de e mined by Rie eld
68
5.3. La ice pa ame e and c ys al g ow h in neu al a mosphe e.
eﬁnemen , aking in o accoun he whole 2θ ange.
In Fig. 5.4a he la ice pa ame e ob ained as a unc ion o empe a u e (XRDs
measu ed a empe a u e and unde s a ic He a mosphe e) and i s de i a i e (Fig. 5.4b),
ha e been also de e mined by he XRD Rie eld eﬁnemen o he B agg peaks, and
ep esen ed oge he wi h he nc-UO2size e olu ion o obse e i s dependence. Also
calcula ed la ice e olu ion o non-s oichiome ic s anda d UO2+x o diﬀe en O/U
a ios due o only he mal expansion, ha e been ep esen ed o compa ison. The
la ice pa ame e o a non s oichiome ic UO2+xis linked o he oxygen con en by he
ela ions o [Lynds e al., 1963]. Also he la ice pa ame e was co ec ed in unc ion
o empe a u e wi h he [Fink, 2000] ela ions eﬂec ed in Eq. 5.3.
2≤O/U ≤2.125 a(nm)=0.54705 −0.0094·O/U
2.1725 ≤O/U ≤2.250 a(nm)=0.54423 + 0.0029·(9 −4·O/U)(5.3)
The main change in he la ice pa ame e occu s be ween RT and 600°C and
o c ys al sizes below 6 nm. The la ice pa ame e suﬀe s a s ong expansion
inc easing s eeply om 0.5417(1) nm o 0.5492(1) nm a 300°C, and hen dec eases
o 0.5485(0) nm. Abo e 600°C, a linea e olu ion o he he la ice pa ame e wi h
empe a u e is obse ed.
(a) (b)
Figu e 5.4: a.) La ice cons an and c ys alli e size a ia ion (cu es only as a guide o eye)
o nc-UO2in unc ion o empe a u e, om in si u HT-XRD measu emen s unde s a ic He
a mosphe e in compa ison wi h la ice e olu ion in unc ion o empe a u es o s anda d UO2
o diﬀe en O/U a ios ob ained by he ela ions o [Lynds e al., 1963], due o only he mal
expansion. b.) Rela i e c ys alli e size and la ice pa ame e s. empe a u e (cu es only as
a guide o eye).
F om, he la ice pa ame e cu e in unc ion o empe a u e, he ex apola ed
linea end up o 600°C is shown by he s aigh do ed line and desc ibed by he
ollowing equa ion a(nm)=0.54439 −0.00007·T, ob aining a alue o he la ice
pa ame e a 20°C o 0.5445 nm, close o he la ice o UO2bulk a RT.
69
Chap e 5. C ys alliza ion and G ain G ow h in (T) o nc-UO2by Aqueous ou e
Rela ing he la ice pa ame e ound in his s udy wi h he [Lynds e al., 1963]
ela ions, a s oichiome y o abou UO2.18 up o 600°C o a ound UO2.17 up o 900°C
has been ound, which is simila o he one epo ed by [Rousseau e al., 2009]. So he
nanoc ys alli es s abilize a O/U 2.17-2.18 a empe a u es abo e 600°C, o in o he
wo ds, a pa icles sizes highe han 6 nm. A XPS da a showing a con ibu ion o
U(VI) and U(IV) in he p ecipi a ed pa icles was epo ed by [Rousseau e al., 2009].
Based on he la ice pa ame e de e mined by XRD, hey concluded hence ha he
s oichiome y was UO2.19, hus desc ibing he sys em as nc-UO2+xand is e y simila
o ou conclusion, oo. Howe e , ema k ha Eq. 5.3 and o he equi alen ela ions a e
alid o bulk compounds; hei applicabili y o nanoc ys als may be s ill open o p oo .
Fig. 5.5 displays he linea he mal expansion (LTE) and he he mal expansion
coeﬃcien (LTEC) o nc-UO2as a unc ion o he empe a u e. The LTE a empe a u e
Twas calcula ed using he ela ion:
LTE =(aT−a0)×100
a0(5.4)
whe e aTis he la ice pa ame e a empe a u e Tand a0is he la ice pa ame e
a 20°C. The LTEC was calcula ed by diﬀe en ia ing he he mal expansion cu e aT
e sus T wi h espec o he empe a u e T:
LTEC =1
a0×δaT
δT (5.5)
The LTE o he nc-UO2is in gene al highe han he one o bulk-UO2[Ma in, 1988]
o all he in e al o empe a u es, wi h a jump a 300°C and a pos e io s abiliza ion
abo e 600°C as one could al eady p edic om he la ice pa ame e ep esen a ion in
unc ion o empe a u e (see Fig. 5.4a).
The LTEC is ini ially highe o nc-UO2 han o bulk-UO2 o empe a u es
below 400°C and ends o s abilize abo e 600°C wi h a alue o 12·10−6°C−1in
ag eemen wi h he alue o he LTEC o bulk-UO2. The oscilla o y ends obse able
o LTEC in nc-UO2a T <900°C can be a ibu e o ansi o y oxida ion- educ ion
eﬀec s. A T ≥600°C i is clea ha he oxygen con en o he ma e ial s abilizes (a
O/U 2.17-2.18) (Fig. 5.4a), a he ime ha he la ice expansion coeﬃcien mee s
he alue o he e e ence bulk phase (la ge g ain) (Fig. 5.5). Fo c ys al sizes >6 nm
he nanoc ys alline ma e ial mee s he he mal-expansion beha iou (i.e. he mal
expansion coeﬃcien ) o bulk (la ge-g ained) UO2. This beha iou was al eady
obse ed in he ep esen a ion o he ela i e la ice pa ame e (see Fig. 5.4b), and
i shows once mo e ha he ab-no mal nano-eﬀec s in he ma e ial a e only o be
expec ed o pa icle sizes below ew ens o nanome e .
I Fig. 5.6 he pa e ns compa ison o nc-UO2as-p oduced (a = 0.5417(1) nm),
nc-UO2a 1200°C (a = 0.5521(0) nm) and nc-UO2a RT a e he mal ea men a
1200°C (a = 0.5473(0) nm) measu ed in si u in he HT-XRD ins umen unde s a ic
He a mosphe e, is shown.
The e is no explana ion o he alue seen o he nc-UO2measu ed a 1200°C
unde s a ic He a mosphe e: an O/U a io close o UO2.17 (see Fig. 5.4a)and hea
70
5.3. La ice pa ame e and c ys al g ow h in neu al a mosphe e.
Figu e 5.5: Linea he mal expansion (LTE) and linea he mal expansion coeﬃcien (LTEC)
o he nc-UO2(cu es only as a guide o eye) in compa ison wi h da a o bulk-UO2 om
[Ma in, 1988].
Figu e 5.6: Pa e ns compa ison o nc-UO2as-p oduced, nc-UO2a 1200°C and nc-UO2a
RT a e he mal ea men a 1200°C measu ed in si u in he HT-XRD ins umen unde
s a ic He a mosphe e.
la ice pa ame e o a = 0.5521(0) nm. In con as , he same he mal ea ed sample
measu ed a e cooling a RT, shows a alue o 0.5473(0) nm (UO2.00) e y simila
om he ypical o bulk-UO2(0.547 nm). The peaks o he nc-UO2a RT a e he
he mal ea men a 1200°C eco e he ypical posi ion o s anda d UO2(see Fig. 5.6).
So, a eco e ing o he c ys al s uc u e has been achie ed wi h empe a u e unde
71

Chap e 5. C ys alliza ion and G ain G ow h in (T) o nc-UO2by Aqueous ou e
s a ic He a mosphe e om he ini ial la ice pa ame e alue o 0.5417(1) nm om
he nc-UO2as-p oduced o 0.5473(0) nm a e he mal ea men a 1200°C. I migh
be ela ed o he s a ic He a mosphe e, which esul ed in a he modynamic equilib ia
be ween H2,H
2O and O2in he gas phase, a ou ing H2du ing he cool down, hus
his migh be he cause.
5.3.3 La ice s ain e olu ion as a unc ion o empe a u e.
The mean s ain, e, in he ma e ial was de e mined by Rie eld eﬁnemen using
he so wa e [HSP-PAN, 2011] and used o cha ac e ize he de o ma ion s a e o he
ma e ial. The analysis o he s ain by Rie eld eﬁnemen me hod is based on he
change o he p oﬁle pa ame e s, compa ed o a s anda d sample. Those a e depending
on he ins umen se ings used o da a collec ion and on he p oﬁle unc ion used o
he eﬁnemen [HSP-PAN, 2011].
The c ys alli e size change wi h empe a u e, al eady shown in Sec. 5.3.1,was
cha ac e ized o wo s eps sepa a ed a 700°C. Below ha empe a u e, he e was a
weak inﬂuence on he c ys alli e size which e ol ed om 2 o 7 nm (measu ed in si u
a empe a u e). Abo e 700°C, he size o he c ys alli e inc eased quasi linea ly bu
d as ically wi h empe a u e, eaching a size o abou 73 nm a 1200°C. In he e olu ion
o he la ice s ain elease wi h empe a u e, wo s eps can be obse ed (Fig. 5.7).
The ﬁ s s ep is isible a T <300°C whe e he s ain is a i s maximum, which could
be ela ed o he dehyd a ion s ep (wa e molecules a ached o he nanoc ys als and
o he ela ed binding-s eng h) isible in he TGA/DTA (Fig. 5.1). The second
s ep is isible in he ange 300-700°C whe e he s ain dec eases, which could be
ela ed o addi ional loss o wa e and oxygen loss (O/U s abiliza ion) and s uc u e
consolida ion. Up o 700°C, he in e nal s ain anishes and he c ys alli e-size s a s
o g ow. I is in e es ing o no e ha he comple e elease o he s ain in he nc-UO2
coincides wi h he onse o he s a ing o he c ys alli e g ow h. So he c ys alli e
g ow h seems o be limi ed by he p esence o he la ice s ain. F om RT un il 700°C
he he mal ene gy is o ally used o emo e he s ain and up o ha empe a u e no
s ain is emaining and he ene gy is used o he g owing o he nanoc ys als.
5.4 La ice pa ame e and c ys al g ow h unde e-
ducing condi ions.
The local s uc u e has been in es iga ed o he nc-UO2as-p oduced and a e
he mal ea men unde educ ion condi ions (A /5%H2), by X-Ray Diﬀ ac ion
(XRD), MAS-NMR, IR and X- ay Abso p ion Spec oscopy (XAS), and compa ed o
bulk-UO2as a e e ence. A combina ion o X- ay Abso p ion Nea Edge S uc u e
(XANES) and Ex ended X- ay Abso p ion Fine S uc u e (EXAFS) was used.
72
5.4. La ice pa ame e and c ys al g ow h unde educing condi ions.
Figu e 5.7: C ys alli e size and s ain o nc-UO2in unc ion o empe a u e. Measu emen s
done a empe a u e unde s a ic He a mosphe e (cu es only as a guide o eye).
5.4.1 C ys al size and la ice pa ame e e olu ion as a unc ion
o empe a u e as p obed by XRD.
As i has been seen om he HT-XRD, he la ice cons an (and c ys al size) o he
ma e ial in he cooled s a e (a RT) a e each diﬀe en maximum empe a u es is
needed (see Fig. 5.6). This allows sepa a ion o he he mal expansion con ibu ion
in he high- empe a u e alues o ob ain cleane cu es o he mal expansion s.
empe a u e and la ice dimension s. c ys al size.
A he ou se o his s udy, a highe conce ning abou he con ol o he O/U a io,
which is no simple, appea ed. The s uc u al in es iga ions p esen ed in he ollowing,
a emp o elimina e his issue as he samples we e hea ed in A /5%H2 o ensu e ha
he O/U = 2.00. The impac o he hea ea men on he mic os uc u e o nc-UO2a
diﬀe en annealing empe a u es was hen s udied unde educing condi ions (dynamic
A /5%H2a mosphe e). As-p oduced o RT, 600°C and 1200°C. The co esponding
XRD da a a e gi en in Fig. 5.8. A hea ing a e o 5°C· min−1unde d y A /5%H2and
annealing o 15 minu es we e used. A 600°C he majo mass loss has al eady occu ed
and no wa e aces should be p esen in he ma e ial as i has been obse ed in he
TGA/DTA (Fig. 5.1). As a e e ence, a s anda d UO2.0(μm c ys al size) sin e ed
a 1600°C unde A /5%H2 o 6 hou s and measu ed du ing he same measu ing
campaign, was also used.
The measu emen s indica ed a well c ys allized single cubic phase wi h a ﬂuo i e
s uc u e (Fm-3m). No e idence o o ho hombic o o he phase was ound. F om
he Rie eld eﬁnemen o he measu ed diﬀ ac og ams, he la ice pa ame e s as well
as he size o he pa icles we e deduced. Reduc ion o he nc-UO2+x owa ds nc-UO2
a e annealing a 600°C and 1200°C was expec ed because he high sensi i i y o he
la ice pa ame e , a, o changes in he oxida ion s a e o U in he hype s oichiome ic
ange, O/M >2. Samples ea ed a inc easing empe a u e (RT, 600°C and 1200°C)
73
Chap e 5. C ys alliza ion and G ain G ow h in (T) o nc-UO2by Aqueous ou e
wi h a sho iso he mal hold o 15 min, exhibi an inc ease in he la ice cons an o
1.02%. P og essing c ys alliza ion o he nc-UO2leaded o pe iodic o de ing o i s
a om and he la ice pa ame e changed om 0.5417(1) nm a RT, o 0.5431(0) nm
and 0.5472(0) nm a 600°C and 1200°C, espec i ely. The c ys al-size inc eased om
3.79 nm (≃4 nm) as-p epa ed, o 9.3 nm (≃9 nm) and 82.16 nm (≃82 nm) ollowing
ea men s a 600°C and 1200°C, espec i ely (Fig. 5.8).
Figu e 5.8: XRD pa e ns o e e ence UO2and aqueous ou e nc-UO2(as-p oduced, a e
he mal ea men unde A /5%H2a 600°C and 1200°C)
A compa ison o he c ys al size and la ice pa ame e o he samples ea ed a
600°C and 1200°C unde dynamic A /5%H2a mosphe e (measu emen a e cooling)
wi h hose unde s a ic He a mosphe e measu ed a empe a u e (see Fig. 5.4a), is
p o ided in Table 5.1. No diﬀe ence in he c ys alli e size was ob ained a 600°C unde
ei he a mosphe e. Bu a no able change is obse ed a 1200°C, whe e a c ys alli e size
o 73.39 nm unde He has been obse ed and a size o 82.16 nm unde A /5%H2. Majo
diﬀe ences ha e been seen in he la ice pa ame e s as a unc ion o he a mosphe e
used wi hou igno ing he ac ha he alues unde He we e measu ed a empe a u e.
These diﬀe ence in he la ice disappea ed once he sample measu ed a empe a u e
is measu ed a e cooling, as i has been seen in Fig. 5.6. In he e olu ion o he la ice
s ain, e, a elease was again obse ed wi h inc easing empe a u es. A e annealing
a 600°C unde A /5%H2, jus he hal o he s ain was p esen being o ally eleased
a 1200°C, as i also happen unde He a mosphe e.
Fig. 5.9 shows TEM images o he nc-UO2(abou 4 nm size) pa icles as-p oduced
and a e being he mal ea ed a 1200°C (abou 120 nm size) unde A /5%H2.The
TEM size obse ed a e ea men is in good ag eemen wi h he a e age alue o
82.2 nm ob ained o he Rie eld eﬁnemen (Fig. 5.8).
74
5.4. La ice pa ame e and c ys al g ow h unde educing condi ions.
Table 5.1: C ys al size and la ice pa ame e o aqueous ou e nc-UO2 ea ed a 600°C and
1200°C unde wo diﬀe en a mosphe e (A /5%H2and He a mosphe e)
A /5%H2a m.aHe a m.b
c ys . size la ice pa am. s ain c ys . size la ice pa am. s ain
(nm) (nm) (%) (nm) (nm) (%)
nc-UO2RT 3.79 0.5417(1) 0.792 1.99 0.5420(10) 1.149
nc-UO2600°C 9.30 0.5431(0) 0.391 5.36 0.5483(5) 0.861
nc-UO21200°C 82.16 0.5472(0) 0.026 73.39 0.5521(0) 0.004
a.) Measu emen a e cooling. b.) Measu emen a empe a u e (HTXRD meas.).
(a) nc-UO2as-p oduced. (b) nc-UO2a e 15 min a 1200°C.
Figu e 5.9: TEM images o he nc-UO2as-p oduced and a e he mal ea men unde
A /5%H2.
5.4.2 O/M a io as a unc ion o empe a u e as p obed by
XANES.
XANES was used o de e mine he oxida ion s a e o U ca ions and he co esponding
mola ac ions and he O/U a ios we e de i ed. The no malized XANES spec a and
he ﬁ s de i a e a he U-L3abso p ion edge o h ee diﬀe en hea ed nanoc ys alline
UO2samples (nc-UO2a RT, 600°C and 1200°C) a e shown in Fig. 5.10, oge he wi h
he e e ence spec a o UIV O2. The expe imen al ea u es a e speciﬁed in Sec. 2.2.3.
The associa ed ene gies o he inﬂec ion poin a abso ion edge and o he whi e-line
(WL), as well as he ene gy shi (ΔE) and he es ima ed oxida ion s a es de i ed om
his s udy, a e gi en in Table 5.2.
A simple obse a ion o he XANES spec a a he U-L3edge immedia ely shows a
end wi h inc easing empe a u e and as xdec eases (UO2+x). The peak o he WL
shi s sligh ly o lowe ene gies and inc eases in in ensi y, and he wi hin he XANES
egions inc ease. The ampli ude dec ease wi h he inc easing empe a u e o he mal
ea men showing a highe s uc u al o de o hese samples.
Fo he samples as-p oduced (RT) and a 600°C, he e is a signiﬁcan diﬀe ence
o shape compa ed o he UIV O2 e e ence, i.e. p esence o a shoulde on he high
75
Chap e 5. C ys alliza ion and G ain G ow h in (T) o nc-UO2by Aqueous ou e
Figu e 5.13: Expe imen al () and ﬁ ed da a (−) o he nc-UO2annealead a 1200°C o
(le ) k3-weigh ed spec a and ( igh ) Fou ie T ans o m a he U-L3edge.
Figu e 5.14: U-O1and U-U1bond dis ances in unc ion o he annealing empe a u e (unde
educing condi ions) and ﬁnale size o he nc-UO2sample (cu es only as a guide o eye).
5.4.3.2 Local s uc u e and alence s a e as p obed by MAS NMR.
The Hahn-echo 17O MAS spec a acqui ed o a ious annealing imes (600°C, 650°C,
700°C, 800°C and 1200°C) a e p esen ed in Fig. 5.15 and some ﬁ s in Fig. 5.16 and
Fig. 5.17. The expe imen al ea u es a e speciﬁed in Sec. 2.2.4. The spec um o he
200°C annealed sample was acqui ed a 10 kHz (as a 1.3 mm p obe was no a ailable
a his ime) he e o e i s analysis has o be done apa om he whole se ies and has
been no he e ep esen ed. In ac , as hese compounds a e pa amagne ic he 17O shi
depends on he spinning speed which induces a sligh hea o he sample. Ne e heless,
ano he s udy on bulk-UIV O2(done in ITU o be published) shows ha he a ia ion
in he peak posi ion is no e y impo an be ween 10 kHz and 55 kHz (∼10 ppm,
small compa ed wi h he peak b oadening). Thus, he shi o 1075 ppm (200°C) can
be compa ed wi h ha ex ac ed o hese se ies (Fig. 5.18). A he con a y, he
b oadening o he peak canno be compa ed wi h ha o he o he s hea ea men as
he Full Wid h a Hal Maximum (FWHM) will dec ease wi h spinning speed due o
he emo ing o pa amagne ic shi aniso opy.
Fo he nex empe a u e o 600°C, a b oad peak o 2478 ppm has been iden iﬁed
82

5.4. La ice pa ame e and c ys al g ow h unde educing condi ions.
Figu e 5.15: S ack o he 17O MAS-NMR o nc-UO2annealed a ﬁ e diﬀe en empe a u es.
a 962 ppm (Fig. 5.16a). By inc easing he empe a u e o only 50°C (a 650°C;
Fig. 5.16b), he MAS spec um o he sample exhibi s wo peaks a 932 and 723 ppm.
On he s a ic spec um acqui ed wi h mo e scans a hi d peak (no he e ep esen ed)
can be iden iﬁed a nea ly 960 ppm. The posi ion o his peak can be ela i ely
compa ed wi h ha o he spinning spec um while he FWHM canno . A 700°C
(Fig. 5.17a), he e a e s ill h ee peaks and he peaks a 732 and 738 ppm a e now
sha pes . Fo he wo las hea ea men s, 800°C (Fig. 5.17b) and 1200°C (Fig. 5.17c),
only wo peaks wi h e y closed shi s a e iden iﬁed.
Two ends a e obse ed in he plo s o he shi s and he FWHM as a unc ion o
empe a u e p esen ed in Fig. 5.18. Unde 700°C, he shi and he FWHM dec ease
ab up ly while abo e his empe a u e hey a e ela i ely cons an . I can be no iced
ha he shi ex ac ed om he spec um acqui ed a 200°C is e y diﬀe en om
ha o he whole se ies. Due o he b oadness o he peak, he p esence o mo e
han one species canno be excluded. A simila end was obse ed by XRD o he
e olu ion o he la ice pa ame e as a unc ion o empe a u e (Fig. 5.4). In ac ,
i inc ease s eeply unde 700°C, hen he e is only a week e olu ion. The e o e, i
seems ha unde his empe a u e he e a e impo an changes on long and sho ange.
Th ee diﬀe en oxygen en i onmen s can be iden iﬁed. The ﬁ s one co esponds
o oxygens ha ing a chemical shi o nea ly 900 ppm (named peak C, g een peak).
These peaks ha e been iden iﬁed up o 650°C. A 650°C, e en i he peak is sha pes ,
he shi is e y simila o ha o he 600°C sample and he e o e he 17O will be
conside ed as he same ype o species. Due o he b oadness o he peak, i is emp ing
o a ibu e his one o 17O in a diso de ed en i onmen . Bu , XRD has shown ha
83
Chap e 5. C ys alliza ion and G ain G ow h in (T) o nc-UO2by Aqueous ou e
17O Chemical Shi (ppm)
-4000 -3000 -2000 -1000 0 1000 2000 3000 4000 5000
* *
(a) 600°C
17O Chemical Shi (ppm)
-5000 -4000 -3000 -2000 -1000 0 1000 2000 3000 4000 5000
(b) 650°C
17O Chemical Shi (ppm) -2000 -1000 0 1000 2000
* *
(c) 700°C
Figu e 5.16: Cha ac e is ic ﬁ s o he spec a o he samples annealed a 600°C, 650°C and
700°C (∗= spinning sidebands; peak A=black;peakB=blue;peakC= g een)
84
5.4. La ice pa ame e and c ys al g ow h unde educing condi ions.
17O Chemical Shi (ppm) -2000 -1000 0 1000 2000
* *
(a) 700°C
17O Chemical Shi (ppm) 400 500 600 700 800 900
* *
(b) 800°C
17O Chemical Shi (ppm) 640 660 680 700 720 740 760 780 800
(c) 1200°C
Figu e 5.17: Cha ac e is ic ﬁ s o he spec a o he samples annealed a 700°C, 800°C and
1200°C (∗= spinning sidebands; peak A=black;peakB=blue;peakC= g een).
85
Chap e 5. C ys alliza ion and G ain G ow h in (T) o nc-UO2by Aqueous ou e
o his ange o empe a u e he size o he UO2is a ound 10 nm (Fig. 5.8 unde
A /5%H2). Mo eo e , p e ious expe imen s done on zeoli es [Zhang e al., 1999]ha e
shown ha he e is an inc ease o he linewid h o he quad upola nucleus (in ha case
27Al) wi h dec easing size o nanopa icles. Indeed, acco ding o [Casabella, 1964] o
well-c ys allized samples he NMR linewid h is g ea ly inﬂuenced by he quad upola
coupling cons an which is due o he local elec ic ﬁeld g adien s in he sample. The
s ong su ace ene gy exis ing on he su ace o he small pa icles will lead o mo e
s ain in he la ice, consequen ly causes he b oadening o he main peak o he
quad upola line. Thus, i can be p oposed ha his ype o oxygen co espond o he
17O in nanoc ys als UO2. Ne e heless, due o he b oadness o he peak, p esence o
o he 17O species canno be excluded.
Figu e 5.18: E olu ion o he 17O shi (le ) and o he ull wid h a hal maximum ( igh )
as a unc ion o c ys alli e size o a ious empe a u es.
The wo second ypes o species appea clea ly om 650°C (Fig. 5.18). The e is
one sha p and one b oad ( hey will be named peaks Aand B espec i ely he ea e ).
These peaks we e ﬁ ed using a Lo en zian o A(black peak on Fig. 5.16 and Fig. 5.17)
and a Gaussian o B(blue peak on Fig. 5.16 and Fig. 5.17) and could be espec i ely
a ibu ed o 17O in a c ys alline phase and in a mo e diso de ed one. I can be
added ha hese wo peaks a e also obse ed a 1200°C meaning ha e en o his
empe a u e i is no s oichiome ic UO2con a y o ha sugges ed by XRD (Fig. 5.8).
Wi h inc easing empe a u e (inc easing o c ys alli e size), he FWHM o he wo
peaks dec ease (Fig. 5.17). As p e iously explained, his is he signa u e o such
c ys alli e size e olu ion. Mo eo e , he shi o peak A eached a minimal alue o
717 ppm a 1200°C. This one is close o he 717 ppm ound o UIV O2-bulk. I s FWHM
is o 5 ppm and co esponds o ha o he c ys alli e ha ing a size abou 80 nm. This
86
5.4. La ice pa ame e and c ys al g ow h unde educing condi ions.
alue is s ill sligh ly bigge han he 3 ppm ound o UIV O2-bulk. Hence, wi h he
shi o peak Aone can say ha he en i onmen a ound he oxygens co esponding o
he bigges c ys alli e size (80 nm) is e y close o ha o UIV O2-bulk. Based on he
FWHM, one can say ha o obse e he signal o c ys alline UO2a size abo e 80 nm
should be eached. This conﬁ ms ha nc-UO2a e ob ained and his is consis en wi h
he la ice pa ame e obse a ion o 0.5472 nm (Fig. 5.8).
5.4.3.3 Local s uc u e and alence s a e as p obed by FTIR.
Se e al samples a he key empe a u es we e analyzed unde he Fou ie T ans o m
In a ed (FTIR) spec ome e (Alpha FT-IR Spec ome e om B ucke ; Sec. 2.2.5).
The in a ed (IR) spec a eco ded o nc-UO2as-p oduced (RT), a 200°C, 600°C and
1200°C unde A /5%H2annealed, as well as UIV O2 e e ence spec a, a e shown in
Fig. 5.19.
In hecaseo nc-UO
2as-p oduced (RT), ou peaks in he ange 400-4000 cm−1
we e obse ed. The abso p ion band o he U-O ib a ion in UO2shows up below
400 cm−1which is ou o he de ec ion limi o he equipmen . The peak a 1625 cm−1
can be assigned o he bending ib a ion o H-O-H bonds o he coo dina ed wa e
[Fuji a e al., 1956], [Sailaja e al., 2002]. This peak disappea s al eady a 200°C. Tha
would be in ag eemen wi h he TGA (Sec. 5.2), we e nea ly no loss o weigh was
obse able a e 600°C (see Fig. 5.1). Some hing simila occu s o he b oad abso p ion
peak a 3400 cm−1, which can be asc ibed o he asymme ic and symme ic s e ching
ib a ions o he H-O-H bonds o coo dina ed wa e . Howe e , his peak educes o
he 200°C and is nea ly disappea ed a 600°C annealed sample. So a 600°C all he OH
would be heo e ically gone aking in o accoun he peaks a 1625 cm−1and 3400 cm−1.
Howe e , he peak a 880 cm−1co esponding also o OH g oups, s ay s ill p esen
a 600°C, o ally disappea ing a 1200°C (Fig. 5.19). The same occu ed o he peak
a 638 cm−1co esponding o mo e oxidised species (UO2+x)[Kim e al., 2009]. Tha
could be an a e ac due o he small size s ill p esen a 600°C (9 nm) (see Table 5.1).
A 1200°C he IR spec a looks like he one o he UIV O2 e e ence and g ains a e
abou 82 nm (Table 5.1).
Tha is also in ag eemen wi h he esul s ob ained by XANES whe e a diﬀe en
elec onic s uc u e a 600°C was obse ed, meanwhile a 1200°C a simila s uc u e
o bulk-UIV O2was ound (see Table 5.2). Also EXAFS is cha ac e ized o a poo
o de ing a 600°C bu en i ely ma ching wi h he bulk-UIV O2oscilla ion pai s a
1200°C (see Table 5.3). In he NMR analysis all he addi ional oxygen si es disappea ed
once a 1200°C anneal bu one which could be due o a su ace eﬀec .
87

Chap e 5. C ys alliza ion and G ain G ow h in (T) o nc-UO2by Aqueous ou e
Figu e 5.19: In a ed spec a eco ded o nc-UO2as-p oduced (RT), a 200°C, 600°C and 1200°C unde A /5%H2annealed. Re e ence spec a o UO2is
also ep esen ed. Inside ampliﬁca ion o he in a ed spec a showing he disappea ing o he peaks a 1625 cm−1and 3400 cm−1wi h inc easing annealing
empe a u es.
88
5.5. nc-UO2long-iso he mal g ain g ow h as p obed by XRD unde neu al and educing condi ions.
5.5 nc-UO2long-iso he mal g ain g ow h as p obed
by XRD unde neu al and educing condi ions.
The as-p oduced nanoc ys als p esen ed a ce ain diso de and la ice s ain which
disappea ed wi h he anneal o he samples, e ol ing o o de ed c ys alline s uc u es
as empe a u e inc eased. This has been p o ed o sho e ime hea ea men s
unde ine and educing a mosphe es (Table 5.1). No mally, ce amic hea ea men s
o ob ain high dense monoli hs ﬁnish up wi h g ain sizes abo e 200 nm. Hence,
iso he mal g ain g ow h kine ics a a gi en empe a u e is an essen ial componen o
de e mine he ﬁnal g ain size, and in consequence o e alua e he pe o mance o such
an inno a i e nuclea uel a e p olonged inse ion a high empe a u es. Al hough
nuclea uel ope a es a abou 500°C a he pe iphe y i can each 1200°C a he
uel pelle cen e. Fo ha eason, i is undamen al o examine he beha iou o
nano- uel mic os uc u e as a unc ion o he ime in a possibly wide empe a u e ange,
o ensu e lack o disp opo iona e g ain g ow h e en a he highes empe a u e in play.
A s udy o he he mal g ow h o hose nanoc ys als o e he whole ange o ini ial
mic os uc u es om amo phous o ully nanoc ys alline was pe o med. Based on
Fig. 5.3, he c i ical empe a u es 500°C, 700°C, 900°C and 1200°C we e selec ed.
Iso he mal g ain g ow h da a a hese empe a u es was de e mined om in-si u
HT-XRD measu emen s unde s a ic He a mosphe e and annealing imes om 0 o
50 h wi h a hea ing a e o 5°C/min. The da a a e shown in Fig. 5.20. Independen
sample ea men s in sepa a e u naces unde dynamic A /5%H2 o dwelling imes o
50 h, 100 h and 200 h, we e also pe o med. Fo each iso he mal dwell empe a u e
one new sample was chosen. Compa ison o he size and la ice pa ame e ob ained o
he bo h ou es, a e shown in Table 5.5.
Figu e 5.20: Iso he mal g ain g ow h o nc-UO2unde He s a ic a mosphe e. Fo each iso he -
mal dwell empe a u e one new sample was used. Measu emen was done in si u in he HT-
XRD de ice a empe a u e. All he cu es ha e been ﬁ using he Eq. 5.6. Fo he one a
1200°C a ﬁ using he Eq. 5.7 was also done.
The in si u HT-XRD de e mina ion o he c ys alli e size was done unde s a ic He
a mosphe e. The expe imen al ins alla ion did no pe mi he use o A /5%H2gas in
89
Chap e 5. C ys alliza ion and G ain G ow h in (T) o nc-UO2by Aqueous ou e
he HT-XRD chambe , which would ha e been necessa y o p ese e he O/U a io o
he sample a a alue a ound 2. The measu emen s we e aken a empe a u e. The
c ys alli e size was de e mined acco ding o in e se o he wid h o he ﬁ s diﬀ ac ion
peak [111], which was moni o ed du ing 50 h while he sample was kep a cons an
empe a u e. F om hese measu emen s in o ma ion on c ys al g ow h as a unc ion
o empe a u e and ime was ob ained (Fig. 5.20). Pos e io XRD-measu emen on
conside a ion o he whole diﬀ ac og am a e 50 h dwell ime a empe a u e and
once he sample was cooled down, was also done (Table 5.5). This ﬁnal measu emen
p o ided da a on c ys al size wi hou he mal b oadening, bu cha ac e is ic o he O/U
ﬁnal a io ha was ob ained unde He a mosphe e. The measu emen o he ex-si u
annealed samples was done unde a dynamic A /5%H2a mosphe e which ensu ed a
ﬁnal s oichiome y UO2.0(UIV ). The measu emen was always done a e -cooling.
Fig. 5.21 shows how g ain g ow h akes place in he ﬁ s hou s o iso he mal hold
o he empe a u es a 500°C, 700°C and 900°C un il a s able a e age g ain size be-
low 100 nm is es ablished a he applied empe a u e, a which ime c ys al g ow h
ceases. This kind o sel -limi ed g ain g ow h was epo ed by [Rupp e al., 2006]and
is desc ibed by he ollowing elaxa ion unc ion:
G−G0=(GL−G0)·(1−e−
τ) (5.6)
whe e Gis he a e age g ain size, G0is he ini ial g ain size, GLis he limi ed g ain
size (when he g ains s op o g ow) and τis he elaxa ion ime ( ime needed o each
he GL).The alueso GLand τob ained om he ﬁ ing desc ibed by he elaxa ion
unc ion Eq. 5.6, a e p esen ed in Table 5.4.
Figu e 5.21: Iso he mal g ain g ow h o nc-UO2unde He s a ic a mosphe e. Fo each iso he -
mal dwell empe a u e one new sample was used. Measu emen was done in si u in he
HT-XRD de ice. All he cu es ha e been ﬁ using he Eq. 5.6.
The diﬀusion coeﬃcien , Di, ha e been so calcula ed by subs i u ion o he alues G0,
GLand τabo e de e mined, in he co esponding app oxima ion by [Rupp e al., 2006]
(Eq. 5.7). A e wa ds, he ac i a ion ene gy o diﬀusion, Qdi , has been ob ained
as a unc ion o iso he mal dwell empe a u e using he A henius law dependence
90
5.5. nc-UO2long-iso he mal g ain g ow h as p obed by XRD unde neu al and educing condi ions.
[Löﬄe and Johnson, 2000]:
Di=(GL−G0)2
4·τ=cons ·e−
Qdi
kBT(5.7)
whe e kBis he Bol zmann cons an (kB=8.6173324 · 10−5eV ·K−1). The
diﬀe en pa ame e s ob ained o he ﬁ ing a each empe a u e, a e shown in
Table 5.4.
Table 5.4: Pa ame e s ob ained om he ﬁ s ollowing he elaxa ion equa ion Eq. 5.6 o he
samples annealed unde He s a ic a mosphe e du ing 50 h.
T(°C) G
0(nm) GL(nm) τ(h) Di(m2/s) R2
nc-UO2500°C 5.41 15.51 9.65 7.34e−22 0.72
nc-UO2700°C 5.41 48.21 4.10 3.10e−20 0.87
nc-UO2900°C 5.41 91.99 0.90 5.80e−19 0.90
nc-UO21200°C 5.41 363.55 18.13 4.91e−19 0.66
S a ing g ain size (G0), limi ed g ain size (GL), elaxa ion ime (τ), diﬀusion coeﬃcien (Di)and
(R2) co ela ion coeﬃcien ob ained om he ﬁ ing.
The de e mined kine ics pa ame e s depend on he empe a u e. The Di alues
inc ease wi h empe a u e, meanwhile he elaxa ion ime (ob ained om he ﬁ ing
wi h Eq. 5.6) diminishes wi h i (Fig. 5.22). Diﬀe en ly, o he sample annealed a
1200°C a much highe elaxa ion ime (see Table 5.4) has been ob ained, no ollowing
he dec easing end wi h empe a u e obse ed o he es o he dwell empe a u es
examined (Fig. 5.22). Fo his sample he g ains seem o con inue o g ow ollowing he
gene alized g ain g ow h equa ion (Eq. 5.8) as al eady obse ed by [Rupp e al., 2006]
o empe a u es abo e 1100°C and exp essed as:
Gn−Gn
0=kn· (5.8)
whe e nis he g ow h exponen and knis a a e cons an (kn=k0·e−
Qdi
RT wi h k0=
cons an , Qdi = ac i a ion ene gy o diﬀusion, R= gas cons an and T= absolu e
empe a u e).
Fo he nc-UO2annealed a 1200°C a g ain g ow h exponen no 2.36 was ob ained
wi h a co ela ion coeﬃcien o 0.77 using he Eq. 5.8. The ﬁ ing cu e is shown
in blue colou in Fig. 5.20. A ﬁnal g ain size o abou 350 nm a e 50 h is also
unde his mechanism ob ained. This is in ag eemen wi h he ange o alues o
adi ional g ain g ow h mechanisms (n=2-4), and in pa icula he pa abolic g ain
g ow h mechanism (n=2.36≃2) [Rupp e al., 2006]. The ﬁ ing line shown in pink
colou , wi h a co ela ion coeﬃcien o 0.66, co esponds o he elaxa ion unc ion
abo e desc ibed (Eq. 5.6), which be e ﬁ s he g ain g ow h kine ics a he lowe
empe a u es examined (T≤900°C).
F om Fig. 5.23 an ac i a ion ene gy o diﬀusion o 0.93 eV wi h a co ela ion
coeﬃcien o 0.90 was de e mined o he empe a u e ange 500-1200°C (conside ing
91
Chap e 5. C ys alliza ion and G ain G ow h in (T) o nc-UO2by Aqueous ou e
In he k3-weigh ed EXAFS spec a (Fig. 5.11a), he oscilla ions and hei ampli ude
inc eased wi h he mal ea men . The 4 nm as-p ecipi a ed sample was e y diﬃcul
o ﬁ wi h a pu e ﬂuo i e s uc u e, as he ﬁ we e non s able and he da a noisy
( ea ed k- ange = 3-8 Å−1). The in ensi y o he FT (Fig. 5.11b) was e y low limi ing
he in e p e a ion o he coo dina ion shell o U-O1. Obse ing he EXAFS esul s
in Table 5.3, he da a we e hea ily dampened a RT whe e a la ge alue o he DW
ac o was ound, meaning a signiﬁcan s a ic diso de o he nc-UO2as-p oduced.
Sho e dis ance o he oxygen shell (U-O bond leng h) was clea ly obse able o he
nc-UO2a RT which did no co espond o any U-oxide. Acco ding o he shape o
he ﬁ s FT peak, i looked like he e we e wo o h ee U-O dis ances ins ead o one.
This was consis en wi h he obse ed la ice con ac ion (0.5417 nm) om XRD a
RT (see Table 5.2).
The 9 nm sample (600°C anneal) showed an in e media e o de ing wi h oscilla ions
clea ly iden iﬁed and ex ending o k = 9 Å−1. The in ensi y o he FT was also low
o his annealed sample, limi ing he ﬁ ing and in e p e a ion o he coo dina ion
shell o U-O1and U-U1 oge he wi h UO2ﬂuo i e s uc u e. S ill a la ge alue o he
DW ac o was ound (Table 5.3). Sho e dis ance was also p esen o he oxygen
shell (U-O bond leng h) in compa ison wi h he e e ence-UIV O2. Howe e he U-U1
bond leng h was close o ha o he bulk-UIV O2, sugges ing ha he U-U1la ice
was mo e o de ed han he O anion subla ice. The U-U1leng hs we e consis en wi h
he less la ice con ac ion (0.5431 nm), in compa ison wi h he nc-UO2as-p oduced,
as p obed by XRD a 600°C anneal.
Ul ima ely, a 1200°C and 82 nm, EXAFS oscilla ions we e simila , i no en i ely
ma ching, hose o he bulk-UIV O2indica ing same cc-s uc u e consolida ion and
subs an ial pa icle g ow h, bo h obse ed in XRD measu emen s (Fig. 5.12). Bo h
shells we e well ﬁ ed wi h Fm-3m s uc u e o his sample (Fig. 5.13) and e y simila
dis ances o e e ence UIV O2s uc u e could be obse ed acco ding o he FT (k- ange
ea ed = 3-12 Å−1). Tha was in ag eemen wi h he simila i y o he XRD da a o
he annealed sample a 1200°C and he bulk-UIV O2(Fig. 5.8). Also consis en wi h
he XANES (see Fig. 5.10) showing no diﬀe en oscilla ion om he ﬂuo i e s uc u e.
The Hahn-echo 17O MAS spec a acqui ed o a ious annealing imes (600°C,
650°C, 700°C, 800°C and 1200°C) we e p esen ed in Fig. 5.15 and some ﬁ s in
Fig. 5.16 and Fig. 5.17. Two ends we e obse ed in he plo s o he shi s and
he FWHM as a unc ion o empe a u e p esen ed in Fig. 5.18. Unde 700°C, he
shi and he FWHM dec eased ab up ly while abo e his empe a u e hey we e
ela i ely cons an . I can be no iced ha he shi ex ac ed om he spec um
acqui ed a 200°C was e y diﬀe en om ha o he whole se ies. Due o he
b oadness o he peak, he p esence o mo e han one species canno be excluded, as
al eady assumed in he EXAFS s udy. A simila end was obse ed by XRD o he
e olu ion o he la ice pa ame e as a unc ion o empe a u e (Fig. 5.4). In ac , i
inc eased s eeply unde 700°C, hen he e was only a week e olu ion. The e o e, i
seemed ha unde his empe a u e he e is impo an changes on long and sho ange.
Th ee diﬀe en oxygen en i onmen s could be iden iﬁed. The ﬁ s one co esponds
o oxygens ha ing a chemical shi o nea ly 900 ppm (named peak C, g een peak).
These peaks ha e been iden iﬁed up o 650°C. Due o he b oadness o he peak, i
was emp ing o a ibu e his one o 17O in a diso de ed en i onmen . Bu , XRD
98

5.6. Resul s and discussion.
has shown ha o his ange o empe a u e he size o he UO2was a ound 10 nm
(Fig. 5.8 unde A /5%H2). The s ong su ace ene gy exis ing on he su ace o
he small pa icles will lead o mo e s ain in he la ice, consequen ly caused he
b oadening o he main peak o he quad upola line. Thus, i could be p oposed
ha his ype o oxygen co esponded o he 17Oinnc-UO
2. Ne e heless, due o he
b oadness o he peak, p esence o o he 17O species could no be excluded.
The wo second ypes o species appea ed clea ly om 650°C (Fig. 5.18). The e
was one sha p and one b oad (named peaks Aand B espec i ely). These peaks could
be espec i ely a ibu ed o 17O in a c ys alline phase and in a mo e diso de ed phase.
Wi h inc easing empe a u e (inc easing o c ys alli e size), he FWHM o he wo
peaks dec eased (Fig. 5.17). As p e iously explained, his was he signa u e o such
c ys alli e size e olu ion. Mo eo e , he shi o peak A eached a minimal alue o
717 ppm a 1200°C. This one was close o he 717 ppm ound o UIV O2-bulk. I s
FWHM was o 5 ppm and co esponded o ha o he c ys alli e ha ing a size abou
80 nm. This alue was s ill sligh ly bigge han he 3 ppm ound o UIV O2-bulk.
Hence, wi h he shi o peak Aone could say ha he en i onmen a ound he
oxygens co esponding o he bigges c ys alli e size (80 nm) was e y close o ha o
UIV O2-bulk. Based on he FWHM, one can say ha o obse e he signal o c ys alline
UO2a size abo e 80 nm should be eached. This conﬁ med ha nc-UO2we e ob ained
being consis en wi h he la ice pa ame e obse ed o 0.5472 nm, as well as he O/M
a io ob ained in he XANES analysis (Table 5.2).
Se e al samples a he key annealing empe a u es we e analyzed unde he FTIR
spec ome e . The IR spec a eco ded o nc-UO2as-p oduced (RT), a 200°C,
600°C and 1200°C unde A /5%H2annealed, as well as UIV O2 e e ence spec a, we e
shown in Fig. 5.19. In he case o nc-UO2as-p oduced (RT), ou peaks in he ange
400-4000 cm−1could be obse ed. They could be assigned o he bending ib a ion o
H-O-H o he coo dina ed wa e , and o a possible mo e oxidised s a e (UO2+x). All o
hem diminished wi h annealing. The peaks assigned o he H-O-H o he coo dina ed
wa e , o ally disappea ed a 600°C. Tha was in ag eemen wi h he TGA, we e
nea ly no weigh o loss was obse able a e 600°C (see Fig. 5.1). Howe e wo o
he peaks ﬁnally disappea ed a 1200°C. Tha could be an a e ac due o he small
size s ill p esen a 600°C (10 nm) (see Table 5.1). Hence, a 1200°C he IR spec-
a looked like he one o he UIV O2 e e ence and g ains we e abou 80 nm (Table 5.1).
Tha was also in ag eemen wi h he esul s ob ained by XANES whe e a diﬀe en
elec onic s uc u e a 600°C was seen, meanwhile a 1200°C a simila s uc u e o
bulk-UIV O2was ound (see Table 5.2). Also EXAFS was cha ac e ized o a poo
o de ing a 600°C bu en i ely ma ching wi h he bulk-UIV O2oscilla ion pai s a
1200°C (see Table 5.3).
Iso he mal e olu ion o he syn hesized nc-UO2was hen pe o med. Iso he mal
g ain g ow h a a gi en empe a u e is an essen ial componen o e alua e he g ain
g ow h kine ics, and in consequence he pe o mance o such an inno a i e nuclea
uel. Diﬀe ences in he g ain g ow h beha iou be ween he mic o- and nano-g ain
o m o he same ma e ial, ha e been al eady epo ed [Moelle and Fech , 1995]
[Na e e al., 2000][Na e e al., 2001][Rupp e al., 2006]. In he ﬁ s case he
ma e ial ollows he gene al g ow h equa ion, bu in he nano-g ain case, he g ain
g ows un il a c i ical ime when he g ain g ow h emains cons an as desc ibed by
99
Chap e 5. C ys alliza ion and G ain G ow h in (T) o nc-UO2by Aqueous ou e
he elaxa ion unc ion Eq. 5.6. Howe e , when he nano-g ain ma e ial is ea ed
abo e a de e mined empe a u e (1100°C [Rupp e al., 2006]), he g ain g ow h ollows
again he gene al g ow h equa ion (Eq. 5.8). This is in ag eemen wi h he esul s
he e obse ed. Fo he empe a u es o 500°C, 700°C and 900°C and a s a ic and
ine a mosphe e o He, he g ain g ow h ook place in he ﬁ s hou s o iso he mal
hold un il a s able a e age g ain size was es ablished a he applied empe a u e, a
which ime c ys al g ow h ceased (Fig. 5.21). Fo he iso he m a 1200°C and a s a ic
a mosphe e o He, he ma e ial had a con inuous g ow h no eaching a cons an g ain
alue in he ﬁ s 50 h (Fig. 5.20).
F om Fig. 5.23 an ac i a ion ene gy o diﬀusion o 0.93 eV o 1.25 eV was ob ained.
Diﬀusion can occu along he g ain bounda y, o i can occu in ag anula ly ( olume
diﬀusion), o because o g ain de ec s. The g ain bounda y diﬀusion is always as e
han he olume diﬀusion, meanwhile he olume diﬀusion occu s wi hin a single
g ain and is only impo an a highe empe a u es. In his case o nc-UO2, he low
ac i a ion ene gies ob ained could be ela ed p edominan ly o g ain bounda y (su ace
and in e ace) diﬀusion as olume diﬀusion con ibu ion exhibi s o highe ac i a ion
ene gies (abo e 4 eV [Sabioni e al., 1998]).
A la ice o abou 0.5472 nm was al eady ound o he samples ea ed a 900°C
a e 50 h dwell ime unde A /H2ob aining a ﬁnal size abou 50 nm (Table 5.5).
The e o e a empe a u e o 1200°C (and in consequence a ﬁnal c ys alli e size o
80 nm) would be, in p inciple, no necessa y o each he ypical la ice pa ame e o
he e e ence la ge-g ained UO2(a=0.5472 nm), as abo e commen ed.
An a e age g ain size o 322 nm was measu ed a e cooling o he hea ea men
a 1200°C a e 50 h dwell unde He (Table 5.5). Taking ha in o accoun , i appea s
ha a empe a u e below 1200°C would be necessa y in he sin e ing p ocess o o he
monoli hs, o a oid ex eme g ow h o he pa icles (>200 nm). Ne e heless o he
nc-UO2samples annealed a 1200°C du ing 50 h unde A /H2dynamic a mosphe e,
a ﬁnal g ain size o 85 nm was measu ed a e cooling. E en a e 200 h dwell ime
a his empe a u e unde educing a mosphe e, a ﬁnal g ain size o 150 nm was seen
(qui e a om he 322 nm obse ed unde He a mosphe e a e 50 h). This diﬀe ence
could be due o he ini ial oxida ion s a e o he nc-UO2samples and hei e olu ion
unde a s a ic He a mosphe e.
As men ioned p e iously, an hype s oichiome ic UO2would p esen a s onge
inc ease o he sel -diﬀusion coeﬃcien s and in he same way aise he mass-ﬂow, he
g ain-bounda y mo ion and he g ain (o c ys al) g ow h will occu . In Fig. 5.24,
he A henius diag am compa ing he ca ion sel -diﬀusion in UO2ﬂuo i e-s uc u e
om la ge-g ain epo ed by [Ma zke, 1987] and nano-g ain om his s udy (samples
annealed unde He s a ic a mosphe e du ing 50 h), has been plo ed. Be ween 20 o de s
o magni ude a 500°C (la ice pa ame e =0.5457 nm; a e age g ain size =18 nm)
and 5 o de s o magni ude a 1200°C (la ice pa ame e =0.5472 nm; a e age g ain
size =322 nm) ha e been ound o he diﬀusion coeﬃcien s be ween bulk-UO2
[Ma zke, 1987] and nc-UO2o his s udy (Table 5.5). Diﬀe ences in he diﬀusi i y in
he g ain bounda ies be ween mic o- and nano-g ain ha e been seen al eady in o he
ﬂuo i e s uc u e me al oxides [Ma in, 2007]. In ac he diﬀe ences in he diﬀusion
coeﬃcien be ween bulk- and nc-UO2a e compa ible wi h an enhancemen o he
diﬀusion p ocesses ei he by a diminishing o he g ain size o by O/U>2 eﬀec s.
100
5.6. Resul s and discussion.
101
Chap e 5. C ys alliza ion and G ain G ow h in (T) o nc-UO2by Aqueous ou e
102
Chap e 6
C ys alliza ion and G ain G ow h
in (T) o nc-UO2by O ganic ou e
6.1 Gene ali ies.
In his chap e he e olu ion o he g ain size and he c ys al s uc u e e olu ion as a
unc ion o empe a u e unde ine and educing a mosphe e o nc-UO2p ecipi a ed
om o ganic phase (see Chap. 4), ha e been in es iga ed. The mog a ime ic analysis
(TGA) o he samples p o ides he s a ing poin o hese in es iga ions, enabling he
iden iﬁca ion o mass losses a gi en empe a u es. The s uc u e o he ma e ial as a
unc ion o empe a u e has been iden iﬁed mainly by XRD and HT-XRD, bu also by
ad anced me hods including XANES and EXAFS.
6.2 The mal e olu ion and mass changes as p obed
by TGA/DTA.
The mog a ime y analysis (TGA) and diﬀe en ial he mal analyses (DTA) we e
employed unde A /5%H2gas a a hea ing a e o 5°C/min o de e mine he he mal
decomposi ion empe a u e and he wa e and o ganic con en o he p oduc (see
Fig. 6.1). The desc ip ion o he ins umen is shown in Sec. 2.5.
Upon hea ing unde A /5%H2, a mass loss o 1 w %is obse ed un il 150°C oge he
wi h a sligh endo he mic peak. This loss o weigh is likely due o ou gassing o esidual
wa e coming om he p ecu so . A second weigh loss o abou 3.5 w %appea s un il
280°C, accompanied by an exo he mic peak a 240°C. A new mass loss appea s un il
485°C which could be due o he esidual ca bon om he p ecu so . Be ween 485 and
1200°C he e is no weigh loss (1 w %) accompanied wi h wide exo he m p obable due
o he hea elease because c ys alliza ion.
6.3 La ice pa ame e and c ys al g ow h in neu al
a mosphe e.
The c ys al g ow h, la ice pa ame e o he nc-UO2 om o ganic ou e (5.52 nm size
and la ice pa ame e a = 0.5430(1) nm; see Sec. 4.4), has been in es iga ed unde ine
103

Chap e 6. C ys alliza ion and G ain G ow h in (T) o nc-UO2by O ganic ou e
Figu e 6.1: TGA and DTA signal o nc-UO2unde A /5%H2a mosphe e.
condi ions (s a ic He a mosphe e) using in si u HT-XRD. The eﬀec o empe a u e
on he c ys alli e size, which is a undamen al pa ame e in he sin e ing p ocess has
been analysed. As i has been seen in Fig. 6.1, no o ganics loss was obse ed a e
500°C. Howe e , o a oid any possible decomposi ion o he nc-UO2o ganic laye in
he s a ic a mosphe e o he HT-XRD chambe , a p e ea men unde O2was applied
(500°C du ing 1 h), ollowed by 2 h unde A /5%H2 o e e se he possible oxida ion
o he pa icles. A e he mal p e- ea men , a size o 37 nm and la ice pa ame e
0.5462(0) nm, we e de e mined. The in si u HT-XRD pa e ns we e acqui ed wi h an
ins umen desc ibed in 2.4.2. The empe a u e ange explo ed was 30 o 1100°C.
The e olu ion wi h empe a u e o he nc-UO2XRD pa e n is shown in Fig. 6.2.
The obse ed eﬂec ions a e assigned o UO2- cc phase s uc u e and o P phase co -
esponding o he hea e pla e. The eﬀec o empe a u e on he peaks can be obse ed
mo e clea ly in Fig. 6.2- igh , which displays he e olu ion o wo main peaks ( he (111)
and (200) eﬂec ions) o he UO2s uc u e. In Fig. 6.2, one obse es a shi in he
peak posi ion o lowe angles, possibly ela ed o a he mal la ice pa ame e expansion.
An eﬀec o he empe a u e is seen in he wid h o he peaks which dec eases wi h
inc easing empe a u e while he in ensi y o he peaks inc eases. This wid h change
was obse ed e en below 700°C which was he highes p e- ea men empe a u e.
This eﬀec could be due o he longe imes (abou 10 h) a empe a u e used in he
HT-XRD which induce o a pe ec ionism o he UO2 cc-s uc u e (highe c ys al-
liza ion). Since he con ibu ion o ins umen al b oadening is independen o he
empe a u e, he b oadening a lowe empe a u es is mainly ela ed o he c ys alli e
size and s ain p esen in he ma e ial, as well as inc ease o he s uc u al o de . Bo h
con ibu ions, c ys al size (p opo ional o cos−1θ; Eq. 5.1) and s ain (p opo ional o
anθ; Eq. 5.2), ha e diﬀe en angula dependences, and a e so sepa able. A s udy o
hose inﬂuences has been pe o med in he ollowing.
104
6.3. La ice pa ame e and c ys al g ow h in neu al a mosphe e.
Figu e 6.2: In si u HT-XRD pa e ns o nc-UO2unde He (le ). The ypical UO2and P
(hea ing pla e) B agg peak posi ions a e also ma ked. The ( igh ) pic u e shows jus he
e olu ion o (111) and (200) peaks o UO2cubic s uc u e as a unc ion o empe a u e.
6.3.1 G ain g ow h as a unc ion o empe a u e unde neu al
a mosphe e.
The c ys alli e size o he nc-UO2has been de e mined by XRD Rie eld eﬁnemen
(see Sec. 2.4) o he B agg peaks (Fig. 6.2), and used also o cha ac e ize he mi-
c os uc u e o he ma e ial. F om hese esul s i is possible o gene a e a uni e sal
ep esen a ion o he c ys alli e size as a unc ion o empe a u e (XRDs measu ed a
empe a u e and unde s a ic He a mosphe e) and epo ed in Fig. 6.3. E en hough,
his in o ma ion is aken as uni e sal, sligh de ia ions may occu , in pa icula due
o dwell imes and empe a u e amps, bu mo e impo an ly due o he a mosphe e
o s a ic He du ing he mal ea men . No able c ys al size a ia ions we e obse ed
abo e 700°C as his was he empe a u e al eady eached du ing he p e- ea men o
he ma e ial. The c ys alli e size change wi h empe a u e shows a slow g ow h up o
700°C, and an in ense g ow h om 37 o 150 nm a 1100°C (see Fig. 6.3).
Fig. 6.4 shows TEM images o he nc-UO2pa icles as-p oduced, a e he p e-
he mal ea men unde O2(500°C) and A /H2(700°C) and a 1000°C. The size
change co esponds o he one measu ed by XRD.
6.3.2 La ice pa ame e and linea he mal expansion coeﬃ-
cien as a unc ion o empe a u e.
The c ys al g ow h o he sample unde ine condi ions (s a ic He a mosphe e) using
in si u HT-XRD, ha e been al eady desc ibed. In addi ion, he a ia ion o he la ice
pa ame e e sus c ys al size and empe a u e, as well as da a on he linea he mal
expansion, a e now epo ed and compa ed o bulk ma e ial UO2. The c ys al s uc u e
105
Chap e 6. C ys alliza ion and G ain G ow h in (T) o nc-UO2by O ganic ou e
Figu e 6.3: E olu ion o he nc-UO2c ys alli e size in unc ion o he empe a u e.
(a) 4 nm as-p oduced (b) 34 nm a 700°C (c) 91 nm a 1000°C
Figu e 6.4: TEM images o he nc-UO2.
o he p ecipi a es was, as he c ys alli e size, de e mined by Rie eld eﬁnemen ,
aking in o accoun he whole 2θ ange.
In Fig. 6.5a he la ice pa ame e ob ained as a unc ion o empe a u e (XRDs mea-
su ed a empe a u e and unde s a ic He a mosphe e) and i s de i a i e (Fig. 6.5b),
ha e been also de e mined by he XRD Rie eld eﬁnemen o he B agg peaks, and
ep esen ed oge he wi h he nc-UO2size e olu ion o obse e i s dependence. Also
he calcula ed la ice e olu ion o non-s oichiome ic s anda d UO2+x o diﬀe en
O/U a ios due o only he mal expansion, ha e been ep esen ed o compa ison. The
la ice pa ame e o a non s oichiome ic UO2+xis linked o he oxygen con en by he
ela ions o [Lynds e al., 1963]. Also he la ice pa ame e was co ec ed as a unc ion
o empe a u e wi h he [Fink, 2000] ela ions al eady eﬂec ed in Eq. 5.3.
An expansion in he la ice pa ame e om 0.5462(0) nm a RT (a e being p e-
ea men ) o 0.5482(0) nm a 300°C, has been de e mined. Abo e his empe a u e,
a linea e olu ion o he he la ice pa ame e wi h empe a u e is obse ed. Rela ing
he la ice pa ame e ound in his s udy wi h he [Lynds e al., 1963] ela ions, a
s oichiome y o UO2.04 up o 300°C o UO2.00 up o 750°C, has been de e mined. The
106
6.3. La ice pa ame e and c ys al g ow h in neu al a mosphe e.
(a) (b)
Figu e 6.5: a.) La ice cons an and c ys alli e size a ia ion o nc-UO2in unc ion o em-
pe a u e (cu es only as a guide o eye), om in si u HT-XRD measu emen s unde s a ic He
a mosphe e in compa ison wi h la ice e olu ion in unc ion o empe a u es o s anda d UO2
o diﬀe en O/U a ios ob ained by he ela ions o [Lynds e al., 1963], due o only he mal
expansion. b.) Rela i e c ys alli e size and la ice pa ame e s. empe a u e (cu es only as
a guide o eye).
nanoc ys alli es s abilize a O/U 2.0 a empe a u es abo e 750°C, o in o he wo ds,
a pa icles sizes >44 nm.
Fig. 6.6 displays he linea he mal expansion (LTE) and he linea he mal
expansion coeﬃcien (LTEC) o nc-UO2as a unc ion o he empe a u e. The LTE
a empe a u e Twas calcula ed using he ela ion 5.4. The LTEC was calcula ed by
diﬀe en ia ing he he mal expansion cu e aT e sus Twi h espec o he empe a u e
T(see Eq. 5.5).
The LTE o he nc-UO2is jus sligh ly highe han he one o UO2bulk
[Ma in, 1988] o all he in e al o empe a u es, as one could al eady p edic om
he la ice pa ame e ep esen a ion in unc ion o empe a u e (see Fig. 6.5a). The
LTEC is ini ially highe o nc-UO2 han o bulk-UO2 o empe a u es below 400°C
and ends o s abilize abo e his empe a u e wi h a alue o 12·10−6°C−1in
ag eemen wi h he alue o he LTEC o bulk-UO2. The oscilla o y ends obse able
o LTEC in nc-UO2can be a ibu e o ansi o y oxida ion- educ ion eﬀec s.
I Fig. 6.7 he pa e ns compa ison o nc-UO2a RT (p e iously ea ed a
500°C du ing 1 h and 700°C du ing 2 h unde O2and A /5%H2, espec i ely)
(a = 0.5462(0) nm), nc-UO2a 1100°C (a = 0.5534(0) nm) and nc-UO2a RT a e
he mal ea men a 1100°C (a = 0.5472(0) nm) (all measu ed in si u in he HT-XRD
ins umen unde s a ic He a mosphe e), is shown.
A 1100°C unde s a ic He a m, an O/U a io o 2.0 (see Fig. 5.4a) and a la ice
pa a ame e o a = 0.5534(0) nm (measu ed a empe a u e), ha e been obse ed.
The same he mally ea ed sample measu ed a e cooling a RT, shows a alue o
107

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