Ni ogen unde cladding o uel pins wi h mixed
u anium-plu onium ni ide*
Aleksey F. G ache 1, Liudmila M. Zabudko1, E geny E. Ma inenko2, Se gey I. Po ollo2,
Anna V. Belyae a3, Feodo N. K yuko 3, Vadim G. Teplo 3, Mikhail V. Skupo 4
1 P o y JSC, 1 Ramenskiy Bl d, 119607 Moscow, Russia
2 IPPE JSC, 1 Bonda enko Sq., 249033 Obninsk, Kaluga Reg., Russia
3 NIIAR JSC, 9 Zapadnoye Sh., 433510 Dimi o g ad, Ulyano sk Reg., Russia
4 VNIINM JSC, 5a Rogo S ., 123098 Moscow, Russia
Co esponding au ho : Se gey I. Po ollo ([email p o ec ed])
Academic edi o : Geo gy Tikhomi o ♦ Recei ed 25 June 2025 ♦ Accep ed 24 Sep embe 2025 ♦ Published 9 Oc obe 2025
Ci a ion: G ache AF, Zabudko LM, Ma inenko EE, Po ollo SI, Belyae a AV, K yuko FN, Teplo VG, Skupo MV (2025)
Ni ogen unde cladding o uel pins wi h mixed u anium-plu onium ni ide. Nuclea Ene gy and Technology 11(3): 225–229. h ps://
doi.o g/10.3897/nuce .11.173059
Abs ac
In gas-bonded uel pins he in e io space is illed wi h helium in o de o p o ide hea emo al om he uel.
Unde i adia ion, as a esul o gaseous ission p oduc s elease, he ini ial gas composi ion unde he uel pin
cladding changes, which leads o a change in i s he mophysical cha ac e is ics. Pos -i adia ion examina ions
ha e shown ha he pa ial p essu e o ni ogen unde he uel pin cladding inc eases wi h uel bu n-up inc ease,
since ni ogen a oms also elease om u anium-plu onium ni ide uel unde he cladding, in addi ion o he
k yp on, xenon and helium ine gases. The mechanisms o elease o ine gases and ni ogen om ni ide a e
di e en : he di usion mechanism o ine gases and he knocking ou o ission agmen s o ni ogen. The
di e ence in he gas elease mechanisms leads o a signi ican quan i a i e di e ence in he gases elease unde
he cladding. The speci ic ni ogen yield unde he uel pin cladding is signi ican ly lowe han he speci ic yield
o o he gases, bu i s p esence is an impo an ac o , since he dissocia ion empe a u e o he mixed ni ide,
as well as he ni iding o he inne su ace o he cladding, depends on he ni ogen p essu e unde he uel pin
cladding. A gene aliza ion and analysis o he esul s o pos -i adia ion examina ions o he ni ogen con en in
he gas mix u e unde he claddings o 87 in es iga ed uel pins a e i adia ion in he BN-600 eac o as pa o
17 expe imen al uel assemblies wi h mixed u anium-plu onium ni ide uel, i adia ed o maximum uel bu n-up
om 3 a . % o 9 a . % is ca ied ou in he pape .
Keywo ds
mixed u anium-plu onium ni ide uel, expe imen al uel pins, expe imen al uel assembly (EFA), combined expe imen al
uel assemble (CEFA) pos -i adia ion examina ion (PIE), ni ogen pa ial p essu e, ission gas p oduc s, cladding ni id-
ing, ine gases
Copy igh G ache AF e al. This is an open access a icle dis ibu ed unde he e ms o he C ea i e Commons A ibu ion License (CC-BY 4.0), which pe mi s
un es ic ed use, dis ibu ion, and ep oduc ion in any medium, p o ided he o iginal au ho and sou ce a e c edi ed.
Nuclea Ene gy and Technology 11(3): 225–229
DOI 10.3897/nuce .11.173059
Resea ch A icle
* Russian ex published: Iz es iya uzo . Yade naya Ene ge ika (ISSN 0204-3327), 2025, no. 2, pp. 40–49.
G ache AF e al.: Ni ogen unde cladding o uel pins226
In oduc ion
In o de o p o ide hea emo al om he uel in
gas-bonded uel pins, he pin in e nal space is illed wi h
helium. Depending on he uel pin design and i s ab i-
ca ion echnology, he quan i y and p essu e o helium
unde he cladding may a y. The elease o gas ission
p oduc s a he i adia ion esul s in he ini ial gas com-
posi ion change unde he uel cladding which changes
he uel pin he mophysical pa ame e s. The e o e, he
s udy o he kine ics and mechanisms o gas elease om
he uel as a esul o i adia ion is one o he mos im-
po an PIE ask. In he Po ollo e al. 2017; G ache e al.
2020 pape s he PIE esul s on he elease o gas ission
p oduc s (Xe and K ) and helium unde he claddings o
u anium and u anium-plu onium ni ide pins a e he
i adia ion in he BR-10 and BN-600 eac o s a e p e-
sen ed. I has been ound ha xenon, and hen helium
and k yp on a e eleased om ni ide uel in he la g-
es quan i ies. Sys emic pos -i adia ion examina ions a
NIIAR JSC ha e shown ha he gas mix u e unde he
i adia ed ni ide uel cladding includes ni ogen as well.
Despi e he ac ha , in pe cen age e ms, he amoun o
ni ogen unde he cladding is no oo la ge compa ed
o o he gases, i is an impo an ac o , since he disso-
cia ion empe a u e o he mixed ni ide, as well as he
ni iding o he inne su ace o he cladding, depend on
he ni ogen p essu e unde he pin cladding.
This pape summa izes and analyzes he esul s o pos -i -
adia ion examina ions o he ni ogen con en in he gas
mix u e unde he claddings o 87 examined uel pins a e
i adia ion in he BN-600 eac o as pa o 17 expe imen al
uel assemblies wi h mixed u anium-plu onium ni ide uel,
i adia ed o maximum uel bu n-up om 3 a . % o 9 a . %.
Resul s o measu emen s o
ni ogen con en unde he ni ide
uel pin cladding o BN-600 eac o
As de ined by speci ica ions, a he ab ica ion he in e nal
space o he BN-600 eac o uel pins wi h ni ide u a-
nium-plu onium uel is illed by helium wi h a olume
con en o no less han 94% unde a mosphe ic p essu e.
Assuming ha he es o he gas mix u e is ai (78% o
ni ogen), he maximum pa ial p essu e o ni ogen un-
de cladding in he ini ial s a e is no expec ed o exceed
4.7·10-3 MPa. The punc u ing, p io o i adia ion, he
cladding o wo “wi ness” uel pins (No. 0050, No. 0420)
om he CEFA-6 combined expe imen al uel assembly
showed ha he pa ial p essu e o ni ogen in hese pins
was 2.8·10-3 and 3.12·10-3 MPa, ha is, he uel pins com-
plied wi h he speci ica ions (Fig. 1).
A he cladding punc u ing o he BN-600 ni ide
pins ca ied ou in NIIAR JSC, he gas composi ion
was also analyzed along wi h measu ing o he o al
gas amoun unde he cladding, including measu emen
o he ni ogen pe cen age. Fig. 1 shows he a e age
alues o he ni ogen pa ial p essu e unde CEFAs pin
claddings oge he wi h he pa ial p essu e o ni ogen
in he CEFA-6 “wi ness” pins, and p esen s he design
ni ogen p essu e in he non-i adia ed uel pin illed
by helium o 94%. Fig. 2 shows he a e age alues o
he pa ial ni ogen p essu e unde he examined EFAs
uel pin cladding.
Resul s o expe imen al da a analysis
Fig. 3 shows he alues o he ni ogen pa ial p essu e
unde he cladding as a unc ion o uel bu n-up. As i can
be seen, he pa ial p essu e o ni ogen unde he clad-
ding inc eases as uel bu n-up g ows. Fig. 4 shows he
change in he pa ial p essu e o ni ogen unde he clad-
ding o examined pins as a unc ion o he ni ogen mass
ac ion in uel (speci ica ion da a).
Figu e 1. A e aged alues o he ni ogen pa ial p essu e unde
he cladding o all ni ide uel pins (CEFAs-1, 2, 3, 6, 7) a e
i adia ion in he BN-600 eac o , and o CEFA-6 “wi ness” uel
pins (No. 0050, No. 0420); * - design alue o ni ogen p essu e
in he 94% helium- illed uel pin.
Figu e 2. A e aged alues o he ni ogen pa ial p essu e unde
he cladding o all EFAs ni ide pins a e i adia ion in he BN-
600 eac o .
Nuclea Ene gy and Technology 11(3): 225–229 227
I ollows om he p esen ed da a ha
• in all cases, wi h he excep ion o uel pins No. 99
o CEFA-7, No. 68 o EFA-17, and No. 107 o EFA-
18, he pa ial p essu e o ni ogen unde he clad-
ding o i adia ed uel pins is less han he ini ial
alue (ni ogen p essu e in “wi ness” uel pins); o
CEFA-6, CEFA -2 and CEFA -3, he pos -i adia ion
ni ogen p essu e dec eased mo e han en old as
compa ed wi h he ini ial p essu e;
• he pa ial p essu e o ni ogen unde he examined
claddings pins inc eases as uel bu n-up inc eases;
• he pa ial p essu e o ni ogen unde he examined
claddings pins does no depend on he mass ac ion
o ni ogen in mixed ni ide uel.
Measu emen s o he ni ogen con en unde he uel
cladding has shown ha , in mos cases, he pos -i a-
dia ion pa ial p essu e is lowe han he ini ial design
p essu e o he p essu e in he CEFA-6 “wi ness” uel
pins (see Figs 1, 2). Speci ically, his is appa en o
uel pins wi h he lowes uel bu n-up alues (CEFA
- 6, CEFA - 2, CEFA - 3). The pos -i adia ion pa ial
p essu e o ni ogen in hese uel pins is an o de o
magni ude smalle han he p essu e o ni ogen a
he uel pin ab ica ion. This means ha he ni ogen
con ained in uel pin in he gas phase in e ac s a he
e y i adia ion beginning p ac ically ully wi h he
uel and/o wi h cladding. In p inciple, bo h o hese
p ocesses a e possible. The ni iding o inne cladding
su ace was obse ed o mos o he examined mixed
ni ide pins (Po ollo e al. 2022). A inc eased empe a-
u es gaseous ni ogen can eac wi h p e-s oichiome -
ic u anium ni ide (Nek aso 1957). An analysis o
speci ica ion da a o he mixed ni ide composi ion in
examined EFAs pins shows ha he ni ogen con en in
mos uel pins is lowe han he exac s oichiome ic
composi ion o 5.55 w . %.
A he same ime, i ollows om Fig. 3 ha he
mixed ni ide bu n-up inc ease leads o an inc eased
amoun o ni ogen unde he cladding. The only sou ce
o ni ogen in he pin is mixed ni ide, in which hal
o he o al a oms numbe s a e ni ogen a oms. One o
he possible mechanisms o he ni ogen a oms elease
is h ough hese being knocked ou by ission ag-
men s o med in ission o u anium o plu onium a oms
(Olande 1976; Lus man 1985). Unlike ine gas a oms
(K , Xe, He), which can mo e eely in u anium ni ide
a a su icien ly high empe a u e, ni ogen a oms a e
loca ed in he c ys al la ice nodes, and hei mo emen
by di usion is limi ed. Ni ogen a oms, which a e lo-
ca ed nea he ee su ace, can be howe e knocked
ou by ission agmen s and pass in o gas unde he
uel cladding. This p ocess is shown schema ically
on Fig. 5, om which i can be seen ha elas ic colli-
sions wi h ission p oduc s can esul in he la ice a -
oms (u anium, plu onium o ni ogen) being knocked
ou he ni ide su ace. The in ensi y o his p ocess is
de ined by many ac o s, he mos impo an o hese
being he ission agmen gene a ion a e ( ission a e),
he pa h leng h o p ima ily knocked ou a oms and he
ni ide open su ace a ea. And he g ea es unce ain y
in he calcula ions o he ni ogen a om knocked ou
a e om i adia ed mixed ni ide is associa ed wi h es-
ima ion o he uel pelle open su ace a ea in he uel
pin. In he ini ial s a e, he e a e mo e han one hund ed
uel pelle s in a uel pin wi h he geome ical su ace
o aling some 250 o 300 cm2. Besides, sin e ed mixed
ni ide pelle s ha e open po osi y, which needs o be
aken in o accoun as well. A he beginning o i adia-
ion a he eac o powe inc ease, uel pelle s a e ag-
men ed which esul s in hei open su ace inc eased
se e al- old. Fu he i adia ion leads o gas- illed
po es o ming wi hin he uel g ains and a he g ains
bounda y. A e a ce ain bu n-up alue is achie ed,
Figu e 3. Pa ial p essu e o ni ogen unde ni ide pin cladding
as a unc ion o uel bu n-up: Blue squa e – CEFA uel pins; Red
ci cle –EFA uel pins.
Figu e 4. Pa ial p essu e o ni ogen unde ni ide pin cladding
as a unc ion o he ni ogen mass ac ion in uel: Blue squa e –
CEFAs uel pins; Red ci cle – EFAs uel pins.
G ache AF e al.: Ni ogen unde cladding o uel pins228
he po es a he g ain bounda ies in eg a e in o chan-
nels ex ending h ough he uel pelle su ace. This ex-
pands g ea ly he su ace h ough which ni ogen a oms
can be knocked ou in o he pin gas space. The es i-
ma ed amoun o ni ogen eleased unde pin cladding
a e he i adia ion in he BN-600 eac o , acco ding o
he dependencies p esen ed in Olande 1976; Lus man
1985, wi h unce ain ies aken in o accoun , i s ai ly
well he measu ed alues o he ni ogen p essu e in
i adia ed uel pins.
The speci ic yield o ni ogen om mixed ni ide a
simila condi ions is many imes less han he speci ic
yield o gas ission p oduc s (xenon and k yp on) o he-
lium (Fig. 6), which indica es once again ha he yield
o ni ogen a oms is only om a he hin su ace lay-
e o uel pelle s. Ne e heless, ha ing ni ogen unde
he uel cladding is impo an in e ms o he mixed
ni ide dissocia ion a high empe a u es. I is known
ha u anium ni ide mel s cong uen ly a a empe a-
u e o 3100 K and a ni ogen p essu e o 2.5·10-1 MPa
and highe , he mel ing empe a u e o u anium-plu o-
nium ni ide being somewha lowe (~3045 K) (Ca a-
jal-Nunez, P ieu , Mana a 2014). A a lowe ni ogen
p essu e, hea ing ni ide o a high empe a u e causes
i o dissocia e. The maximum empe a u e o mixed
ni ide a no mal eac o ope a ion does no exceed
1500 - 1600 °С, his i ing he equilib ium ni ogen
p essu e abo e u anium ni ide equal o ~6·10-9 MPa
(Hayes e al. 1990). Adding ni ogen o a closed sys-
em, such as uel pin, up o p essu e o ~2·10-3 MPa
will shi he u anium ni ide decomposi ion eac ion
owa ds highe empe a u es, and equilib ium will be
eached in he sys em a a much highe empe a u e o
~2400 °C (Hayes, Thomas, Peddico d 1990). Sca ce
da a show ha he ni ogen p essu e abo e mixed ni-
ide a low empe a u es is highe han ha abo e u a-
nium ni ide, while his di e ence is smalle a a highe
empe a u e (Rogozkin e al. 2003). One can he e o e
s a e ha o in ac uel pins wi h a uel bu n-up o ≥ 8
a % he elease o ni ogen om mixed u anium-plu-
onium ni ide is an addi ional ac o ha p e en s
i s dissocia ion.
Conclusion
The measu emen s p esen ed in his pape o he ni-
ogen con en unde he cladding o expe imen al ni-
ide uel pins i adia ed in he BN-600 eac o ha e
shown ha he pos -i adia ion pa ial p essu e o ni-
ogen in i adia ed uel pins is in mos cases lowe
han he ini ial design p essu e alue. This means ha
he ni ogen gas con ained in uel in e ac s p ac ically
ully a he e y beginning o i adia ion wi h he uel
and/o he cladding.
A he same ime, an analysis o he pos -i adia-
ion examina ion esul s show ha he pa ial p essu e
o ni ogen unde he cladding o examined uel pins
is highe a highe uel bu n-up, since he i adia ion
esul s in ni ogen a oms elease om mixed ni ide
unde he cladding in addi ion o ine gases (k yp on,
xenon and helium). The elease mechanisms o in-
e gases and ni ogen di e g ea ly: i is a di usion
mechanism o ine gases and knocking ou by ission
agmen s o ni ogen. The di e ence in he gas e-
lease mechanisms leads o a g ea e quan i a i e di -
e ence o gas elease unde he cladding. The speci ic
yield o ni ogen unde he pin cladding is much lowe
han ha o o he gases. Howe e , o in ac uel pins
wi h a bu n-up o ≥ 8 a .% he elease o ni ogen om
mixed u anium-plu onium ni ide is an addi ional ac-
o ha p e en s i s dissocia ion.
Figu e 5. Diag am o he a om knocking ou om he c ys al
su ace laye by ission agmen s (Lus man 1985).
Figu e 6. Speci ic yield o k yp on (Red squa e) and ni ogen
(Blue ci cle) om mixed ni ide uel a e i adia ion in he
BN-600 eac o .
Nuclea Ene gy and Technology 11(3): 225–229 229
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