Isolation of Pu-isotopes from environmental samples using ion chromatography for accelerator mass spectrometry and alpha spectrometry

Elsevier Editorial System(tm) for Analytica Chimica Acta Manuscript Draft Manuscript Number: Title: Isolation of Pu-isotopes from environmental samples using ion chromatography for accelerator mass spectrometry and alpha spectrometry Article Type: Original Paper Section/Category: SEPARATION METHODS Keywords: Plutonium; AMS; alpha spectrometry; TEVA Corresponding Author: Physicist Elena Chamizo, Corresponding Author's Institution: Centro Nacional de Aceleradores First Author: Elena Chamizo Order of Authors: Elena Chamizo; María del Carmen Jiménez-Ramos, PHD Student; Lukas Wacker, doctor; Ignacio Vioque, Doctor; Ana Calleja, PHD student; Rafael García-Tenorio, professor; Manuel García-León, professor Abstract: A radiochemical method for the isolation of plutonium isotopes from environmental samples, based on the use of specific chromatography resins for actinides (TEVA®, Eichrom Industries), has been set up in our laboratory and optimised for their posterior determination by alpha spectrometry (AS) or accelerator mass spectrometry (AMS). The proposed radiochemical method has replaced in our lab a wellestablished one based on the use of a relatively un-specific anion-exchange resin (AG® 1X8, Biorad), because it is clearly less time consuming, reduces the amounts and molarities of acid wastes produced, and reproducibly gives high radiochemical yields. In order to check the reliability of the proposed radiochemical method for the determination of plutonium isotopes in different environmental matrixes, twin aliquots of a set of samples were prepared with TEVA and with AG 1X8 resins and measured by AS. Some samples prepared with TEVA resins were measured as well by AMS. As it is shown in the text, there is a comfortable agreement between AS and AMS, which adequately validates the method. Isolation of Pu-isotopes from environmental samples using ion chromatography for accelerator mass spectrometry and alpha spectrometry E. Chamizo a*, M.C. Jiménez-Ramos b, L. Wacker c, I. Vioque b, A. Calleja b, M. García-León a,b, R. García-Tenorio b a Centro Nacional de Aceleradores (CNA), Avda. Thomas A. Edison , 41092, Sevilla, Spain. b Applied Nuclear Physics Group, University of Sevilla, P.O Box 1065, 41080, Sevilla, Spain. c Institute of Particle Physics, ETH Hönggerberg, CH-8093 Zürich, Switzerland. * Cover Letter 1 Isolation of Pu-isotopes from environmental samples using ion chromatography for accelerator mass spectrometry and alpha spectrometry Abstract A radiochemical method for the isolation of plutonium isotopes from environmental samples, based on the use of specific chromatography resins for actinides (TEVA®, Eichrom Industries), has been set up in our laboratory and optimised for their posterior determination by alpha spectrometry (AS) or accelerator mass spectrometry (AMS). The proposed radiochemical method has replaced in our lab a well-established one based on the use of a relatively un-specific anion-exchange resin (AG® 1X8, Biorad), because it is clearly less time consuming, reduces the amounts and molarities of acid wastes produced, and reproducibly gives high radiochemical yields. In order to check the reliability of the proposed radiochemical method for the determination of plutonium isotopes in different environmental matrixes, twin aliquots of a set of samples were prepared with TEVA and with AG 1X8 resins and measured by AS. Some samples prepared with TEVA resins were measured as well by AMS. As it is shown in the text, there is a comfortable agreement between AS and AMS, which adequately validates the method. 1.- Introduction Plutonium is an anthropogenic element that has found its way into the environment since the beginning of the nuclear age in 1945. Among the different actinides released due to nuclear activities, Pu deserves a special attention from the point of view of radioecology and toxicology. The main source of Pu to the environment were the atmospheric nuclear weapons tests carried out mainly in the 1950’s and the 1960’s, which introduced 0.3 TBq of Pu in a global scale [1]. Additional amounts of this element have been released at a local scale as a consequence of civil nuclear reactor accidents (as the Chernobyl accident), releases from nuclear reprocessing facilities (as Sellafield, UK; La Hague, France; Mayak PA, Russia) and nuclear weapons accidents (Palomares, Spain; Thule, Greenland). * Manuscript 2 Among the four major plutonium isotopes (241Pu, 238Pu, 239Pu and 240Pu), three are alpha-emitters: 238Pu (T1/2=87.7 y, Eα=5,499 MeV (72%)), 240Pu (T1/2=6564 y, Eα=5,168 (76%)) and 239Pu (T1/2=24110 y, Eα=5,157 (73,3%)). Conventionally, they are determined in environmental samples by alpha spectrometry (AS). However, the two most abundant plutonium isotopes, 240Pu and 239Pu, cannot be quantified independently in low-level samples due to their very similar alpha-energy emissions (ΔE=11 keV). AS usually informs about the joined activity of 239Pu and 240Pu. In many cases, this is enough for a simple monitoring or surveillance program. Nevertheless, the study of the ratio of 240Pu and 239Pu offers unambiguously information about the origin of the plutonium. The 240Pu/239Pu atomic ratio ranges from 0.03 to 0.06 for weapon-grade plutonium, and reaches a value of 0.4 for reactor-grade plutonium. This ratio can be quantified by mass spectrometry techniques (MS): either thermal ionisation mass spectrometry (TIMS) [2], inductively coupled mass spectrometry (ICP-MS) [3], or accelerator mass spectrometry (AMS) [4]. Among them, AMS offers very low detection limits (~1 µBq for both 239Pu and 240Pu), making it possible to study these isotopes in any environmental compartment. On the other hand, limited information on the Pu source term can be gained by measuring the 238Pu/239+240Pu activity ratio by AS. We routinely perform in our laboratory the determination of 238Pu and 239+240Pu in a wide variety of environmental matrixes by AS. Recently, we have gained as well experience in the field of the determination of 239Pu and 240Pu by AMS [5]. In both cases, it is necessary to apply a radiochemical procedure to isolate the plutonium fraction and adapt it to optimum physicochemical conditions for the final measurement: a) the electro-deposition of plutonium onto stainless steel disks for its determination by AS or b) the dispersion of the plutonium in a metal/metal oxide matrix for its measurement by AMS. In this paper, we describe a radiochemical method, based on the use of the extraction chromatography resin TEVA (Eichrom Industries), for the isolation of the plutonium from different environmental matrixes. The following points have been studied: recovery yields, alpha-peak resolution in AS, and the magnitude of the U, Th and Dy decontamination factors as potential interferences for AMS or AS. The necessary validation was performed by applying the TEVA method and the conventional one, based on the AG 1-X8 resin (Biorad), to different samples, and comparing the obtained results by AS. Also some independent aliquots were prepared with the 3 TEVA method and measured by AMS. That way, the validity of the recently adapted TEVA method for both AS and AMS has been checked. A description of the TEVA method is given in Section 2, its main characteristics and advantages are highlighted in Section 3 and, finally, the results obtained in the validations exercises are shown and discussed in Section 4. 2.- Experimental 2.1.- Standards, resins, equipments and samples The samples have to be spiked with a known amount of 242Pu both for AS and for AMS measurements. This spike is necessary in both techniques for the determination of plutonium concentrations by the isotope-dilution method: 239+240Pu activity in AS, or 239Pu and 240Pu activities in AMS. In AS, this spike can be used as well to estimate the chemical recovery yields of the procedure. The standard solution of 242Pu was supplied by NPL, England, and was diluted to obtain stock solutions with a concentration of 20.9 mBq/ml (144 pg/ml) in a 2M HNO3 matrix. The extraction chromatography resin used was TEVA (Eichrom Industries), which has been proved to be a good option for the separation of tetravalent actinides (Pu4+, Np4+ and Th4+) in 2-4 M nitric acid from Am3+ and U6+ [6]. Ready-to-use mini-columns (diameter: 7 mm; length: 8 cm) packed with 2 ml of resin (100-150 µm mesh) were used for these analyses. The AS system used for the measurements was an alpha spectrometer Alpha Analyst (Canberra) with eight independent chambers that can work in parallel. Each chamber was equipped with a 450 mm2 passivated implanted planar silicon detector (PIPS, 18 keV of nominal resolution). The measurements were performed by placing the samples at a distance of 1.5 mm from the detector. In these conditions a counting efficiency of 34% was achieved [7]. . The AMS measurements were performed with the ETHZ/PSI compact AMS system Tandy adapted for the measurement of actinides. Briefly, the plutonium was extracted from the Cs+ sputtering ionsource as PuO-, stripped to Pu3+ with an Ar stripper gas at a terminal voltage of 330 kV, and counted with a gas ionisation detector with 1.3 MeV of energy. This detector features with a 3x3 mm2 4 silicon nitride entrance window, which is 40 nm thick for optimal separation of the plutonium ions from the 2+ molecular fragments with the same M/q ratio [4]. The ICP-MS determinations, carried out to study the magnitude of the U, Th and Dy decontamination factors, were done with an Agilent-7500C mass spectrometer. The analysed samples were the followings: a) artificially traced water, b) ashes of wood, c) superficial soils collected in Palomares (Spain) and d) superficial soils with fallout plutonium. Some of these samples were used either in the decontamination factors study or in the validation exercises. 2.2.- Proposed TEVA radiochemical method Sample pre-treatment ( 20 h) A known amount of 242Pu (1 ml of the 20.9 mBq/ml solution for AS, and 20 μl of the same solution for AMS) was added to each sample (water, soils, wood) as a spike, together with some ml of diluted HCl for its homogenisation. The solid samples were dried at 70ºC for 2 hours. Afterwards, they were ashed in a muffle furnace to remove the organic matter in two sequential steps: firstly at 150ºC during 2 hours, and finally at 550ºC during 6 hours. The water sample was directly evaporated to dryness. Once cooled, the residues were placed in a teflon beaker and 20 ml of 8M HNO3 and 2 ml of H202 were added. After heating at 90ºC for 1 hour, 1 ml of concentrated HF was added in order to enhance the quantitative extraction of the refractory plutonium that may be present, particularly in the analysed soils from Palomares. The mixture was then heated for 5 hours, adding 8M HNO3 to keep the volume at 20 ml. The supernatant was thereby isolated by centrifugation (4000 rpm, 10 minutes). The solid residue was discarded. The supernatant was then evaporated to dryness and the resulting residue was dissolved with 1 ml 8M HNO3 and 10 ml of distilled water, giving a 0.7M HNO3 solution. The plutonium was then subjected to a redox cycle: reduction to Pu3+ with Fe2+, adding 0.1 g of FeSO4, and oxidation to Pu4+, adding 8 ml of 8M HNO3 and 0.2 g of NaNO2. At these conditions, the plutonium forms a 5 negatively charged nitrate complex, Pu(NO3)62-, which is retained in the column. The obtained solution (about 19 ml in a 3 M HNO3 medium) was finally conditioned to a 0.5M Al(NO3)3 medium by adding 3 g of Al(NO3)3-9H20, to complex the oxalates that reduce the retention of the plutonium in the column [6]. The sample was then loaded into the extraction chromatography resin TEVA. Separation by TEVA resins (4 h) After the conditioning of the commercially pre-packed 2 ml TEVA resin with 10 ml of 3M HNO3, the sample solution was loaded onto the column. At these conditions, the Pu is stick to the resin, and most of the U and matrix elements are eluted (see posterior discussion). The resin was then washed successively with three different acid solutions: firstly with 10 ml of 3M HNO3, secondly with 40ml of 2M HNO3 to assure the removal remaining traces of U and, finally, with 15 ml of 6M HCl to elute the Th. In a last step the Pu was eluted with 20 ml 0.5 M HCl. The Pu fraction was then ready for the preparation of the Pu source in the form required for each technique. Preparation of the Pu source The final plutonium fraction has to be adapted to an optimum physicochemical medium for each technique. In the case of AS, the plutonium in our laboratory is conventionally deposited onto a stainless-steel disk, following a well-established electrodeposition method [8]. For AMS the plutonium, as plutonium oxide, needs to be dispersed in a metallic matrix for its measurement in a Cs+ sputtering ion-source. To prepare the AMS target, 0.5 mg of Fe3+ (as Fe(NO3)3) was added to the final plutonium fraction from the TEVA resin (20 ml of 0.5 M HCl) as a carrier. This solution was evaporated to dryness (5 h). The residue was baked at 800ºC in a muffle furnace for 3 hours to convert the plutonium and iron to oxides (Pu3O8, Fe2O3). Finally, 2 mg of pure aluminium powder were mixed with the oxides and pressed into a 1 mm diameter aluminium cathode. That way, the sample was ready to be measured by AMS [4, 9]. A complete flow-diagram of the analytical TEVA procedure proposed in this paper for Pu determination in environmental samples is shown in Fig. 1. 6 2.3. - Conventional method This radiochemical procedure is fully described in [7]. Briefly, it consists of the following steps: Step 1.- The samples are ashed (in order to remove the organic matter) and then wet-oxidised to put Pu-isotopes and other elements in dissolution. Step 2.- After filtration, the dissolved sample is passed through an AG® 1-X8 anion exchange resin, from where after several elutions (in order to remove interfering elements), the Pu-isotopes are stripped in a very purified solution. Step 3.- The Pu-isotopes are finally electrodeposited onto stainless-steel planchets. The Pu sources so obtained are then ready for measurement. 3.- Results and Discussion U, Th and Dy decontamination factors In order to check the reliability of the TEVA method for both AS and AMS, the quality of the plutonium fraction from the TEVA resin has been studied. In both cases, the achievement of a purified plutonium fraction is important: in AMS, the PuO- current extracted in the sputtering ion source improves with the concentration of Pu in the target; in AS, the resolution of the energy spectra is closely related to the thickness of the electrodeposits. Moreover, special attention must be paid to the decontamination factors achieved for some trace elements that may be potential plutonium interferences in AS (228Th and 241Am) or AMS (238U and 160Dy). As long as alpha spectrometry is concerned, it is important to remove traces of 241Am (T1/2=432.2 y, Eα=5.486 MeV (85%)) and 228Th (T1/2=1.91 y, Eα=5.420 MeV (73%)) in the final plutonium fraction, as their main alpha-emissions interfere with the ones of 238Pu (T1/2=87.7 y, Eα=5.499 MeV (72%)) in the energy spectra. In the case of the compact AMS system Tandy, 238U may reach the detector even when the machine is tuned for 239Pu, due to straggling on the residual gas in the vacuum system or due to instrumental 1 Table 1 Sample AG 1X8 TEVA Soil-1 115 68 Soil-2 60 36 Soil-3 70 44 Water 50 35 Table 2 Sample Pu recovery yield (%) TEVA method Pu recovery yield (%) AG 1-X8 method Water 76 61 Soil-1 74 54 Soil-2 62 55 Soil-3 68 51 Ashes 60 39 Table 3 AS Sample 239+240Pu (Bq/kg) TEVA method 239+240Pu (Bq/kg) AG 1X8 method Water 35.0 ± 2.2 Bq/m336.0 ± 2.1 Bq/m3 Ashes 0.08 ± 0.01 Bq/kg 0.08 ± 0.01 Bq/kg Soil-1 (Palomares) 49.4 ± 1.0 Bq/kg 46.5 ± 2.3 Bq/kg Soil-2 (Fallout) 2.40 ± 0.15 Bq/kg 2.37 ± 0.09 Bq/kg Soil-3 (Fallout) 1.48 ± 0.08 Bq/kg 1.44 ± 0.07 Bq/kg Table 1-4 2 Table 4 AS AMS Sample 239+240Pu (Bq/kg) TEVA method 239+240Pu (Bq/kg) AG 1X8 method 239+240Pu (Bq/kg) TEVA method Soil-2 (Fallout) 2.40 ± 0.15 2.37 ± 0.09 2.65 ± 0.14 Soil-3 (Fallout) 1.48 ± 0.08 1.44 ± 0.07 1.53 ± 0.09 Soil-4 (Fallout) 1.55 ± 0.06 1.50 ± 0.05 Soil-5 (Palomares) 0.06 ± 0.01 0.049 ± 0.005 Figure 1 Click here to download high resolution image Figure 2 Click here to download high resolution image Figure 3 Click here to download high resolution image Figure 4 Click here to download high resolution image