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Determination of Plutonium, Uranium and Americium/Curium Isotopes in Environmental Samples with Anion Exchange, UTEVA, Sr and DGA Resin

Determination of Plutonium, Uranium and Americium/Curium Isotopes in Environmental Samples with Anion Exchange, UTEVA, Sr and DGA Resin

Proc. Radiochim. Acta 1, 189–194 (2011) / DOI 10.1524/rcpr.2011.0034 © by Oldenbourg Wissenschaftsverlag, München

Determination of , and / in environmental samples with anion exchange, UTEVA, Sr and DGA resin

By M. H. Lee∗, J. H. Park and K. Song

Nuclear Chemistry Research Division, Korea Atomic Research Institute, P.O. Box 105, Yuseong, Daejeon, 305-353, Korea

(Received December 15, 2009; accepted in revised form November 12, 2010)

Actinides / / Environmental samples / Recently, several studies have reported on combined pro- Anion exchange / Extraction cedures for the determination of in soils or sediment samples with extraction chromatographic materi- als such as the TRU, Sr and TEVA Spec resins [1–4]. These Summary. This study presents a quantitative sequential ra- methods are somewhat difficult to apply for very low levels diochemical separation method for the Pu, U, Sr and Am/Cm of fallout radionuclides without using large amounts of ex- isotopes with an anion exchange resin, UTEVA resin, Sr resin traction chromatographic resin. Therefore, it is necessary to and DGA resin in environmental samples. After the radionu- develop improved determination methods for with clides were leached from samples with 8 M HNO ,thePu,U, 3 lower detection limits, faster turnaround time and reduced Sr and Am/Cm isotopes were sequentially adsorbed on the anion exchange column, UTEVA column connected with Sr analysis costs. It is also desirable that the method does not Spec column and DGA column. The Pu isotopes were puri- generate any appreciable amount of mixed waste or haz- fied from other through the anion exchange column, ardous wastes. In order to meet these challenges, a proced- and the uranium isotopes were separated from other nuclides ure has been developed to provide sequential determination through the UTEVA column. Also, 90Sr was separated from of plutonium, uranium, americium, curium and radiostron- other hindrance elements such as Ca2+,Ba2+ and Y3+ with tium in environmental and samples. By se- the Sr Spec column. Finally, Am/Cm fractions were purified quentially analyzing the entire sample, it is possible to avoid with the DGA and anion exchange resins. After α source repeating the extra and unnecessary /dissolution / preparation for the purified Pu, U and Am Cm isotopes with processes, which are quite time consuming and costly. Min- the micro-coprecipitation method, the Pu, U and Am/Cm imum detectable activity (MDA) is also achieved by ana- isotopes were measured by an alpha spectrometry. Strontium- 90 was measured by a low level liquid scintillation counter. lyzing the whole sample, rather than dividing it into small The radiochemical procedure for Pu, U and Am nuclides subsamples for each individual analysis [5]. investigated in this study has been validated by application to In order to reduce the analysis cost and turnaround time IAEA Reference soils. in conventional methods, a sequential determination for Pu, U, Sr and Am nuclides in environmental and radioactive waste samples has become increasingly important. The aim 1. Introduction of this study was to develop a rapid and sensitive analyt- The major nuclear tests conducted in the 50s and ical procedure for the sequential determination of critical early 60s caused worldwide contamination. The determin- man-made radioisotopes in environmental and radioactive ation of radionuclides resulting from global fallout and dis- waste samples based on the alpha spectrometry following charges from nuclear facilities is obviously important in radiochemical separation by exchange and extraction view of the public health and public concern about the chromatography after decomposition of sample matrix. The hazards of pollutants in the environment. For sequential analytical method for Pu, U, 90Sr and 241Am nu- obtaining reliable data for the activity concentration of ra- clides investigated in this study has been validated by appli- dionuclides, the analysis of plutonium, uranium, americium, cation to IAEA Reference materials. curium and radiostrontium at extremely low levels in the en- vironment requires a large amount of samples. The methods of analysis of radionuclides have always been time consum- 2. Method development ing regarding sample treatment and the radiochemical sepa- ration procedures of large amounts of sample material. In an Most of the radiochemical protocols follow three main emergency situation a rapid and reliable analytical method steps: sample decomposition, radiochemical separation, and of radionuclides is desirable. source preparation before measurement. A flow chart of the analytical procedure for Pu, U, 90Sr and 241Am isotopes in *Author for correspondence (E-mail: [email protected]). the nitric medium is shown in Fig. 1. 190 M. H. Lee, J. H. Park, and K. Song

Fig. 1. Separation scheme of the Pu, U, Sr and Am/Cm isotopes in envi- ronmental samples.

2.1 Sample decomposition The UTEVA, Sr and DGA resin columns were obtained as cartridges containing 1 g of each resin from Eichrom After soil or sediment (10–30 g) was weighed into a porce- company. Small size (50–100 μm) resin was em- lain dish and ashed in a muffle furnace with a gradual heat- ◦ ployed along with a extraction system. The UTEVA, ing program up to 550 C to eliminate organic , the Sr and DGA resin cartridges were stacked on the vacuum sample was transferred to a Teflon beaker. A strontium car- jar from top to bottom, in that order. The resins were condi- rier (about 1 ppm) and spikes of 242Pu, 232Uand243Am trac- tioned with 10 mL of 8 M HNO . The sample solution eluted ers (about 0.1 Bq) were added to ensure isotopic equilibrium 3 from the anion exchange column was then loaded onto the with the analyte nuclides. In the acid method, the cartridge at a rate of 0.5mL/min. Beakers and columns were ashed samples were dissolved in 30 mL of 8 M HNO with 3 washed with 2 × 5mLof8MHNO . a stirring on a hot plate. In the total decomposition method, 3 After this step, the cartridges were separated. The the ashed samples were dissolved in 10 mL of concentrated UTEVA column containing U isotopes was washed with HNO and 10 mL of HF (48%), and evaporated to a dryness. 3 5 mL of 8 M HCl. This rinse converted the resin to the chlo- Dissolution in HNO /HF was repeated and the sample was 3 ride system and removed a small amount of Np isotopes. evaporated again to a dryness. To remove and isotopes, 20 mL of 5 M The residue was dissolved with 30 mL of 8 M HNO . 3 HCl – 0.05 M oxalic acid was added into the column. The The sample solution was filtered through a membrane filter uranium isotopes were eluted with 15 mL of 0.01 M HCl. (0.2 μm pore size). About 2 mL of 0.5 M NaNO was added 2 After the Sr Spec column was separated from the stacked into the solution to adjust the of the Pu(IV). columns, it was washed with 10 mL of 3 M HNO3 –0.05 M oxalic acid for removing the trace levels of Pu and Np iso- topes passed from the front columns. The columns were 2.2 Radiochemical separation washed with an additional 5 mL of 8 M HNO3 to remove To shorten the radiochemical separation steps of the con- the alkaline earth interferences such as K1+,Ca2+,and 2+ ventional analysis of Pu and Sr isotopes, an anion exchange Ba . Finally, Sr was stripped with 10 mL of 0.05 M HNO3. column was connected with a Sr Spec column [6, 7]. In this The DGA columns were rinsed with 3 mL of 1 M HNO3, study, U, Sr and Am radionuclides were sequentially pu- followed by 10 mL of 0.1 M HNO3 to remove interferences. rified with UTEVA [4, 8], Sr and DGA resins [9, 10]after The DGA columns were stripped with 15 mL of 0.25 M HCl purifying Pu isotopes with anion exchange resin. to elute americium and curium. The residue was dissolved

The sample solution with an 8 M HNO3 medium was in 20 mL of 1 M HNO3 – 93% CH3OH with gentle heating passed through a pre-conditioned anion exchange resin and stirring. The column for removing rare earth elements (Bio-Rad, 100–200 Mesh) column (inner diameter; 10 mm, (REEs) was prepared from Bio Rad AG 1 × 4 resin (mesh; resin bead length; 120 mm) with 8 M HNO3 at the rate of 100–200, inner diameter; 1 cm, column length; 1.2cm).The 0.5ml/minute. The column was then washed with 20 mL column was conditioned with 100 mL of 1 M HNO3 – 93% of 8 M HNO3 to remove U isotopes. The effluent (passing methanol. Samples were loaded onto the columns followed and washing solution) was reserved for sequential separation by washing with 50 mL of 1 M HNO3 – 93% methanol. of U, Sr and Am radionuclides. Columns were washed with Americium is adsorbed on the column together with REEs 20 ml of 9 M HCl to desorb the Th. Finally, Pu isotopes were and traces of Pb and U, etc., while any remaining traces of eluted with 20 mL of 0.1MNH4I–12MHCl. Fe pass through the column. The column was washed with Determination of Pu, U and Am/Cm isotopes in environmental samples with anion exchange, UTEVA, Sr and DGA resin 191

90 80 mL of 0.1MHCl–0.5MNH4SCN – 80% methanol to For a chemical yield of Sr, 1 mL was taken from the pu- remove REEs and traces of U isotopes. A wash with 20 mL rified Sr solution, and the concentration of stable Sr element of 1 M HNO3 – 93% methanol was performed. Am was with an ICP-AES was measured. The remaining the purified stripped with 20 mL of 0.5MHCl. Sr solution was transferred to low polyethylene vial and mixed with 11 mL of liquid scintillation cocktail. Strontium-90 was analyzed by liquid scintillation count- 2.3 Source preparation of Pu, U, Am and Sr nuclides ing [16]. A tracer level (about 1 Bq) of 242Pu, 233U, 243Am and 244Cm was used for comparing properties of electrodeposition and micro-coprecipitation. After electrodeposition [11, 12]and 3. Results and discussion micro-coprecipitatiion [13–15], the 242Pu, 233U, 243Am and 3.1 Decomposition of sample matrix 244Cm isotopes were measured by alpha spectrometry. The purified Pu, U and Am fractions were evaporated to Complete decomposition of the soil matrix is important for dryness. The residue was dissolved in 1 mL of concentrated radiochemical analysis in environmental samples, because

HNO3 and evaporated to dryness. The purified isotopes were destruction of soil matrices makes it possible to help iso- coprecipitated with fluoride [15]. After source topic exchange and convert the nuclides to an ionic form preparation, the Pu, U and Am isotopes were measured by that can undergo chemical reactions. As presented in Ta- alpha spectrometry. bles 1 and 2, the activity concentrations of 239,240Pu and 238U

Table 1. Activity concentrations of 239,240 Pu in the IAEA-375 reference materials.

Activity concentration of 239,240 Pu a (Bq/kg)

Separation Decomposition Sequential Recommended Mean Chemical method method determination values concentration yield (%)

Anion exchange Acid leaching Possible 0.30 0.27 ± 0.10 b 74 ± 5 b (Bio-Rad) [17] (8 M HNO3) Anion exchange Total decomposition Possible 0.30 0.33 ± 0.09 77 ± 4 c (Bio-Rad) (con. HNO3 + HF) Extraction Total decomposition Possible 0.30 0.29 ± 0.06 73 ± 5 chromatography (con. HNO3 + HF) (TEVA) [18] Solvent extraction Total decomposition Difficult 0.30 0.35 ± 0.08 65 ± 7 (TOPO) [19] (con. HNO3 + HF) a: Number of aliquots analyzed is 3; b: Error is 1σ; c: This paper.

Table 2. Activity concentrations of 238U in the IAEA-375 reference materials.

Activity concentration of 238 U a (Bq/kg)

Separation Decomposition Sequential Recommended Mean Chemical method method determination values concentration yield (%)

Extraction Acid leaching Possible 24.4 20.3 ± 1.5 c 81 ± 6 c chromatography (8 M HNO3) (UTEVA) b Extraction Total decomposition Possible 24.4 25.2 ± 1.678± 4 chromatography (con. HNO3 + HF) (UTEVA) b Anion exchange Total decomposition Possible 24.4 26.4 ± 1.972± 4 (Bio-Rad) [20] (con. HNO3 + HF)

Solvent extraction Total decomposition Difficult 24.4 26.9 ± 2.178± 6 (TBP) [21] (con. HNO3 + HF) a: Number of aliquots analyzed is 3; b: This paper; c: Error is 1σ. 192 M. H. Lee, J. H. Park, and K. Song

Table 3. Activity concentrations of 90Sr in the IAEA-375 reference materials.

Activity concentration of 90Sr a (Bq/kg)

Separation Decomposition Sequential Recommended Mean Chemical method b method determination values concentration yield (%)

Sr Spec Acid leaching Possible 108 115 ± 11.4 c 79 ± 7 c (8 M HNO3) Sr Spec Total decomposition Possible 108 103 ± 9.474± 5 (con. HNO3 + HF) a: Number of aliquots analyzed is 3; b: This paper; c: Error is 1σ.

Table 4. Activity concentrations of 241Am in the IAEA-375 reference materials.

Activity concentration of 241Am a (Bq/kg)

Separation Decomposition Sequential Recommended Mean Chemical method method determination values concentration yield (%)

Extraction Acid leaching Possible 0.13 0.15 ± 0.02 d 82 ± 6 d chromatography (8M HNO3) (DGA + AE b ) c Extraction Total decomposition Possible 0.13 0.12 ± 0.01 85 ± 5 chromatography (con. HNO3 + HF) (DGA + AE b ) c Extraction Total decomposition Possible 0.13 0.14 ± 0.01 79 ± 6 chromatography (con. HNO3 + HF) (TRU + AE) [6] a: Number of aliquots analyzed is 3; b: Anion exchange resin; c: This paper; d: Error is 1σ. with total decomposition method were close to the recom- IAEA-375 reference material with the anion exchange, the mended value reported by the IAEA. However, the activity extraction chromatography and solvent extraction method concentrations of 238U with an acid leaching method were were close to the recommended value reported by the IAEA. a little lower than the recommended value reported by the However, the analytical cost of the anion exchange method IAEA. This means that the acid leaching method is not is cheaper than the extraction chromatography method. enough to completely leach the uranium in the soil matrix. Also, it is difficult to determine sequentially activity con- Therefore, to analyze U isotopes in the soil, total depompo- centrations for Pu, U, Sr and Am isotopes with the solvent sition method must be used, though the total depomposition extraction method. method is time consuming and requires expensive Teflon As presented in Table 2, activity concentrations of 238Uin beakers. the IAEA-375 reference material with different separations The activity concentrations of 90Sr and 241Am with the method were close to the recommended value reported by acid leaching method were close to the recommended value the IAEA, except for those with the acid leaching method. reported by the IAEA, as shown in Tables 3 and 4.Also, For the samples contaminated with high activity concentra- the activity concentrations and chemical yields of 90Sr and tion of Pu and Am isotopes, the determination of uranium 241Am with the acid leaching method were similar to those isotopes with the anion exchange method or solvent extrac- with the total decomposition method. This means that with tion method is insufficient to completely separate U isotopes 90 241 8MHNO3, Sr and Am isotopes which is from Pu and Am isotopes [22]. Therefore, to obtain pre- from241Pu, were easily leached into acid solution from the cise and accurate data on the U isotopes in highly contam- soil matrix. inated samples, it is necessary to completely separate the uranium isotopes from transuranium elements with the ex- traction chromatography method. 3.2 Separation of Pu, Sr, U and Am isotopes The separation of the actinides from environmental sam- Activity concentrations of 239,240Pu in the IAEA-375 refer- ples, especially those in the trivalent state like 241Am has ence material with different separations method were pre- been a complicated and time-consuming task. Recently, sented in Table 1. Activity concentrations of 239,240Pu in the the chromatographic resins for extraction such as TRU, Determination of Pu, U and Am/Cm isotopes in environmental samples with anion exchange, UTEVA, Sr and DGA resin 193

UTEVA, and Diphonix resins have given an to develop fast and simple separation methods for many ra- dionuclides [23–25]. The Diphonix resin shows high affinity towards Am(III), however, its elution is not as easy as with DGA Resin. The TRU and DGA resins have been reported as being suitable for separation of Am(III) with a high reten- tion factor. The activity concentrations of 241Am using ex- traction chromatographic method with DGA and TRU resins were close to the recommended value reported by the IAEA, asshowninTable4. Also, the chemical yield for 241Am with the DGA resin was close to that with the TRU resin, though the retention factor for Am(III) on the DGA resin in 8 M

HNO3 medium is higher than that on the TRU [26]. In spite of the Am isotopes are purified from other hindrance nu- clides such as Pu and U isotopes with the DGA or TRU resins in the soil sample, it is necessary to remove REEs in the Am fraction with the anion exchange resin or TEVA resin. In this study, the REEs were removed with the anion Fig. 3. Energy resolutions using electrodeposition and microcoprecipi- exchange resin, since the cost of the anion exchange resin tation methods (error; 1σ). is cheaper than that of the TEVA resin. However, with a ra- dioactive waste sample that contains very low level of the rare earth elements, it is sufficient to purify the Am isotopes AsshowninFig.3,theα-peak resolution for the ac- with DGA or TRU resins. tinides with the electrodeposition methods was found to be in the range of 20 keV to 30 keV. The α-peak resolution for 3.3 Source preparation for measuring Pu, U and Am the actinides with the electrodeposition methods was bet- isotopes ter than that with micro-coprecipitation methods. Especially, There are a number of α source preparation methods, the α-peak resolution for the actinides with La coprecipi- such as direct evaporation, electrodeposition, and micro- tation method was too large to distinguish alpha peaks, so coprecipitation; this paper compares the electrodeposition that it is difficult to use for α source preparation. However, method and the micro-coprecipitation method. As shown in though the α-peak resolution with Nd or Ce coprecipitation Fig. 2, recoveries of the actinides (242Pu, 232U, 243Am, 244Cm) method is over 30 keV, the peak of 241Am was distinguished with electrodeposition methods and micro-coprecipitation from 243Am and 244Cm, as shown in Fig. 4. Also, for analyz- methods were over 90%. Recoveries of the actinides with ing 241Pu by a LSC or 239Pu and 240Pu by thermal ionization Talvitie‘s method (Electrodep-1) [11] were similar to those mass spectrometry (TIMS) after measuring alpha peaks by with Lee‘s method (Electrodep-2) [12]. However, for prepar- alpha spectrometry, in the electrodeposition method, it is ation of an α source, Lee‘s method is more convenient than necessary to remove Fe or Ni dissolved from an elec- Talvitie‘s method, because Talvitie‘s method requires accu- troplating plate with an resin. However, in rate control of pH and that the deposition solution is free of the micro-coprecipitation method, the Pu isotopes are meas- and organic materials. Also, recoveries of the actinides ured directly by a LSC or a TIMS after dissolving the Pu with the Nd coprecipitation method were similar to those isotopes from the membrane filter. Therefore, the micro- with the Ce and La coprecipitation methods. coprecipitation method with the Nd and Ce element is more useful for the alpha source preparation than the electrode- position method, because electrodeposition requires rather

Fig. 2. Recoveries using electrodeposition and microcoprecipitation Fig. 4. Alpha spectra of Am and Cm isotopes with NdF3 coprecipita- methods (error; 1σ). tion method in the radioactive waste sample. 194 M. H. Lee, J. H. Park, and K. Song elaborate equipment which is difficult to maintain and is 241Am and Pu radionuclides in soil samples. J. Radioanal. Nucl. plagued with problems, such as current fluctuations and pH Chem. 226, 279 (1997). changes during an electrodeposition. 7. Tavcar, P., Jakopic, R., Benedik, L.: Sequential determination of 241Am, 237Np, Pu radioisotopes and 90Sr in soil and sediment sam- ples. Anal. Chim. Slov. 52, 60 (2005). 8. Croudace, I., Warwick, P., Taylor, R., Dee, S.: Rapid procedure for 4. Conclusions plutonium and uranium determination in soils using a borate fu- sion followed by ion-exchange and extraction chromatography. In this study, we developed a quantitative sequential sep- Anal. Chim. 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