Appelblad, P.

Nygren, U.

Roos, P.

Sjögren, A.

Skipperud, L.

The aim of the reference material in the Norcmass-project was to produce a lowlevel (&lt;&lt;1mBq/g of 239Pu) sample of sufficient amount to allow individual laboratories to perform several tests without risk of using up the material. Although there are several reference materials available (eg IAEA) few have 239Pu/240Pu data and almost none have 237Np/239Pu-data. Those who have (eg IAEA-384) have very high concentrations and are not useful for testing analytical methods designed for low-level measurements where a large sample mass may be required. The reference material consist of the top 10cm of 2mm sieved soil pooled together from 12 different Danish locations collected during 2003. The Soil was blended and sieved through 0.6 and finally through a 0.4 mm sieve. A total amount of 17 kg soil was produced. Several aliquots of the material was subject to analysis by alpha spectrometry and ICP-MS. The material contain 239+240Pu at a concentration of 0.24 ± 0.01 mBq/g and a 240Pu/239Pu atom ratio of 0.19 ± 0.006. The ratio 237Np/239Pu was determined to 0.32 ± 0.01

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Downloaded from orbit.dtu.dk on: Dec 19, 2017NKS-NORCMASS reference material for analysis of Pu-isotopes and 237Np by massspectrometryRoos, Per; Nygren, U.; Appelblad, P.; Skipperud, L.; Sjögren, A.Publication date:2006Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Roos, P., Nygren, U., Appelblad, P., Skipperud, L., & Sjögren, A. (2006). NKS-NORCMASS reference materialfor analysis of Pu-isotopes and 237Np by mass spectrometry. (NKS-135). Nordisk kernesikkerhedsforskningNorrænar kjarnöryggisrannsóknirPohjoismainen ydinturvallisuustutkimusNordisk kjernesikkerhetsforskningNordisk kärnsäkerhetsforskningNordic nuclear safety research NKS-135 ISBN 87-7893-197-5 NKS-Norcmass reference material for analysis of Pu-isotopes and 237Np by mass spectrometry Per Roos1, Ulrika Nygren2, Petra Appelblad2Lindis Skipperud3 & Anna Sjögren3 1Risø National Laboratory, Denmark 2Swedish Defence Research Agency, FOI, Sweden 3Norwegian University of Life Sciences, Norway April 2006 Abstract The aim of the reference material in the Norcmass-project was to produce a low-level (<<1mBq/g of 239Pu) sample of sufficient amount to allow individual labora-tories to perform several tests without risk of using up the material. Although there are several reference materials available (eg IAEA) few have 239Pu/240Pu data and almost none have 237Np/239Pu-data. Those who have (eg IAEA-384) have very high concentrations and are not useful for testing analytical methods designed for low-level measurements where a large sample mass may be re-quired. The reference material consist of the top 10cm of 2mm sieved soil pooled together from 12 different Danish locations collected during 2003. The Soil was blended and sieved through 0.6 and finally through a 0.4 mm sieve. A total amount of 17 kg soil was produced. Several aliquots of the material was subject to analysis by alpha spectrometry and ICP-MS. The material contain 239+240Pu at a concentration of 0.24 ± 0.01 mBq/g and a 240Pu/239Pu atom ratio of 0.19 ± 0.006. The ratio 237Np/239Pu was determined to 0.32 ± 0.01. Key words Norcmass, ICP-MS, Mass spectrometry NKS-135 ISBN 87-7893-197-5 Electronic report, April 2006 The report can be obtained from NKS Secretariat NKS-775 P.O. Box 49 DK - 4000 Roskilde, Denmark Phone +45 4677 4045 Fax +45 4677 4046 www.nks.org e-mail nks@nks.org NKS-B Norcmass reference material for analysis of Pu-isotopes and 237Np by mass spectrometry Purpose Although there are several reference materials available (eg IAEA) few have 239Pu/240Pu data and almost none have 237Np/239Pu-data. Those who have (eg IAEA-384) have very high concentrations and are not useful for testing analytical methods designed for low-level measurements. They are also often of different composition (eg IAEA-384 being a coral sample) than ordinary soils and are usually distributed in small amounts preventing testing at the 10-50g analytical scale. The aim of the reference material in the Norcmass-project was to produce a low-level (<<1mBq/g of 239Pu) sample of sufficient amount to allow individual laboratories to perform several tests without risk of using up the material. The main disadvantage with the current reference material is the very few laboratories capable of providing data on the sample. Origin and preparation The top 10cm of 2mm sieved soil from 12 different Danish locations collected during 2003 were pooled at Risoe Research Centre, Denmark. The Soil was further sieved through 0.6 and finally through a 0.4 mm sieve and coarsely mixed by hand. Following a single homogenization of all soil for 30 minutes in a large volume mixer a total of 17 kg soil remained. A second batch of 8.5 kg soil with the same origin was prepared simultaneously but was mixed separately and therfore not added to the larger soilsample. It is however expected that this smaller soilsample have an almost identical composition as the larger one. Analysis A total of 512.9g of the sieved and mixed reference soil was ashed in a large glass tray at 550 ºC for 12h. The weight of ashed soil was recorded and a total of 47.3g ash subsampled for Pu-analysis by alpha spectrometry. The remaining sample was leached in 500 ml aqua regia over night, filtered through GF/A glass fiber filters and the leached ash thoroughly washed repeatedly with 1M HCl until a total volume of 1.5 litre. After precipitation using ammonia, amphoteric elements were removed by 6M NaOH and the final Fe(OH)3 precipitate washed two times with weak ammonia. Plutonium was separated from the soil by a two-column ion exchange method The so far purified sample was diluted to a total of 32.98g in 1M HNO3. From this 0.488g was taken (SampleR1) for U-analysis and two 10g samples (10.030g and 10.008g) was transferred to plastic bottles and sent to two of the other participating laboratories together with 1.2kg of the original reference soil. The remaining 12.45g of solution was finally purified by a TTA-extraction procedure and the final solution diluted to 5ml with 1 M HNO3. From this 0.5ml (Sample R2) was taken for U-Pu analysis. The remaining 4.5ml was diluted to 10ml (Sample R3) by deionized water (ELGA-water) and used for the Pu-isotopic measurement. This final sample thus represents a total of 72.77g ashed soil or 78.46g dry soil. Mass-spectrometric analysis At Risoe a PlasmaTrace2 (PT2) HR-ICP-MS with an ultrasonic nebuliser was used. Analysis was performed for uranium and plutonium in the three samples R1-R3. Tuning of the instrument before analysis was performed using a 0.5 ppb uranium solution. Instrument settings R1 R2 R3 Sample uptake rate [ml min-1] 0.45 0.45 0.45 Cool gas flow [l min-1] 13 13 13 Auxiliary gas flow [l min-1] 1.4 1.4 1.4 Nebuliser gas flow [l min-1] 1.03 1.03 1.03 Nebuliser heater temperature [ºC] 140 140 140 Nebuliser condenser temperature [ºC] 3 3 3 Detector Multiplier Multiplier Multiplier Dwell time:238U [ms] 2 2 2 Dwell time:239Pu [ms] 50 50 50 Dwell time:240Pu [ms] 50 50 50 Points per peak 20 20 20 Peak width 2 2 2 Scans 1 1 1 Sweeps 75 75 15 Resolution 350 350 350 The reference soil sample (R3) in a 10 ml volume was continously pumped into the USN and 15 separate measurements of 238U, 239Pu and 240Pu was performed in the peak jumping mode. Results The total peak counts for each isotope in sample R3 is given in Table 2 below . Table 2: Raw data counts for each isotope measured. The analysis of sample R1 showed that the levels of uranium already was sufficiently low to make the plutonium analysis without having any large corrections to the mass 239 due to tailing and UH+. Sample R2 showed that after the TTA-extraction the uranium was removed to the extent where the dominating source of uranium is the chemicals themselves. This was concluded from nearly identical uranium amounts found in the R2-sample and the 8M HNO3 used for backextraction from TTA.The fraction of counts in mass 239 due to the presence of uranium was measured to be 3.2 10-5 which means that the correction to mass 239 in this case was less than one count. Due to the low count rates no dead-time corrections was applied to any of the isotopes. A standard U-solution (U112a) was used to monitor mass-fractionation. This was found to be insignificant and no corrections have been applied to the raw data. Alpha-spectrometric measurements Run No. 238U 239Pu 240Pu Atom 240Pu/239Pu 1 1223 5000 911 0.18220 2 1180 5310 990 0.18644 3 1143 5368 923 0.17194 4 1082 5196 955 0.18380 5 1135 5111 964 0.18861 6 1058 5185 957 0.18457 7 1069 5140 979 0.19047 8 1117 5174 938 0.18129 9 980 4937 975 0.19749 10 1081 5004 967 0.19325 11 1031 5013 959 0.19130 12 991 5080 968 0.19055 13 1012 4985 894 0.17934 14 958 5179 966 0.18652 15 1020 4899 948 0.19351 Atom ratio: 0.1868±0.0065 (1 std.dev) The four subsamples of ashed material used for alpha spectrometry were separated for plutonium on a single ion exchange column. The samples were counted for 15 days on PIPS-detectors. Sample Mass of dry soil 239+240Pu 238Pu/239+240Pu [mBq/g] K1 12.38 0.237±0.009 0.032±0.007 K2 9.60 0.249±0.012 0.037±0.009 K3 18.67 0.220±0.007 0.031±0.005 K4 10.41 0.247±0.011 0.032±0.007 Average 0.238±0.0135 0.033±0.0027 Unfortunately there is an inverse correlation between Pu-activity and sample mass, which indicates incomplete isotopic exchange between tracer and analyte. The samples were measured before equilibrium had been attained between 228Th and 224Ra so even if the very small 238Pu peak was visibly separated from the tiny 228Th peak the 238Pu/239+240Pu values should be treated with some care. Measurements of 237Np The same soil as was used for the Pu reference material was also used for the 237Np analysis. Eight subsamples of 10g each were analysed for 237Np and Pu-isotopes using 242Pu as tracer for both elements. The results are given in table 4 below. Sample Atom 240Pu/239Pu Atom 237Np/239Pu 239+240Pu via ICP-MS 239+240Pu via alpha spectrometry Soil-1 0.1908 0.283 0.240 0.251 Soil-2 0.1818 0.362 0.246 0.239 Soil-3 0.1969 0.329 0.239 0.238 Soil-4 0.1965 0.318 0.238 0.235 Soil-5 0.1895 0.294 0.227 0.231 Soil-6 0.1887 0.304 0.239 0.238 Soil-7 0.1928 0.316 0.240 0.246 Soil-8 0.1790 0.371 0.240 0.233 Atom ratio: 0.1895 ± 0.023 0.322 ± 0.01 0.238 ± 0.018 0.238 ± 0.087 Determination of 240Pu/239Pu in NORCMASS reference soil using ICP-SFMS at laboratory #1 The reference soil was prepared at Risö and sent to FOI together with a Pu-extract prepared from approximately 100 g of soil. Sample preparation Soil: About 100g of the soil-sample was ashed at 650°C for 12h. After ashing, two different approaches were used for treatment of the soil prior to Pu-separation; lithium-borate fusion and acid leaching. For the fusion, 8 g of the ashed sample was mixed with a lithium-borate flux at a sample:flux ratio of 3:1 in a carbon crucible. The mixture was melted for 15 min at 1050°C and the melt was dissolved in 1.4M HNO3. Silica was removed by precipitation with PEG2000, the actinides co-precipitated with calciumoxalate and Pu was separated using a 2ml TEVA column. Pu was eluted from the resin using 5 ml 0.1% HEDPA which was used for ICP-MS analysis. For the acid leaching, 40g of the ashed soil was mixed with 8M HNO3 for 4 h. The sample was filtered and the leaching repeated once. The leachates were combined, evaporated and redissolved in 3M HNO3. Pu was separated in the same way as the fused sample. Extract: The extract from Risö was further purified using a 1 ml TEVA column. Pu was eluted from the resin using 5 ml 0.1% HEDPA which was used for ICP-MS analysis. Determination of Pu using ICP-SFMS The instrument used is an Element2 (ThermoElectron) ICP-sector field MS equipped with a “Twister” cyclonic spray chamber and a conical nebuliser. Dead-time, mass-discrimination and UH+ formation was monitored using IRMM184 (ref mtrl containing U of natural composition) in 0.1% HEDPA. The raw-data from the analysis of the samples were subsequently corrected for dead-time and UH formation. No significant effect of mass-discrimination was found and hence no correction was made for this. Table 5 Instrument and data acquisition settings for the ICP-SFMS determinations Instrument settings Argon flow rates Cooling gas 14 l min-1 Auxiliary gas 1 l min-1 Nebuliser gas ~0.95 l min-1 RF power 1300W Sample cone Nickel, 1.1 mm orifice diameter Skimmer cone Nickel, 0.8 mm orifice diameter Sample uptake ~0.2 ml min-1 Data acquisition settings Resolution (m/∆m) 300 Detection mode Pulse counting Data acquisition mode E-scan Quantitative determination of Am and Pu Isotopes 238U+, 239Pu+, 240Pu+, 241Pu+, 242Pu+ and 244Pu+ Sample time 10 ms for 239Pu+, 240Pu+ and 241Pu+, 3 ms for the other ions Mass window 5% Samples per peak 100 Number of scans 1500 Settling time 1 ms (default minimum) Magnet mass 238.050 u Monitoring of dead-time, mass discrimination and UH+ formation Isotopes and polyatomic ions 235U+, 238U+ and 238U1H+ Sample time 3 ms for 238U+, 5 ms for 235U+ and 10 ms for 238U1H+ Mass window 5% Samples per peak 100 Number of scans 1500 Settling time 1 ms (default minimum) Magnet mass 235.043 u Results The results of the analyses are shown in table 6. The uncertainties of the results were calculated according to ISO/GUM using the software GUM Workbench®. All reported uncertainties are expanded uncertainties with a coverage factor of two (k = 2). The 240Pu/239Pu ratio for the soil-samples prepared using LiB-fusion and acid-leaching corresponds well with each-other. It can also be noticed that the uncertainty of the result from the LiB-fusion is only slightly higher than the one obtained for the leached sample, although the net intensity is over 12 times lover for the fused sample. This is due to a relatively high amount of U remaining in the Pu-fraction from the acid-leached sample, which leads to a quite large correction of the 239Pu signal for 238U1H+. The contribution from this correction to the combined uncertainty of the ratio is about 50%, while the corresponding contribution from the UH correction for the fused sample is only 0.5%. The determined 240Pu/239Pu atomic ratio for the extract prepared at Risö is lower than the ratios obtained for the two aliquots of the soil sample. The uncertainty of this result is also lower than the uncertainties for the other results primarily due to a larger net intensity of the Pu-isotopes. Both the results from the soil prepared at FOI are significantly different from the result for the Risö-extract at the 95% confidence level. The reason for this bias is not clear. A source of uncertainty in determination of 240Pu/239Pu is the correction for contribution from 238U1H, sand since the level of 238U was high in the leached sample this could be an explanation for a biased result. However, since the result for the fused sample where the level of 238U was low corresponds well with the result for the leached sample this is not a likely explanation. Further analysis need to be made before the discrepancy can be explained. Table 6 Results of Pu-determination in NORCMASS reference soil Sample I_net_239Pu I_net_240Pu N_240Pu/N_239Pu Uc (k=2) Uc (cps) (cps) (%)Fused soil 55.1 11.8 0.215 0.021 9.8Acid leached soil 680 146 0.215 0.015 7.0Risö extract 2286 428 0.187 0.004 2.0 Results obtained from laboratory #2 are without details and only final results are presented (Table 7 below). The ‘Norc soil’ data are from preparations of soil samples in the lab#2 laboratory while ‘Norc extract’ data refer to the pre-separated aqueous solution sent out. 240Pu/239Pu ratio % stdv Stdv Norc soil 1 0,11 12 0,016 Norc soil 2 0,12 13 0,02 Norc extract 0,193 2 0,004 Table 7. Summary of determinations of 240Pu/239Pu atom ratio in Normass reference soil (Leached soil solution prepared and pre-separated at Risoe) 240Pu/239Pu Lab #1 0.187 ± 0.004 Lab #2 0.193 ± 0.004 Lab #3 0.187 ± 0.006 Kelley et.al. ’Roskilde soil’ (Sci.Tot. Env. 237/238, 1999, 483-500) 0.1904 ± 0.0012 Table 8 Determinations from ’Norcmass’ soil (data for reference soil prepared in the individual laboratories) 240Pu/239Pu Lab #1 0.215 ± 0.021 Lab #2 0.11 ± 0.02 Lab #3 0.189 ± 0.0062 Table 9 Comments to the results The pre-separated soil solution prepared at Risoe before sending out to the two other laboratories contained Pu from approximately 100g soil. This is more than the individual laboratories used in their own leaching/dissolution of the soil and therefore the Pu-signal intensity in this sample was higher. Furthermore, the solution was pre-purified at Risø and the final sample might therefore be expected to be cleaner than the individual soil leach/dissolution samples performed. The difference in results between the solution and raw soil samples is therefore due to both sample size (signal intensity) and sample purity. Only one of the laboratories managed to perform the 237Np analysis so no comparison can be done on this subject. Even if the reference material was expected to be homogenous it should be kept in mind that that is was pooled from several Danish locations and there may thus be a risk, even though small, that 240Pu/239Pu ratio as well as the 237Np/239Pu ratio varies in the sample due to either incomplete mixing or due to the presence of individual fall-out particles. Variation in the fall-out ratios are well known and for the Nordic countries published values are given in the table below. Table 10. Expected variation in fallout ratio 239Pu/240Pu & 237Np/239Pu in the Scandinavian countries. From Kelley et.al. (Sci.Tot. Env. 237/238, 1999, 483-500) 240Pu/239Pu 237Np/239Pu Reykjavik 0.1801 ± 0.0006 0.474 ± 0.01 Bergen 0.1738 ± 0.0006 0.391 ± 0.009 Oslo 0.1790 ± 0.0007 0.447 ± 0.009 Roskilde 0.1904 ± 0.0012 0.531 ± 0.013 Bibliographic Data Sheet NKS-135 Title NKS-Norcmass reference material for analysis of Pu-isotopes and 237Np by mass spectrometry Author(s) Per Roos1, Ulrika Nygren2, Petra Appelblad2, Lindis Skipperud3 and Anna Sjögren3 Affiliation(s) 1Risø National Laboratory, Denmark. 2Swedish Defence Research Agency, FOI, Sweden. 3Norwegian University of Life Sciences, Norway. ISBN 87-7893-197-5 Electronic report Date April 2006 Project/Sub Project NKS-B / Norcmass No. of pages 9 No. of tables 10 No. of illustrations 2 No. of references 0 Abstract The aim of the reference material in the Norcmass-project was to produce a low-level (<<1mBq/g of 239Pu) sample of sufficient amount to allow individual laboratories to perform several tests without risk of using up the material. Although there are several reference materials available (eg IAEA) few have 239Pu/240Pu data and almost none have 237Np/239Pu-data. Those who have (eg IAEA-384) have very high concentrations and are not useful for testing analytical methods designed for low-level measurements where a large sample mass may be required. The reference material consist of the top 10cm of 2mm sieved soil pooled together from 12 different Danish locations collected during 2003. The Soil was blended and sieved through 0.6 and finally through a 0.4 mm sieve. A total amount of 17 kg soil was produced. Several aliquots of the material was subject to analysis by alpha spectrometry and ICP-MS. The material contain 239+240Pu at a concentration of 0.24 ± 0.01 mBq/g and a 240Pu/239Pu atom ratio of 0.19 ± 0.006. The ratio 237Np/239Pu was determined to 0.32 ± 0.01. Key words Norcmass, ICP-MS, Mass spectrometry Available on request from the NKS Secretariat, P.O.Box 49, DK-4000 Roskilde, Denmark. Phone (+45) 4677 4045, fax (+45) 4677 4046, e-mail nks@nks.org, www.nks.org.

NKS-NORCMASS reference material for analysis of Pu-isotopes and 237Np by mass spectrometry

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NKS-NORCMASS reference material for analysis of Pu-isotopes and <sup>2</sup><sup>3</sup><sup>7</sup>Np by mass spectrometry

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