Final Report for CCRI(II)-S2 CCRI(II)-S2. S. Nour*, K. G. W. Inn, L. R. Karam *University of Maryland, USA; NIST, USA

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1 Activity measurements of a suite of radionuclides ( 90 Sr, 137 Cs, 210 Pb, 210 Po, 228 Ra, 234 U, 235 U, 238 U, 238 Pu, 239,240 Pu) in soil reference material (Rocky Flats II) CCRI(II)-S2 Abstract S. Nour*, K. G. W. Inn, L. R. Karam *University of Maryland, USA; NIST, USA In 2005, the CCRI decided that a comparison undertaken from 2002 to 2007 by the NIST (SIM) in the development of a new soil (Rocky Flats II) standard reference material (SRM) was sufficiently well constructed that it could be converted into a supplementary comparison under CCRI(II), with comparison identifier CCRI(II)-S2, so as to support calibration and measurement capability (CMC) claims for radionuclide measurements in reference material (specifically, low calcium-content soils). Previous comparisons of radionuclides have been of single or multiple nuclides in non-complex matrices and results of such could not be extended to support capabilities to measure the same nuclides in reference materials. The results of this comparison have been reported to the participants, and have been used to determine the certified reference value of the SRM. The key comparison working group (KCWG) of the CCRI(II) has approved this approach as a mechanism to link all the results to certified reference values in lieu of the key comparison reference value (KCRV) of these specified radionuclides in this type of matrix (soil) so as to support CMCs of similar materials. 1. Introduction In the field of radionuclide metrology (radioactivity measurements), a particular issue has arisen with regards to CMCs of reference materials (soils, organic matrices, natural waters, etc.), most of which have not been subject to either key or supplementary comparisons. While the measurement of their contributing radionuclides (such as 137 Cs) have been compared, and such comparisons are used to support the CMCs of said nuclide even in a reference material, the comparison of the reference materials themselves offers very specific and often recalcitrant difficulties. In addition to the preponderance of a vast variety of reference materials, many of which are considered by only one laboratory, how such material is to be handled (sampling) and prepared for analysis (i.e., procedures used to extract the nuclides of interest from the matrix quantitatively) present potential problems for any kind of comparison. Radioactivity contamination in the environment and its subsequent transport have been long recognized as serious issues. Particularly after the Chernobyl incident in 1986, the balance of anthropogenic and naturally-occurring radionuclides throughout the Northern Hemisphere has changed, presenting various problems for environmental clean-up, waste transport and storage, and import/export of a variety of manufactured goods across national borders. In the United States, several governmental agencies have responsibilities to provide effective remediation of contaminated lands to allow for general use. To support quality radioactivity measurements of environmental samples, the International Committee for Radionuclide Metrology (ICRM) Low-Level Measurement Techniques Working Group recommended a soil-based reference material that could be used by the metrology and environmental monitoring communities as a well-defined matrix in the development and validation of environmental radiochemistry methods and for performance evaluation exercises. The Rocky Flats II soil standard reference material was developed through an intercomparison with 12 participating laboratories from four different countries, including two NMIs. 1/13

2 2. Participants Two NMIs, and an additional ten laboratories, participated in a comparison of one or more of the nuclides in the Rocky Flats II soil reference material matrix, and provided results in the agreed-upon format. Laboratory details are given in Table 1. The comparison was piloted by the NIST. Table 1. Details of participants in the CCRI(II)-S2 supplementary comparison* Institution Full name Country Regional metrology organization BNG British Nuclear Group Sellafield Ltd. United Kingdom EURAMET CEMRC Carlsbad Environmental Monitoring & Research Center United States SIM EML Environmental Measurements Laboratory United States SIM FSU Florida State University United States SIM GSF National Research Center for Environment and Health, Institute of Radiation Protection Germany EURAMET IAEA International Atomic Energy Agency Austria --- LANL Los Alamos National Laboratory United States SIM NIST National Institute of Standards and Technology United States SIM OSU Oregon State University United States SIM RESL Radiological and Environmental Sciences Laboratory United States SIM SRNL Savannah River National Laboratory United States SIM WHOI Woods Hole Oceanographic Institution United States SIM * Bold indicates signatory to CIPM MRA 3. Material and methods used in comparison 3.1 Material The material was obtained from Rockwell International s Rocky Flats Plant (RFP) in north-central Colorado, USA, by the National Institute of Standards and Technology (NIST) of the US Department of Commerce and by the Environmental Measurements Laboratory (EML) currently of the US Department of Homeland Security. The material was first coarsely sieved in the field to remove rocks larger than about 1.5 cm diameter. After air drying, the soil was blade milled twice and then pulverized with a pancake style air jet mill to an average particle diameter of 8 μm. More than 99 percent, by weight, of the particles were less than 20 μm in diameter. The soil was then V-cone blended to optimize homogeneity and bottled in polyethylene bottles. The final bottled material was sterilized with > 50 kgy of 60 Co radiation to satisfy export regulations and to increase shelf-life time. The elemental composition of the soil was determined by semi-quantitative x-ray fluorescence (XRF) analysis. 2/13

3 Table 2: Elemental Composition Based on Semi-quantitative X-Ray Fluorescence (XRF) Analysis H Element Percent by mass (%) Element Percent by mass (%) Si 36 Cl Al 4.5 Cr Fe 2.6 Cu Mg 0.29 Ga <0.001 Ca 0.40 Ni Na 0.65 Pb K 1.7 Rb Ti 0.20 Sr P 0.07 V Mn Y C 1.5 Zn S 0.02 Zr 0.02 H Estimated relative combined standard uncertainty for each reported concentration is -33 % to +50 %. When nonvolatile radionuclides were to be determined, participants were instructed to dry samples of the material at 40 o C for 24 hours prior to weighing. Laboratories were also advised to determine volatile radionuclides (e.g., 210 Po, 137 Cs, 210 Pb, 212 Pb and 214 Pb) on material as received (with separate samples dried as described to obtain a correction factor for moisture); correction for moisture content was made to the data for volatile radionuclides before comparing with the values given by this report. This approach limited the loss of these radionuclides during drying [1]. The mass lost on drying is typically less than 4 percent. Several (twenty three) bottles of the prepared soil were examined for gamma-ray heterogeneity through measurement of emission rates by counting in a 5-in (12.7 cm) NaI(Tl) detector coupled to a multichannel analyzer. The count rates from each measurement were analyzed for statistical difference for ten selected energy regions, and no detectable heterogeneity was observed. This material was also measured for alpha-particle emitting radionuclides using sample sizes of 1 gram to 100 grams; there are variations of results due to sample size. Based on over 100 plutonium and 241 Am measurements, it was concluded that the material contained hot particles. Participants were advised that a sample size of 5 grams to 10 grams should be used for radiochemical analysis, and a sample size of 30 grams to 100 grams for gamma isotopic analysis. Statements of uncertainty, tolerance limits, and ranges of reported results incorporate the effects of heterogeneity. 3.2 Methods Each laboratory was instructed to use its radiochemical and detection methods of choice, and to provide data for those nuclides they are experienced in analyzing. The various methods used are indicated in Table 3. NIST-calibrated tracer solutions ( 243 Am, 242 Pu, 232 U and 229 Th), to be used as internal reference for both internal calibration and extraction efficiency determinations, were provided by NIST. 3/13

4 Table 3. Measurement methods of the participants of CCRI(II)-S2 Laboratory Radiochemical Method(s) Detection Method(s) RESL, IAEA, EML, RESL, WHOI FSU, LANL, NIST, SRNL, EML, RESL, WHOI, OSU, BIL- GSL FSU, NIST, SRNL FSU, NIST, SRNL, RESL, BIL-GSL FSU, RESL, CEMRC, NIST, SRNL, EML CEMRC, NIST, SRNL, EML FSU, RESL, CEMRC, NIST, SRNL, EML FSU, GSF, IAEA RESL, LANL, CEMRC, NIST, SRNL, EML, WHOI, OSU, BIL-GSL FSU, GSF, IAEA, RESL, LANL, CEMRC, NIST, SRNL, EML, WHOI, OSU, BIL-GSL Acid leach (any combination of HNO 3, HCl, HF, HClO 4 ); Fusion/total decomposition Non-destructive Non-destructive Not analyzed by any laboratory Non-destructive Calcinations, acid leach (any combination of the following HNO 3, HCl, HF, HClO 4 ); Fusion/total decomposition Calcinations, acid leach (any combination of the following HNO 3, HCl, HF, HClO 4 ); Fusion/total decomposition Calcinations, acid leach (any combination of the following HNO 3, HCl, HF, HClO 4 ); Fusion/total decomposition Acid leach (any combination of HNO 3, HCl, HF, HClO 4 ); Fusion/total decomposition Calcinations, acid leach (any combination of the following HNO 3, HCl, HF, HClO 4 ); Acid leach (any combination of HNO 3, HCl, HF, HClO 4 ); Fusion/total decomposition Beta-particle counter Germanium gamma-ray Germanium gamma-ray Germanium gamma-ray Silicon surface-barrier alpha-particle Silicon surface-barrier alpha-particle Silicon surface-barrier alpha-particle, ICP-MS, AMS Silicon surface-barrier alpha-particle Silicon surface-barrier alpha-particle Radionuclide 90 Sr 137 Cs 210 Pb 210 Po 228 Ra 234 U 235 U 238 U 238 Pu 239,240 Pu 4. Results Due to the complex nature of the material compared, and the variability in sample preparation and analysis among the participating laboratories, the comparison reference values are taken as reference values for each radionuclide individually. In other words, the comparison reference values are radionuclide-specific. The determination of the reference value of each radionuclide in the Rocky Flats soil material was accomplished by using the radioanalytical results from each laboratory that measured the specific radionuclide. Twelve laboratories world-wide participated in this comparison and reported their final results to NIST, including the two NMIs listed in Table 1. The results from all 12 laboratories (and two from one NMI) were used in the calculation of the reference values. 4/13

5 Table 4. Measurement results of radionuclides in Rocky Flats soil from participating NMIs. NMI IAEA, Siebersdorf (1998) IAEA, Siebersdorf (2004) Nuclide Massic Activity Reported Value * Expanded Uncertainty (k = 2) mbq 238 Pu ,240 Pu Sr Pu ,240 Pu Sr NIST 238 Pu ,240 Pu U U U Cs Ra Pb * The laboratory value represents the mean of five replicate measurements. The reported uncertainty was expanded by a coverage factor of k = Calculation of the Massic Activity Value: the Reference Value Results for the radionuclides analyzed in this comparison are given in Table 5 and Appendix 1. For each radionuclide, the mean value is calculated for each laboratory together with expanded uncertainty (k = 2). Some laboratories did not report uncertainties for their mean values, and in these cases, uncertainties were calculated by NIST as a standard deviation from the reported results (2 sigma). In Appendix 1, the solid line represents the reference value (RV) for that radionuclide, and dashed lines are the associated uncertainty for the mean (k = 2). Different techniques used in analysis have been marked in figures when applicable (such as decomposition of matrix by acid leach or total dissolution, or non destructive methods by gamma spectroscopy). The method of choice for the calculation of the RV (see Table 3) was the mean of the individual laboratory means. Table 5. Massic Activity Reference Values for CCRI(II)-S2. Reference date 1 April 1998 Radionuclide Half-life Used* Mean ± U (k = 2) 95/95 Tolerance Limits [] () 90 Sr (28.79 ± 0.06) a 10.5 ± to Cs (30.07 ± 0.03) a 21.6 ± to Pb (22.20 ± 0.22) a 58.0 ± to Ra ** (5.75 ± 0.03) a 74.9 ± to U (4.468 ± 0.003) 10 9 a 40.4± to U (7.04 ± 0.01) 10 8 a 1.88 ± to U (4.468 ± 0.003) 10 9 a 39.6 ± to Pu (87.7 ± 0.1) a ± to /13

6 Half-life Used* Mean ± U (k = 2) 95/95 Tolerance Limits Radionuclide [] () 239,240 (24110 ± 30) a Pu 16.8 ± to 26.8 (6561 ± 7) a * Half lives taken from Evaluated Nuclear Structure Data File (ENSDF), April The stated uncertainty is the standard combined uncertainty ** Radium-228 activity values are based on measurements of its 228 Ac daughter 4.2 Calculation of the Uncertainty of the Reference Values Although all laboratories provided uncertainties on their values, not all reported complete uncertainty budgets for all radionuclides. Since this comparison was done with the objective of reference material certification, and the reference values were to be based on calculations of central laboratory values, uncertainty budgets were not requested from the participants. However, for this comparison, an example uncertainty budget (from the NIST) is shown in Table 5. Table 5. Uncertainty components for Rocky Flats soil. Relative uncertainty of output quantities (%) Radio- Calibra- Sample Radiochem. Source Coun- Spectrum Mass Tracer Blank Total nuclide tion Prep Prep ting Analysis 238 Pu &240 Pu U U U Th Th Th Gammas Contributions to the uncertainty for measurements of these types of material include: instrument calibration, weighing (gravimetric), sample dryness, nuclear data (half life, energy), tracer measurement uncertainties (negligible), chemical yields, and counting statistics. Of all the potential contributions to the uncertainty, only counting statistics and spectrum analysis methods are considered to be significant for these types of measurements in reference materials. The standard combined uncertainties (u c ) for each laboratory value were computed by incorporating components from three sources: 1) the estimated standard deviation of the mean of the laboratory mean values, 2) the k = 1 uncertainty associated with the radiochemical tracer SRMs used, and 3) Type B scientific judgment. The uncertainty components were combined in quadrature as specified by the GUM [2] and elsewhere [3]. The expanded uncertainties (U) for each of the massic activity reference values were estimated by combining the standard deviation of the mean of the laboratory mean values expanded with the Students t-statistics and expanded uncertainty (k = 2) contributions from radiochemical tracer SRMs and scientific judgment; U is taken at the 95 percent confidence interval. Expanded uncertainties were computed using the Welch-Satterthwaite coverage factor and are given at k = 2. The laboratories mean values are normally distributed for the massic activity reference value for a given radionuclide (i.e., the data is a group of values that are normally spread about their mean). The mean, A, used for the massic activity reference value, is calculated by averaging all the laboratories means using the equation: 6/13

7 A = 1 n n i= 1 A i (1) for which A i are the means of the individual laboratories, and n is the number of laboratories. The standard deviation (S) of laboratory means is a measure of how close an individual laboratory s mean, A i, is to the final mean, A, and is calculated as: 2 ( Ai A ) i s = n 1 (2) The expanded uncertainty (U) of the massic activity reference value ( A ) is given at the 95 % confidence interval based on Student t-distribution: S U = t (3) n for which t is the value of the t statistic for the number of laboratories. Table 6 includes some t values based on the GUM: Table 6. Student s t statistics Confidence Intervals Number of observations 90 % 95 % 99 % /13

8 Table 7. Uncertainty components for CCRI(II)-S2 Radionuclide Relative uncertainty of output quantities % Standard deviation with the Students t- statistics Tracer Scientific judgment Relative expanded uncertainty (k = 2) % mbq 90 Sr Cs Pb Ra U U U Pu ,240 Pu Degrees of equivalence In general, the degree of equivalence of a given measurement is the degree to which it is consistent with the comparison reference value [4], and is indicated for each radionuclide and for each NMI in Appendix 2. The degree of equivalence of a particular NMI, i, with the reference value (RV) of a specific radionuclide in this matrix is expressed as the difference (D i ) between the NMI s result, A i, and the RV: D i = A i RV (4) together with the associated expanded uncertainty (k = 2) of D i, U i, given by the expression: U i = 2u Di (5) where u D is the square root of the quadratic sum of all measurement uncertainty components [5]. Similarly, the degree of equivalence between any two NMIs (i and j) can be expressed as the difference, D ij, in their results: with the expanded uncertainty of D ij given by U ij : D ij = D i D j = A i - A j (6) U ij 2 = u i 2 + u j 2 2u(A i,a j ) (7) where obvious correlations between the NMIs, such as the traceable calibrations of the NISTprovided tracer radionuclides, would be subtracted using the covariance, u(a i,a j ). However, as the uncertainty components in these measurements have already been taken into account, the fact that the tracers used were calibrated by NIST (a negligible contribution to the total uncertainty in any case) has not been considered again in the determination of the equivalence uncertainty. 8/13

9 5. Results of comparison Results for the radionuclides analyzed in this comparison are given in Table 5 and Appendix 1. For each radionuclide, the mean value is calculated for each laboratory together with expanded uncertainty (k =2). In Appendix 1, the solid line represents the mean (i.e., the reference value) for that radionuclide and dotted lines are the associated uncertainty for the mean (k = 2). All reported values were considered in the determination of the reference value (no results were considered as outliers). Despite the interest in the user community for this, as is exhibited by a high rate of participation by non-nmis in the comparison, only NIST (the pilot laboratory) and IAEA (Siebersdorf, which is responsible for providing measurement support for a variety of laboratories around the world) were CIPM MRA-signatory participants. Therefore, few degrees of equivalence between labs can be presented. 6. Conclusions A supplementary comparison, with comparison identifier CCRI(II)-S2, was undertaken by two NMIs which analyzed a suite of radionuclides in soil from Rocky Flats in the USA, NIST SRM-4353A. Radionuclides included in this comparison were 90 Sr, 137 Cs, 210 Pb, 228 Ra, 234 U, 235 U, 238 U, 238 Pu, and 239,240 Pu. Some laboratories reported results for additional radionuclides; however, since certified reference values currently exist for only the listed radionuclides, these others are not included in these comparison results. 7. References 1 R. Bock, A Handbook of Decomposition Methods in Analytical Chemistry, International Textbook Company, Limited. T. & A. Constable Ltd., Great Britain, International Organization for Standardization (ISO), Guide to the Expression of Uncertainty in Measurement, Available from the American National Standards Institute, 11 West 42 nd street, New York, NY 10036, USA (Listed under ISO miscellaneous publications as ISO Guide to the Expression 1993 ). 3 Taylor, B.N. and Kuyatt, C.E., Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results, NIST Technical Note 1297, Available from the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402, USA. 4 CIPM MRA: Mutual recognition of national measurement standards and of calibration and measurement certificates issued by national metrology institutes, International Committee for Weights and Measures, 1999, 45 pp. 5 Ratel, G., Evaluation of the uncertainty of the degree of equivalence, 2005, Metrologia 42, /13

10 Appendix 1. Comparison results of each nuclide Pu FSU GSF IAEA RESL LANL CERMC IAEA NIST SRNL EML RESL WHOI OSU BIL-GSL 239,240 Pu FSU GSF IAEA RESL LANL CERMC IAEA NIST SRNL EML RESL WHOI OSU BIL-GSL 137 Cs FSU LANL NIST SRNL EML RESL WHOI OSU BIL- GSL 10/13

11 238 U FSU RESL CERMC NIST SRNL EML RESL 234 U FSU RESL CERMC NIST SRNL EML RESL 235 U CERMC NIST SRNL RESL 11/13

12 90 Sr IAEA IAEA EML RESL WHOI 228 Ra 80 q/g mb FSU NIST SRNL RESL BIL-GSL 210 Pb FSU LANL NIST 12/13

13 Appendix 2. Comparison results for each NMI participant 90 Sr 210 Pb 137 Cs 228 Ra 234 U 235 U 238 U 238 Pu 239,240 Pu Lab, i D i U i D i U i D i U i D i U i D i U i D i U i D i U i D i U i D i U i IAEA NIST Pu Lab i Lab j IAEA D ij U ij NIST ,240 Pu Lab i Lab j IAEA D ij U ij NIST