K. G. W. Inn

Evaluation of a sequential extraction method for determining actinide fractionation in soils and sediments

Abstract A sequential chemical extraction method for the determination of the geochemical fractionation of Am, Pu, and U was evaluated rigorously on a single marine sediment standard ( IAEA -135). Partitioning of actinides was operationally defined for five reagent fractions and stable element analyses (Al, Ca, Fe, Mn, Ti, etc.) were performed to assist in establishing the phase specificity of the extractions. The method produces, in general, results that agree to within a one standard deviation interval for replicate measurements for each fraction. Actinide readsorption, examined by a double-spiking technique, was found to be significant in some cases. Ethylenediamine-tetraacetic acid (EDTA) was examined as a hold-back reagent to lessen the effect of readsorption but seriously compromised the phase selectivity of the extractants. In addition, the use of NaOCl for the destruction of organic matter was found to dissolve carbonates out of sequence and is not recommended for carbonate-bearing materials.

The urgent requirement for new radioanalytical certified reference materials for nuclear safeguards, forensics, and consequence management

A multi-agency workshop was held from 25 to 27 August 2009, at the National Institute of Standards and Technology (NIST), to identify and prioritize the development of radioanalytical Certified Reference Materials (CRMs, generally provided by National Metrology Institutes; Standard Reference Materials, a CRM issued by NIST) for field and laboratory nuclear measurement methods to be used to assess the consequences of a domestic or international nuclear event. Without these CRMs, policy makers concerned with detecting proliferation and trafficking of nuclear materials, attribution and retribution following a nuclear event, and public health consequences of a nuclear event would have difficulty making decisions based on analytical data that would stand up to scientific, public, and judicial scrutiny. The workshop concentrated on three areas: post-incident Improvised Nuclear Device (IND) nuclear forensics, safeguard materials characterization, and consequence management for an IND or a Radiological Dispersion Device detonation scenario. The workshop identified specific CRM requirements to fulfill the needs for these three measurement communities. Of highest priority are: (1) isotope dilution mass spectrometry standards, specifically 233U, 236gNp, 244Pu, and 243Am, used for quantitative analysis of the respective elements that are in critically short supply and in urgent need of replenishment and certification; (2) CRMs that are urgently needed for post-detonation debris analysis of actinides and fission fragments, and (3) CRMs used for destructive and nondestructive analyses for safeguards measurements, and radioisotopes of interest in environmental matrices.

GEOCHEMICAL PARTITIONING OF ACTINIDES USING SEQUENTIAL CHEMICAL EXTRACTIONS : COMPARISON TO STABLE ELEMENTS

We have developed a sequential extraction technique for determining the geochemical partitioning of Am, Pu, and U in soils and sediments. Stable element analyses were combined with radiometric measurements to determine the most probable geochemical host phases of these actinides in reference sediment IAEA-135.241 Am results indicate an association with carbonate minerals and organic matter. The extraction profile of238U was similar to that of refractory elements Al, Ti, and K.239/240Pu data suggest a fractionation of Pu into Fe-bearing phases of varying solubility. The reproducibility of the method was quite good (replicates agreed to within 10% at a 95% confidence level).

Determination of 239Pu/240Pu isotopic ratio by high-resolution alpha-particle spectrometry using the ADAM program

A novel analysis program to unfold alpha-particle energy spectra was introduced, demonstrated and validated using radiochemically processed test sources, which contained different amounts of (239)Pu and (240)Pu. A high-resolution alpha spectrometer was used for data acquisition. The software known as ADAM unfolds the spectra using nuclide-specific decay data as a constraint. The peaks can have different shapes and the software can also cope with the coincidences between alpha particles and electrons/photons. In the present paper, the (239)Pu/(240)Pu activity ratios from alpha spectrometry agreed, within the stated uncertainties, with the reference values. Number of counts in the (239,240)Pu peak group must be larger than 100 to obtain reliable values when using semiconductor detector of energy resolution FWHM=10.6 keV.

A blueprint for radioanalytical metrology CRMs, intercomparisons, and PE

A workshop was held from 28 February to 2 March 2006 at the National Institute of Standards and Technology (NIST) to evaluate the needs for new directions for complex matrix reference materials certified for radionuclide content, interlaboratory comparisons and performance evaluation (PE) programs. The workshop identified new radioanalytical metrology thrust areas needed for environmental, radiobioassay, emergency consequence management, and nuclear forensics, attribution, nonproliferation, and safeguards.

NIST Radiochemistry Intercomparison Program: a Summary of Four-year Performance Evaluation Study

This paper summarizes the methods currently applied by the National Institute of Standards and Technology (NIST) for the preparation and verification of environmental and radiobioaasay performance evaluation materials for the traceability testing of low-level radioactivity measurements. The evaluation of measurement quality of the all participants indicated that over 90% of the intercomparison results complied with ANSI N42.22 and/or HPS N13.30 standards. Overall, the differences from the NIST values observed for all test nuclides and matrices are less than -7%, with a typical measurement precision better than 10% (1 s). The dependency of measurement quality on the test radionuclides, sample matrices, and radiochemical methods is identified by statistical analyses.

Optimizing the removal of carbon phases in soils and sediments for sequential chemical extractions by coulometry

We have developed a coulometric technique to optimize the removal of the carbonate and organic fractions for sequential chemical extractions of soils and sediments. The coulometric system facilitates optimizing these two fractions by direct real-time measurement of carbon dioxide (CO2) evolved during the removal of these two fractions. Further analyses by ICP-MS and alpha-spectrometry aided in interpreting the results of coulometry experiments. The effects of time, temperature, ionic strength and pH were investigated. The sensitivity of the coulometric reaction vessel/detection system was sufficient even at very low total carbon content (< 0.1 mol kg-1). The efficiency of the system is estimated to be 96% with a standard deviation of 8%. Experiments were carried out using NIST Standard Reference Materials 4357 Ocean Sediment (OS), 2704 Buffalo River Sediment (BRS), and pure calcium carbonate. Carbonate minerals were dissolved selectively using an ammonium acetate-acetic acid buffer. Organic matter was then oxidized to CO2 using hydrogen peroxide (H2O2) in nitric acid. The carbonate fraction was completely dissolved within 120 min under all conditions examined (literature suggests up to 8 h). For the OS standard, the oxidation of organic matter self-perpetuates between 45 and 50 degrees C, a factor of two less than commonly suggested, while organic carbon in the BRS standard required 80 degrees C for the reaction to proceed to completion. For complete oxidation of organic matter, we find that at least three additions of H2O2 are required (popular methods suggest one or two).

The NIST Natural Matrix Radionuclide Standard Reference Material Program for Ocean Studies

In 1977, the Low-level Working Group of the International Committee on Radionuclide Metrology met in Boston, MA (USA) to define the characteristics of a new set of environmental radioactivity reference materials. These reference materials were to provide the radiochemist with the same analytical challenges faced when assaying environmental samples. It was decided that radionuclide bearing natural materials should be collected from sites where there had been sufficient time for natural processes to redistribute the various chemically different species of the radionuclides. Over the succeeding years, the National Institute of Standards and Technology (NIST), in cooperation with other highly experienced laboratories, certified and issued a number of these as low-level radioactivity Standard Reference Materials (SRMs) for fission and activation product and actinide concentrations. The experience of certifying these SRMs has given NIST the opportunity to compare radioanalytical methods and learn of their limitations. NIST convened an international workshop in 1994 to define the natural-matrix radionuclide SRM needs for ocean studies. The highest priorities proposed at the workshop were for sediment, shellfish, seaweed, fish flesh and water matrix SRMs certified for mBq per sample concentrations of 90 Sr, 137 Cs and 239 Pu + 240 Pu. The most recent low-level environmental radionuclide SRM issued by NIST, Ocean Sediment (SRM 4357) has certified and uncertified values for the following 22 radionuclides: 40 K, 90 Sr, 129 I, 137 Cs, 155 Eu, 210 Pb, 210 Po, 212 Pb, 214 Bi, 226 Ra, 228 Ra, 228 Th, 230 Th, 232 Th, 234 U, 235 U, 237 Np, 238 U, 238 Pu, 239 Pu + 240 Pu, and 241 Am. The uncertainties for a number of the certified radionuclides are non-symmetrical and relatively large because of the non-normal distribution of reported values. NIST is continuing its efforts to provide the ocean studies community with additional natural matrix radionuclide SRMs. The freeze-dried shellfish flesh matrix has been prepared and recently sent to participating laboratories for analysis and we anticipate receiving radioanalytical results in 2000. The research and development work at NIST produce well characterized SRMs that provide the worlds environment-studies community with an important foundation component for radionuclide metrology.

The National Bureau of Standards fresh water lake sediment environmental-level radioactivity standard reference material

The National Bureau of Standards has, for the past five years, been developing natural-matrix, environmental-level radioactivity Standard Reference Materials in large quantities to be available to users over a ten year time period. These materials have been found to be useful for the evaluation of radiochemical methods and analysis, as interlaboratory comparison materials, and as quality assurance materials. To date, six Standard Reference Materials have been issued: River Sediment, Human Lung, Human Liver, Rocky Flats Soil-1, Freshwater Lake Sediment, and Peruvian Soil. The concentrations of twenty radionuclides have been certified in these materials.

A bone ash standard for90Sr,210Pb,210Po, uranium and the actinides

A bone ash standard for a number of radionuclides is required as a quality control sample in dosimetry studies with bone as the critical organ. The procedures that were used to prepare a candidate bone ash standard are given with some initial encouraging measurements that the candidate sample will meet the requirements of NIST standard reference manual.