Spatially-resolved uranium isotopic analysis of contaminated scrap metal using laser ablation multi-collector ICP-MS.

Laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) was applied to the detailed investigation of the uranium (U) isotopic composition (234U, 235U, 236U, and 238U) of five contaminated scrap metal samples found within the European Union. Pressed pellets of the two certified U isotopic reference materials CRM U-020 and CRM U-030 were included in the measurement protocol for mass bias correction, calculation of the ion counter gains and for quality assurance. Since the investigated samples had low U content (0.15-14.3 wt%) compared to typically analysed pure U compounds (>60 wt%), the applied experimental parameters had to be adjusted. Spatially-resolved U isotopic information was obtained by line scan analysis of each sample. While other analytical techniques used typically in nuclear forensic investigations, such as γ-spectrometry and thermal ionisation mass spectrometry (TIMS) yielded average U isotopic compositions of the entire sample, LA-MC-ICP-MS provided substantial added value, highlighting the inhomogeneous distribution of U isotopes within various scrap metal samples. Analysis of individual particles via secondary ion mass spectrometry (SIMS) confirmed the large range of 235U enrichment levels in heterogeneous scrap metal samples. Four out of five scrap metal samples contained 236U (∼0.05-∼0.11 wt%), indicating the presence of reprocessed U. Taken together, LA-MC-ICP-MS analysis provided fast and accurate spatially-resolved U isotopic information without consuming or altering the scrap metal samples, a key feature for nuclear forensics investigations.

Intercomparison of the Radio-Chronometric Ages of Plutonium-Certified Reference Materials with Distinct Isotopic Compositions.

An intercomparison of the radio-chronometric ages of four distinct plutonium-certified reference materials varying in chemical form, isotopic composition, and period of production are presented. The cross-comparison of the different 234U/238Pu, 235U/239Pu, 236U/240Pu, and 241Am/241Pu model purification ages obtained at four independent analytical facilities covering a range of laboratory environments from bulk sample processing to clean facilities dedicated to nuclear forensic investigation of environmental samples enables a true assessment of the state-of-practice in "age dating capabilities" for nuclear materials. The analytical techniques evaluated used modern mass spectrometer instrumentation including thermal ionization mass spectrometers and inductively coupled plasma mass spectrometers for isotopic abundance measurements. Both multicollector and single collector instruments were utilized to generate the data presented here. Consensus values established in this study make it possible to use these isotopic standards as quality control standards for radio-chronometry applications. Results highlight the need for plutonium isotopic standards that are certified for 234U/238Pu, 235U/239Pu, 236U/240Pu, and 241Am/241Pu model purification ages as well as other multigenerational radio-chronometers such as 237Np/241Pu. Due to the capabilities of modern analytical instrumentation, analytical laboratories that focus on trace level analyses can obtain model ages with marginally larger uncertainties than laboratories that handle bulk samples. When isotope ratio measurement techniques like thermal ionization mass spectrometry and inductively coupled plasma mass spectrometry with comparable precision are utilized, model purification ages with similar uncertainties are obtained.

Analytical considerations in the determination of uranium isotope ratios in solid uranium materials using laser ablation multi-collector ICP-MS.

Validated analytical measurement protocols for the fast and accurate determination of the uranium (U) isotopic composition (234U, 235U, 236U, 238U) of solid nuclear materials were developed employing ns-laser ablation (LA) coupled to multi-collector ICP-MS. The accuracy of the analytical procedure was assured by frequent (n = 65) analysis of a pressed pellet of certified isotopic reference material CRM U-030 (∼3 wt% 235U). The expanded uncertainty (k = 2) for the n(235U)/n(238U) ratio was as low as 0.05%, rising to 0.62% and 1.09% for n(234U)/n(238U) and n(236U)/n(238U) ratios, respectively. LA-MC-ICP-MS measurements of a pressed pellet of certified isotopic reference material CRM U-020 (∼2 wt% 235U) before analysis of each sample allowed calculation of the ion counter gains and mass bias correction. Both individual spot analysis and line scan analysis were used to measure n(234U)/n(238U), n(235U)/n(238U), and n(236U)/n(238U) ratios in two low-enriched UO2 pellets from the fourth Collaborative Materials Exercise (CMX-4), four seized low-enriched UO2 pellets intercepted from illicit trafficking and one metal sample consisting of depleted U. LA-MC-ICP-MS results of all investigated samples matched well with U isotope ratios obtained by thermal ionisation mass spectrometry (TIMS). This independent confirmation of the LA-MC-ICP-MS results by TIMS underpinned the high quality of generated analytical data. Acquisition of several thousand data points within a couple of minutes during line scan analysis yielded detailed information on the spatial distribution of the U isotopic composition of selected UO2 pellets, revealing straightforwardly their (in-)homogeneity on the µm-scale. Calculating skewness and half width of the frequency distributions of the n(235U)/n(238U) amount ratio allowed the quantitative assessment of the (in-)homogeneity of the investigated samples. This information allows drawing conclusions on the starting materials used for the production of the pellets. From a nuclear forensics perspective, LA-MC-ICP-MS provides quick, accurate results on the spatial distribution of major and minor U isotopes while preserving the sample i.e. piece of evidence, essentially intact.

Measurement of the 231Pa/235U ratio for the age determination of uranium materials.

The paper describes the age (production date) determination of uranium reference materials using the 231Pa/235U ratio. Direct addition of 237Np in secular equilibrium with its 233Pa daughter was chosen instead of the regular milking of 237Np to avoid possible loss of Pa. Sample preparation consists of a fast, one-step procedure. The developed method using ICP-MS for the measurement of 231Pa is more precise than alpha spectrometry and is applicable for freshly produced low-enriched uranium materials. The measured ages are in good agreement with the reported production dates, thus the 231Pa/235U chronometer can be applied for validation of 230Th/234U in nuclear forensics and safeguards.

Methodology for the Preparation and Validation of Plutonium Age Dating Materials.

The present work describes a method for the preparation and validation of plutonium age dating reference materials. The test samples prepared in this context could be used to validate experimental protocols for determining the production date of plutonium via the 234U/238Pu, 235U/239Pu, 236U/240Pu, and 241Am/241Pu chronometers. The starting material was prepared using reactor-grade plutonium, which was purified using a dedicated method to guarantee high Pu recovery, while maximizing U and Am separation efficiencies. The U and Am separation factors were determined by the addition of high-amounts of 233U and 243Am spikes and their remeasurement in the final product. The prepared material is intended for quality control and assessment of method performance in nuclear forensics and safeguards.

Plutonium age dating (production date measurement) by inductively coupled plasma mass spectrometry.

This paper describes rapid methods for the determination of the production date (age dating) of plutonium (Pu) materials by inductively coupled plasma mass spectrometry (ICP-MS) for nuclear forensic and safeguards purposes. One of the presented methods is a rapid, direct measurement without chemical separation using 235U/239Pu and 236U/240Pu chronometers. The other method comprises a straightforward extraction chromatographic separation, followed by ICP-MS measurement for the 234U/238Pu, 235U/239Pu, 236U/240Pu and 238U/242Pu chronometers. Age dating results of two plutonium certified reference materials (SRM 946 and 947, currently distributed as NBL CRM 136 and 137) are in good agreement with the archive purification dates.

IRMM-1000a and IRMM-1000b uranium reference materials certified for the production date. Part I: methodology, preparation and target characteristics.

The paper describes the preparation and production of the reference materials, IRMM-1000a and IRMM-1000b, certified for the production date based on the 230Th/234U radiochronometer in compliance with ISO Guide 34:2009. The production date of the reference materials corresponds to the last separation of 230Th from 234U, i.e. when the initial daughter nuclide content in the material was finally removed. For the preparation low-enriched uranium was used, which was purified using a unique methodology to guarantee high U recovery and Th separation efficiency. The CRM is intended for calibration, quality control, and assessment of method performance in nuclear forensics and safeguards.

Remeasurement of (234)U Half-Life.

The half-life of (234)U has been measured using a novel approach. In this method, a uranium material was chemically purified from its thorium decay product at a well-known time. The ingrowth of the (230)Th daughter product in the material was followed by measuring the accumulated (230)Th daughter product relative to its parent (234)U nuclide using inductively coupled plasma mass spectrometry. Then, the (234)U decay constant and the respective half-life could be calculated using the radioactive decay equations based on the n((230)Th)/n((234)U) amount ratio. The obtained (234)U half-life is 244 900 ± 670 years (k = 1), which is in good agreement with the previously reported results in the literature with comparable uncertainty. The main advantages of the proposed method are that it does not require the assumption of secular equilibrium between (234)U and (238)U. Moreover, the calculation is independent from the (238)U half-life value and its uncertainty. The suggested methodology can also be applied for the remeasurement of the half-lives of several other long-lived radionuclides.

Uranium from German Nuclear Power Projects of the 1940s--A Nuclear Forensic Investigation.

Here we present a nuclear forensic study of uranium from German nuclear projects which used different geometries of metallic uranium fuel. Through measurement of the (230)Th/(234)U ratio, we could determine that the material had been produced in the period from 1940 to 1943. To determine the geographical origin of the uranium, the rare-earth-element content and the (87)Sr/(86)Sr ratio were measured. The results provide evidence that the uranium was mined in the Czech Republic. Trace amounts of (236)U and (239)Pu were detected at the level of their natural abundance, which indicates that the uranium fuel was not exposed to any major neutron fluence.

Development and validation of a methodology for uranium radiochronometry reference material preparation.

The paper describes a methodology for a reference material preparation to be used for the determination of the production date (i.e. the time elapsed since the last chemical processing) of uranium materials based on the (230)Th/(234)U radiochronometer. The reference material was prepared from highly enriched uranium by a complete separation of thorium decay products, thus zeroing the initial daughter nuclide concentration at known time. The complete elimination of thorium from the starting material was verified by gamma spectrometric measurements and by addition of a (232)Th tracer to the material and its re-measurement in the final product after the separation. The validation of the methodology was carried out subsequently by comparing the ingrown daughter nuclide (230)Th and the measured (230)Th/(234)U ratio after recorded times following the last chemical separation with the calculated values obtained on the basis of their respective half-lives. The prepared reference material can be used as a quality control material for age determination of uranium in nuclear forensics and safeguards as well as for method validation.

Determination of rare-earth elements in uranium-bearing materials by inductively coupled plasma mass spectrometry.

A novel and simple analytical procedure has been developed for the trace-level determination of lanthanides (rare-earth elements) in uranium-bearing materials by inductively coupled plasma sector-field mass spectrometry (ICP-SFMS). The method involves a selective extraction chromatographic separation of lanthanides using TRU resin followed by ICP-SFMS analysis. The limits of detection of the method proposed is in the low pg g(-1) range, which are approximately two orders of magnitude better than that of without chemical separation. The method was validated by the measurement of reference material and applied for the analysis of uranium ore concentrates (yellow cakes) for nuclear forensic purposes, as a potential application of the methodology.