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In 2017, the Nuclear Forensics International Technical Working Group organized their fifth Collaborative Materials Exercise (CMX-5). The exercise samples were two uranium dioxide fuel pellets manufactured from the same starting materials by different processes to have similar bulk isotopic composition, but different spatial uranium isotopic distributions. Sets of identical materials were sent to all participating laboratories, who then utilized their existing nuclear forensic capabilities to independently analyse fuel pellets and identify similarities and differences of the materials’ characteristics. Here we present and compare the ability of different analytical techniques to spatially resolve uranium isotopic heterogeneity in the uranium oxide fuel pellets.

The Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident in Japan resulted in a major release of radionuclides into the environment. Compared to other radionuclides, few studies have investigated the fate of actinides in the environment. Accordingly, this research investigates the Pu composition in soil samples collected in paddy fields before and after the accident. Furthermore, the vertical distributions of Pu and U isotopic signatures, along with 137Cs activities, were measured in a sediment core collected in the Mano Dam reservoir, in the Fukushima Prefecture. Changes in the relative contributions of the major actinide sources (global fallout or FDNPP derived fallout) were investigated in sediment deposited in the reservoir. The distinct peak observed for all Pu isotope ratios (240Pu/239Pu, 241Pu/239Pu and 242Pu/239Pu) and for 137Cs concentrations in the sediment core was attributed to the Fukushima fallout, and coincided with the maximum atomic contribution of only 4.8 ± 1.0% of Pu from the FDNPP. Furthermore, 236U/238U ratios measured in the sediment core remained close to the global fallout signature indicating there was likely no U from the FDNPP accident detected in the sediment core. More research is required on the environmental dynamics of trace actinides in landscapes closer to the FDNPP where there are likely to be greater abundances of FDNPP-derived Pu and U.

Numerous applications require the precise analysis of U isotope relative enrichment in sample amounts in the sub-nanogram to picogram range, among those are nuclear forensics, nuclear safeguards, environmental survey and geosciences. However, conven-tional thermal ionization mass spectrometry (TIMS) yields U combined ionization and transmission efficiencies (i.e ratio of ions detected to sample atoms loaded) less than 0.1 or 2% depending on the loading protocol, motivating the development of sources capable of enhancing ionization. The new prototype cavity source TIMS at ETH offers improvements from 4 to 15 times in com-bined ionization and transmission efficiency compared to conventional TIMS, yielding up to 5.6 % combined efficiency. Uranium isotope ratios have been determined on reference standards in the 100 pg range bound to ion-exchange or extraction resin beads. For natural U standards, n(235U)/n(238U) ratios are measured to relative external precisions of 0.5 to 1.0 % (2RSD, 2

Capabilities of laser ablation coupled to a time of flight ICP-MS (‘icpTOF’, Tofwerk) for direct isotopic analysis of U micrometric reference particles deposited on carbon disks were demonstrated. Thanks to the very high temporal resolution which allows quasi simultaneous measurement of all nuclides, the icpTOF is well-suited for the acquisition of the very short transient signals produced by laser ablation of μm-sized particles. With appropriate corrections of the background due to high peak tailing of the 238U isotope and selection of signals within the detector's linear range, excellent internal and external precisions (0.2–0.4%) for measurement of the 235U/238U ratios were obtained in the most favorable cases (same abundances for the two isotopes). These precisions are better than the values reported until now in the literature, even with laser ablation coupled with multi-collector ICP-MS (MC-ICP-MS). For particles from other reference materials with low abundances of 235U, equal or close to the one of natural U, precisions for 235U/238U ratio measurements (4–5%) were not as good as the ones reported for MC-ICP-MS, but better than the ones obtained with single collector ICP-MS. Minor isotope ratios (234U/238U and 236U/238U) were also successfully measured, although with limited precision and accuracy when the abundances of the minor isotopes are very low, in the 10−5 range. This work shows that laser ablation coupled to ICP-TOF-MS has a great potential for ultra-precise isotope ratio measurements provided that the ablated area contains enough analyte and that abundances between isotopes are of the same order of magnitude.

Rationale A multi-collector inductively coupled plasma (MC-ICP) mass spectrometer coupled to a UV ns-laser ablation (LA) system was used to measure uranium isotopic ratios (234 U/238 U, 235 U/238 U and 236 U/238 U) in single uranium particles of various sizes and isotopic compositions, including home-made sub-micrometric natural uranium particles of narrow size distribution (415 ± 60 nm). Methods The LA-ICP mass spectrometer was operated in wet plasma conditions thanks to simultaneous injection of the laser aerosol and water vapor through a desolvating nebulizer. The isotopic ratios were corrected for mass bias and gain factors between detectors. The 236 U/238 U ratios were also corrected for the presence of 235 U hydrides and tailing of the 238 U+ peak. Results 236 U/238 U ratios were successfully measured in micrometer-sized particles from the NBS U050 certified standard material with a 236 U/238 U ratio of ~5 × 10-4 . The analysis of 77 natural uranium sub-μm-sized particles yielded a very good trueness with respect to the expected 234 U/238 U and 235 U/238 U ratios, while the measurement errors for single particles ranged from -2.7% to +2.1% for 235 U/238 U and from -17% to +33% for the 234 U/238 U ratios. Their relative combined standard uncertainties ranged from 3.3% to 32.8% and from 0.4% to 4.0% for 234 U/238 U and 235 U/238 U ratios, respectively. In addition, extremely low detection limits, in the attogram range, were achieved. Conclusions This study demonstrates that coupling of a ns-laser ablation system with a MC-ICP mass spectrometer allows measurements of the isotopic composition in natural uranium particles of a few hundreds of nm with very good trueness, average combined standard uncertainties of ~1% for 235 U/238 U ratios and 12% for 234 U/238 U ratios, and detections limits of a few ag for minor isotopes.

The results of a joint effort by expert nuclear forensic laboratories in the area of age dating of uranium, i.e. the elapsed time since the last chemical purification of the material are presented and discussed. Completely separated uranium materials of known production date were distributed among the laboratories, and the samples were dated according to routine laboratory procedures by the measurement of the 230Th/234U ratio. The measurement results were in good agreement with the known production date showing that the concept for preparing uranium age dating reference material based on complete separation is valid. Detailed knowledge of the laboratory procedures used for uranium age dating allows the identification of possible improvements in the current protocols and the development of improved practice in the future. The availability of age dating reference materials as well as the evolvement of the age dating best-practice protocol will increase the relevance and applicability of age dating as part of the tool-kit available for nuclear forensic investigations.

In an inter-laboratory measurement comparison study, four laboratories determined 230Th–234U model ages of uranium certified reference material NBL U050 using isotope dilution mass spectrometry. The model dates determined by the participating laboratories range from 9 March 1956 to 19 October 1957, and are indistinguishable given the associated measurement uncertainties. These model ages are concordant with to slightly older than the known production age of NBL U050.

The design of a prototype thermal ionization cavity (TIC) source attached to the mass analyser of a MAT262 thermal ionization mass spectrometer is presented. In addition to a detailed calculation and experimental verification of the ion optics, the new design includes several innovative features that set it apart from previously reported TIC sources. The goal is to employ this new ion source for high-efficiency isotope ratio analysis. For uranium adsorbed to a single resin bead, an overall efficiency gain of about a factor of 10 compared to the same sample type analysed by state-of-the-art conventional thermal ionization mass spectrometry (TIMS) on a Triton instrument is reported (ions detected per atom loaded). First attempts at isotope ratio analysis of micrometre sized uranium oxide particles, as is commonly done for nuclear safeguards, seem to confirm the enhancement in overall efficiency and could help with the analysis of the minor uranium isotopes on such samples. (C) 2018 Elsevier B.V. All rights reserved.

International audience; The $^{236}$U/$^{238}$U isotope ratio is a widely used tracer, which provides information on source identification forsafeguard purposes, nuclear forensic studies and environmental monitoring. This paper describes an originalapproach to determine $^{236}$U/$^{238}$U ratios, below 10$^{−8}$, in environmental samples by combination of ICP-MS/MS for $^{236}$U/$^{238}$U ratio and multiple collector ICPMS measurements for $^{235}$U/$^{238}$U and $^{234}$U/$^{235}$U isotope ratios.Since the hydride form of U$^+$ (UOH$^+$) is less prone to occur than UH$^+$, we were focused on the oxidisedforms of uranium in order to reduce hydride based-interferences in ICP-MS/MS. Then, in-cell ion-moleculereactions with O$_2$ and CO$_2$ were assessed to detect the uranium isotopes in mass-shift mode (Q1: U$^+$ → Q2:UO$^+$). The performances in terms of UO$^+$ sensitivity and minimisation of hydride form of UO$^+$ were evaluatedusing five different desolvating systems. The best conditions, using an Apex $\Omega$ or an Aridus system, produceduranium oxide hydride rate ($^{235}$U$^{16}$O1H$^+$/$^{235}$U$^{16}$O+) of about 10$^{-7}$ with O$^2$ in the collision cell.The method was validated through measurements of two certified IRMM standards with $^{236}$U/$^{238}$U isotoperatio of 1.245×10$^{17}$ and 1.052×10$^{-8}$, giving results in agreement with certified reference values. The relativestandard deviations on seven independent measurements for each standard were respectively of 1.5% and6.2%. Finally, environmental samples corresponding to sediments from the radioactive contamination plumeemitted by the Fukushima Daiichi Nuclear Power Plant accident were analysed after a well-established uraniumchemical separation procedure. $^{236}$U/$^{238}$U atomic ratios between 1.5×10$^{-8}$ and 7×10$^{-9}$ were obtained with a level accuracy lower than 20%.

In this paper, we will describe and indicate the performance of a new method based on the use of femtosecond laser ablation (fs-LA) coupled to a quadrupole-based inductively coupled plasma mass spectrometer (ICP-QMS) for analyzing the isotopic composition of micrometer-size uranium particles. The fs-LA device was equipped with a high frequency source (till 10 kHz). We applied this method to 1–2 μm diameter-uranium particles of known isotopic composition and we compared this technique with the two techniques currently used for uranium particle analysis: Secondary Ionization Mass Spectrometry (SIMS) and Fission Track Thermal Ionization Mass Spectrometry (FT-TIMS). By optimizing the experimental conditions, we achieved typical accuracy and reproducibility below 4% on 235 U/ 238 U for short transient signals of only 15 s related to 10 to 200 pg of uranium. The detection limit (at the 3 sigma level) was ~ 350 ag for the 235 U isotope, meaning that 235 U/ 238 U isotope ratios in natural uranium particles of ~ 220 nm diameter can be measured. We also showed that the local contamination resulting from the side deposition of ablation debris at ~ 100 μm from the ablation crater represented only a small percentage of the initial uranium signal of the ablated particle. Despite the use of single collector ICP-MS, we were able to demonstrate that fs-LA-ICP-MS is a promising alternative technique for determining uranium isotopic composition in particle analysis.

In this article, the coupling of a UV-fs-LA system with a sector-field ICP-MS for isotopic analysis of individual sub micrometric and micrometric uranium particles, especially for the measurement of minor isotopes (234U and 236U), is discussed. Ablation of uranium particles results in short (a few seconds) and highly noisy signals with sudden and extremely brief (

International audience; The isotope drift encountered on short transient signals measured by multicollector inductively coupled plasma mass spectrometry (MC-ICPMS) is related to differences in detector time responses. Faraday to Faraday and Faraday to ion counter time lags were determined and corrected using VBA data processing based on the synchronization of the isotope signals. The coefficient of determination of the linear fit between the two isotopes was selected as the best criterion to obtain accurate detector time lag. The procedure was applied to the analysis by laser ablation-MC-ICPMS of micrometer sized uranium particles (1–3.5 μm). Linear regression slope (LRS) (one isotope plotted over the other), point-by-point, and integration methods were tested to calculate the 235U/238U and 234U/238U ratios. Relative internal precisions of 0.86 to 1.7% and 1.2 to 2.4% were obtained for 235U/238U and 234U/238U, respectively, using LRS calculation, time lag, and mass bias corrections. A relative external precision of 2.1% was obtained for 235U/238U ratios with good accuracy (relative difference with respect to the reference value below 1%).

In this study we describe a method for uranium dating (i.e. determination of the date of the last chemical purification undergone by the material) by measurement of the 230Th/234U ratio, applicable to sub-microgram quantities. The chosen protocol (AG1x8 resin in hydrochloric acid medium) has been tested on separation microcolumns (100 μl). This ‘microchemistry’ technique considerably limits the risks of contamination by reagents or the environment. Thorium extraction efficiencies were greater than 90 % and reproducible. The quantities of 230Th introduced by the chemical purification procedure were negligible. Using an ultra-sensitive inductively coupled plasma mass spectrometry measurement technique, detection limits of the order of femtograms (10−15 g) of 230Th were obtained. The complete procedure, chemical separation and isotope measurement, was successfully tested and validated on a few micrograms of uranium.

In this paper we describe the application of laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) coupling to particle analysis, i.e., the determination of the isotopic composition of micrometric uranium particles. The performances of this analysis technique are compared with those of the two reference particle analysis techniques: secondary ion mass spectrometry (SIMS) and fission track-thermo-ionization mass spectrometry (FT-TIMS), based on the measurement of the isotopic ratios of 235U/238U in particles present in an inter-comparison particulate sample. The agreement of the results obtained using LA-ICP-MS with target values and with the results obtained using FT-TIMS and SIMS was good. Accuracy was equivalent to that of the other two techniques (±3 % deviation). However, relative experimental uncertainties present with LA-ICP-MS (7 %) were higher than those present with FT-TIMS (4.5 %) and SIMS (3 %). Furthermore, measurement yield of LA-ICP-MS coupling was close to that obtained with the same quadrupole ICP-MS for the measurement of a liquid sample (~10−4), but lower than that obtained with FT-TIMS and SIMS, respectively, by a factor of 10 and 20, although the particles analyzed using LA-ICP-MS were most likely smaller (diameter ~0.6 μm, containing 4–7 fg of 235U). Nevertheless, thanks to the brevity of the signals obtained, the detection capacity for low isotopic concentrations by LA-ICP-MS coupling is equivalent to that of FT-TIMS, although it remains well below that of SIMS (×15). However, with more sensitive double focusing ICP-MS, performances equivalent to those achieved using SIMS could be obtained.

We compare the analytical performance of a modern quadrupole-based ICP-MS (“X-Series”, Thermo-Electron, Winsford, UK) with a single-collector double-focusing sector-field ICP-MS (“Axiom”, VG Elemental, Winsford, UK) for uranium isotopic measurements in environmental samples. We focus on the precision and accuracy obtained with both instruments for the 235U/238U isotopic ratios and on the abundance sensitivity that is a key parameter for low 236U/238U isotopic ratios measurements. We observe that isotopic measurements are more precise accurate with the “X-Series” than with the “Axiom”. Besides, we demonstrate that the “Axiom’s” higher abundance sensitivity limits its capability to measure 236U/238U ratio below a few ppm.