The Collaborative Materials Exercise (CMX) is organized by the Nuclear Forensics International Technical Working Group, with the aim of advancing the analytical capabilities of the participating organizations and providing feedback on the best approaches to a nuclear forensic investigation. Here, model nuclear fuel materials from the 5th CMX iteration were analyzed using a NanoSIMS 50L (CAMECA) in order to examine inhomogeneities in the U-235/U-238 ratio and trace element abundance within individual, micrometer scale particles. Two fuel pellets were manufactured for the exercise and labelled CMX-5A and CMX-5B. These pellets were created using different processing techniques, but both had a target enrichment value of U-235/U-238 = 0.01. Particles from these pellets were isolated for isotopic and trace element analysis. Fifteen CMX-5A particles and 20 CMX-5B particles were analyzed, with both sample types displaying inhomogeneities in the U isotopic composition at a sub-micrometer scale within individual particles. Typical particle diameters were similar to 1.5 to 41 mu m for CMX-5A and similar to 1 to 61 mu m for CMX-5B. The CMX-5A particles were shown to be more isotopically homogeneous, with a mean U-235/U-238 atom ratio of 0.0130 +/- 0.0066. The CMX-5B particles showed a predominantly depleted mean U-235/U-238 atom ratio of 0.0063 +/- 0.0094, which is significantly different to the target enrichment value of the pellet and highlights the potential variation of U-235/U-238 in U fuel pellets at the micrometer scale. This study details the successful application of the NanoSIMS 50L in a mock nuclear forensic investigation by optimizing high-resolution imaging for uranium isotopics.
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