The Collaborative Materials Exercise (CMX) is organized by the Nuclear Forensics
International Technical Working Group (ITWG), 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 235U/238U ratio and trace element abundance within individual, micron-scale particles. Two fuel pellets were manufactured for the exercise and labelled as CMX-5A and CMX-5B. These pellets were created using different processing techniques, but both had a target enrichment value of 235U/238U = 0.01. Particles from these pellets were
isolated for isotopic and trace element analysis. Fifteen CMX-5A particles and twenty CMX 5B particles were analyzed, with both sample types displaying inhomogeneities in the U isotopic composition at a sub-micron scale within individual particles. Typical particle diameters were ~ 1.5 ‒ 41 μm for CMX-5A, and ~ 1 – 61 μm for CMX-5B. The CMX-5A particles were shown to be more isotopically homogeneous, with a mean 235U/238U atom ratio of 0.0130 ± 0.0066. The CMX-5B particles showed a predominately depleted mean 35U/238U 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 235U/238U in U fuel pellets at the micron 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.