LAUSR

Until recently, the analysis and identification of nuclear fuel pellets in the context of a nuclear forensics investigation have been mainly focused on macroscopic characteristics such as fuel pellet dimensions, uranium enrichment, and other reactor-specific features. Here we report microscale isotopic heterogeneity observed in different fuel pellet fragments that were characterized in situ by NanoSIMS. The materials analyzed include fuel fragments obtained as part of the Collaborative Materials Exercise (CMX-4) organized by the Nuclear Forensics International Technical Working Group (ITWG), as well as a fuel pellet fragment from a commercial power reactor. While the commercial fuel pellet showed a homogeneous 235U/238U ratio across the sample (within analytical error), NanoSIMS imaging of the CMX-4 fuel pellet fragments showed distinct microscale variations in the uranium isotopic composition. The average 235U enrichment was 2.2 and 2.9 % for the two samples respectively, however, the measured 235U/238U ratios varied between 0.0081 - 0.035 (0.79 - 3.3 at. % 235U) and 0.0090 - 0.045 (0.89 - 4.3 at. % 235U). The measurement of the 236U in one of the CMX-4 samples suggested the presence of at least three uranium oxide powders of different isotopic composition ('source terms') used in the production of the pellets. These variations were not detected using the conventional bulk/macroscopic techniques applied to these materials. Our study highlights the importance of characterizing samples on the microscale for heterogeneities that would have otherwise been overlooked and demonstrates its potential use in guiding further nuclear forensic analysis.