Microscale Isotopic Variation in Uranium Fuel Pellets with Implications for Nuclear Forensics

• Published in: Analytical chemistry (Washington) Vol. 91; no. 18; pp. 11598 - 11605
• Main Authors: Kips, Ruth, Weber, Peter K, Kristo, Michael J, Jacobsen, Benjamin, Ramon, Erick C
• Format: Journal Article
• Language: English
• Published: Washington American Chemical Society 17-09-2019; American Chemical Society (ACS)
• Subjects: Analytical chemistry; Chemistry; Composition; Enriched fuel reactors; Error analysis; Forensic science; Forensic sciences; Fragments; Heterogeneity; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Isotopes; Mass spectrometry; Mass spectroscopy; Nuclear fuels; Nuclear reactors; Pellets; RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEAR CHEMISTRY; Reactors; Secondary ion mass spectrometry; Uranium; Uranium oxide; Uranium oxides; Variation
• ISSN: 0003-2700; 1520-6882
• DOI: 10.1021/acs.analchem.9b01737

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 nanoscale secondary ion mass spectrometry (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. Although 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 enrichments were 2.2 and 2.9% for the two samples; however, the measured 235U/238U ratios varied between 0.0081 and 0.035 (0.79–3.3 atom % 235U) and between 0.0090 and 0.045 (0.89–4.3 atom % 235U). The measurement of 236U in one of the CMX-4 samples suggested the use of at least three uranium oxide powders of different isotopic compositions (“source terms”) 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 otherwise be overlooked and demonstrates the potential use of NanoSIMS in guiding further nuclear forensic analysis.