Assessment of uranium inhomogeneity and isotope imaging for nuclear forensics

Laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) was used to study the heterogeneity of solid uranium materials. Using spatially resolved isotope ratio measurement, the degree of isotopic homogeneity at micron-level could be assessed in nuclear samples for nuclear safeguards and forensics purposes. LA-MC-ICP-MS was applied for two UO2 pellets which had been distributed in the 5th Collaborative Materials Exercise (CMX-5) organized by the Nuclear Forensics International Technical Working Group (ITWG). The two materials in question have very similar U-235 isotope abundance (around 1%), but very different U-235 isotopic heterogeneity. Although both UO2 pellets were prepared from identical source materials (mixture of depleted 0.3% and low-enriched 4.3% U-235), their different production routes resulted in observable differences at micron-scale. By the spatially resolved measurement the number of constituting components (feed materials) with different uranium enrichment and their n(U-234)/n(U-238) and n(U-235)/n(U-238) isotope ratios could be determined. Imaging can help assess the lateral dimensions of the individual components (grains) and may indicate a certain production route. This information on the inhomogeneity, the possible inference on the production process and isotopic composition of the constituents are highly valuable in nuclear forensics investigations for determining the origin of an unknown nuclear material.