Spatial distribution of uranium isotopes in solid nuclear materials using laser ablation multi-collector ICP-MS

摘要

Employing laser ablation multi-collector ICP-MS (IA-MC-ICP-MS), its potential for evaluating the homogeneity of solid uranium (U) bearing materials was investigated. To this end, the n(U-235)/n(U-238) ratio of two low-enriched U certified reference materials (CRMs), i.e. powdered standard reference material U-010 (similar to 1 wt% (235) U) and a UO2 pellet of CRM 125-A (similar to 4 wt% U-235) was determined using line scan analysis. Four spots of 5 mu m diameter each were ablated per second with the LA system moved at a speed of 20 mu m s(-1) for several minutes. Experimental and certified U isotope ratios matched perfectly, unequivocally validating the accuracy of the applied analytical methodology. In addition, the narrow and symmetric frequency distribution of the n(U-235)/n(U-238) ratio of both CRMs confirmed their homogeneity with respect to the U isotopic composition. Utilizing LA-MC-ICP-MS line scan analysis of two similar UO2 pellets (similar to 1 wt% U-235) from the 5th Collaborative Materials Exercise (CMX-5) revealed diverse inhomogeneity regarding their n(U-235)/n(U-238) ratio. Although both UO2 pellets used for the CMX-5 exercise were prepared from identical source materials, their different production routes yielded largely contrasting frequency distributions of the n(U-235)/n(U-238) ratio. While U-235 isotope abundance ranged from 0.75%-1.6% for the first pellet, this value fluctuated between 0.45% and 3.0% for the second pellet. This specific information sheds additional light on the production process of these materials and is helpful in nuclear forensics investigations when determining the origin of unknown nuclear material. The depleted nature (similar to 0.4 wt% U-235) of two seized U metal samples was established through LA-MC-ICP-MS analysis of the n(U-235)/n(U-238) ratio. As black and yellow regions within both U bearing samples had identical U isotopic composition, the observed colour differences might arise from different degrees of U oxida