Reactor fuel characterization: Evaluation of multiple techniques for expedited analysis of 235U/238U isotopic ratios in U-10Mo

Abstract

Uranium (U) enrichment measurements for the U.S. High Performance Research Reactor (USHPRR) conversion program are needed to certify the fuel is <20% 235U (tolerance range: 19.55%–19.95%). The main thrust of this work was to determine whether thermal ionization mass spectrometry (TIMS) and multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) U isotopic measurements can be made without chemically separating U from molybdenum (Mo) and other trace metal impurities. Although the data TIMS produces is considered the most accurate and precise, it is not typically used for isotopic analysis of U alloyed with 10 wt% Mo (U-10Mo) materials because of the complex sample preparation required prior to analysis. Results indicate that relative uncertainties on measurements made on chemically separated and unseparated samples was less than 0.03% (2 standard deviations) for this sample set with <0.06% trace metal impurities with no observable biases stemming from the presence of 10% Mo. This methodology provides relief from the bottleneck of separations chemistry, which discourages the use of TIMS and MC-ICP-MS in the case of 235U/238U ratios for U-10Mo, and allows for more rapid, highly accurate and precise determination of the 235U content in final fuel foils by TIMS. Additionally, samples were isotopically characterized for laser ablation multi-collector inductively coupled mass spectrometry (LA-MC-ICP-MS) by both TIMS and MC-ICP-MS in order to utilize them as in-house matrix matched reference materials in the absence of a standard.