Re-certification of the CRM 125-A UO₂ fuel pellet standard for uranium isotopic composition

The U.S. Department of Energy New Brunswick Laboratory (NBL) has been tasked to develop certified reference materials (CRMs) tailored for the demands of modern analytical methods in the field of nuclear forensics. As part of this effort, several existing uranium CRMs are being characterized for additional attributes. One of these materials is the CRM 125-A uranium oxide (UO ₂) pellet assay and isotopic standard (4% enriched in ²³⁵U), which is being developed as a CRM for U-Th age determinations. Reliable U isotopic composition data with relatively small uncertainties are essential for use as a uranium age standard. Therefore, re-characterization of the uranium isotopic composition of CRM 125-A was deemed necessary due to poorly constrained minor U abundances (i.e., ²³⁴U and ²³⁶U) resulting from instrumental limitations of the mass spectrometer used at the time of original certification (1996-1997). The analytical work presented in this study was undertaken with the purpose of reducing the uncertainties of certificate values for uranium isotope-amount ratios. Six randomly selected CRM 125-A pellets were characterized for the uranium isotopic abundances by thermal ionization mass spectrometry (TIMS). The re-certified major ratio n( ²³⁵U)/n(²³⁸U) of 0.042301(25) was determined using the total evaporation (TE) and modified total evaporation (MTE) methods. The re-certified minor isotope ratios n(²³⁴U)/n(²³⁸U) and n(²³⁶U)/n(²³⁸U) of 0.00039130(38) and 0.0000040754(47), respectively, were measured by MTE and a conventional Faraday cup analysis method using internal normalization. Additionally, the material was checked for the presence of ²³³U using a secondary electron multiplier (SEM) equipped with a retarding potential quadrupole (RPQ). No ²³³U was observed within the instrumental detection limit. The homogeneity of CRM 125-A was confirmed by the absence of any statistically significant unit-to-unit variation in the uranium isotope amount ratios. The new values measured in this study represent a considerable refinement of the older data, in particular for the minor ratios, with uncertainties that are significantly smaller than those cited in the original certificate. The results of the characterization analyses are presented along with an explanation of the uncertainty estimates, which are compliant with the Guide to the Expression of Uncertainty in Measurement (GUM).