A direct-AMS multi-isotope survey of uranium ore concentrates

Abstract

Uranium ore concentrate (UOC) is an important nuclear material of interest for Canada. A large scale analytical program is being led by the Directorate of Security and Safeguards (DSS) of the Canadian Nuclear Safety Commission (CNSC) to establish a reference dataset of UOCs that have passed through and/or are currently under Canadian regulatory control. Isotopic ratios are among the signatures being captured under the reference dataset. Accelerator Mass Spectrometry (AMS) has been used for the measurement and assessment of 236U/238U and 187Os/188Os, respectively. Furthermore, since UOCs have uranium typically concentrated to ≥70% by weight, a direct-AMS assay method is possible wherein the samples can be measured without time consuming chemical digestion and processing. Using this direct-AMS approach, several related ratios (231Pa/238U, 230Th/238U, 226Ra/238U) were also assessed within the data acquisition sequence used for measuring the 236U/238U ratios, and 185Re, 187Re and 187Os, 188Os, 191Ir and 193Ir in the sequence for the 187Os/188Os ratios. The sum of these results can be shown together in a “bar-code” pattern to strengthen the capability for UOC source identification. Unexpectedly large 236U/238U ratios (up to ≥1 × 10−7) have been found in several UOC samples. The 187Os/188Os ratio has also been shown for the first time to be a viable supplementary signature of UOCs. This work shows that the direct-AMS method has the potential to become an effective tool for nuclear forensics provenance assessment applications with UOCs. The implications of the results, and the need for further refinement of the sputter target preparation, as well as the Cs+ sputter ion source itself, are also discussed.