Determining precisely the oxidation states and local structures of aged PuO2 and Pu-doped UO2 samples

In the present proposal, a combination of High Energy-Resolution Fluorescence Detected X-ray absorption near edge structure (HERFD-XANES) and Extended X-ray Absorption Fine Structure (EXAFS) measurements are sought on the U and Pu M4 and L3 edges at BM20 to determine precisely the oxidation states and local structures of aged PuO2 samples and Pu-doped UO2 samples. Such structures form after high burn-up in Mixed Oxide (MOX) fuels and are referred to as high burnup structures (HBS) in spent nuclear fuel (SNF). We have previously shown using HERFD-XANES at BM20 that UO2 NPs undergo relatively rapid oxidation under ambient conditions, which was not anticipated. This proposal builds on those results by examining similar phenomena in PuO2 and Pu-doped UO2, both relevant to MOX spent fuel. In particular, we hypothesize the occurrence of higher valence states of Pu and U corresponding to hydroxide-like phases, which HERFD-XANES and EXAFS measurements will be used to confirm.