In the present proposal, HERFD-XANES and EXAFS measurements are sort at BM20 to determine the chemical state and mechanism of incorporation of caesium in UO2 nanoparticles. Such nanoparticles are found after a high burn-up time of UO2 and referred to as high burnup structures (HBS) which is a part of spent nucleal fuel (SNF). Scanning electron microscopy (SEM) and Powder X-ray diffraction (PXRD) measurements suggests incorporation of caesium into the crystal lattice. However, the mechanism for incorporation, particularly the local chemical structure is unclear and will affect their stability in SNF disposal scenario, hence HERFD-XANES and EXAFS measurement are sought on the U M4-edge and Cs L3-edge to determine changes. This work has implications for the storage and safety of SNF particularly in the context of the “instance release fraction” in which caesium is a part of and to draw appropriate conclusions for the storage of SNF.

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.