Comparative U, Np and Pu M edge high energy resolution X-ray absorption spectroscopy (HR-XANES) investigations of model and genuine active waste glass

Understanding the long term behavior of vitrified nuclear waste requires a full and detailed characterization of the materials including their characteristics as synthesized and after exposure to groundwater. Genuine radioactive waste glass has a complex chemical composition. Therefore we take a simplified approach by investigating and comparing the oxidation states of U, Pu and Np in high level waste (HLW) glass sampled from the VEK vitrification process (VEK glass) and in model glasses. The model glasses doped with U and Pu have the same borosilicate glass frit composition as the VEK glass, whereas the model glass doped with Np has a base glass composition (R7T7) typically used for vitrification of HLW in France. U/Pu/Np M4/M5 edge high energy resolution X-ray absorption near edge structure (HR-XANES) spectroscopy technique [1] is applied to characterize the An oxidation states in model and genuine VEK HLW glass. The HR-XANES analyses suggest predominant existence of U(VI) and Pu(IV) in the HLW and the model glasses as expected from the oxidative vitrification conditions. Weak changes in U oxidation state as a function of the U loading (1.2 – 5 wt% UO2) are discussed on the basis of U M4 edge HR-XANES and X-ray photoelectron spectroscopy (XPS) results. ... mehrOne significant difference found between the model and the genuine HLW glasses is the strong radiation damage induced in the HLW glass by the soft X-ray beam (position of the U M4 edge: 3.73 keV) which was not observed for the U doped model glasses and the previous L3 edge investigations of the HLW glass sample. The dominant U(VI) oxidation state is reduced almost by 50% to U(IV) within 5 h of measurement. The complex chemical composition of the HLW glass leads to different local U atomic environments compared to the model glass as found by EXAFS investigations. EXAFS results confirm that U in the HLW glass is coordinated by Al/Si neighbors in the second coordination sphere, whereas no neighboring atoms are observed at this distance for the model glass. Differences in results obtained for the Np oxidation state for Np doped asprepared and leached R7T7 borosilicate model glasses and the HLW glass will be presented and discussed.