Synthesis and Characterisation of Mixed Actinide Oxide Materials for Radioactive Waste Disposition

In this study, we directly evidence the defect chemistry of mixed actinide oxide materials containing Gd³⁺, within the context of understanding the basic fundamental chemistry of actinide waste immobilisation matrices. Materials were prepared by two different routes (dry mixing and oxalic wet precipitation) and characterised to quantify the incorporation rate and distribution of Gd within the actinide oxide matrix. The thermal treatment of the oxalic powders was refined to improve the uptake of Gd by the final material. Besides measuring the general characteristics of the final sintered materials, such as grain size, density and crystallographic parameters, the defect chemistry was determined using Raman spectroscopy and through analysis of U M₄-edge X-ray absorption near edge spectra, collected in high energy resolved fluorescence detection mode (HERFD-XANES). These data revealed that, despite containing the same amount of Gd, the incorporation method between the two synthesis routes differed. Materials prepared <i>via</i> the wet co-precipitation route had defect bands typically associated with the formation of oxygen vacancies, and the HERFD-XANES data evidenced the presence of U⁵⁺. Both observations are consistent with the charge-balance mechanism required when Gd³⁺ is incorporated on the actinide oxide lattice sites. Conversely, the materials prepared by the dry mixing route did not evidence the presence of U⁵⁺, and the defect bands attributed to oxygen vacancies were significantly reduced.