Insight into the Cationic Charge Distribution in U_1-PuAmO_2± Mixed Oxides

Uranium-plutonium mixed oxides, containing few mol % of Am, are currently studied as fuel for Sodium Fast Reactors. The study of the O/M ratio of these fuels is of main interest, as its variation can induce issues for reactor safety. For this purpose, four U_1-PuAmO_2± samples (0.235 ≤ y ≤ 0.39 and 0.005 ≤ z ≤ 0.02) were studied using electron probe microanalysis (EPMA), X-ray powder diffraction (XRD), and X-ray absorption spectroscopy (XAS) experiments. EPMA analyses revealed a homogeneous matrix phase with few U- and Pu-rich agglomerates. A matrix phase of fluorite structure (face-centered cubic), as well as a small contribution of a UO₂ phase of the same structure, was evidenced for all of the samples by XRD. The O/M ratios of the (U,Pu)O_2± matrix were calculated based on the obtained lattice parameters. XANES experiments highlighted the simultaneous presence of U⁵⁺ and Pu³⁺/Am³⁺ for all of the samples, indicating a charge compensation mechanism even for stoichiometric samples. Based on the EPMA and XRD results, it was evidenced that this coexistence of U⁴⁺, U⁵⁺, Pu³⁺, Pu⁴⁺, and Am³⁺ was not originating from the U- and Pu-rich agglomerates. It was found to be rather a peculiarity of the U_1-PuAmO_2± matrix phase, discussed with the help of thermodynamic calculations performed on the U-Pu-Am-O system.