Actinide oxidation state and O/M ratio in hypostoichiometric uranium-plutonium-americium U0.750Pu0.246Am0.004O2-x mixed oxides

Innovative americium-bearing uranium-plutonium mixed oxides U1-yPuyO2-x are envisioned as nuclear fuel for next generation Sodium-cooled Fast neutron Reactors (SFRs). The Oxygen-to-Metal (O/M) ratio, directly related to the oxidation state of cations, affects many of the fuel properties. Thus, a thorough knowledge of its variation with the sintering conditions is essential. The aim of this work is to follow the oxidation state of uranium, plutonium and americium, and so the O/M ratio, in U0.750Pu0.246Am0.004O2-x samples sintered for 4 h at 2023 K in various Ar + 5% H2 + z vpm H2O (z = ~15, ~90 and ~200) gas mixtures. The O/M ratios were determined by gravimetry, XAS and XRD and evidenced a partial oxidation of the samples at room temperature. Finally, by comparing XANES and EXAFS results to that of a previous study, we demonstrate that the presence of uranium does not influence the interactions between americium and plutonium and that the differences in the O/M ratio between the investigated conditions is controlled by the reduction of plutonium. We also discuss the role of the homogeneity of cation distribution on the mechanisms involved into the reduction process.