Reassessing the solid/liquid phase transitions of U1--Pu Am O2± mixed oxides

• First determination of O/M ratio variation during melting for U 1-y-z Pu y Am z O 2±x • No influence of up to 2 mol.% Am content on the Tsolidus of U 1-y-z Pu y Am z O 2±x • Melting behavior of U 1-y-z Pu y Am z O 2±x depends on the initial stoichiometry For safety reasons, it is crucial to understand the melting behavior of U 1-y-z Pu y Am z O 2±x (0.2 ≤ y ≤ 0.4 and 0 ≤ z ≤ 0.06) mixed oxide fuels for Sodium Fast Reactors. In this study, melting temperature measurements were conducted on seven U 1-y-z Pu y Am z O 2±x samples (with 0.235 ≤ y ≤ 0.44 and 0.005 ≤ z ≤ 0.02) using a laser heating technique. Thanks to the addition of oxygen gauges to the setup, the variation of the oxygen partial pressure inside the chamber during the experiment, and thus the O/M ratio (M = U+Pu+Am), was recorded for U 1-y-z Pu y Am z O 2±x for the first time. This study discusses the influence of the Pu and Am content, as well as the O/M ratio on the melting behavior of U 1-y-z Pu y Am z O 2±x . A decrease in the solidus temperature was observed with increasing Pu content, whereas no significant influence of the Am content on the solidus temperature was noted. Regarding the influence of the O/M ratio, the hypotheses used to calculate its variation are discussed. It was demonstrated that only the molten pool reacted with the surrounding atmosphere. Two distinct melting behaviors were observed depending on the initial O/M ratio of the fuels. For initially hypo-stoichiometric fuels, oxidation of the samples was observed during melting, as demonstrated by thermodynamic calculations. On the contrary, for initially stoichiometric or slightly hyper-stoichiometric fuels, reduction was occurred during the melting.