Thermodynamic assessment of the uranium–oxygen system

A thermodynamic assessment of the uranium–oxygen system is presented. A consistent set of experimental data is selected among the numerous data in the literature on the phase diagram and oxygen chemical potential. The thermodynamic properties of the phases are described using the compound energy model with ionic constituents for the solids and an ionic two-sublattice model for the liquid. For the uranium dioxide, the structure is described using three sublattices, one for the cations U 3+, U 4+ and U 6+, one for the normal site of oxygen ions, and one for the interstitial oxygen ions. Vacancies are included in both oxygen sublattices. In this first approach, the homogeneity ranges of the U 4O 9− y and U 3O 8− y compounds are not represented. A set of consistent model parameters that describes both the phase diagram and the oxygen chemical potential data in the whole composition range is thus obtained. The description of this basic binary system will be used to calculate higher order systems such as O–U–Zr and Fe–O–U which are important for simulating severe nuclear accidents.