Progress in the thermodynamic modelling of the O–U binary system

Abstract The fuel of civil nuclear plants, UO 2 , melts at 3120 K. During an hypothetical severe accident, urania, submitted to high temperatures and various oxygen potentials, presents a wide non-stoichiometry range: the melting temperature of UO 2± x , related to oxygen potential, decreases in all cases. In this scenario, urania could react with other materials, firstly zircaloy, and the melting temperature of (U, Zr)O 2± x still decreases. That is why the critical assessment of the O–U binary system including the non-stoichiometry range of urania, is a major step to a correct thermodynamic modelling of multicomponent systems for nuclear safety. The very numerous experimental information has been compiled and analysed. The associate model was used for the liquid phase, and a sublattice model for UO 2± x ; U 4 O 9− y , U 3 O 8 and UO 3 were treated as stoichiometric. Phase diagram and thermodynamic properties have been calculated from the optimised Gibbs energy parameters. The calculated consistency with the experimental ones is quite satisfactory.