Thermodynamic assessment of the U–La–O system

Highlights: • CEF model of the Gibbs energy for fluorite U_1−yLa_yO_2±x and rhombohedral ULa₆O_12−x and (U,La)₈O₁₆ phases. • Two sublattice ionic liquid model of the U–La–O phase. • Tentative Gibbs energy function for the rhombohedral U₂La₅O₁₅ phase. • Computed phase diagrams for U–La–O, UO₂–LaO_1.5, U–La, and La–O. - Abstract: The CALPHAD methodology was used to develop a thermodynamic assessment of the U–La–O system. The solid solution and liquid phases are described with the compound energy formalism (CEF) and the partially ionic two-sublattice liquid model, respectively. Experimental thermodynamic and phase equilibria data published in the open literature were then used in optimizations to develop representations of the phases in the system that can be extended to include other actinide and fission products for multi-component models. The models that comprise this assessment very well reproduce experimentally determined oxygen potentials and the observed phase relations for the U–La–O system