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h i g h l i g h t s CEF model of the Gibbs energy for fluorite U 1Ày Ce y O 2±x and (U,Ce) 3 O 8 phases. Two sublattice ionic liquid model of the UeCeeO phase. Comparison of computed phase diagrams and calculated thermochemistry for UeCeeO and UePueO. Uranium Cerium Oxygen Compound energy formalism Calphad Oxygen potential Phase equilibria a b s t r a c tA thermodynamic assessment of the UeCeeO system was performed by combining the existing Calphad assessments of the UeO and CeeO binaries. A compound energy formalism representation for the fluorite U 1Ày Ce y O 2±x and a partially ionic two-sublattice liquid model for UeCeeO melt were developed to describe the ternary solutions. Oxygen potentials for U 1Ày Ce y O 2±x for 0.05, 0.20, 0.30, and 0.50 Ce metal fractions were measured from thermogravimetric analysis and used, along with other thermodynamic data reported in the literature, to optimize the adjustable parameters of the models for U 1Ày Ce y O 2±x and the UeCeeO ternary liquid. In addition, the thermochemical behavior of U 1Ày Ce y O 2±x and U 1Ày Pu y O 2±x was compared in order to assess the suitability of using Ce as a surrogate for Pu in MOX fuel.

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