Thermodynamic modeling of the U3O8-x solid solution phase

Abstract

It is important to accurately characterize the equilibrium behavior of U3O8 during UO2 nuclear fuel fabrication as the determination of the oxygen to uranium ratio in UO2 powders and pellets is dependent on U3O8 stoichiometry. Prior thermodynamic modeling efforts have treated U3O8 as stoichiometric; however, it is experimentally known that the phase exhibits hypostoichiometric behavior at temperatures >850 K. The goal of this work was to model U3O8 as a solid solution using the compound energy formalism and optimize the model to experimental data according to the CALPHAD methodology. Results of the U3O8-x model assessment were visualized by recalculating the previously assessed U–O binary phase diagram replacing a stoichiometric representation of U3O8 with a U3O8-x solid solution model. Moreover, a recommendation is made to revise the U3O8 to U conversion factor used in the ASTM C 1453 standard based on the literature and updated thermodynamic representation for U3O8-x presented in this work.