Thermodynamic modeling of the U 3 O 8-x solid solution phase

Abstract

It is important to accurately characterize the equilibrium behavior of U 3 O 8 during UO 2 nuclear fuel fabrication as the determination of the oxygen to uranium ratio in UO 2 powders and pellets is dependent on U 3 O 8 stoichiometry. Prior thermodynamic modeling efforts have treated U 3 O 8 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 U 3 O 8 as a solid solution using the compound energy formalism and optimize the model to experimental data according to the CALPHAD methodology. Results of the U 3 O 8-x model assessment were visualized by recalculating the previously assessed U–O binary phase diagram replacing a stoichiometric representation of U 3 O 8 with a U 3 O 8-x solid solution model. Moreover, a recommendation is made to revise the U 3 O 8 to U conversion factor used in the ASTM C 1453 standard based on the literature and updated thermodynamic representation for U 3 O 8-x presented in this work.