Vapor pressures and vapor compositions in equilibrium with hypostoichiometric uranium dioxide at high temperatures

Abstract

Thermodynamic functions of the gaseous species, thermodynamic functions of the condensed phase, and an oxygen-potential model have been combined to calculate the vapor pressures and vapor compositions in equilibrium with condensed-phase UO 2-x for 1500⩽T⩽6000 K and 0⩽ x ⩽0.5. A method for extending the oxygen-potential model of Blackburn to the liquid region has been derived and evaluated. Results of these calculations show that the oxygen-to-uranium ratio of the vapor is larger than that of the condensed phase with which it is in equilibrium for most of the ranges of T and x of interest. Near 6000 K the vapor is very oxygen-rich so that the composition of the condensed phase would be changed considerably by even a few percent vaporization. In general, the vapor in equilibrium with UO 2-x is poorly approximated at UO 2(g); the species U, UO, UO 3, O 2, and O each have higher partial pressures than UO 2 for some conditions. The calculated total pressures at high temperatures are in good agreement with those recommended by the International Working Group on Fast Reactors (IWGFR).