Part I: Thermodynamics of Uranium Oxidation in Support of Kinetics Model for Operating Defective Fuel Elements

Abstract

The accidental exposure of UO₂ to heavy water has chemical consequence on the O/U ratio leading to the potential formation of other oxide phases. In order to provide boundary conditions for kinetical models for defective fuel element oxidation, it is essential to have a self-consistent set of thermodynamic properties for the U-O system. This must include all phases and in particular treat nonstoichiometry in UO_2+x. The treatment presented brings together all data in a way, which emphasizes replication of solubilities and 3 phase invariant conditions displayed in the U-O binary phase diagram.

The current work treats the centrally important solid and liquid UO_2+x phases as a homogeneous equilibrium among the formal components UO₂, UO and UO₃. For the hyper-stoichiometric oxide (x>0), a simple non ideal mixing expression for the mixing of UO₂ and UO₃ brings the oxygen partial pressure into close agreement with recent reviews on the relationship between O/U ratio, temperature and oxygen partial pressure. For hypo-stoichiometric UO_2+x (x<0), no departure terms from ideal mixing are necessary. As in two recent comprehensive reviews, all other oxide phases are treated as stoichiometric; minor adjustments to the enthalpies of formation (within experimental uncertainty) have been sufficient to ensure self-consistency among potentially co-existing phases. The metallic uranium liquid phase makes allowance for dissolved atomic oxygen. The treatment is considered valid from 25°C to 3000°C and provides explicit expressions for partial oxygen pressure for all phase combinations in this range. A key feature in the modeling approach for the U-O system has been the provision to connect it with thermodynamic treatments (existing or under development) for other compounds and phases.