UO₂ Fuel Oxidation

Abstract

A thermodynamic analysis of the uranium-oxygen system is used to determine the equilibrium state of oxidized fuel as a function of the atmospheric oxygen potential (i.e., hydrogen-to-steam partial pressure ratio) as required for the development of fuel-oxidation kinetics models. Annealing experiments have been performed to investigate the oxidation kinetics of fuel in a steam atmosphere at high temperature as well as the fuel-reduction kinetics. Mathematical treatments of these phenomena, coupled with heat and mass transport theory, can be used to describe the oxidation behaviour of uranium dioxide fuel in operating defective rods for water-cooled nuclear reactors. Model predictions are consistent with the observed oxygen-to-metal ratios seen in commercial defective rods as measured post-test using a coulometric titration technique. Such models can be further extended to higher temperature using a phase-field approach to simulate centreline melting in defective fuel. Fuel oxidation effects in air and water at low temperature are also discussed.