UO2 Oxidation Behaviour in Air and Steam With Relevance to Fission Product Release

Abstract

This study of UO₂ behaviour is part of a program in support of the modelling of fission product release during postulated accidents in CANDU reactors. Some experiments were performed in air at 500-1650°C and others in steam at 1000-1650°C. Oxidation of UO₂ in air at 500°C proceeded by grain boundary attack, producing sub-grain sized fragments of U₃O₈ in a process which was controlled by oxygen depletion in the gas phase. Air oxidation at 900-1200°C was controlled by solid state diffusion of oxygen until volume expansion produced cracking of the oxide and accelerated kinetics. Oxidation of UO₂ in air at 1650°C produced hyperstoichiometric UO₂. Significant mass losses due to UO₂ volatilization were observed in steam, and to a greater extent in high temperature air. Grain growth during steam exposure was very rapid at 1500°C and above. Implications of these effects are discussed with relevance to fission product releases.