U02 oxidation behaviour in air and steam with relevance to fission product releases

This study of U02 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 degrees Celsius and others in steam at 1000-1650 degrees Celsius. Oxidation of U02 in air at 500 degrees Celsius 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 degrees Celsius was controlled by solid state diffusion of oxygen until volume expansion produced cracking of the oxide and accelerated kinetics. Oxidation of U02 in air at 1650 degrees Celsius produced hyperstoichiometric U02. Significant mass losses due to U02 volatilization were observed in steam, and to a greater extent in high temperature air. Grain growth during steam exposure was very rapid at 1500 degrees Celsius and above. Implications of these effects are discussed with relevance to fission product release