High temperature oxidation of UO2 in steam-hydrogen mixtures

An experimental study of UO2 oxidation in pure steam and in H2O/Ar/H2 mixtures was conducted in a continuously recording thermogravimetric apparatus in the temperature range 1273-1623 K at one atmosphere system pressure. Two surface reaction models were utilized in the analysis of the data: the phenomenological model assumes the reaction rate to be proportional to the deviation of the oxygen concentration in the solid from the equilibrium value established by the ambient gas. Analysis of the results show that the phenomenological model does not fit the experimental data for pure steam, but the fit is good for H2O/Ar/H2 mixtures. The mechanistic model is based on the rate-controlling step of water dissociation at the solid surface. It provides a very good fit to all of the data. As long as the pressure is 1 atm, either of the models can be used to predict fuel oxidation in severe accidents because the ambient gas invariably contains hydrogen produced by Zircaloy corrosion. However, the kinetics of oxidation in high-pressure steam-hydrogen mixtures is still uncertain. Evidence of uranium volatilization and preferential etching at the grain boundaries at high temperatures were observed. (orig.)