A review of the oxidation of uranium dioxide at temperatures below 400 C

A critical review of the extensive literature on the air oxidation of UO2 at temperatures below 400 C is presented. The key parameters that affect the rate of UO2 oxidation are examined systematically and their importance to the reaction rate is evaluated. The formation of U3O7/U4O9 on unirradiated UO2 powders follows the discrete-layer mechanism and displays diffusion-controlled kinetics. In contrast, U3O8 formation on unirradiated UO2 displays sigmoidal 'nucleation-and-growth' kinetics. Low-temperature oxidation of used fuel tends to proceed by rapid grain-boundary oxidation followed by simultaneous intragranular oxidation throughout the sample. The best estimates of the activation energy for the formation of U3O7/U4O9 are 95.7 kJ mol-1 for UO2 powders, 98.6 kJ mol-1 for sintered pellets and ∝106 kJ mol-1 for used fuel. The activation energy for the formation of U3O8 is temperature dependent. The best estimate of the activation energy below ∝325 C is 154 kJ mol-1, but all the kinetic data incorporate substantial approximations so that further study is required to properly predict the behaviour of used fuel under low-temperature (<150 C) dry-air storage conditions, based on high-temperature (200 to 350 C) laboratory data. (orig.)