A REVIEW OF THE OXIDATION OF URANIUM-DIOXIDE AT TEMPERATURES BELOW 400-DEGREES-C

A critical review of the extensive literature on the air oxidation of UO2 at temperatures below 400 degrees C is presented, The key paramete rs that affect the rate of UO2 oxidation are examined systematically a nd their importance to the reaction rate is evaluated. The formation o f U3O7/U4O9 on unirradiated UO2 powders follows the discrete-layer mec hanism and displays diffusion-controlled kinetics. In contrast, U3O8 f ormation 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 an 95.7 kJ mol(-1) for UO2 powd ers, 98.6 kJ mol(-1) for sintered pellets and similar to 106 kJ mol(-1 ) for used fuel. The activation energy for the formation of U3O8 is te mperature dependent. The best estimate of the activation energy below similar to 325 degrees C is 154 kJ mol(-1), but all the kinetic data i ncorporate substantial approximations so that further study is require d to properly predict the behaviour of used fuel under low-temperature (<150 degrees C) dry-air storage conditions, based on high-temperatur e (200 to 350 degrees C) laboratory data. (C) 1998 Elsevier Science B. V.