Formation of uranium trioxide hydrates on UO2 fuel in air-steam mixtures near 200deg C

The oxidation of unused CANDUTM UO2 fuel in air-steam mixtures in a closed system at 200 and 225deg C is described. At moisture levels above about 50% of saturation, UO3 hydrates (mainly dehydrated schoepite, UO3.xH2O, x≅0.8) are formed within 20 days at 200deg C, and 5 days at 225deg C; U3O8 also occurred in some cases. Schoepite, UO3.2H2O, was detected in a number of experiments, but was probably formed as a product of secondary hydration during cooling or subsequent handling. The large volume change upon conversion of UO2, to UO3 hydrates creates potential for damage of used fuel with defected cladding, when stored in moist air. The actual extent of damage requires further investigation. In most experiments, reaction was limited to general surface oxidation. The mechanism of reaction appears to be dissolution and precipitation of uranium(VI) species within a liquid-like film of adsorbed moisture on the UO2 surface. This is analogous to some atmospheric corrosion processes on metals, such as rusting of iron. (orig.)