MICROSTRUCTURES OF CORROSION FILMS ON UO2 FUEL OXIDIZED IN AIR-STEAM MIXTURES AT 225-DEGREES-C

Oxidation of unirradiated UO2 fuel specimens in aerated steam above ab out 70% saturation (relative humidity) at 225 degrees C yields uniform ly textured or granular corrosion-product layers consisting of U3O8 an d dehydrated schoepite (DS; UO3 . xH(2)O, x similar or equal to 0.8). These fine-grained products (0.5 to 2.0 mu m) contrast with the much l arger DS crystals that are formed in aerated liquid water around 200 d egrees C. At lower steam partial pressures (30 to 70% saturation), onl y scattered crystals of these oxidation products are formed, also with dimensions < 2 mu m. Both the uniform films and the sparser microcrys talline products are thought to form by dissolution and precipitation of uranium within a thin film (a few molecules thick) of adsorbed mois ture.