Oxidation of spent fuel in air at 175 to 195°C

Abstract

Abstract Air oxidation tests were conducted on four LWR spent fuels at 175 and 195°C to determine the effects of variations in burnup, gas release, and grain size on oxidation of fuel in a potential repository. Weight gains were measured and samples were examined by XRD, ceramography, and TEM. The fuel characteristics were found to affect only the initial oxidation behavior. Following transient behavior related to differences in grain-boundary susceptibility to oxidation, all four fuels progressed toward a metastable state at an oxygen-to-metal (O/M) ratio near 2.4. Unlike nonirradiated UO 2 under similar conditions, the fuels oxidized preferentially along grain boundaries and formed U 4O 9 rather than U 3O 7. Cubic U 4O 9, the sole oxidation product observed at times to ∼38000 h, persisted to oxygen supersaturations as high as UO 2.4 with no detectable U 3O 8 formation. Growth of the U 4O 9 into the UO 2 grains followed parabolic kinetics with an activation energy of 26.6 kcal/mol. Based on reaction kinetics, the time to completely oxidize LWR spent fuel to U 4O 9 at 95°C would be greater than 2000 years.