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

Oxidation tests in dry air were conducted on four LWR spent fuels at 175 degrees and 195 degrees C to determine the effect of the fuel characteristics on the oxidation state likely to exist at the time leaching occurs in a potential repository. Weight changes were measured and samples were examined by XRD, ceramography, TEM, and TGA. Despite local variations in the grain boundary susceptibility to oxidation, all four fuels progressed toward an apparent endpoint at an oxygen-to-metal (O/M) ratio of 2.4. The sole oxidation product was U4O9+x, a cubic phase structurally related to UO2 but with a slightly smaller lattice constant. The growth of the U4O9+x from the grain boundaries into the UO2 grains followed parabolic kinetics and had an activation energy of 26.6 kcal/mol. Based on the kinetics, the time required at 95 degrees C to completely oxidize LWR spent fuel to U4O9+x would be at least 2000 yr. The next oxidation product to form after the U4O9+x phase may be U3O8, but no U3O8 or other dilatational oxidation product has been detected in these accelerated tests conducted up to 25,000 h