NONUNIFORM OXIDATION OF LWR SPENT FUEL IN AIR

Oxidation of light-water-reactor (LWR) spent fuel fragments in air at 175-degrees-C was investigated by ceramography, X-ray diffractometry, and transmission electron microscopy. Enhanced oxidation of the original fuel pellet edge was observed in a typical oxidized fragment and the local structures were analyzed in micro-core samples. The oxidation proceeded by advance U4O9 formation along grain boundaries, grain boundary microcracking, and subsequent U4O9 growth into enveloped UO2 grains. No U3O7 or higher oxidation products were detected even near fragment edges where UO2 conversion was complete. The tendency of LWR spent fuel to oxidize along grain boundaries has been attributed to short circuit oxygen penetration through fission gas microbubbles concentrated on the boundaries. Enhanced grain boundary oxidation of the pellet edges in low-gas-release LWR fuel is further attributed to temperature-related variations in bubble spacing, and is seen as a complicating factor in attempts to model postirradiation oxidation behavior.