Thermodynamic Assessment of UO₂ Pellet Oxidation in Mixture Atmospheres under Spent Fuel Pool Accident

Abstract

For an analysis of the oxidation behavior of UO 2 nuclear fuel pellet under a loss of water coolant accident in a spent nuclear fuel pool of an LWR, thermodynamic assessments of UO 2 oxidation were carried out under various atmospheric conditions. In a steam atmosphere, it was assessed that UO 2 would not be fully oxidized into U 3 O 8 due to the relatively lower oxygen partial pressure, while UO 2 will be fully oxidized into U 3 O 8 in an air atmosphere. In an air and steam mixture atmosphere, the UO 2 oxidation was dominantly affected by the air volumetric fraction, because of the relatively higher oxygen partial pressure of air. In addition, the effect of H 2 volumetric fraction on the oxygen partial pressure under a mixture atmosphere was calculated, and it was revealed that UO 2 pellet oxidation could be reduced above the critical value of H 2 volumetric fraction.