Grain Boundary Corrosion and Alteration Phase Formation During the Oxidative Dissolution of UO2 Pellets

Abstract

ABSTRACTThe alteration behavior of UO2 pellets following their reaction under unsaturated drip-test conditions, at 90°C, for time periods of up to 10 years has been examined by solid phase and leachate analyses. Sample reactions were characterized by preferential dissolution of grain boundaries between the original press-sintered UO2 granules comprising the samples, development of a polygonal network of open channels along the intergrain boundaries, and spallation of surface granules that had undergone severe grain boundary corrosion. The development of a dense mat of alteration phases after two years of reaction trapped loose granules, resulting in reduced rates of particulate uranium release. The paragenetic sequence of alteration phases that formed on the present samples was similar to that observed in surficial weathering zones of natural uraninite (UO2) deposits, with alkali and alkaline earth uranyl silicates representing the long-term solubility-limiting phases for uranium in both systems.