The 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.