Oxidative corrosion of spent UO2 fuel in vapor and dripping groundwater at 90°C

Oxidative dissolution of spent UO2 fuel in vapor and dripping groundwater at 90 degrees C occurs via general corrosion at fragment surfaces. Dissolution along fuel-grain boundaries is also evident in samples contacted by the largest volumes of groundwater, and corroded grain boundaries extend at least 20 or 30 grains deep (> 200 mu m), possibly throughout mm-sized fragments. Apparent dissolution of fuel along defects that intersect grain boundaries has produced 50 to 200 nm-diameter dissolution pits that penetrate 1-2 mu m into each grain, giving rise to a "worm-like" texture along fuel-grain boundaries. Sub-micrometer-sized fuel shards are common between fuel grains and may contribute to the reactive surface area of fuel exposed to groundwater. Outer surfaces of reacted fuel fragments develop a fine-grained layer of corrosion products adjacent to the fuel (5-15 mu m thick). A more coarsely crystalline layer of corrosion products commonly covers the fine-grained layer, the thickness of which varies considerably among samples (from less than 5 mu m to greater than 40 mu m) The thickest and most porous corrosion layers develop on fuel fragments exposed to the largest volumes of groundwater. Corrosion-layer compositions depend strongly on water flux, with uranyl oxy-hydroxides predominating in vapor experiments, and alkali and alkaline earth uranyl silicates predominating in high drip-rate experiments. Low drip-rate experiments exhibit a complex assemblage of corrosion products, including phases identified in vapor and high drip-rate experiments.