The corrosion of spent UO2 fuel in synthetic groundwater

Leaching of high burnup BWR fuel for up to 3 years showed that both U and Pu attain saturation rapidly at pH 8.1, giving values of 1-2 mg/l and 1 μg/l respectively. The leaching rate for Sr-90 decreased from about 10-5/d to 10-7/d but was always higher than the rates for U, Pu, Cm, Ce, Eu and Ru. Congruent dissolution was only attained at pH values of about 4. When reducing conditions were inposed on the pH 8.1 groundwater by means of H2/Ar in the presence of a Pd catalyst, significantly lower leach rates were attained. The hypothesis that alpha radiolytic decomposition of water is a driving force for UO2 corrosion even under reducing conditions has been examined in leaching tests on low burn-up (low alpha dose-rate) fuel. No significant effect of alpha radiolysis under the experimental conditions was detected. Thermodynamically the calculated uranium solubilities in the pH range 4-8.2 generally agreed well with the measured ones, although assumptions made for certain parameters in the calculations limit the validity of the results. (orig.)