Long-term performance of spent fuel waste forms

This report investigates the leaching/dissolution characteristics of spent fuel in simulated groundwaters. Experiments evaluated the leaching of unirradiated UO2 in simulated groundwaters both in the presence and absence of an external radiation field. Other experiments characterized the dissolution behavior of spent fuel under the same conditions as for UO2. Leach data were interpreted in terms of rates and mechanisms of radionuclide release, thus providing insight on leach behavior under realistic repository conditions. Uranium release from both unirradiated and irradiated UO2 appeared to be solubility controlled, with only minor leachate compositional effects on the uranium concentrations. When leached in the presence of a radiation field, unirradiated UO2 produced elevated uranium concentrations only in the brine leachates. The leaching of both cesium and iodine from spent fuel was rapid and linear with the surface-area-to-volume (S/V) ratio. Similar to cesium and iodine, strontium release was nearly proportional to S/V. However, unlike cesium and iodine, strontium release was strongly affected by leachate composition, with greater release associated with lower pH. A model describing the kinetics of spent fuel/UO2 dissolution was developed, complementing the experimental studies. For fixed values of the time constant and kinetic exponent, the model predicts that the effect of water chemistry on spent fuel dissolution is largely limited to affecting the saturation concentration of uranium in the aqueous solution. This result contrasts sharply with results obtained previously for glass dissolution. 59 refs., 34 figs., 13 tabs