Assessing the concept of direct disposal of used nuclear fuel in a geological vault requires a model to predict the dissolution rate of UO2 in groundwater. A solubility-limited model can be used to calculate the dissolution rate of UO2 fuel under non-oxidizing conditions. When the oxidative dissolution of UO2 is an irreversible process, a kinetic model is more suitable to describe the dissolution of UO2 under oxidizing conditions. Experimental studies were carried out using electrochemical techniques and X-ray photoelectron spectroscopy, XPS, to determine criteria for selecting the appropriate model for estimating used-fuel dissolution rates as a function of the redox conditions in the vault at the time of container failure. UO2 electrodes were subjected to prolonged (>1000 min) potentiostatic oxidation, and the rate of oxidation and dissolution of UO2 fuel was investigated as a function of the applied potential. UO2 oxidation was also carried out by the products of water radiolysis and studied as a function of dose rate, total dose and solution chemistry.
These studies show that significant oxidative dissolution of UO2 appears possible for potentials more positive than -100 mV vs SCE in solutions with a pH close to that of the deep groundwaters, i.e., from 6 to 10. A kinetic model, which takes into account the mechanism of UO2 oxidation, is more appropriate to estimate dissolution rates of UO2 fuel for redox conditions more oxidizing than -100 mV vs SCE.