Very high burnup spent fuel corrosion & leaching under hydrogen conditions

This paper concerns a very high burnup UO2 spent nuclear fuel leaching experiment performed under high hydrogen pressure conditions (5 MPa H2 initially) in simplified granitic groundwater. Following an initial release after start-up the results demonstrate that the concentrations of redox sensitive elements such as Tc, U, Np and Pu stabilize at the solubility limits of their reduced oxide forms. The release of non-redox sensitive elements such as Cs also ceased within a year. Together these observations indicate that the hydrogen overpressure resulted in reducing conditions that suppresses fuel matrix corrosion. Following an initially rather rapid release of fission gases Kr and Xe, the release also ceases towards the end of the experiment, although at a surprisingly high released fraction of the inventory. In this experiment the fuel was cut from the rod, crushed in a vice and pre-leached under semi aerated conditions for 9 days before transitioning to conditions of high hydrogen overpressure and the resulting apparent inhibition of further corrosion or dissolution of the fuel matrix. These results are in contrast to our previous experiments with other portions of fuel from the same rod under similar hydrogen conditions, but with extensive dry milling and fuel corrosion in aerated conditions, prior to the transitions to hydrogen conditions. The results thus tentatively support the conclusion that preceding oxidation of the fuel surfaces may limit the potential for hydrogen to suppress fuel corrosion and dissolution.