On the effects of fission product noble metal inclusions on the kinetics of radiation induced dissolution of spent nuclear fuel

Radiation induced oxidative dissolution of UO ₂ is a key process for the safety assessment of future geological repositories for spent nuclear fuel. This process is expected to govern the rate of radionuclide release to the biosphere. In this work, we have studied the catalytic effects of fission product noble metal inclusions on the kinetics of radiation induced dissolution of spent nuclear fuel. The experimental studies were performed using UO ₂ pellets containing 0%, 0.1%, 1% and 3% Pd as a model for spent nuclear fuel. H ₂O ₂ was used as a model for radiolytical oxidants (previous studies have shown that H ₂O ₂ is the most important oxidant in such systems). The pellets were immersed in aqueous solution containing H ₂O ₂ and HCO 3 − and the consumption of H ₂O ₂ and the dissolution of uranium were analyzed as a function of H ₂ pressure (0–40 bar). The noble metal inclusions were found to catalyze oxidation of UO ₂ as well as reduction of surface bound oxidized UO ₂ by H ₂. In both cases the rate of the process increases with increasing Pd content. The reduction process was found to be close to diffusion controlled. This process can fully account for the inhibiting effect of H ₂ observed in several studies on spent nuclear fuel dissolution.