Water corrosion of spent nuclear fuel: radiolysis driven dissolution at the UO₂/water interface

X-ray diffraction has been used to probe the radiolytic corrosion of uranium dioxide. Single crystal thin films of UO₂ were exposed to an intense X-ray beam at a synchrotron source in the presence of water, in order to simultaneously provide radiation fields required to split the water into highly oxidising radiolytic products, and to probe the crystal structure and composition of the UO₂ layer, and the morphology of the UO₂/water interface. By modeling the electron density, surface roughness and layer thickness, we have been able to reproduce the observed reflectivity and diffraction profiles and detect changes in oxide composition and rate of dissolution at the Ångström level, over a timescale of several minutes. A finite element calculation of the highly oxidising hydrogen peroxide product suggests that a more complex surface interaction than simple reaction with H₂O₂ is responsible for an enhancement in the corrosion rate directly at the interface of water and UO₂, and this may impact on models of long-term storage of spent nuclear fuel.