The influence of deposited films on the anodic dissolution of uranium dioxide

Corrosion product deposits formed over long periods of time could exert a considerable influence on the corrosion rate of used nuclear fuel under permanent disposal conditions. To simulate the build up of such deposits, the oxidative dissolution of UO2 (nuclear fuel) has been studied under constant current conditions in sodium chloride (pH = 9.5) solutions containing silicate. Currents in the range 1-300 nA cm−2 (normalized to the geometric area of the electrode surface) were applied in an attempt to simulate rates as close as experimentally feasible to those anticipated under disposal conditions. The deposits were characterized using scanning electron microscopy, energy dispersive X-ray spectroscopy and Raman spectroscopy. At high currents (⩾20 nA cm−2) very high potentials (∼0.5 V vs. SCE) were achieved and surface deposits were formed at localized sites on the electrode surface. Raman analyses indicated that these deposits were hydrated uranyl silicates. Their localization was shown to be due to the formation of acidified sites on an otherwise passive surface as a consequence of uranyl ion hydrolysis underneath the deposit. At these sites the local current density was considerably higher than the nominally applied current density. The fraction of the surface covered by a deposit increased as the applied current decreased, leading to a decrease in the extent of acidification. Measurements as a function of applied current density established a potential of ∼0.25 V (vs. SCE) as a threshold below which acidification did not occur despite the formation of a deposit. When the current was reduced to 1-2 nA cm−2, the potential (∼0.11 V (vs. SCE)) approached the range of corrosion potentials measured in aerated solutions. These values are well below the threshold potential. Since the maximum corrosion current densities anticipated under actual disposal conditions are <1 nA cm−2, the prospects for acidification leading to enhanced corrosion and radionuclide release rates are very remote.