Dissolution of mixed uranium thorium oxides under geological disposal conditions
Uranium dioxide alone offers enough complexity to provide the subject matter for 50 years of dissolution studies, not least due to uranium’s redox sensitivity and the orders of magnitudes between the solubility of U(IV) and U(VI). The addition of plutonium dioxide as a solid solution opens up a whole new realm of intrigue. How does plutonium affect the oxidation of uranium? How does plutonium affect interactions with radiolytic species of the surface? The aim of this PhD project is to piece together answers to these questions to build a picture of the dissolution mechanism for mixed actinide oxides, specifically in the environments that materials of this type will be exposed to during the long term storage of spent nuclear fuel. Due to the restrictions on the use of plutonium, except on nuclear-licensed premises, thorium is used as a plutonium surrogate. Studying thorium simplifies the experiment since the influence of the additional actinide can be observed separately to the production of radiolytic oxidants and the dissolution of end member pellets can be studied in the same environment. This work extends to developing methods that better simulate spent fuel by continuously delivering hydrogen peroxide using a syringe pump. It has been found that homogeneously mixed oxide pellets suppress the uranium dissolution by a factor of 10, as compared to end member uranium dioxide pellets, without affecting the consumption of hydrogen peroxide. The percentage of consumed hydrogen peroxide leading to dissolution is found to be dependant on the initial hydrogen peroxide concentration. Therefore, the batch dissolution experiment with continuous additions improves the simulation of (U,Pu)O2 samples where the initial peroxide concentration is 0 mol.L−1. Less homogeneously mixed oxide pellets dissolved less homogeneously, with the height of uranium rich regions decreasing relative to the thorium rich regions as visible by stereographic reconstruction of the surface using Scanning Electron Microscopy. Blank experiments provided evidence that the dissolution rate increase coincided with the onset of background disproportionation of hydrogen peroxide in the solution away from the pellet surface.
dissolution, geological disposal facility, hydrogen peroxide, mixed oxide, nuclear waste
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