Event Timeslots (1)

Day 2 – June 21
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This presentation will outline the results of recent batch dissolution experiments of UO2, U0.5Th0.5O2, U0.7 Th0.3O2 and ThO2 pellets, in synthetic ground waters doped with hydrogen peroxide under anoxic conditions. The oxide powders have been fabricated by oxalic coprecipitation and thermal decomposition then pressed into 5mm pellets and sintered at 1600 degrees.

Mixed oxide fuel containing uranium and plutonium dioxide needs a disposal plan. Deep geological disposal is internationally recognised as the safest long-term solution. However, it is expected that eventually groundwater will break through the multiple barriers of a deep geological repository and contact the fuel. So, it is important to understand how mixed oxide fuel dissolution may differ from uranium-based spent nuclear fuels. Experiments with plutonium can only be conducted on licensed sites with stringent safety protocols. These experiments are limited in number, so it is necessary to perform complementary experiments on lower activity model fuels. Uranium-thorium dioxide has been selected to model Pu(IV) homogenously distributed in the uranium dioxide matrix.

The main dissolution mechanism of the uranium matrix in groundwater solutions involves the oxidation of U(IV) to the more soluble U(VI) and the subsequent removal of U(VI) by carbonate ions. There is very little oxidation in the anoxic conditions underground, particularly considering the reductants introduced by the dissolution of steel canisters. However, radiolytic oxidants will be produced by the interaction of radiation from the spent nuclear fuel and water. These radiolytic oxidants are produced close to the surface of the fuel and contribute to the oxidative dissolution of uranium dioxide. The dominant radiolytically produced oxidant is hydrogen peroxide [1]. In the presented batch experiments, hydrogen peroxide was added to the solutions initially. The consumption of hydrogen peroxide was observed in samples taken throughout the study using UV/vis spectroscopy. The uranium concentrations in the solution were followed using ICP-MS as a measure of the dissolution rate. The dissolution yield was calculated by subtracting the peroxide consumption in background effects unrelated to the pellet. The dissolution yield is independent of surface area and allows comparison to (U, Pu)O2 dissolution data.

Mixed oxide fuels are more radioactive and so will have a higher radiolytic production rate than uranium-based spent nuclear fuels. There are concerns that this will accelerate the dissolution, but these concerns are countered by evidence that the chemistry of plutonium mitigates these effects [2]. This mitigation depends on enhanced autocatalytic decomposition of the peroxide and the formation of a ‘protective’, less soluble plutonium-rich surface on the altered mixed oxide fuel as uranium is released. No thorium has been detected in the prewash, dissolution or rinse solutions of the presented batch dissolution experiments. Therefore, hydrogen peroxide consumption in the ThO2 dissolution study sets the baseline for the autocatalytic decomposition rate.