Role of fluoride ions on the uranium oxidation mechanism in highly alkaline solutions

This research is focused on understanding uranium oxidation to predict the behavior of uranium contaminated Mg alloy waste in confinement matrices. The environment of the matrix proposed for the magnesium encapsulation, based on geopolymer material, imposes basic conditions and the presence of fluoride ions (corrosion inhibitor of the Mg alloy). This work demonstrates the opposite behavior of the hydroxide and fluoride ions on the uranium oxidation: the fluoride ions induce the corrosion of uranium, whereas the hydroxide ions protect the metal against corrosion by making an anionic barrier at the uranium/solution interface. Two mechanisms are proposed, depending on the [OH−]/[F−] ratio. In absence of fluoride ions, or if [OH−]/[F−] > 1, the mechanism corresponds to a single step, the charge transfer characteristic of the UO2/UO2+x system. Uranium metal is thus protected from corrosion by the oxides layer. By contrast, if [OH−]/[F−] < 1, uranium is corroded, the fluoride ions diffuse through the oxides layer and cause its desquamation. This phenomenon has to be taken into account for the confinement of uranium contaminated Mg waste in geopolymers matrices containing fluoride ions.