UO2 dissolution in aqueous halide solutions exposed to ionizing radiation

In this work, we have experimentally studied UO2 dissolution in pure water and in 1 M aqueous solutions of either Cl- or Br- exposed to gamma-radiation. It has previously been found that high ionic strength can facilitate adsorption of dissolved UO?+ on UO2 surfaces. The adsorption is also affected by the solution pH relative to the point of zero charge of UO2. In our experiments, Br3 -was observed in 1 M Br- solution exposed to gamma-radiation. Experiments confirmed that Br3 -can quantitively oxidize UO2. XPS and UPS were used to characterize potential surface modifications after exposure. The XPS results show that the UO2 surfaces after exposure to gamma-radiation in pure water and in 1 M aqueous solutions of either Cl- or Br- were significantly oxidized with U(V) as the dominating state. U 4f7/2 and O 1 s spectra of the UO2 surface after exposure to gamma-radiation in pure water demonstrates the formation of uranyl peroxide secondary phases. UPS results indicate that there is a large percentage of U(VI) on the ultra-thin outer layer of UO2 after exposure to gamma-radiation in 1 M aqueous solutions of Br- and Cl-, and 100 % of U(VI) in the pure water case.