UO2 dissolution in the presence of hydrogen peroxide at pH > 11

The dissolution of non irradiated UO2 was studied in the presence of hydrogen peroxide (10(-4) mol dm(-3)) at alkaline pH (11, 11.5, 12, and 13). Both hydrogen peroxide and uranium concentration in solution were determined as a function of time. The H2O2 consumption was modelled considering a pseudo first order reaction, the rate constants obtained were 0.126 +/- 0.004 h(-1), 0.126 +/- 0.003 h(-1), 0.078 +/- 0.002h(-1), and 0.056 +/- 0.002h(-1) at pH 11, 11.5, 12, and 13, respectively. The uranium concentrations measured at the end of the experiments were close to the solubility of sodium uranate (Na2U2O7). X-ray photoelectron spectroscopy showed that the surface of the solid was more oxidized at lower pH, indicating that at this pH the limiting step of the oxidative dissolution process is the dissolution of the U(VI) formed on the surface. At more alkaline pH values, the rate limiting step would be the oxidation of the UO2 surface.