Effects of on the kinetics of UO2 oxidation by H2O2

The effect of HCO 3 - on the kinetics of UO ₂ oxidation by H ₂O ₂ in aqueous solution has been studied using powder suspensions where the concentration of H ₂O ₂ was monitored as a function of time. By varying the UO ₂ surface to solution volume ratio second order rate constants were obtained for HCO 3 - concentrations ranging from 0 to 100 mM. The second order rate constant increases linearly with HCO 3 - concentration from 0 to approximately 1 mM. Above 1 mM HCO 3 - the rate constant is 4.4 × 10 ⁻⁶ m min ⁻¹ independent of [ HCO 3 - ]. This indicates that the kinetics of the reaction depends on both oxidation and dissolution below 1 mM HCO 3 - while at higher concentrations it is solely governed by oxidation. Hence, the rate constant obtained at HCO 3 - concentrations above 1 mM is the true rate constant for oxidation of UO ₂ by H ₂O ₂. The results also imply that the reaction between HCO 3 - and oxidized UO ₂ on the UO ₂ surface (i.e. HCO 3 - facilitated dissolution) is limited by diffusion (ca 10 ⁻³ m min ⁻¹ in the present system). Furthermore, the experimental results were used to estimate the oxidation site density of the powder used (126 sites nm ⁻¹) and the rate constant for dissolution of UO 2 2 + from the UO ₂ surface (7 × 10 ⁻⁸ mol m ⁻² s ⁻¹).