Oxidation and dissolution rates of UO2(s) in carbonate-rich solutions under external alpha irradiation and initially reducing conditions

Abstract

This paper describes the effects of aqueous carbonate concentrations on the UO2 oxidation and dissolution rates under an alpha beam irradiation in a cyclotron. As solid samples, UO2 colloids were synthesized by nano-particle precipitation. The specific surface area obtained is 85.3 m2/g. All aqueous solutions were buffered in the pH range 8<pH<9. The observation of particles under TEM and X-ray diffraction showed well-crystallized UO2. Molecular radiolysis products such as H2O2 and carbonate degradation products where analysed. Solution analysis of dissolved uranium and of radiolytically generated H2O2 allowed to deduce dissolution rates of UO2 as well as H2O2 consumption rates (deduced from the H2O2 concentrations in presence of UO2 colloids and those in irradiated blank solutions). In absence of carbonate, the dissolution rates of UO2 are much lower than the H2O2 consumption rates. The difference is explained by the oxidation of uranium(IV) at the UO2/solution interface (e.g. formation of U3O7). In contrast, in the presence of carbonate (up to 4×10-2 mol/L), the dissolution rates of UO2 are similar to H2O2 consumption rates. In this case the oxidized uranium is complexed with carbonate ions resulting in highly soluble species as indicated by the geochemical model PHREEQC calculations [UO2(CO3)3 4 -]. The high carbonate concentration did not affect the H2O2 formation/consumption rates. Radiolytic decomposition of carbonate ions and formation of organic radiolytic degradation products were negligible. The results indicate that under the present experimental conditions, UO2 oxidation by H2O2 and UO2 dissolution are connected as a sequential reaction: for a given H2O2 concentration, UO2 oxidation occurs at a constant rate. At low carbonate concentration, the dissolution of oxidized UO2 is slower than the oxidation rate (hence oxidation is not rate limiting), while at high carbonate concentrations all oxidized UO2 becomes immediately dissolved and UO2 oxidation becomes rate limiting for UO2 dissolution. This mechanism is in agreement with that proposed in the Matrix Alteration Model (MAM).