Kinetics of dissolution of Th0.25U0.75O2 sintered pellets in various acidic conditions

Abstract

In order to gain insights into the kinetics of dissolution of Gen(IV) MOX fuels, sintered pellets of Th0.25U0.75O2 were prepared as surrogates. The study of the kinetics of dissolution was achieved to determine the impact of the incorporation of 25 mol % of thorium in UO2 on the rate and on the mechanism of dissolution. Especially, various acidic media were used to evidence changes occurring in the rate of the limiting step and to establish specific rate laws. For most of the investigated dissolution media the dissolution reaction was found to be congruent. The homogeneity of the Th0.25U0.75O2 solid/solutions allowed both elements to be recovered at the same rate. The way of synthesis by oxalate co-precipitation, then conversion in mixed oxide, allowed the quantitative dissolution of the elements. In aerated hydrochloric acid solutions, the kinetics of the overall dissolution reaction appeared to be controlled by adsorption and desorption of protons on activated surface sites. The dissolution rate determined in aerated sulfuric acid solution was enhanced compared to hydrochloric acid solution of the same acidity. This result was interpreted in terms of formation of sulfate ions surface complexes that favor the detachment of the actinides from the mixed oxide solid solution. In nitric acid solutions, the dissolution of the Th0.25U0.75O2 pellets followed several successive steps. During this first steady state period, the kinetics of the overall dissolution reaction appeared to be controlled by the oxidation of U(IV) by HNO3 at the solid/solution interface. A rate law was established by adding a redox contribution to the proton-promoted surface contribution defined for aerated hydrochloric acid solutions. Then, an increase of the normalized dissolution rate was observed, which was attributed to the simultaneous increase of the specific surface area of the pellet and of the HNO2 concentration in solution.