Effect of cement water on UO_2 solubility

摘要

To assess the stability of spent fuel in the highly alkaline chemical environment of the Belgian Supercontainer design, static leach experiments were performed with depleted UO_2 and ~(238)Pu-doped UO_2 at different SA/V ratios for 1.5 years in cement waters (11.7＜ pH ＜ 13.5) at ambient temperature and under argon atmosphere. The influence of the calcium concentration on the uranium release was also investigated. While the ultrafiltered U(Ⅳ) concentration was 10~(-9)-10~(-8) mol.L~(-1) and independent of the pH, the U(Ⅵ) release from the UO_2 surface was enhanced by the OH~- concentration, leading to soluble U concentrations up to 10~(-5) mol.L~(-1) at high SA/V and under the influence of radiolysis. Together with the high Ca concentration, this can lead to the formation of Ca-U(Ⅵ) colloids as precursor of Ca-U(Ⅵ) secondary phases, decreasing the soluble U concentration. The precipitation of Ca-U secondary phases was however not clearly evidenced by surface analyses.