Investigation of radiolysis in cement pastes immobilizing simulated evaporator concrentrates

Abstract The waste management of radioactive evaporator concentrates (EC) in deep geological repositories requires safe immobilization in a solid matrix, such as cement composites. Radiolysis occurs inside the hydrated cement paste due to irradiation of either free or bonded water. As a result, the generation of hydrogen and the consequent pressure build-up within the material may pose safety risks within the geological repository. Three kinds of cement pastes sealed in steel containers were irradiated by gamma source 60Co to achieve absorbed dose of 1.75 MGy simulating the conditions inside a repository. Specific binder matrix suitable for cement composites immobilizing EC marked as “NP” was studied, as well as the actual simulated immobilized waste (model concentrate) in the binder marked as “NP_C” and a reference paste “PC”. Results showed an increase in pressure of generated gas from the irradiated samples up to 100 kPa. After 120 days of irradiation the reference paste reached steady state, while NP and NP_C continued in the rising trend. However, the absolute values of NP_C were lower than the values of the reference paste, probably due to the presence of dissolved salts in the model concentrates, especially nitrates. The maximum value reached does not present safety risks within the repository, moreover the presence of certain salts in the concentrate has a positive effect on the decrease of the rate and the final values of the pressure build-up.