Studies on the Effect of Concrete on the Chemistry in a Repository for Radioactive Waste

Abstract: In Sweden, the disposal method for long-lived low- to intermediate-level radioactive waste is proposed to be the long-term storage of the waste in a concrete repository deep in a geological formation. The use of concrete and steel for the waste containment will affect the chemical conditions inside the repository and in the host rock.

In this thesis the expected chemical conditions and their importance on the repository performance are reviewed. The proposed function of the barrier system is described in terms of waste dissolution, diffusion and sorption.

The alkaline degradation of cellulose in the waste will produce the complexing agent gluco-isosaccharinic acid (ISA). Experiments made to study the impact of ISA on radionuclide solubility and sorption in cementitious systems are reported. The results show that ISA have a large effect on actinide solubilities but a little effect on their sorption onto concrete, due to the strong sorption of ISA onto concrete.

Groundwater will leach alkalihydroxides and portlandite from the concrete barriers. The interactions of the leaching water with the surrounding host rock may influence the properties for radionuclide diffusion in the rock. Primary minerals may dissolve and secondary minerals may precipitate. Experiments with diffusion of alkaline water and radionuclides in rock show that these reactions are very slow in intact rock, but faster at fresh surfaces of the rock. The results also show that the composition of the alkaline water can have a influence on the diffusion of cations in geological media.