Retardation of Tracers in Crystalline Rock Sorption and Matrix Diffusion of Alkali Metal and Alkaline Earth Metal Tracers in Laboratory and Field Experiments

Doctoral thesis, 2000

The safety of geological repositories for radioactive or environmentally hazardous waste is dependent on several engineered and natural barriers. One of the natural barriers is the surrounding bedrock, in which water-conducting rock fractures may act as transport pathways in the event of a leakage from the engineered barriers. Radionuclides that are transported with the groundwater flow may be significantly retarded by sorption onto minerals and by diffusion into pores in the rock matrix and will thus have more time for radioactive decay. Thus the barrier function of the bedrock is of great importance in a safety assessment of geological repositories. The aim of this work was to gain greater understanding of the processes controlling the diffusive transport of some slightly to moderately sorbing tracers used in experiments at different size scales in heterogeneous crystalline rock. In this work, the sorption and diffusion behaviour of some selected radionuclides of the alkali metals and alkaline earth metals were studied. The experiments were performed at different scales ranging from crushed and intact rock specimens in the laboratory to in situ tracer migration experiments over a distance of about 5 m in a single fracture at the Äspö Hard Rock Laboratory in Sweden. Studies of the rock porosity and porosity distribution were included in the laboratory experiments. The results of the laboratory experiments were applied in modelling the field experiments. It was found that the relative order of sorption strength for the different tracers was consistent between experiments performed at the different scales. A solid phase size dependency in the absolute values of the sorption distribution coefficients was observed and interpreted in terms of induced sample disturbances. Deviations from an "ideally" fast and reversible sorption behaviour were indicated for some tracers in both the in situ and the laboratory experiments. Furthermore, rock heterogeneity was indicated to be of importance for the diffusion behaviour in the laboratory experiments, and the possible influence of these effects on the interpretation of the field experiments is discussed in this thesis. Transport modelling of the field experiments showed that diffusive mass transfer is required for the interpretation of the tracer breakthrough curves. However, more laboratory data on the diffusion properties in the fracture materials are needed to confirm the modelling results.