Present status of the study on radionuclide diffusion in barrier materials

Yamaguchi, Tetsuji ; Nakayama, Shinichi 

When long-lived radionuclides are transported by groundwater through fractures in the rock mass surrounding a radioactive waste repository, their diffusion into pores in the rock matrix and ensuing sorption onto mineral surfaces are expected to retard their transport along the pathways. We have characterized the pore structure of the Japanese Inada biotite granite and confirmed that Fick's diffusion law can be applied to the transport of aqueous species in granite. Effective diffusivity was determined by the through-diffusion method for cationic, anionic and actinide complex species to explore the mechanism of diffusion of the aqueous species. The results of this study enable us to provide a scientifically sound basis for radionuclide diffusion in granite for performance assessment of geological disposal. Future studies should emphasize understanding the diffusion mechanisms in low-permeability engineered barrier materials, data acquisition on long-term degradation of the materials and quantifying uncertainties associated with long-term mass transport analysis.