Diffusion of Water in Bentonite

Because of the low permeability and the high sorption coefficient, compacted sodium bentonite has been proposed as an effective backfill material in the repository of high-level radioactive waste. The only possible mechanism of nuclide transport in bentonite must be the diffusion. The diffusivity has been considered to have a close relationship with the retardation factor and the self-diffusivity of water. Then, we performed the experiments to determine the self-diffusivity of water in bentonite by using tritiated water as a tracer. The obtained self-diffusivity can be written as Da= (3.4 ± 0.2) × 10-6 [cm²/s] under the condition of temperature T= 25°C and compacted density of bentonite ρ= 2.7 [g/cm³]. It was revealed quantitatively that the self-diffusivity of water in bentonite is much smaller than that of free water (2.13 × 10-5 [cm²/s] at T= 25°C). Also discussed is the analysis of experimental results on the basis of a new solution derived for the diffusion in one dimensional finite zone