Diffusivities for ions through compacted Na-bentonite with varying dry bulk density

Diffusivity of ions of radioactive species is one of the important factors for designing radwaste repositories that employ clay minerals as a backfill material. In this study, diffussion of Cs, Sr, and Cl ions through compacted bentonite of various densities has been studied, using a diffusion cell. The measured diffusivities of Cs, Sr, and Cl ions decreased as the density of bentonite increased. The diffusivity of Cs ion decreased from 1.00 × 10−11 m2/s to 1.13 × 10−13 m2/s as the clay dry bulk density increased from 1.0 to 2.0 g/cm3. Sr and Cl ions also showed similar diffusion behavior. The diffusivity for Sr and Cl ions varied from 1.93 × 10−11 to 3.13 × 10−12 m2/s and from 2.09 × 10−10 to 5.71 × 10−12 m2/s, respectively, in the same ran clay density. The migration of Cs and Sr ions through the compacted bentonite was found to be dominated by surface diffusion. Density dependence of Cs ion diffusivity was stronger than that of Sr ion.