Abstract: The groundwater at the near site of the geological disposal repository of high-level radioactive waste may affect the barrier system in the disposal repository and may reduce the safety and stability of the disposal repository. In order to study the diffusion law of salt ions in the groundwater into the buffer backfill system in the disposal repository, the spontaneous imbibition, adsorption and diffusion of Gaomiaozi (GMZ) bentonite from Inner Mongolia in Ca²⁺ solution was carried out in laboratory under static non-external loading condition. The spontaneous diffusion of Ca²⁺ in bentonite samples with different dry densities and initial saturation was analyzed based on soil microstructure and classical diffusion theory. The results showed that under the condition of the same initial saturation of bentonite, the retardation coefficient increases with the increase of dry density, while the diffusion capacity of Ca²⁺ decreases. When the dry density of bentonite was the same, the retardation coefficient and diffusion capacity of Ca²⁺ decrease with the increase of initial saturation.

Key words: HLW geological disposal, Na-bentonite, spontaneous diffusion, diffusion coefficient, the second law of Fick