Abstract
AbstractCompacted bentonite is to be used as a component of an engineered barrier system to retard the migration of radionuclides in the geological disposal of radioactive waste. In such an environment, montmorillonite in compacted bentonite might be altered to illite due to the hydrothermal reactions caused by the decay heat of radionuclides. In the present study, the diffusion and retention behaviour of Cs in compacted montmorillonite containing illitewas investigated using through-diffusion experiments. The experimental results showed that the flux of Cs attributed to the surface diffusion was independent of the sorption of Cs on illite, indicating that the Cs sorbed on illite was immobile or considerably less mobile than the Cs sorbed on montmorillonite. Consequently, the illite content in compacted bentonite is expected to enhance the sorption capacity of Cs without causing surface diffusion.
Highlights
In the geological disposal of radioactive waste, compacted bentonite will be used as a component of the engineered barrier system to retard the migration of radionuclides from waste packages
The difference in diffusion and sorption behaviours of radionuclides due to the illite generated in compacted bentonite is, one of the issues to be investigated in terms of the safety assessment (Ohnuki et al, 1994; Ahn et al, 1995)
0.5 mol/dm3 NaCl solution containing 1 × 10–4 mol/dm3 non-radioactive CsCl, the concentration profiles of 137Cs showed almost the same trend except for the sample containing 50% illite. These results indicate that the illite content contributed to the high concentration of 137Cs in the compacted montmorillonite samples
Summary
In the geological disposal of radioactive waste, compacted bentonite will be used as a component of the engineered barrier system to retard the migration of radionuclides from waste packages. The increase in illite content can, be expected to have a positive impact on the performance of the EBS, unless the extent of alteration of montmorillonite is large enough to affect significantly the permeability and sorption capacity of the compacted bentonite. For Cs, enhanced diffusivities have been obtained in clay rocks (Van Loon et al, 2004; Melkior et al, 2005; Wersin et al, 2008) and in compacted bentonite (Muurinen et al, 1985; Kim et al, 1993; Suzuki et al, 2007; Sawaguchi et al, 2013) This phenomenon is often referred to as the surface diffusion effect, which has been explained by the increase in mobile cations concentration due to the sorption of cations on the sites to which sorbed cations are not fixed. The possibility of surface diffusion of Cs sorbed on illite contained in compacted montmorillonite was discussed
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