## 6 Discussion  
### Mineral Types of Buffer Materials in Engineering Barrier Systems  
The buffer material of the EBS in the radioactive waste repository mainly comprises clay with low permeability as the sealing material of the waste container. The clay with low permeability provides very high tightness, which minimizes the percolation of groundwater and provides a flexible environment for the disposal container for mitigating the impact of external forces from the host rock. Low permeability clays must possess the ability to expand to support the surrounding rock and heal voids that may be generated from strains imposed by external tectonic forces or internal processes, including temporary shrinkage caused by drying. The further required characteristic is that the buffer material must be chemically compatible with the canister material and the surrounding host rock with its groundwater chemical species, and that it must be mainly inorganic to eliminate the risk of producing organic colloids that can carry radionuclides or create new forms of life [6]. The protection function of the buffer material can protect the canisters from mechanical, geochemical, and hydrogeological conditions, which can be predicted and can  
Figure 15: Concentration breakthrough curves as a function of time for radionuclides with and without porosity correction: (**A**) I-129; (**B**) Ni-59; (**C**) Sr-90; (**D**) Cs-137. adversely affect the canisters. The process protects the container from external influences that may endanger the safety of the entire containment of spent nuclear fuel and related radionuclides. It can constrain and retard the release of radionuclides during canister failure [6,8]. Since the late 1980s, waste management agencies in different countries have proposed various sealing materials; some use cementitious materials, but most of them consider bentonite. For example, ANDRA in France and ENRESA in Spain employ nonsodic smectites or the mixture of expansive clay and aggregate in different proportions. As for aggregates, AECL in Canada, DOE in the United States, JNC in Japan, and ANDRA in France and ONDRAF/NIRAS in Belgium have employed crushed granite, crushed basalt, zeolite and quartz, and quartz and graphite, respectively. A buffer material containing an inert aggregate can increase the thermal conductivity of a barrier, improve the mechanical resistance of compacted blocks, and reduce the material cost. Although most studies have considered that the emplacement of the EBS buffer layer will be performed in the form of precompacted blocks, other systems are also proposed. Among them, the combination of high-density bentonite pellets and powdered bentonite affords a barrier density equivalent to the compacted blocks when the mixture of the groundwater and buffer material reaches a saturated state and the density of the entire buffer zone is homogenized [29,52]. In 1977, [PERSON] proposed that the clay material of the EBS buffer layer should exhibit low hydraulic conductivity and provide support to the canisters, yielding the best isolation for the embedded sealing of canisters and the backfilling of drifts and shafts. Additionally, the effect of the buffer materials on ion adsorption and nuclide diffusion was identified as an important property, and a comprehensive and systematic laboratory study was conducted to determine suitable clay materials. The research mainly focused on montmorillonite-rich clay because of its particularly low hydraulic conductivity and high homogenization potential. However, [PERSON] performed more extensive research in 1999 to analyze the sealing capacity of other clay materials. The procedure was performed on compacted samples under the 100 MPa pressure, and the following conclusions were drawn [19,53,54,55]:  
* In addition to the MX-80 developed by SKB as a buffer material, the following clay types are possible buffer candidates: saponite (magnesium-rich smectite), mixed-layer (smectite-illite) Friedland Ton, kaolinite, and palygorskite.