## 6 Discussion
### Using SCM to Estimate Kd for Performance Assessment  
In the past, the traditional method for the determination of the retardation factor (R\({}_{\text{f}}\)) of a nuclide released from a repository involved sampling the groundwater and geological materials of the disposal site. The samples were then used to carry out adsorption experiments and estimate the nuclide Kd. After comparing the experimentally derived Kd value with data from the scientific literature, a set of recommended Kd values were generated for the safety assessment of the disposal site. The Kd value is the ratio of the mass of the nuclide adsorbed or precipitated onto the minerals per unit of dry soil mass, S, to the concentration of the nuclide in the liquid, C. Equilibrium conditions and complete reversibility are assumed to exist between the soil minerals and the nuclides in aqueous solution. Kd can be expressed as Kd = S/C. R\({}_{\text{f}}\) is defined as R\({}_{\text{f}}\) = 1 + (\(\rho_{\text{b}}\)/n\({}_{\text{e}}\))Kd = v\({}_{\text{p}}\)/v\({}_{\text{c}}\), where \(\rho_{\text{b}}\) is the bulk density of the porous medium, n\({}_{\text{e}}\) is the effective porosity of the medium at saturation, v\({}_{\text{p}}\) is the velocity of the water traveling through the porous medium, and v\({}_{\text{c}}\) is the velocity of the contaminant traveling through the porous medium. Many studies has pointed out that Kd values can be measured from many adsorption processes; however, experiments have shown that Kd values are not constant, and that they depend on the soil properties. As the geological material of the disposal system is a dynamic system, the Kd values will not remain constant over the long term. The soil properties affecting the Kd value include the soil texture (sand, loam, clay or organic soil), the soil organic matter content, the soil pH, the soil solution ratio, the solution or pore water concentration, and the presence of competing cations and adsorption sites for surface complexation. Because of the soil characteristics, the distribution coefficients of radionuclides in various soils may differ by several orders of magnitude. Table 12 lists the Kd values of nuclides in four representative soil types [58,59] and compares these values with the Kd values estimated in the case study (Table 6). Bulk MX-80 contains 65-75% montmorillonite, 10-14% quartz, 5-9% feldspar, 2-4% mica and chlorite, 3-5% carbonates and chlorite, and 1-3% heavy minerals. Therefore, montmorillonite and quartz are the main components and the dominant reactants in bentonite [60]. It can be seen from Table 12 that the variation of the Kd values is very large in sand, loam, clay and organic soils. Bentonite is mainly composed of montmorillonite clay and quartz sand; thus, the case study estimation in this study is between the Kd values of clay and sand proposed in the literature. Because the montmorillonite content of bentonite is 65-75%, the Kd value is estimated to be 28.6-33.0 cm\({}^{3}\)/g. Thus, the Kd value can be deduced by applying the chemical thermodynamic data of SCM and the chemical properties of minerals. Coupled surface complexation reactions in reactive chemical transport models can be used to simulate the spatiotemporal characteristics of Kd distribution caused by changing chemical conditions. In this study, the simulation of SCM was used to analyze the adsorption data measured in the laboratory, and a quantitative relationship between the measured Kd and the nuclide adsorption degree and environmental parameters (such as the pH, component concentration and density of the adsorption sites) was established. In this way, the simulation of SCM can provide scientific support for the measured Kd, and can increase the confidence of the Kd estimation. The potential uses of SCM for performance assessment and safety analysis in radioactive waste disposal include the following:  
1.