## 6 Discussion
### Transformation of Smectite into Illite in Bentonite  
[PERSON] et al. [68] systematically investigated the kinetics for the conversion of a Na-saturated montmorillonite to a mixed-layer smectite/illite as a function of KCl concentration from 0.1 to 3 mol/L over a temperature range of 25 to 325 \({}^{\circ}\)C at 500 bar in cold seal pressure vessels using gold capsules. The smectite illitization rate can be described by a simple empirical rate equation for a Na-rich solution as follows:  
\[-\frac{dS_{sm}}{dt}=A\cdot\exp\left(\frac{-Ea}{RT}\right)\cdot\text{C}_{1}\cdot S _{sm}{}^{2} \tag{23}\]  
\[\text{C}_{1}=[\text{K}^{+}]-\frac{[\text{K}^{+}]}{2}\times\frac{[\text{Na}^{+ }]}{2.3} \tag{24}\]  
where \(S_{sm}\) is the fraction of smectite layers in the \(I/S_{sm}\), \(t\) is the time in seconds, \(A\) is the frequency factor of 8.08 \(\times\) 10\({}^{-4}\) sec\({}^{-1}\), \(Ea\) is the activation energy of 28 kcal/mole, \(R\) denotes the gas constant of 1.987 cal/deg-mole, \(T\) represents the temperature in Kelvin. [K\({}^{+}\)] and [Na\({}^{+}\)] are the concentrations of potassium and sodium ions in the pore water of granite in the potential radioactive waste disposal site of Taiwan, which are 9.22 \(\times\) 10\({}^{-5}\) mol/L and 1.53 \(\times\) 10\({}^{-3}\) mol/L, respectively.  
Figure 16 shows the ratio of smectite in the illitization process. Within 10,000 years, less than 0.05% of smectite is transformed to illite. This is because high temperatures do not remain within the buffer material for extended periods.  
[PERSON] [6] stated that the long-term function of the buffer zone mainly involves the stability of smectite-rich clay, which is determined by the reaction controlled by thermodynamics. This denotes that the occurrence of mineral transformation relies on the stability conditions of the interacting minerals and the pore water under the main temperature and temperature gradient conditions. Moreover, the processes affecting the expected importance of the buffer zone must be determined, such as the evolution of the buffer clay in partial and fully saturated water in contact with canisters exposed to high temperatures and groundwater for long durations. [PERSON] [6] presented the analysis results and stated that about 100% of the original smectite transforms to illite at 100 \({}^{\circ}\)C. Furthermore, the typical buffer temperature history of SKB spent fuel canisters is (a) 100-150 \({}^{\circ}\)C in the first 100 years, (b) an average temperature of 50-100 \({}^{\circ}\)C after 500 years, and (c) less than 50 \({}^{\circ}\)C in the next 1000 years, converting 15% of the initial smectite into illite in 1500 years. [PERSON] [6] also indicated that the activation energy is uncertain, and the acquisition of potassium is difficult to define and deduce. For the activation energy of about 20 kcal/mol, the model denotes that the buffer undergoes significant degradation by conversion to illite within 100 years. This would be disastrous, and smectite clays like montmorillonite would be discontinued as buffer material. Finally, they proposed that [6] the real meaning of activation energy and whether the theoretical conversion model is correct should be determined. Although some studies have attempted this, not many examples exist, mainly from the natural environment, that exhibit temperatures and temperature gradients similar to those in the repository from the smectite clay sediment environment [6].  
Herein, when the decay heat is controlled below 100 \({}^{\circ}\)C, only about 0.05% of the initial montmorillonite content is converted to illite in 10,000 years. The maximum temperature at point A at the junction of the canister and buffer material was about 91.3 \({}^{\circ}\)C, which occurred in the sixth year (Figure 12). Thus, controlling the decay heat effect temperature below 100 \({}^{\circ}\)C in the buffer material is a very important EBS design condition.