## 2 Materials and Methods
### Conceptual Model  
Considering the cylindrical geometry of the bentonite layer around the heater and striving to save computational resources, a 1D model has been selected for the analysis of coupled THC processes in the bentonite (Figure 2). The bentonite layer of 0.69 m thickness starts at the coordinates corresponding to the heater surface (z = 0.45 m) and ends at the coordinates corresponding to the bentonite-granite interface (z = 1.14 m). The bentonite in the model was discretized into finite elements to solve the coupled PDEs with the finite element method. Non-uniform discretisation was applied following [18], as indicated in Table 1. The duration of numerical simulation was set 18 years in line with the FEBEX in situ test duration. Bentonite is a complex porous medium. As it contains large quantities of montmorillonite (for example, 61-95 wt.% in the MX-80 bentonite, 92 \(\pm\) 4 wt.% in the Febx bentonite (smectite-illite mixed-layer) [20]), a mineral that has a significant impact on bentonite properties. The structure of montmorillonite is a unit made of an alumina octahedral sheet surrounded by two silica tetrahedral sheets [21]. Due to substitution of trivalent Al for tetravalent Si and divalent cations (e.g., Mg, Fe) for trivalent Al in the montmorillonite structure, a permanent negative lattice charge (the cation exchange capacity, CEC) persists [21]. The cations located in spaces between adjacent particles (interlayer spaces) compensate this charge [22]. The other type of reactive sites relevant in montmorillonite are surface hydroxyl groups existing along the edges of the clay platelets (\"edge\" or \"broken bond\" sites). According to [23], these sites provide \(\sim\)10% of the CEC and can protonate and deprotonate so that the concentrations of neutral, protonated and deprotonated edge sites change as a function of pH. Saturation of bentonite with porewater of a particular \begin{table}
\begin{tabular}{c c} \hline \hline
**Range** & **Mesh Size, m** \\ \hline
0.45 m–0.46 m & 0.000625 \\
0.46 m–0.49 m & 0.0025 \\
0.49 m–0.59 m & 0.005 \\
0.59 m–1.00 m & 0.01 \\
1.00 m–1.10 m & 0.005 \\
1.10 m–1.13 m & 0.0025 \\
1.13 m–1.14 m & 0.00625 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Discretisation. Figure 2: Model geometry. composition will impose the re-distribution of dissolved species between the porewater and those bonded on montmorillonite. Apart from this main mineral, bentonite contains small amounts of other minerals as impurities, such as calcite, gypsum, and quartz. If impurities are not chemically inert, they can influence the chemical state of bentonite by mineral precipitation/dissolution processes and in turn affect its physical properties. In the long-term perspective, the precipitation of illite and dissolution of montmorillonite might also take place. [PERSON] et al. [12] analysed the illitisation in the FEREX in situ test and concluded that illitisation would continue for 50 years but would not proceed. The major controlling factor in this case is chemical conditions, while temperature plays a secondary role. [PERSON] et al. [13] concluded that consideration of smectic dissolution via kinetic law was significant in terms of the increase in aluminium concentration. The concentrations of other dissolved cations increased slightly, and concentrations of anions and neutral species showed no sensitivity to smectic dissolution [13]. Considering the outcomes from previous studies [12; 13], illitisation and smectic dissolution were not considered in the current study. Apart from dissolved species of K, Na, Ca, Mg, and C, the inflowing granite porewater will contain dissolved gases, among which CO2 is important. It could, depending on its level, impose CO2 transport into or out of bentonite and affect the pH, concentration of dissolved HCO3-, and calcite precipitation/dissolution.