## 3 Evaluation and Application of a Thermodynamic Database (TDB) in Geochemical Models
### Radionuclide Speciation and Solubility  
The radionuclides selected for the geochemical simulation were Thorium-232, Neptunium-237 and Plutonium-242, which have a long half-life and are safety-relevant materials for radioactive waste in Taiwan. We referred to the test calculations from THEREDA [50] and used the thermodynamic database (THEREDA_PIT_PHRC_R-07.dat) to simulate the radionuclide speciation and solubility for the solid compounds Th(OH)\({}_{4}\), Np(OH)\({}_{4}\) and Pu(OH)\({}_{4}\) in NaCl, CaCl\({}_{2}\), Na\({}_{2}\)CO\({}_{3}\) and KHCO\({}_{3}\) aqueous solution conditions (Tables 1-3) [50] using the PHREEQC model. Additionally, we also used the PHREEQC model, and used the NEA TDB (NEA_TDB_phreeqc_Nov2018.dat) to simulate radionuclide speciation and solubility for the solid compounds Th(OH)\({}_{4}\), Np(OH)\({}_{4}\) and Pu(OH)\({}_{4}\) in the groundwater (Table 4) of subsurface systems of potential radioactive waste disposal sites in Taiwan. The simulation results showed that the main dissolved ions of Th(OH)\({}_{4(8)}\) are Th\({}^{4+}\)(aq) and Th(OH)\({}_{4(aq)}\), and the total dissolved concentration of Th\({}^{4+}\) can vary with the pH, as shown in Figure 3. The main dissolved ions of NpCl\({}^{3+}\)(aq), Np(OH)\({}^{3+}\)(aq), Np(OH)\({}_{2}\)\({}^{2+}\)(aq), Np(OH)\({}_{3}\)\({}^{+}\)(aq), Np(OH)\({}_{4(aq)}\) and Np\({}^{4+}\)(aq) have a valence of 4, and the redox formations NpO\({}^{2+}\)(aq), NpO\({}_{2}\)(OH)\({}_{4(aq)}\) and NpO\({}_{2}\)(OH)\({}_{2}\)\({}^{-}\)(aq) have a valence of 5. The total dissolved concentration of Np\({}^{4+}\) in relation to the pH is shown in Figure 4. The main dissolved ions of Pu(OH)\({}_{4(8)}\) are PuCl\({}^{3+}\)(aq), Pu(OH)\({}_{4(aq)}\), Pu(OH)\({}_{3}\)\({}^{+}\)(aq), Pu(OH)\({}_{2}\)\({}^{2+}\)(aq), Pu(OH)\({}^{3+}\)(aq) and Pu\({}^{4+}\)(aq). The total dissolved concentration of Pu\({}^{4+}\) also varied with the pH (Figure 5). There are 16 aqueous complexation formations for Th(OH)\({}_{4(8)}\), 30 aqueous complexation formations for Np(OH)\({}_{4(8)}\), and 24 aqueous complexation formations for Pu(OH)\({}_{4(8)}\) in the dissolution reaction of a solid compound. Figure 6 showed the total dissolved concentrations of the solid compounds Th(OH)\({}_{4}\), Np(OH)\({}_{4}\) and Pu(OH)\({}_{4}\), as calculated using the groundwater quality of a potential radioactive waste disposal site in Taiwan, which can be used as the initial concentration data of radionuclides in the adsorption/desorption interactions of surface complexation reactions between nuclides and bentonite after its release from the waste canister to the engineer barrier. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Solution No.** & **Solid Compound** & **NaCl (mol/kgw)** & **CaCl2** (mol/kgw)** & **pH** & **pe** & **HCl (mol/kgw)** & **NaOH** (mol/kgw)** & **Na2 CO3** (mol/kgw)** \\ \hline
1 & Th(OH)4(5) & 4.0 & & 6.72 & 4.0 & \(10^{-4}\) & \\ \hline
2 & Th(OH)4(5) & 4.0 & & 6.72 & 4.0 & \(10^{-5}\) & \\ \hline
3 & Th(OH)4(5) & 5.26 & 5.38 & 4.0 & & 0.0010 & \\ \hline
4 & Th(OH)4(5) & 5.26 & 5.38 & 4.0 & & 0.0175 & \\ \hline
5 & Th(OH)4(5) & & 7.00 & 4.0 & & 0.1 & 0.25 \\ \hline
6 & Th(OH)4(5) & & 7.00 & 4.0 & & 0.1 & 1 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Initial conditions of the aqueous solution for the speciation and solubility calculation of Th(OH)4(5). \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Solution No.** & **Solid Compound** & **NaCl (mol/kgw)** & **CaCl2** (mol/kgw)** & **pH** & **pe** & **HCl (mol/kgw)** & **NaOH** (mol/kgw)** & **KHCO3** (mol/kgw)** \\ \hline
7 & Np(OH)4(5) & 4.0 & & 6.72 & \(-5.0\) & \(2\times 10^{-4}\) & \\ \hline
8 & Np(OH)4(5) & 4.0 & & 6.72 & \(-5.0\) & \(10^{-5}\) & \\ \hline
9 & Np(OH)4(5) & & 5.26 & 5.38 & \(-5.0\) & & 0.0010 & \\ \hline
10 & Np(OH)4(5) & & 5.26 & 5.38 & \(-5.0\) & & 0.04 & \\ \hline
11 & Np(OH)4(5) & & & 7.00 & \(-5.0\) & & & 0.03 \\ \hline
12 & Np(OH)4(5) & & & 7.00 & \(-5.0\) & & & 0.3 \\ \hline \hline \end{tabular}
\end{table}