## 2 Safety Assessment Model
### Chemical Thermodynamic Database  
For the solubility evaluation, fundamental chemical thermodynamic data such as formation constants and solubility product constants were collected from the literature [11; 18]. The latest version of the JAEA thermodynamic database (JAEA-TDB) [11] was selected as the reference data in the present work. Moreover, the formation constants and solubility product constants of Tc, Am, U, and Pu that have been recently updated from the OECD/NEA Chemical Thermodynamics Vol. 14 [18] were additionally considered for the comparative study between the thermodynamic databases. Therefore, in the present work,  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Radionuclide** & **Inventory (Bq)** & **Half-Life (Year)** \\ \hline Fe-55 & 4.67 \(\times\) 10\({}^{15}\) & 2.73 \\ Co-58 & 6.55 \(\times\) 10\({}^{14}\) & 8.1 \(\times\) 10\({}^{-3}\) \\ Co-60 & 1.03 \(\times\) 10\({}^{15}\) & 5.27 \\ Ni-59 & 3.78 \(\times\) 10\({}^{13}\) & 7.60 \(\times\) 10\({}^{4}\) \\ Ni-63 & 2.71 \(\times\) 10\({}^{15}\) & 1.00 \(\times\) 10\({}^{2}\) \\ Sr-90 & 3.20 \(\times\) 10\({}^{12}\) & 2.89 \(\times\) 10\({}^{1}\) \\ Nb-94 & 9.72 \(\times\) 10\({}^{10}\) & 2.03 \(\times\) 10\({}^{4}\) \\ Tc-99 & 7.85 \(\times\) 10\({}^{11}\) & 2.11 \(\times\) 10\({}^{5}\) \\ Ce-144 & 6.81 \(\times\) 10\({}^{11}\) & 7.81 \(\times\) 10\({}^{-1}\) \\ Gross-alpha * & 6.22 \(\times\) 10\({}^{11}\) & - \\ \hline \hline \end{tabular}  
* U-238, Pu-239, Am-241, and Cm-244 were considered as representatives of gross-alpha radionuclides.  
\end{table}
Table 1: Radionuclide inventories employed in the present work taken from the literature [17].  
two different sets of the chemical thermodynamic database, i.e., JAEA-TDB as reference data and amended JAEA-TDB reflecting the updates of the OECD/NEA Chemical Thermodynamics Vol. 14 (which will be referred to as updated TDB in the following sections) were employed to assess the influence of those updated data on the solubility evaluation and exposure dose calculation process. The chemical thermodynamic data of selected radionuclides, e.g., Tc, Am, U, and Pu are represented in Table 1 in Appendix A. The authors note that the thermodynamic database established by the Lawrence Livermore National Laboratory [19] was alternatively adopted for the case of Ce, of which data is not available in the JAEA-TDB.  
For the thermodynamic calculation, the specific ion interaction theory (SIT) [10,20] was used as an ionic strength correction model, because the validity range of the other ionic strength correction models, e.g., [PERSON] model, Debye-Huckel model, etc., are relatively narrower than that of SIT [21]. According to the SIT formulism, the activity coefficients of (charged) aqueous species (\(\gamma_{i}\)) are calculated as described in Equations (1) and (2):  
\[\log\gamma_{i}=-z_{i}^{2}D+\Sigma\epsilon_{ij}m_{j} \tag{1}\]  
\[D=\frac{0.509\sqrt{l_{m}}}{1+1.5\sqrt{l_{m}}} \tag{2}\]  
where \(z_{i}\) indicates the charge of the given ionic species, \(D\) represents the Debye-Huckel term at T = 25 \({}^{\circ}\)C, \(\epsilon_{ij}\) means the specific ion-interaction coefficient for a pair of oppositely charged ions, \(m_{\mathrm{j}}\) corresponds to the molal concentration of the ion \(j\), and \(l_{m}\) is the molal ionic strength. Specific ion interaction coefficients used in the thermodynamic calculations performed in the present work were taken from the literature [11,18].