## 3 Evaluation and Application of a Thermodynamic Database (TDB) in Geochemical Models
### Evaluation of a Thermodynamic Database (TDB)
The nuclides and toxic chemical elements are outlined as follows: (1) long half-life nuclides and actinide daughter nuclides over 1000 years, including C, Al, Cl, K, Ca, Mn, Fe, Ni, Se, Rb, Zr, Nb, Mo, Tc, Pd, Sn, I, Cs, Ba, Sm, Ho, Hf, Pb, Ra, Th, Pa, U, Np, Pu, Am and Cm (Be, La, Bi and Ac will be included in the future); (2) short half-life nuclides, including CO, Sr, Ag and Eu; and (3) toxic chemical elements, including B, Cr, As, Cd and Sb. The basis for the development of the ThermoChimie database includes a pH range of 5-14, the redox stability range of water, a temperature of 15-90 \({}^{\circ}\)C, and an ionic strength between very low (0.001 mol/kgw) and 3 mol/kgw. The data selection process must consider both temperature and ionic strength correction. In many cases, there is not enough data to perform these corrections, and strategies have been developed to overcome this limitation [32; 46]. The purpose of the THEREDA chemical database is to simulate the geochemical evolution of radioactive and chemical toxic waste disposal sites. In the process of modeling, it is necessary to check the internal consistency of the thermodynamic data (including the adsorption data) of chemical reactions between nuclides and the surrounding rock. The built-in databases provided by different geochemical models are inconsistent, such as the activity coefficient equation used in the data generation process and the experimental data of the equilibrium constant. Therefore, using different chemical databases to compare geochemical simulation results may have different results. This is due to the incomplete and conflicting data in the existing chemical databases, the different validities of the solution composition (ionic strength) and model limitations, and the lack of sorption data. THEREDA is a collaborative project that has addressed these challenges. The aim of the project is to establish a consistent and quality-assured database for all of the safety-relevant nuclide and toxic element, temperature and pressure ranges, with a focus on high-concentration brine systems. The database uses the Pitzer equation to calculate the activity coefficient applicable to such conditions. The chemical elements and nuclides in the THEREDA database are as follows: H, C, O, Na, Mg, Al, Si, P, S, Cl, K, Ca, Se, Sr, Tc, Cs, Nd, Th, U, Np, Pu, Am and Cm. Part of the THEREDA project involves the collection and evaluation of uranium chemical reaction data. The aquatic chemical and thermodynamic conditions for U (VI) and U (IV) are crucial for the geochemical modeling of disposable-site-relevant systems. The THEREDA chemical database established the correction of the activity coefficients for U, Na, K, Mg, CA, Cl, S, and C in high-salinity aqueous solutions using the Pitzer equation. The THEREDA database does not have the same strict quality requirements for thermodynamic data as the NEA TDB. Currently, THEREDA is jointly planned, implemented and further developed by the research institute engaged in the final disposal of radioactive waste and its potential host rock [41]. The NEA TDB has provided high-quality and accurate chemical thermodynamic data used in the safety assessment of deep geological repositories for the international radioactive waste management community. The scenario analysis and assessment of radionuclide migration paths to the environment are essential for the performance assessment of geological disposal. The safety assessment of radioactive waste disposal is based on the use of a chemical thermodynamics database for the numerical simulation of the speciation. The NEA mentioned [47] that their TDB has become the worldwide standard for geochemical modeling in the field of geological disposal. However, the applicability of the NEA TDB data is limited to low-to-medium ionic strength environments and temperatures of less than 100 \({}^{\circ}\)C. Additionally, the NEA TDB data do not cover all of the species and thermodynamic quantities required for safety assessment.