## 1 Introduction  
For the reliable safety assessment of a radioactive waste repository, a systematic understanding of the migration behavior of radionuclides is essential, because a key criterion for an appropriate disposal strategy is the capability to isolate the harmful substances [1]. Various geochemical reactions such as aqueous complexation [2, 3], solubility [4, 5, 6], sorption [7, 8], etc. can be quantitatively predicted by using relevant chemical thermodynamic data such as formation constant, solubility product constant, etc. Therefore, for the precise estimation of the chemical behavior of various radionuclides, it is of cardinal importance to employ the latest, updated, and comprehensively reviewed thermodynamic data for the relevant chemical thermodynamic calculation. Since 1992, OECD/NEA has been establishing a reliable thermochemical database [9, 10], also called as OECD/NEA-TDB, based on the in-depth reviews and surveys on the various literature data, fulfilling the requirements of a proper geochemical modelling of the chemical behavior of radionuclides in natural environments. Moreover, various institutes such as JAEA [11], PSI/Nagra [12], and Andra [13] have also been developing customized chemical thermodynamic databases on the basis of OECD/NEA-TDB as a reference standard database. In particular, the thermodynamic database established by JAEA (JAEA-TDB) is being continuously updated and amended to provide further reliable and up to date chemical thermodynamic data. Moreover, OECD/NEA-TDB has been recently updated and revised for several major radionuclides such as U, Np, Pu, Am, and Tc, and is expected to have a remarkable influence on various customized chemical thermodynamic databases. According to the updated thermodynamic database, solubility and species distribution of aqueous radionuclides as primary parameters for the safety assessment model can be influenced based upon the chemical thermodynamic equilibrium, affecting source term of nuclear waste, migration/retardation behavior of relevant radionuclides, and eventually exposure doses from various radionuclides disposed in the waste repository. However, since the impact of those updated chemical thermodynamic databases on the primary parameters, i.e., solubility and species distribution, as well as exposure dose under natural environmental conditions can only be quantitatively evaluated by considering practical and conditional natural groundwater systems. Relevant thermodynamic calculations that take the information of those systems into account is essentially needed in this respect. The RESRAD-OFFSETITE [14] is a computational code for the safety assessment model developed by Argonne National Laboratory (ANL) on the basis of various hydrogeochemical parameters and radionuclide transport pathways such as direct external exposure, inhalation, ingestion, etc. The latest version of RESRAD-OFFSETITE enables improved assessment of radionuclide migration in accordance with the radionuclide solubility as an input parameter. Therefore, the combination of RESRAD-OFFSETITE code, solubility data, and species distribution derived from the relevant thermodynamic calculation is required to assess the impact of the updated chemical thermodynamic database on the exposure dose result and safety assessment model. In the safety assessment modeling, the application of thermodynamic estimation is necessary for the calculation of radionuclides' solubility and relevant chemical species distribution to achieve more reliable prediction of the migration behavior of radionuclides. With respect to the computational thermodynamic estimation, two different approaches, the law of mass action (LMA) and the Gibbs energy minimization (GEM), were considered. The LMA approach is based on the equilibrium constant and mass balance function, while the GEM approach uses the Gibbs free energy function to derive the solution.