## 6 Discussion
### Application of Geochemical Modeling for the Performance Assessment of Radioactive Waste Disposal  
Knowledge on the species distribution of radionuclides is required in order to understand the state equilibrium and geochemical reactions at a disposal site. The radionuclide compositions and groundwater quality data can form the basis for reactive chemical transport simulation for the determination of the transport mechanisms and migration phenomena of radionuclides. In order to better represent and predict the chemical reaction and behavior of radionuclides in radioactive waste repositories, laboratory tests, field experimental studies and geochemical simulation studies should be drawn upon. Despite the significant improvements made in geochemical models, there is still potential for further development. Before the execution of a geochemical model simulation, the following items should be established: computer code, a database of thermodynamic data, the kinetics of chemical species, and measured data of the chemical and physical properties in the study area. Accordingly, this study proposes that the following questions should be considered: (1) Are the chemical analysis data sufficiently accurate to support simulation studies? (2) Does the TDB used in the program include data on the key solution species and reactions of surface complexation models, which occur between chemical species and minerals? (3) Are the equilibrium constant values of crucial reactions in the TDB sufficiently accurate to support a clear and unambiguous simulation conclusion? (4) Is the ionic strength level sufficiently low to ensure the appropriate applicability of the activity coefficient model in the geochemical program? On the basis of the above questions, the following recommendations are proposed for future research. (1) Before establishing a detailed geochemical model, researchers must determine which model to develop; because most computer models are based on the assumption of chemical equilibrium, it is important that the assumption of chemical reaction equilibrium (in disposed radioactive waste) should be evaluated and confirmed using kinetic simulation models. The time required for a reaction to reach equilibrium can be calculated using a kinetic model; subsequently, it can be decided whether the assumption can be justified. The problem must be thoroughly understood before the simulation, followed by suitable simplification (if possible) to create an appropriate conceptual and geochemical model. The appropriate simplification of the conceptual simulation not only reduces the problem's complexity considerably but also enables the achievement of the desired results. Therefore, before the establishment of a geochemical model, the following points should be considered: (a) the species and components to be included in the model; (b) the types of geochemical reactions to be included in the model; (c) whether chemical reaction kinetics are included in the model or a local equilibrium hypothesis is used; and (d) whether any other processes need to be included in the simulation. (2) Activity coefficient calculation: the selection of a suitable chemical TDB is crucial in the generation of an appropriate species activity coefficient. The ion-specific parameters provided in a TDB are highly crucial for the applicability of the modified Debye-Huckel equation. Because thermodynamic data affect the species distribution pattern generated in the model simulation, the simulation results will reflect the accuracy of the activity coefficient. For example, in a dilute solution (\(l<0.1\) mol/kgw), if the TDB is not accurate, the calculated ionic strength will deviate. This may lead to erroneous activity coefficients of high-charge species. Thus, the careful selection of TDBs is necessary. [PERSON] developed a theoretical model of an electrolyte solution based on the Debye-Huckel equation, which is highly effective in calculating the activity coefficients of high-concentration mixed electrolyte solutions.