## 4 Discussion
A significant change in the updated formation constant of Ca\({}_{2}\)UO\({}_{2}\)(CO\({}_{3}\))\({}_{3}\)(aq) species from log K\({}^{0}\) = 29.22 \(\pm\) 0.26 [11] to log K\({}^{0}\) = 30.80 \(\pm\) 0.40 [18], indicating the increase of log K\({}^{0}\) by almost an order of magnitude, allowed for the domination of the aqueous groundwater system by Ca\({}_{2}\)UO\({}_{2}\)(CO\({}_{3}\))\({}_{3}\)(aq) species, according to the updated TDB. On the other hand, the relative abundance of Sr ion bound U species became negligible owing to the remarkable decrease in the formation constant, from log K\({}^{0}\) = 26.86 \(\pm\) 0.50 [11] to log K\({}^{0}\) = 25.90 \(\pm\) 0.20 [18], of SrUO\({}_{2}\)(CO\({}_{3}\))\({}_{3}\)\({}^{2-}\) species. Furthermore, electrically neutral U species other than Ca\({}_{2}\)UO\({}_{2}\)(CO\({}_{3}\))\({}_{3}\)(aq) such as Mg\({}_{2}\)UO\({}_{2}\)(CO\({}_{3}\))\({}_{3}\)(aq) and Sr\({}_{2}\)UO\({}_{2}\)(CO\({}_{3}\))\({}_{3}\)(aq), which were newly included in the updated TDB, did not show substantial abundance in U species distribution on account of either an exceptionally weak formation constant (for Mg case) or a very low aqueous concentration of the metal ion in the groundwater system (for Sr case). Even though the exposure dose results for both thermodynamic databases are similar to each other, the updated TDB still provides an insight into the precise exposure dose estimation in terms of the distribution coefficient (Kd). In particular, in case of U, the predominant aqueous species was considerably changed from a negatively charged species, CaUO\({}_{2}\)(CO\({}_{3}\))\({}_{3}\)\({}^{2-}\), to an uncharged species, Ca\({}_{2}\)UO\({}_{2}\)(CO\({}_{3}\))\({}_{3}\)(aq), according to the selection of the thermodynamic database. Therefore, the distribution coefficient of U against the  
Figure 2: Relative distribution of aqueous (**a**) Am and (**b**) U species predicted by using JAEA-TDB and updated TDB, respectively. Chemical species less than 2% in their abundances are merged as ”others”. mineral surface, e.g., aluminosilicate minerals, which are negatively charged in natural groundwater systems [26], can also be dramatically changed because the electrostatic repulsion between the U species and the mineral surface will be decreased significantly. Figure 3 represents the exposure dose results of U-238 calculated with updated TDB at various distribution coefficients, unambiguously indicating that the distribution coefficient acts as a key parameter that can promote/increase or delay/decrease the peak time and peak value of the exposure dose, respectively. In comparison to the default Kd value of RESRAD-OFFSETE code (50 cm\({}^{3}\)/kg for U-238) against geologic barriers in repository sites, the enhanced Kd value of 500 cm\({}^{3}\)/kg, set to be 10 times higher than the default value, presents a significant delay in the release rate of U-238 into the biosphere due to the remarkably enhanced retardation capacity. On the other hand, the result obtained with a relatively decreased Kd value of 5 cm\({}^{3}\)/kg, 10 times lower than the default value, was contrary to that predicted with the enhanced Kd value, revealing a remarkably increased and accelerated exposure dose value and peak time. From this perspective, it provides a hint towards further precise evaluation of the exposure dose by means of achieving a reliable estimation of the distribution coefficient. The current safety assessment model based on the single-valued distribution coefficient for each particular zone in the repository site might not precisely reflect the fundamental geochemical parameters, e.g., pH, ionic strength, aqueous ion concentrations, which vary over the site.