## 4 Discussion  
The notable similarities between exposure dose results obtained with two different thermodynamic databases were mainly due to the relatively minor influences of newly added and/or updated chemical species in the solubility data. In this regard, the relative abundance of aqueous chemical species of various radionuclides, which can influence the solubility data and other input parameters, was quantitatively calculated from the relevant equilibrium reactions. As shown in Figure 2a, species distributions of Am indicated the predominant presence of AmSiO(OH)\({}_{3}\)\({}^{2+}\) together with AmCO\({}_{3}\)\({}^{+}\) and Am\({}^{3+}\), and revealed almost identical compositions with respect to two different sets of thermodynamic databases. The almost unchanged species distribution of Am under the given groundwater system results from a relatively weak affinity between Am and CI\({}^{\prime}\)/SO\({}_{4}\)\({}^{2-}\) ions under the given groundwater system as well as a trivial change in the updated formation constants of AmSiO(OH)\({}_{3}\)\({}^{2+}\) species from log K\({}^{0}\) = \(-\)(1.68 \(\pm\) 0.18) [11] to log K\({}^{0}\) = \(-\)(1.67 \(\pm\) 0.20) [18]. The behaviors of solubility data and aqueous species distributions of most of the other radionuclides presents nearly unchanged results for both JAEA-TDB and updated TDB, which were comparable to those of the Am case. For the Tc case, even though various chemical species such as TcCl\({}_{5}\)\({}^{-}\), TcCl\({}_{6}\)\({}^{2-}\), and Tc2 OCl\({}_{10}\)\({}^{4-}\) were newly included in the updated TDB, no significant change in solubility and species distribution was identified for similar reasons to the case of Am. For the case of Pu, the solubility limiting phase was determined to be PuO\({}_{2}\)(cr), with exceptional stability of log K\({}^{0}_{\rm sp}\) = \(-\)(64.0 \(\pm\) 0.5) [11,18] in aqueous systems compared with the other Pu solid phases, for both calculations with two different TDBs. According to the thermodynamic calculation, a relatively minor change in Pu solubility was identified because (i) there was no update for the solubility product constant of PuO\({}_{2}\)(cr) and the other tetravalent aqueous Pu species; (ii) the oxidation state of Pu under the given groundwater system was expected to be tetravalency; and (iii) the stability of the other oxidation states, i.e., Pu(III), Pu(IV), and Pu(V), was remarkably low so that the abundance of newly added or updated Pu species in the species distribution was identified as being very minor. Figure 1: Exposure dose from various radionuclides derived by using solubility data estimated from (a) JAEA-TDB and (b) updated TDB as the source term. On the contrary, in the case of U in particular, remarkable discrepancies in solubility data and aqueous species distribution were identified between thermodynamic calculation results obtained with JAEA-TDB and updated TDB. For all cases, the chemistry of U in natural groundwater systems was mostly governed by ternary Mg/Ca-UO\({}_{2}\)-CO\({}_{3}\) complexes because of exceptionally strong chemical affinity among earth alkaline metal ions, UO\({}_{2}\)\({}^{2+}\) free ion, and CO\({}_{3}\)\({}^{2-}\) ion [3; 22; 23; 24; 25]. Moreover, thermodynamic calculations using JAEA-TDB revealed that negatively charged CaUO\({}_{2}\)(CO\({}_{3}\))\({}^{2-}\) species presents predominantly under the given system, whereas a remarkable formation of uncharged Ca\({}_{2}\)UO\({}_{2}\)(CO\({}_{3}\))\({}_{3}\)(aq) species was expected based on the updated TDB (see Figure 2b), even in the same groundwater condition. A slight increase in U solubility for the calculation with the updated TDB (see Table 3) is mainly due to the further formation of Ca\({}_{2}\)UO\({}_{2}\)(CO\({}_{3}\))\({}_{3}\)(aq), which can provide additional stability for the dissolved U species according to the remarkably increased formation constant of Ca\({}_{2}\)UO\({}_{2}\)(CO\({}_{3}\))\({}_{3}\)(aq) selected in the updated TDB compared with that in the JAEA-TDB.