## 6 Discussion
### Effects of Porosity Change on Radionuclides Transport  
Figure 15 shows that the porosity changes significantly affect the migration concentration and the retardation of radionuclides in buffer materials. Four key radionuclides (I-129, Ni-59, Sr-90, and Cs137) were used to analyze the porosity change effects on radionuclide transport due to decay heat. The solubility limit and half-life of the four radionuclides were compared using the data in Table 5. I-129 has no solubility limit and the distribution coefficient is zero; that is, no chemical precipitation formation and no adsorption in bentonite occurs, and the half-life is up to \(1.57\times 10^{7}\) years. The initial inventory of Ni-59 is large, i.e., it is larger than that of other radionuclides, and the half-life is also long, up to 1.01 \(\times\) 10\({}^{5}\) years. Ni-59 can be adsorbed by the buffer material, and its K\(d\) value is greater than that of Cs-137 and Sr-90. Cs-137 has a short half-life of 30.17 years and can be adsorbed by the buffer material. Moreover, its K\(d\) value is greater than that of Sr-90. Both Cs-137 and I-129 have no solubility limit; that is, no precipitation formation occurs in the simulation process. Sr-90 has a solubility limit, and precipitates will be formed. The half-life of Sr-90 is also short, only 28.79 years. Based on the solubility limit, half-life, and distribution coefficient for I-129, Ni-59, Sr-90, and Cs137, simulation analyses were conducted for unmodified and modified porosities. Figure 15 clearly shows the simulation concentration of radionuclides at the junction of near-field and host rock fracture. The simulation concentration of modified porosity is greater than that of unmodified porosity. It occurs after 1, 1671, 63, and 172 years for the I-129, Ni-59, Sr-90, and Cs137 radionuclides, respectively. Clearly, the use of modified porosity affords more conservative results and approaches the real situation of the radionuclide release concentration. The concentration simulation results of I-129 and Ni-59 radionuclides show that the concentration difference between the unmodified and modified porosities is obvious after a long simulation time (Figure 15A,B). The half-life of Sr-90 and Cs-137 is short, and the concentration difference between the unmodified and modified porosities is small after a long time (Figure 15C,D). The reason for the difference between I-129, Ni-59, Sr-90, and Cs137 is that among the investigated radionuclides, only I-129 has no adsorption value and Ni-59, Cs-137, and Sr-90 have adsorption value. Therefore, Ni-59, Sr-90, and Cs137 nuclides develop the adsorption formation in bentonite and retard within the buffer material. Furthermore, the concentration breakthrough curve in Figure 15 shows that the concentration of radionuclides is higher for the unmodified porosity simulation at the beginning modeling time and then the concentration of radionuclides for the modified porosity simulation exceeds it with time. In the evaluation results within 20,000 years, the concentrations of I-129, Ni-59, Sr-90, and Cs137 did not reach the solubility limit of nuclides, which is the condition for the formation of nuclide precipitation. Montmorillonite is a hydrated mineral containing interlayer water, which is an intrinsic part of the mineral. The interlayer water possesses different chemical and thermodynamic properties because of the groundwater in porous media, and its content can be up to 25% of the mass of the hydrated minerals [39]. The exact amount is a function of numerous physical and chemical variables, such as temperature, pressure, relative humidity, layer charge, and solution salinity. Therefore, the total water content of montmorillonite in bentonite or smectite-bearing sediments obtained using traditional physical property measurements or geophysical parameters (such as seismic velocity or neutron density) cannot be directly used to evaluate porosity unless the interlayer water of montmorillonite is corrected.