Distribution coefficients for buffer material for the safety assessment in the second progress report of the geological disposal of high-level radioactive waste

In this research, the distribution coefficients (Kds) of the radionuclides were determined for the buffer material to be used in the safety assessment of the 'Second Progress Report on Research and Development for the Geological Disposal of HLW in Japan'. The 18 elements (i.e. Se, Zr, Nb, Tc, Pd, Sn, Cs, Sm, Pb, Ra, Ac, Th, Pa, U, Np, Pu, Am, and Cm) are evaluated. In the setting of the Kd, basically Kd derived from the apparent diffusion coefficient (Da) is adopted because Da is measured by using compacted bentonite. In the derivation of Kd from Da, effective diffusion coefficient (De) is needed, however measured De value is limited. Therefore, considering the speciation and ionic charge in the expected porewater composition, De of HTO is generally used, however, for Cs, the measured De value of Cs, and for Se, the De value of iodine as analog of anion were used respectively. For the redox sensitive elements, experimental data conducted under reducing conditions are only used for the reducing disposal environments. For each element, Da data were reviewed, then conservative Kd value derived from Da was selected considering the variations. The Kd data obtained batch experiments were also reviewed and correlation with environmental conditions, such as ionic strength, were discussed, and it is reflected to the correction of Kd for saline porewater. For some elements, Kd values derived from mechanistic sorption model were also discussed. (author)