An estimation of the effective diffusion coefficients and distribution coefficients of Iodine in granodiorite and sandstone saturated by synthetic cement pore water and synthetic groundwater

Cementitious materials will be used expensively in the TRU waste repository. The cementitious materials will keep the high pH condition (pH<12) in the pore water of repository and the surrounding environment for long period. Thus it is necessary to obtain the data measured under the high pH conditions for the performance assessment of the TRU waste repository. However the number of the available data are limited. On the other hand, iodine-129 is an important nuclide because of its long half-time and poorly-sorbing nature. Therefore the effective diffusion coefficients De and distribution coefficients Kd of iodine in both crystalline rock and sedimentary rock were estimated in this study. Granodiorite and sandstone were selected as an example of crystalline rock and that of sedimentary rock respectively. Through diffusion experiment were carried out in an argon atmosphere. The diffusivity of iodine in both type rocks saturated by synthetic cement pore water was reduced with increasing experimental period. The analysis of experimental results suggested that this phenomenon was caused by decrease of De values. For granodiorite, the De values obtained in this study using synthetic cement pore water were 2 orders of magnitude smaller than literature values which obtained at neutral pH. For sandstone, the De values using synthetic cement pore water were 1-2 orders smaller than literature values, however those using synthetic groundwater were consistent with literature values. SEM observation of sandstone after the diffusion experiment showed that the secondary mineral coated the surface of sandstone. These observation results suggested that decrease of De value in case of synthetic cement pore water was ascribed to the precipitation of secondary mineral. In this study, Kd values of iodine on sandstone were 1.7 x 10-3 - 3.0 x 10-3 m3/kg in synthetic groundwater. However Kd values of other cases could not be estimated. (author)