Diffusion and Migration of Ions in Sedimentary Rock Matrix: Behaviour of Cesium and Iodide in Micropores of Sandstone
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Diffusion and Migration of Ions in Sedimentary Rock Matrix: Behaviour of Cesium and Iodide in Micropores of Sandstone Haruo SATO and Tamotsu MURAOKA, Japan Nuclear Cycle Development Institute 4-33 Muramatsu, Tokai-mura, Naka-gun, Ibaraki-ken, 319-1194, JAPAN ABSTRACT In-diffusion experiments for Cs+ and I- in sandstone were performed as a function of ionic strength of porewater ([NaCl]=0.01, 0.51M) and tracer concentration ([CsI]=7.5E-5, 1.5E-2M) together with the measurements of the fundamental properties of sandstone, and apparent diffusivities (Da) and rock capacity factors (α) for Cs+ were obtained. The obtained Da-values for Cs+ were scarcely dependent on [NaCl], but increased with increasing [Cs+]. This trend is consistent with that of α-values, indicating that although effective diffusivity and distribution coefficient scarcely depend on [NaCl], those depend on [Cs+]. The concentration profiles of I- were all in breakthrough. Although this indicates that I- diffusion is faster than that of Cs+, the concentration profiles of I- may have been lower than those for blank samples. INTRODUCTION In safety assessment for the geological disposal of high-level radioactive waste in Japan, it is important and essential to reliably treat radionuclide migration in the geosphere, many studies concerning radionuclide diffusion have been therefore conducted to date [e.g. 1]. Particularly, for sedimentary rocks, since fundamental data and information for diffusion are quite limited, we have systematically acquired data for radionuclide diffusion and have performed an understanding of diffusion mechanism for porous media. Rocks are often classified as fractured and porous media from the viewpoints of the transport and modeling of radionuclides [2]. Fractured media are found in crystalline rocks such as granite, and the fractures are considered to be the principal pathways for radionuclide migration. Porous media are generally treated by assuming that a continuously and homogeneously distributed pore space exists in sedimentary rocks, such as sandstone, mudstone, tuff, etc., in which no fractures develop. The processes that radionuclides and ions diffuse in the rock matrixes occur through porewater, and those radionuclides and ions take place various interactions at the surface of minerals composing the rock matrixes in the diffusion processes. For distribution coefficients (Kd) onto rocks, in the second progress report (H-12 report) [3], retardation has been evaluated by Kd-values obtained from batch experiments, based on the judgment that the pore sizes are generally large enough compared with ionic sizes. However, actually, inconsistency is often found between Kd-values obtained from batch experiments and those calculated from diffusion experiments, and Kd-values from batch experiments tend to be larger than those calculated from diffusion experiments. As this cause, although the differences in specific surface area of samples used between batch and diffusion experiments, an exposure of active surface by crushing in
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