Effects of microstructure of clay on diffusion behavior of radionuclides in buffer materials (II)

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Diffusion behavior of radionuclides in compacted bentonite plays an important role in the performance assessment of bentonite buffer material in geological disposal of high-level radioactive waste. Microstructure of bentonite is considered to be one of the key parameters to affect on the diffusion behavior. In this study, therefore, nondestructive, three-dimensional images of the internal microstructures of compacted bentonite samples in dry and water-saturated states were examined with the Microfocus X-ray computerized tomography (Micro-CT). It was confirmed that, for dry bentonite samples, clear high-resolution, three-dimensional images could be obtained by Micro-CT. In addition, such microstructures can easily be quantitatively evaluated if the image data are analyzed with computer graphics. For water-saturated bentonite samples, structural changes in bentonite grains were found. This suggests that outer part of bentonite grains swelled and formed a gel that occupied the vacancies between grains, whereas the inner part of the grains did not change significantly in the water-saturated process. For the mixtures of montmorillonite and silica sand, the effect of silica sand on the diffusion behavior and microstructure of montmorillonite was studied. The diffusion coefficients of HTO and sodium ions in the mixtures of montmorillonite and silica sand were determined.The apparent diffusion coefficients of sodium ions and HTO in the mixture samples were almost the same as those in the pure montmorillonite samples at the same partial dry densities. This suggests that the effect of the silica sand on the diffusion coefficients could be negligible. However, activation energy dependence on the partial dry density for the diffusion of sodium ions in the mixture samples was found to be different from that in the pure montmorillonite samples. This implies that the diffusion process of sodium ions can be altered by the addition of silica sand. Based on the basal spacing ...