Effects of microstructure of clay on diffusion behavior of radionuclides in buffer materials

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, two kinds of montmorillonite (major clay mineral of bentonite) with different particle sizes were prepared, and characterized with several methods. In addition, the apparent and effective diffusion coefficients of HTO, Cl-, and Cs+ were determined using the montmorillonite samples with different particle sizes and dry densities. In the sample characterization, the specific surface areas of montmorillonite samples with different particle sizes were determined by the BET and the EGME methods, and the particle size distributions of each sample were analyzed by laser diffraction/scattering particle size analysis. Microstructure of the samples was also observed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The BET method gave a higher specific surface area of the fine grained sample than of the coarse sample, while the EGME method gave same values for both samples. The laser diffraction/scattering particle size analysis using ethanol as a dispersion medium gave different particle size distributions, but when the samples were dispersed in water with Na6(PO3)6, the particle size distributions were similar. These findings indicate that the montmorillonite layers, which compose the montmorillonite particles, have the same size, even if the particle sizes of the samples are different. In the diffusion experiments, it was found that the apparent diffusion coefficients of HTO and Cl- for the fine grained sample were higher than for the coarse grained sample at two dry densities, 1.0 and 1.8 Mg m-3, while the opposite particle size effect was observed for Cs+ ions. These findings cannot be explained by changes in the distribution coefficients, as the effective diffusion coefficient, which is independent of the distribution factor, was also affected by the grain size of montmorillonite. The new concepts for diffusion process, including the diffusion on external surfaces and interlayer, are needed for accounting the experimental results that were obtained in this study and cannot be explained by the pore water diffusion model. (author)