Study on microstructure of clay and its effects on sorption and diffusion behavior of nuclides in geological disposal of nuclear waste

点击翻译按钮获取中文摘要

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 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 diffusion coefficients of HTO, Np(V)ions, Cs^+ ions, Cl^- ions, Ca^<2+> ions and He gas were determined using the montmorillonite samples with different particle sizes and dry densities. It was found that the montmorillonite layers, which compose the montmorillonite particles, have the same size, even if the particle sizes of the samples are different. In addition, the apparent diffusion coefficients of HTO and Cl^- for the fine grained sample were found to be higher tthan 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 the conventional diffusion model, "pore water diffusion model". The activation energies for diffusion of Np(V), Cl^- and Ca^<2+> ions seemed to be closely related to the basal spacings of the montmorillonite. This suggests the contribution of the diffusion of ions in the interlayer of the montmorillonite. The contribution of a surface diffusion is also suggested for diffusion of Na^+ ions in the mixture of montmorillonite and silica sand. The sorption behavior of Se and Np were studied for Na-and Fe(II)-montmorillonite. The effects of exchangeable cations on the sorption coefficients were found in this study.