Study on migration behavior of radionuclides in engineered clay barrier

Hokkaido University*

As a basic research for geological disposal of high-level radioactive wastes, long-term durability of clay buffer materials was studied from the viewpoint of performance of clay as barrier, which would be changed when exchangeable cations (Na ions) of Na-montmorillonite were replaced with Ca ions. The apparent diffusion coefficients of Cs ions were determined as functions of the diffusion temperature and the ionic equivalent fraction of Ca ions. The apparent diffusion coefficients were almost constant at higher temperatures, 323 and 313 K, while they decreased with increasing ionic equivalent fraction at lower temperatures, 298 and 288 K. The activation energy increased from 34 kJ mol to 38 kJ mol as the ionic equivalent fraction increased from zero to 0.75, after which it increased steeply and reached 49 kJ mol at the ionic equivalent fractions of 0.94 and 1.0. These findings cannot be explained by the pore water or surface diffusion model. Interlayer diffusion is suggested as the most probable explanation for the diffusion of Cs ions. Corrosion experiments were carried out using neutron-activated iron foils and bentonite specimens with different drying periods to evaluate the effects of oxidation of pyrite on the corrosion rates of iron and the apparent diffusion coefficients of the corrosion products in bentonite. There was a decrease in the quantity of pyrite in the bentonite during drying, together with a decrease in pH, and an increase in SO concentration in bentonite-water suspension. These results indicate that the oxidation of pyrite proceeded gradually during the drying process. It was found that the average corrosion rate of iron and the apparent diffusion coefficients of iron corrosion products in the compacted bentonite increased with the drying period. It is likely that the decrease of pH caused by the oxidation of pyrite affects the corrosion rates and the apparent diffusion coefficient.