Abstract
Cs+ has been considered as an importnat radionuclide for performance assessment of high-level radioactive waste disposal. It is expected that migration of Cs+ in rock would be retared by sorption on clay minerals. Sorption of Cs+ on clay minerals, however, might compete with other cations such as NH4+ and K+ in groundwater. Therefore, it would be quite important to esimate the reaction controlling the NH4+ concentration in groundwater for evaluation of Cs+ migration in rock. In the present study, thermodynamic calculations were conducted to estimate the reaction controlling the NH4+ concentration in the Horonobe groundwaters. As the results, the Horonobe groundwaters were plotted on the mineral stability field of muscovite similar to illite which is a dominant clay mineral in the Horonobe sedimentary rocks, and additionally an activity ratios of H+ for NH4+ and K+ for groundwaters were plotted on the line of slope 1. These results suggest that the NH4+ concentration in the Horonobe groundwaters could be controlled by an ion exchange reaction between NH4+ and K+ on illite in the sedimentary rocks.
