Diffusion mechanism of cesium ions in compacted montmorillonite

For the safety assessment of geological disposal of high-level radioactive waste, the diffusion behavior of Cs+ ions in compacted Na- and Ca-montmorillonite mixtures was studied. The apparent diffusion coefficients of Cs+ ions were determined as functions of the diffusion temperature and the ionic equivalent fraction of Ca2+ 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 energies for diffusion of Cs+ ions were also determined from the temperature dependence of the apparent diffusion coefficients at each ionic equivalent fraction. The activation energy increased from 34 kJ mol−1 to 38 kJ mol−1 as the ionic equivalent fraction increased from zero to 0.75, after which it increased steeply and reached 49 kJ mol−1 at the ionic equivalent fractions of 0.94 and 1.0. These findings cannot be explained by the pore water or surface diffusion models. Interlayer diffusion is suggested as the most probable explanation for the diffusion of Cs+ ions.