Activation Energy for Diffusion of Cesium in Compacted Sodium Montmorillonite

Compacted bentonite is a promising candidate for engineered barriers in geological disposal of high-level radioactive waste(') (').It is, therefore, important to study the migration behavior of radioactive nuclides in compacted bentonite.A pore water diffusion model, in which nuclides are considered to diffuse through pore water in the compacted bentonite, has been widely adopted to explain the migration behavior of the n ~c l i d e s ( ~) ( ~) .Re~earches(~)-(~) have pointed out that other diffusion paths, closely related to clay surfaces, could be dominant in highly compacted bentonite for highly sorbing cations such as cesium ions.However, the diffusion model, including surface diffusion, has not been fully established.The activation energy for the diffusion of nuclides in compacted bentonite is an important parameter to determine the diffusion processes.Few reports(10)(") are available on the activation energy, and these have indicated that the activation energies for the diffusion in compacted bentonite are nearly the same as those for diffusion in free water.In a previous study(''), we have determined the activation energy for the apparent selfdiffusion of sodium ions in compacted sodium montmorillonite, which is a major clay mineral in bentonite.That study found a lower activation energy than that for the diffusion of sodium ions in free water for montmorillonite specimens with low dry density, while there were higher activation energies for montmorillonite specimens with high dry density.In the present study, the activation energies for the apparent diffusion of cesium ions were determined for compacted montmorillonite with different dry densities.