Effect of particle size on the diffusion behavior of some radionuclides in compacted bentonite

For performance assessment of bentonite buffer material in geological disposal of high-level radioactive waste, the particle size of bentonite and its effect on the diffusion behavior of radionuclides in compacted bentonite were studied. Bentonite samples with different particle sizes were prepared, and characterized by the BET and EGME methods for specific surface areas, by the laser diffraction/scattering particle size analysis for particle size distribution, and by SEM observations. Apparent and effective diffusion coefficients of tritiated water (HTO), Cl- ions and Cs+ ions in compacted bentonite were also determined using bentonite samples with different particle sizes. With HTO and Cl- ions, there were higher diffusion coefficients in fine grained samples, but opposite particle size effects were observed with Cs+ ions. These findings cannot be explained by the conventional pore water diffusion model, and suggest a different diffusion process for Cs+ ions and also at higher dry density region. (C) 1999 Elsevier Science B.V. All rights reserved.