Self-diffusion of sodium ions in compacted sodium montmorillonite

Diffusion of sodium ions through compacted sodium montmorillonite in a water-saturated state was studied to obtain fundamental information for performance assessments of geological disposal of high-level radioactive waste. Basal spacings obtained from X-ray diffraction measurements indicated a decrease in the interlamellar spacing with increasing dry density of the montmorillonite; the three-water-layer hydrate was observed at low dry density (less than or equal to 1.3 Mg/m(3)), and the two-water-layer hydrate was observed at high dry density (greater than or equal to 1.6 Mg/m(3)), whereas both were observed at dry densities between 1.4 and 1.5 Mg/m(3). Activation energies from 14.1 to 24.7 kJ/mol were obtained from the temperature dependence of the self-diffusion coefficients of sodium ions. Activation energies lower than that for the diffusion of sodium ions in free water were found for montmorillonite specimens with dry densities of less than or equal to 1.2 Mg/m(3), while higher activation energies were observed at dry densities greater than or equal to 1.4 Mg/m(3). The pore water diffusion model, the general model used for migration of nuclides, is based an geometric parameters; however findings cannot be explained by only the changes in the geometric parameters. Possible explanations for the dry density dependence of the activation energy are changes in the temperature dependence of the distribution coefficients of sodium ions on the montmorillonite, changes in the diffusion process with an increase in dry density, or both.