Effect of sodium nitrate on the diffusion of Cl- and I- in compacted bentonite

For safety assessment of TRU waste disposal, the effective diffusion coefficients (De) for Cl- and I- in compacted bentonite (Kunigel V1) were determined as a function of NaNO3 concentration, ranging from 0.01 to 5 mol/dm3. The De values for Cl- and I- increased from approximately 10-12 to 10-11 m2/s with increasing NaNO3 concentration. The capacity factor α, indicative of the effective porosity, also increased with increasing NaNO3 concentration. The maximum α values of 0.21 for Cl- and 0.24 for I- at 5 mol/dm3 NaNO3 concentration were lower than the total porosity of compacted bentonite (0.40), suggesting an anion exclusion effect. Therefore, the increases in the De values were interpreted in terms of the decrease in anion exclusion. The De values for Cl- and I-, normalized by their diffusivities in bulk water, were found to increase in proportion to the α values in a log-log plot, while the De values in porous media generally increase in proportion to the total porosity. This relationship between the log De and the log α is considered to be derived from the lower effective porosities for Cl- and I- than the total porosity in compacted bentonite due to anion exclusion. (author)