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 (D_e) 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 D_e values for Cl^- and I^- increased from approximately 10 ^-12 to 10^-11 m²/s with increasing NaNO ₃ 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 D_e values were interpreted in terms of the decrease in anion exclusion. The D_e 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 D_e values in porous media generally increase in proportion to the total porosity. This relationship between the log D _e 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.