Some Methodological Modifications of Determination of Diffusion Coefficients in Compacted Bentonite

Abstract

The results of ³H, ³⁶Cl and ¹³⁷Cs diffusion experiments through compacted bentonite using a new design of diffusion cell and a new methodology of diffusion coefficients evaluation are presented. The diffusion cell was made from the stainless steel and enables to connect it directly to the input and/or output reservoirs without any tubing. The evaluation of diffusion coefficients utilizes a compartmental model developed in the environment of the GoldSim transport code. It enables to determine diffusion coefficients for various types of boundary conditions, including also input and output filters. The influence of the diffusion through filters on the determined values of both effective (D_e) and apparent (D_a) diffusion coefficients was numerically demonstrated for the through diffusion method. This effect is most important for D_a, the value of which would be underestimated using standard ways of evaluation for neutral and positively charged species, mainly in the case of high effective porosity.

The comparison of standard and the newly developed method of evaluation of diffusion coefficients showed a significant influence of diffusion in filters for HTO. Contrary to the standard method of evaluation, the evaluation taking into account filters showed here no difference between total and effective porosity. The effect of filter resistance was negligible for Cl^-, especially at high dry density of compacted bentonite, due to the anion exclusion effect. The numerical model developed enabled to determine D_a values of Cs⁺ from the concentration change in the inlet reservoir.