Topical report on the effect of the ionic strenght on the diffusion accessible porosity of Boom Clay

The diffusion accessible porosity is a key transport parameter for each migrating radionuclide. This diffusion accessible porosity is directly influenced by changes of the thickness of the electrical double layer (EDL). Most of the perturbations linked to the geological disposal of High and Medium Level Radioactive waste will lead to a change of the ionic strength and thus influence the EDL. This report gives an overview of all the experiments that have been carried out to study the effect of ionic strength on the diffusion accessible porosity of Boom Clay. Besides this experimental overview, this report also intends to explain the observed variations and evolutions of physico-chemical as well as transport related parameters, with known soil processes and mechanisms. For iodide, an increase in ionic strength leads to an increase of the diffusion accessible porosity. A subsequent decrease of the ionic strength towards the undisturbed Boom Clay pore water level, restores the diffusion accessible porosity to its initial value. These observations are in agreement with what is expected: a collapse of the electrical double layer diminishes the effect of anion repulsion. The influence of the changes in ionic strength on the apparent diffusion coefficient is not significant. The two fold anisotropy of the apparent diffusion coefficient between vertical and horizontal stratified Boom Clay remains. For tritiated water, no significant variations of the diffusion accessible porosity with ionic strength were observed. This observation was expected, as ionic strength variation is not supposed to have an effect on the diffusion accessible porosity of neutral molecules. As for iodide, no effect on the apparent diffusion coefficient was observed. The two fold anisotropy between vertical and horizontal stratified Boom Clay remains.