Mobility of sorbing cations through clay-rich media insight given by sorption/diffusion experiments performed under partially-saturated conditions
Résumé
Clay-rich formations are currently considered by several countries as potential host-rocks for high-level and intermediate-level long-lived radioactive waste repositories. These geological formations have high retention properties and a very low permeability, limiting the transport of radionuclides to the slow process of diffusion. However, the implementation of such repositories could induce hydric, mechanical and chemical perturbations within the host-rock and also the engineered components (e.g. packed clay barrier). For instance, the corrosion of the canisters should generate gases, mainly hydrogen, leading to a partial dehydration of the surrounding materials.Under these conditions, the migration of some radionuclides could be affected, thus modifying the clayey barrier performances in regard to the radioactive waste containment. The present study aims to tackle the issue of the cation mobility in clay-rich materials under partial water saturation. Several studies have shown that sorbing cations seem to diffuse faster than predicted by a simple pore diffusion model. Some authors attribute this enhanced diffusion to surface diffusion mechanisms occurring within the electrical double layer (EDL) developed from the negatively charged clay mineral surfaces [see e.g. 1].The proposed attack angle is to study the effect of the saturation on cation enhanced diffusion and sorption because dehydration restricts the diffusion pathways within the porous media. For that purpose, an original experimental approach was developed [2] and used for performing diffusion and batch experiments under partially-saturated conditions in intact and compacted crushed clay-rich materials, i.e. the Callovo-Oxfordian (COx) claystones. The studied cations, sodium, strontium and caesium, were chosen for their distinct sorption affinity, and uncharged tritiated water (HTO) was selected as a diffusion reference. As illustrated in Figure 1, results from the diffusion experiments show thatCation enhanced diffusion appears to be inhibited around 0.8-0.85 of saturation degree,Sorption evolution with saturation is not straightforward and depends on the chemical element.Batch experiments on intact rock are currently performed to refine the sorption data set. All the results will be discussed regarding the type of the porous media (intact or compacted crushed), the saturation degree and the radionuclides involved. They will be modelled considering the different approaches from literature used for quantifying the surface diffusion phenomenon.