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Experimental study on diffusion of tritiated water and anions under variable water-saturation and clay mineral content: comparison with the Callovo-Oxfordian claystones

Abstract The diffusion of tritiated water and anionic species was studied through unsaturated compacted materials with variable clay content. These materials were designed to be analogous to the Callovo-Oxfordian claystone, a potential host-rock for disposal of high-level radioactive wastes. The diffusion parameters under such conditions were determined using modified through-diffusion cells in which the suction is generated by the process of osmosis. This device leads to saturation degree values ranging from about 70 to 100%. The results show that the diffusion through the unsaturated samples is clearly slower than that in the fully saturated samples, with steady-state fluxes decreasing by a factor up to 10 for tritiated water and up to 16 for anionic species. This tritiated water diffusivity (De/D0) decrease is similar to that obtained in the Callovo-Oxfordian claystone samples, whereas it is lower than that obtained in claystone samples for anionic species. This difference can be accounted for by the distinct hydro-mechanic behaviour of these materials, the pore-volume of which depends on their rigidity and clay content when hydrating. Finally, these diffusive behaviours have been modelled by means of an excluded-volume expansion of Archie's second law, taking into account a critical water saturation below which no tracer can diffuse.