Influence of temperature on HTO and ^{36}$cl- diffusion in bentonite and callovo-oxfordian clays
Résumé
The Callovo-Oxfordian formation at Bures in the Paris Basin (France) has been chosen as a potential host rock for deep radioactive waste disposal. Bentonite MX80 is proposed as buffer barrier surrounding the canisters. These materials have very low permeability and diffusion is likely to be the predominant mechanism transfer governing radionuclide migration. High level waste disposal will induce a temperature increase in these barriers. In a dilute solvent, this effect is described by the Stokes-Einstein law D$^ \infty$$_{IS}$ = k T /6 $\pi$ $\eta _S$= S r$_I$ whith D$^ \infty$$_{IS}$ the diffusion coefficient of a solute I infinitely diluted, k the Boltzmann constant, T the temperature, r$_I$ the ionic radius and $\eta _S$ the viscosity of the solvent. The effective diffusion coefficient of solutes in porous media is linked to the diffusion coefficient in free-water, porosity, tortuosity and constrictivity. Moreover, the effective diffusion coefficients of halides are reduced by anionic exclusion. In compacted clays, the physical properties of the interstitial water is strongly affected by liquid-solid interfaces. As a matter of facts, the use of Stokes-Einstein law in order to evaluate the effect of temperature on diffusion coefficients in such media is questionable.
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