Determination of the Diffusion Coefficient of Ionic Species in Boom Clay by Electromigration: Feasibility Study

Migration studies are necessary for the performance assessment of a geological disposal of HLW. Classical diffusion experiments on Boom Clay take long time because of the excellent retention characteristics. To accelerate the migration of ionic species, an electrical field is used to reduce the experimental time drastically. The electromigration technique has also the advantage that the apparent molecular diffusion coefficient can be derived by two independent methods. First, it can be calculated from the dispersion coefficient, knowing the dispersion length and convection velocity. The dispersion length for Boom Clay was experimentally determined as 8.5 10^-5 m. Secondly, from the Einstein relation between the velocity of the migrating species and the apparent molecular diffusion coefficient. But corrections are necessary for electro-osmotic flow. The electro-osmotic mobility was experimentally determined as 1.4 10^-9 m²/Vs. Both methods were used to determine the apparent molecular diffusion coefficient of Sr-85. Both methods result in an apparent molecular diffusion coefficient of 6.3-7.8 10^-12 m²/s which is validated by pure diffusion experiments. The feasibility of the technique is further demonstrated for Na-22, I-131 and HTO. The excellent agreement with the apparent molecular diffusion coefficients obtained by classical diffusion tests clearly demonstrates the feasibility of the electromigration technique for the determination of diffusion coefficients.