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 geologi cal 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 expe rimental time drastically. The electromigration technique has also the adva ntage that the apparent molecular diffusion coefficient can be derived by t wo independent methods. First, it can be calculated from the dispersion coe fficient, knowing the dispersion length and convection velocity The dispers ion length for Boom Clay was experimentally determined as 8.5 10(-5) m. Sec ondly, from the Einstein relation between the velocity of the migrating spe cies and the apparent molecular diffusion coefficient. But corrections are necessary for electro-osmotic flow. The electro-osmotic mobility was experi mentally determined as 1.4 10(-9) m(2)/Vs. Both methods were used to determ ine the apparent molecular diffusion coefficient of Sr-85. Both methods res ult in an apparent molecular diffusion coefficient of 6.3-7.8 10(-12) m(2)/ s which is validated by pure diffusion experiments. The feasibility of the technique is further demonstrated for Na-22, I-131 and I-ITO. The excellent agreement with the apparent molecular diffusion coefficients obtained by c lassical diffusion tests clearly demonstrates the feasibility of the electr omigration technique for the determination of diffusion coefficients.