The assessment of electromigration as a new technique to study diffusion of radionuclides in clayey soils

For the performance assessment study of a geological disposal of High Level Waste (HLW) in clayey formations, migration studies are essential. For low permeability soils (clays), classical diffusion studies take a very long time. In order to reduce the experimental time, we propose an electrical field as driving force to accelerate the migration of ionic species. This paper reports the assessment of the electromigration technique as a powerful new and fast technique for migration studies. The apparent molecular diffusion coefficient can be derived by two independent methods using the migration parameters obtained from an electromigration experiment, namely the apparent dispersion coefficient and the apparent convection velocity. First, it can be calculated from the velocity of the migrating species by the Einstein relation. But, corrections are necessary for electroosmotic flow. The apparent electroosmotic mobility is experimentally determined as 2.2·10⁻⁹ m²/Vs. Second, it can be calculated from the relation between the apparent dispersion coefficient and the total apparent convection velocity. But it is necessary to know the dispersion length of the medium. The dispersion length for Boom Clay is experimentally determined as 8·10⁻⁵ m. Because of the serious reduction in time, it becomes possible to run series of experiments at different electrical fields to obtain averaged values for the apparent molecular diffusion coefficient according to the two methods. Experiments at different electrical fields have another advantage: the intercept of the linear relationship between the total apparent convection velocity and the apparent dispersion coefficient gives the apparent molecular diffusion coefficient. The apparent molecular diffusion coefficients obtained for ⁸⁵Sr, ¹³¹I and HTO are respectively 0.8·10⁻¹¹, 15·10⁻¹¹, and 24·10⁻¹¹ m²/s. These values are confirmed by pure diffusion experiments. 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.