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 t ime. In order to reduce the experimental time, we propose an electrical fie ld as driving force to accelerate the migration of ionic species. This pape r reports the assessment of the electromigration technique as a powerful ne w and fast technique for migration studies. The apparent molecular diffusio n coefficient can be derived by two independent methods using the migration parameters obtained from an electromigration experiment, namely the appare nt dispersion coefficient and the apparent convection velocity. First, it c an be calculated from the velocity of the migrating species by the Einstein relation. But, corrections are necessary for electroosmotic flow. The appa rent electroosmotic mobility is experimentally determined as 2.2 . 10(-9) m (2)/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 le ngth 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 experi ments at different electrical fields to obtain averaged values for the appa rent molecular diffusion coefficient according to the two methods. Experime nts 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 Sr -85, I-131 and HTO are respectively 0.8 . 10(-11), 15 . 10(-11), and 24 . 1 0(-11) m(2)/s. These values are confirmed by pure diffusion experiments. Th e excellent agreement with the apparent molecular diffusion coefficients ob tained by classical diffusion tests clearly demonstrates the feasibility of the electromigration technique for the determination of diffusion coeffici ents. (C) 1999 Elsevier Science B.V. All rights reserved.