Diffusion experiments with compacted powder/pellets clay mixtures

Abstract A mixture (1:1) of pellets and powdered FoCa bentonite is being investigated as a possible sealing material to be used in geological disposal. One of the possible advantages of this mixture is the fact that it can be used to backfill irregular-shaped volumes, obtaining a homogeneous material when saturation is reached. Because the migration of radionuclides in compacted clayey materials is mainly controlled by diffusion, the determination of radionuclide diffusion coefficients is of key importance to assess the feasibility of such type of barrier. Through-diffusion (TD) experiments, with a constant concentration gradient condition, were carried out with powders and powder/pellet mixtures at different densities using, as a tracer, the neutral and conservative molecule, tritiated H 2 O (HTO). These experiments aimed to determine the effective diffusion coefficients and the tracer-accessible porosity, and to analyse the possible differences in the transport behaviour in both systems. After water saturation, the powder/pellet mixture appeared very homogeneous. The difference in the diffusion coefficients for HTO, measured for the powder or for the pellet/powder mixture at the same density, was negligible. Accessible porosity for HTO was roughly equal to the total theoretical porosity of the clay samples. Additional experiments in the powder/pellets mixture were done with the anionic species iodide. At the same clay density, the effective diffusion coefficients for iodide were between one and two orders of magnitude lower than those obtained with HTO. Furthermore, only a small fraction of the total porosity was accessible for iodide, showing the existence of anion exclusion effects. Additional evidence for anion exclusion was found in an experiment in which the clay was saturated with water traced with 36 Cl − . In this experiment, where the clay is adsorbing water and advection is actually the main transport mechanism, an important increase of the tracer concentration in the saturation water was observed.