Very dense smectite clay surrounding canisters with high-level radioactive waste effectively stops migration of radionuclides due to flowing porewater, however, does not stop their movement by diffusion. The paper explains how the diffusion rate of released waste ions depends on the microstructural constitution referring to the mechanisms in diffusive transport and the sorption by the clay matrix of varying density. In general, diffusion refers to the transport of elements by action of random motions and works to eliminate distinct discontinuities in concentration. For smectite clay this process is complex since interlamellar diffusion is a 2-dimensional process on the microscale and takes place in diffuse electrical double-layers on the basal planes of the clay crystallites. This is in contrast with pore diffusion which is a 3-dimensional process. By making use of microstructural parameters one can distinguish the different contributions to bulk diffusion migration and create a basis for theoretical modelling of diffusive ion migration.
Keywords: Smectite clay, canisters, diffusion, montmorillonite, ion migration, clay microstructure.
ISSN: 1792-9040 (Print)