Transport of Radionuclides in Smectite Clay

Smectite clay is used for sealing purposes in repositories for radioactive waste because of its excellent isolating properties. Typical examples are grouts injected into rock fractures for sealing the host rock, and blocks of highly compacted clay powder for plugging shafts and tunnels and for embedding metal containers with waste. The most important clay materials are bentonites, which usually have the smectite species montmorillonite as the major clay mineral. Such clays have a low hydraulic conductivity and a swelling potential that makes them fill up and seal off the space in which they are placed. They also have a significant cation exchange capacity and, at least for some ion species, a low diffusion transport capacity. The migration of radionuclides is of course of great importance for the performance of repositories for high-level radioactive waste, a matter that will be dealt with here. This chapter starts by describing the nature of smectitic clays and by giving examples of bentonite seals, and proceeds with explaining microstructural processes that control physical properties like hydraulic and gas conductivities and swelling ability. The chapter ends by outlining the basics of the longevity of smectite clay materials and by describing the performance of smectitic seals under repository conditions.