Longevity of bentonite as a nuclear waste barrier depends on mineralogical stability of the smectite component of the clay as well as on its ability to perform as a buffer in the repository for long periods of time. Processes affecting the stability of smectites under the geochemical and thermal conditions in a high-level nuclear waste repository can be construed by examining the geological analogues and the results of the hydrothermal experiments. The physical and chemical properties of bentonite seem to be related to the nature of smectites in them. Smectites represent a range of expandable layer silicates of Al, Mg, Fe and other transition metals. Their thermal, hydrothermal, or solution stabilities vary from one smectite species to another. This is also true to a certain extent for the physical properties of bentonites, i.e. swelling, permeability, plasticity and adsorption.
Swelling ability and permeability of naturally altered (metabentonites) and steam-treated bentonites are badly damaged. Temperature distribution in a clay barrier that was simulated in situ under the repository conditions showed a sharp drop from 175°C at the clay/canister contact to 81°C at a distance 65 mm away. Microstructures of bentonites were noticeably altered after hydrothermal treatment at 200cC.