The work targeted some of the key issues and processes associated with bentonite that are likely to be of greatest importance for the selection of an appropriate disposal concept and the development of the safety case for a Geological Disposal Facility (GDF). The following themes were included: temperature dependence of bentonite performance; effect of radiation on bentonite performance; effect of bentonite on porewater evolution, and subsequent effects on glass evolution, wasteforms and canister corrosion; effects of groundwater and waste compositions on bentonite stability and swelling; effect of canister corrosion products on bentonite; bentonite erosion and piping; and the influence of bentonite composition on performance. The report 'Bentonite: a review of key properties, processes and issues for consideration in the UK context', which can be downloaded from the NDA bibliography, makes the following key points:
- The evolution of bentonite under the conditions expected in a Geological Disposal Facility (GDF) is complex, and the processes affecting it are coupled, perhaps strongly.
- Most aspects of bentonite performance are likely to be specific to both the site (both groundwater flow rate and composition) and the particular bentonite that will be used.
- The use of bentonite imposes constraints on a number of aspects of GDF design, such as maximum allowable temperatures and the choice of other barrier materials.
- Issues associated with the performance of bentonite are of international concern, and, as a result, a great deal of work has been carried out and a significant knowledge base exists, which the UK can draw on.
Simplified ‘ball and stick’ model of an idealised smectite crystal structure