Sealing materials for a deep geological repository: evaluation of swelling pressure and hydraulic conductivity data for bentonite-based sealing materials proposed for use in placement rooms

Several countries are planning to safely contain and isolate used nuclear fuel in a deep geologic repository. Similar to other countries, Canada's Nuclear Waste Management Organization (NWMO) proposes to place used fuel in corrosion-resistant containers deep underground as part of a multiple barrier approach. Key components of the sealing systems are bentonite-based and must maintain low hydraulic conductivity and an ability to swell when in contact with free groundwater. Swelling pressure ( Ps) and hydraulic conductivity ( k) data for the specific bentonite product known as MX80 bentonite under low (<15 g/L) through high (>325 g/L) total dissolved solids’ (TDS) pore fluids that simulate groundwaters that could be encountered in sedimentary and crystalline rock geospheres are reviewed, and information from ongoing NWMO studies of behaviour is presented. These data are statistically evaluated to establish the best-fit and prediction limit relationships between compacted dry density and Ps and k as they are influenced by porefluid composition. They also allow identification of dry density and(or) TDS (salinity) conditions where Ps and k may not meet defined performance. From these analyses, it is expected that bentonite can be placed such that on achieving fluid saturation and density equilibration, it meets the requirements set for placement room fill.