Backfilling and sealing materials for a Deep Geological Repository

NWMO is considering use of densely compacted bentonite clay and clay-sand blends in the sealing systems proposed for both sedimentary and crystalline host media for Deep Geological Repository (DGR) concepts. As part of developing a safety case, reliable material performance parameters need to be available for backfilling and/or sealing components. Development of sealing concepts and safety assessment models require many system inputs (e.g. material composition, as-placed density, degree of water saturation and groundwater composition). Knowing how each will affect system performance and how they will change with time is also critical. While some parameters can be estimated from existing data, it is important to identify what degree of variability observed in these data can be attributed to materials used and what is due to laboratory technique(s). Ultimately, linkage of laboratory measurements to actual field performance requires that uncertainties be reduced so as to separate system performance issues from natural variability. Golder Associates Ltd. and the Royal Military College of Canada (RMC) have been investigating the behaviour of three bentonite-based materials (70-30 bentonite-sand mixture and two 100% bentonite materials with dry densities of >1.7, >1.4 and >1.7 Mg/m³ respectively), through the conduct of specialized laboratory tests. All test specimens are constructed from identical bentonite and sand source materials. With source material variation controlled, test replication provides a measure of what scatter in parameter values might be reasonably expected. From this it should be possible to develop behavioural parameters for use in modelling system behaviour that are both representative and conservative. Parameters of particular interest and importance to the sealing materials include mineralogical composition, swelling behaviour, swelling pressure developed under confinement, hydraulic conductivity, gas permeability, water retention and deformation parameters. These measurements require use of very specialized testing equipment and are made more challenging by the groundwater compositions being used in these tests (0-335 g/L TDS). (author)