Bisulphide transport through highly compacted bentonite clay: Model verification, validation, and sensitivity study

ABSTRACT: The Canadian nuclear waste management organization (NWMO) has proposed a deep geological repository (DGR) concept as a long-term solution for disposal of Canada's used nuclear fuel. The DGR is designed to isolate radionuclides by using engineered barrier system (EBS) which includes copper coated used fuel container (UFC) surrounded by highly compacted bentonite (HCB). EBS will be emplaced 500 m deep in a low permeability host rock (either crystalline or sedimentary). Although the copper coating is thermodynamically stable in oxygen-free environments, it is susceptible to microbiologically influenced corrosion (MIC) during anoxic conditions. Depending on site-specific conditions (e.g., host rock type, groundwater chemistry, microbial growth conditions), bisulphide (HS-) produced by sulphate reducing bacteria near the rock-bentonite interface could diffuse slowly through HCB to the UFC surface and corrode the copper coating. This transport is affected by the transient conditions in the DGR, such as saturation, temperature, microbial and geochemical conditions which are all interconnected and, therefore, requires a robust numerical model to understand possible transport mechanisms. Towards this goal, a variably saturated, non-isothermal model has been developed in COMSOL Multiphysics to examine bisulphide transport through the HCB under a range of conditions anticipated in Canada's DGR. Two areas with different geospheres have remained in NWMO siting program, therefore, two possible rock types have been considered in a sensitivity study (crystalline or sedimentary). This study showed how the saturation and transport behaviour varied between the different rock types due to differences in permeability and groundwater characteristics. These findings support the ongoing efforts in planning for Canada's DGR as they provide valuable insight into the expected EBS performance under a wide range of conditions.