Previous Canadian safety assessments have focussed on radionuclide transport from a deep geologic repository under steady climate and groundwater conditions The purpose of this study was to address potential impacts associated with glacial cycles. The effects of glaciation were addressed using a three-dimensional (3-D) numerical model of a hypothetical repository in the Canadian Shield. This study investigated: (1) the impact on the hydrogeological system of a representative glacial cycle, consisting of temperate (present-day climate), permafrost, ice cover and proglacial lake periods, and, (2) the impact of multiple glacial cycles on the long-term transport of radionuclides from a hypothetical defective container. The model included a representative set of fractures across the 13 km x 20 km model domain.
Transient groundwater flow results show that hydrogeological conditions are profoundly changed during climate cycles, with median groundwater velocities within the repository footprint varying over two orders of magnitude. Glaciation induced pressure gradients are rapidly propagated horizontally and vertically through the highly connected high permeability fracture system and through hydromechanical coupling using a simplified one-dimensional strain hydromechanical model. Although the glacial cycles do modify the flow field extensively, the cumulative impact on radionuclide transport of repeated cycles of advance and retreat tend to effectively “cancel out”, leading to a general plume structure that is not substantially different from the steady-state flow and transport model. Glacially induced overpressures can persist for thousands of years after the glacier itself has retreated, and this effect impacts transport to the surface.
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