Eastern Section Meeting

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Hydrogeologic Modelling of the Bruce DGR site

Abstract

A Deep Geologic Repository (DGR) for low and intermediate level radioactive waste has been proposed by Ontario Power Generation for the Bruce nuclear site in Ontario, Canada. The DGR is to be constructed at a depth of about 680 m below ground surface within the argillaceous Ordovician limestone of the Cobourg Formation. The analyses provide a framework for the assembly and integration of the site-specific geoscientific data and explain the factors that influence the predicted long-term performance of the geosphere barrier. The density-dependent flow and transport model, FRAC3DVS-OPG, and the two-phase gas and water flow model, TOUGH2-MP, were used. The regional-scale domain encompasses an 18,500 km2 region extending from Lake Huron to Georgian Bay. The analyses also included a site-scale numerical model, with a surface area of approximately 400 km2, an approximately east to west cross-sectional model of the Michigan Basin, and one-dimensional models for two-phase modelling. Pressure data from the DGR site investigation boreholes indicate that the Cambrian sandstone and the Niagaran Group in the Silurian are over-pressured relative to density corrected hydrostatic levels. The Ordovician sediments are significantly under-pressured. A requirement of both abnormal overpressures and underpressures is low hydraulic conductivity in either the formation in which the abnormal pressures are observed or in the overlying and underlying formations. The overpressure in the Cambrian was described in the numerical modelling study by differences in fluid density across the Michigan Basin and in surface topography. The most likely cause of the under-pressures in the Ordovician sediments is the presence of a discontinuous gas phase in the rock. The TOUGH2-MP analyses support this conclusion. Paleohydrogeologic analyses that included mechanical loading and honoured field derived parameters could not describe the underpressures. The low advective velocities in the Cobourg and other Ordovician units, estimated in the numerical modelling, result in solute transport that is diffusion dominant with Peclet numbers less than 0.003 for a characteristic length of unity. Solute transport in the Ordovician limestone and shale was diffusion dominant in all simulations. With a mean life expectancy for the DGR estimated to be in excess of 150 Ma, the Bruce nuclear site has the necessary attributes for the proposed DGR for Canada's low and intermediate level radioactive waste.