Hydrogeologic Modelling to Assess Conditions Related to OPG's Proposed Deep Geologic Repository in Tiverton, Ontario
Sykes, Eric Alexander
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A Deep Geologic Repository (DGR) for Low and Intermediate Level (L&IL) 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 660 m below ground surface within the argillaceous Ordovician limestone of the Cobourg Formation. The objective of this thesis is to develop a regional-scale geologic conceptual model for the DGR site and to describe modelling using FRAC3DVS-OPG that provides a basis for the assembly and integration of site-specific geoscientific data. The numerical model is used to explain and illustrate the influence of conceptual model, parameter and scenario uncertainty on predicted long-term geosphere barrier performance. The modelling also provides a framework for hydrogeologic and geochemical investigations of the DGR, serves as a basis for exploring potential anthropogenic and natural perturbations to the DGR system, and demonstrates the long-term stability of the deep system. In the geologic framework of the Province of Ontario, the Bruce DGR is located west of the Algonquin Arch within the Bruce Mega-Block at the eastern edge of the Michigan Basin. Well logs have been used to define the structural contours at the regional and site scale of the up to 37 units that may be present above the Precambrian crystalline basement rock. The regional scale domain is restricted to a region extending from Lake Huron to Georgian Bay. While the selection of a larger domain might decrease the contribution of boundary condition uncertainty to any uncertainty in any site-scale performance measure, it significantly increases the contribution of the uncertainty in the spatial characterization to the uncertainty of the selected measure. From a hydrogeologic perspective, the domain can be subdivided into three horizons: a shallow zone characterized by the units of the Devonian; an intermediate zone comprised of the low permeability units of the Silurian and the shale units of the upper Ordovician; and a deep groundwater domain or zone characterized by units, such as the Cobourg formation, with stagnant water having high total dissolved solids concentrations that can exceed 200g/l. Hence, the conceptual model of the Bruce DGR site required the development of constitutive models that relate the fluid density and viscosity to the fluid total dissolved solids (TDS), temperature and pressure. The regional-scale hydrogeologic modelling will help demonstrate that at the proposed repository horizons, there are low energy gradients and that the combination of the low permeabilities and gradients will result in diffusional groundwater systems with favourable retardation properties.