Modelling of the fifth case study glaciation and erosion scenarios

This technical report presents the results of detailed numeric modelling of the effects of glaciation on a hypothetical deep geologic repository (DGR) for used fuels situated in a sedimentary formation. The Nuclear Waste Management Organization (NWMO) has undertaken post-closure safety assessments of hypothetical DGRs in both crystalline and sedimentary rock formations (NWMO 2012; NWMO 2013). The safety assessment for a DGR in sedimentary formations documented the behaviour of reference and sensitivity cases associated with the normal evolution of the repository system under constant climate conditions, with a qualitative discussion of the potential effects of glaciation on calculated impacts. However, a complete safety assessment requires a quantitative assessment of the impact of glaciation events. Two scenarios are considered in this report: (1) a glaciation only scenario, and (2) an erosion scenario, which accounts for erosion occurring during glaciation events. The base glaciation scenario first describes the evolution of the groundwater flow regime in a three-dimensional domain at the sub-regional scale, extending several tens of kilometres around the repository location, for eight glacial cycles over a one million year (1 Ma) period. A performance metric is developed describing the time required for radionuclides to be transported from the repository location by advective-dispersive and diffusive processes over multiple glacial cycles with a transient groundwater flow regime. Multiple sensitivity cases are conducted to quantify the effect of variations in geosphere parameters, assumed model initial and boundary conditions, and transport process. Additional two-dimensional simulations assess the impact of possible methane presence as a gas phase within the low-permeability host formations. Subsequently, a 3D transport model is developed encompassing a limited local domain immediately surrounding the repository. Model boundary conditions are extracted directly from the 3D transient flow model results. The transport model discretization and property assignment includes all repository features such tunnels, shafts, placement rooms and engineered barrier system (EBS) components as well as excavation damaged zones (EDZ) surrounding all mined features. A defective container provides a single radionuclide source (the long-lived and radiotoxic isotope ¹²⁹Iodine, or I-129) and a water-supply well is the point of biosphere impact. Performance metrics are activity transport rates through geologic formations between the repository and the water supply aquifer and uptake at the water supply well. Simulations are conducted for several source and well location combinations and for a limited number of geosphere and numeric parameter sensitivity cases, including a disruptive event case describing shaft-seal failure. The erosion scenario extends the base glaciation scenario to incorporate removal of surface geologic material during each glacial cycle. A total of 100m of overburden and bedrock are removed over the 1 Ma performance period in two different manners: 1) uniform erosion (UE), where an equal amount of material is removed over the entire surface of the domain, and 2) valley erosion (VE), where glaciers incise a 100m deep and 15 km wide valley located directly over the repository. Results from the sub-regional scale groundwater flow and the local scale transport models were compared to the glaciation only case. In all cases, the transport behaviour of the low-permeability sedimentary geosphere proved extremely robust, with virtually no changes in performance metrics for glaciation or erosion cases when compared to constant climate cases with steady-state groundwater flow. (author)