Ontario Power Generation (OPG) is proposing to build a Deep Geologic Respository (DGR) for Low and Intermediate Level Waste (L&ILW) near the existing Western Waste Management Facility at the Bruce site in the Municipality of Kincardine, Ontario. The Nuclear Waste Management Organization (NWMO), on behalf of OPG, is currently preparing an Environmental Impact Statement (EIS) and Preliminary Safety Report (PSR) for the proposed repository. This involves investigation of the site’s geological and surface environmental characteristics, conceptual design of the DGR, and technical studies to demonstrate the operational and long-term safety of the proposed facility. A preliminary postclosure safety assessment (SA) was undertaken in 2008 and 2009. Consistent with the guidelines for the preparation of the EIS for the DGR and the regulatory guide on assessing the long-term safety of radioactive waste management, the SA evaluated the DGR’s performance and its potential impact on human health and the environment through pathway analysis of contaminant releases, contaminant transport, receptor exposure and potential effects. Consideration was given to the expected long-term evolution of the repository and site following closure (the Normal Evolution Scenario) and four disruptive (“what if”) scenarios (Human Intrusion, Severe Shaft Seal Failure, Open Borehole, and Extreme Earthquake), which considered events with uncertain or low probability that could disrupt the repository system. Conceptual and mathematical models were developed and then implemented in a range of software tools including AMBER, to provide estimates of impacts such as dose, FRAC3DVS, for detailed 2D and 3D groundwater flow and transport calculations, and T2GGM, a code that couples the Gas Generation Model (GGM) and TOUGH2 and models the generation of gas in the repository and its subsequent 2D transport through the geosphere. Calculations have been undertaken to assess the impact of radionuclides on human and non-human biota and the impact of non-radioactive species on humans and the environment. The results indicate that the DGR system provides a high level of postclosure safety.