OPG's Deep Geologic Repository For Low And Intermediate Level Waste - Safety Assessment

Abstract

Ontario Power Generation (OPG) is proposing to build a Deep Geologic Repository (DGR) for Low and Intermediate Level Waste near the existing Western Waste Management Facility (WWMF) at the Bruce nuclear site in the Municipality of Kincardine, Ontario. The safety of the proposed DGR was assessed under both preclosure and postclosure conditions.

Under normal operation, there will be some release of tritium and carbon-14 as off-gassing from the waste packages, as currently occurs at WWMF, until underground rooms are closed off. The maximum dose to the public due to normal DGR operations is small, similar to the impacts that would be expected from WWMF for similar amounts of low and intermediate level waste.

Accidents during operations were assessed for potential release of radionuclides or non-radiological elements in the wastes. Although unlikely, credible accidents include fires involving packages, package breach accidents, failure of the ventilation system, and inadequate shielding. The assessment results indicate that effects on a member of the public at the Bruce nuclear site boundary are well below criteria. Overall, both WWMF experience as well as the DGR-specific analyses, indicate that the wastes can be handled and emplaced without undue risk to workers or the general public.

The postclosure safety was quantitatively assessed through considering a range of potential future scenarios. Specifically, the assessment considered the expected evolution of the DGR system and the site with time (i.e., the Normal Evolution Scenario) and the potential impacts of unlikely events leading to penetration of barriers and loss of containment (i.e., Disruptive Scenarios or "What If" Scenarios). The safety assessment was conducted in a systematic manner, consistent with Canadian Nuclear Safety Commission Regulatory Guidance G-320 and with international best practice.

The postclosure assessment calculations for the Normal Evolution Scenario indicate that the DGR system provides effective containment of the emplaced waste. Most radionuclides decay within the repository or the deep geosphere. The amount of contaminants eventually reaching the surface is very small, such that calculated impacts are orders of magnitude less than the public and environmental criteria. The isolation afforded by the location and design of the DGR limits the disruptive events potentially able to bypass the natural barriers to a small number of very unlikely situations. Even if these events were to occur, the analysis shows that the contaminants in the waste would continue to be contained effectively by the DGR system such that the risk criterion is met.