Detailed Modelling for the Postclosure Safety Assessment of OPG’s DGR.

As part of the postclosure safety assessment undertaken for the proposed Deep Geologic Repository (DGR) for Low and Intermediate Level Waste (L&ILW) at the Bruce nuclear site, calculations were undertaken to evaluate the repository’s potential postclosure impacts. Impacts were evaluated for a Normal Evolution Scenario, describing the expected long term evolution of the repository and site following closure, and for several Disruptive Scenarios, which consider events that could lead to possible penetration of barriers, abnormal degradation, and/or loss of containment. The postclosure modelling was conducted using both detailed models with three-dimensional representation of the repository geometry, and with an overall (system) assessment-level model. The purpose of the detailed modelling described in this paper was to evaluate postclosure performance in terms of repository pressures, repository resaturation levels, mass flow rates of gas at various levels in the shaft, groundwater flow and radionuclide transport through the saturated geosphere and shaft seals, and capture rates of radionuclides by a hypothetical water supply well. The results of the detailed modelling were used to inform overall assessment-level (system) modelling that was performed using the compartmental modelling code AMBER and which is described in a companion paper. Two separate detailed modelling studies were undertaken: 1) generation of gas in the repository, repository resaturation, and transport of gas through the geosphere and shaft sealing system was simulated using T2GGM, a modified version of the TOUGH2 gas transport code with coupled gas generation, and 2) transport of groundwater and radionuclides through the saturated geosphere and shaft seals was simulated with FRAC3DVS-OPG. The gas generation model (GGM) incorporated within T2GGM was developed expressly for the L&ILW waste that will be present at the DGR. GGM calculates generation and consumption of oxygen, hydrogen, carbon dioxide, methane, hydrogen sulphide and nitrogen from degradation of the various organic and metallic waste streams. Results from the Normal Evolution Scenario groundwater model show that radionuclide transport is diffusion dominated and very slow, with virtually no transport beyond the immediate vicinity of the repository. Results from gas modelling indicate that the repository will take hundreds of thousands of years to resaturate, and that there will be no gas flow within the shaft for the Reference Case and most Normal Evolution sensitivity cases. In the few cases where gas flow in the shaft does occur, it is restricted to lower portions of the shaft. No gas enters the shallow groundwater system. The gas modelling also indicates that in most cases, repository pressures will equilibrate near the expected steady-state in-situ pressure. In no cases do the gas pressures exceed the lithostatic pressure.