Impact Modelling for the Postclosure Safety Assessment of OPG's DGR

Abstract

As part of the safety assessment for the proposed DGR, calculations were undertaken to evaluate the repository's potential post closure impacts. Impacts were evaluated for a Normal Evolution Scenario, describing the expected long-term evolution of the repository and site following closure, and four Disruptive Scenarios, which consider events that could lead to possible loss of containment.

An assessment-level (system) model was implemented in AMBER, a compartment modelling code, that represents radioactive decay, waste package degradation, potential contaminant transport through the repository, sealed shafts, geosphere and surface environment, and the associated impacts. The model used input from detailed models implemented in the FRAC3DVS-OPG and T2GGM codes for the repository saturation, gas generation, and groundwater and gas flow processes.

Both safety and performance indicators were calculated to assess the potential impact of the DGR. Safety indicators include radiation dose to humans and environmental concentrations of radionuclides and non-radioactive hazardous substances. Performance indicators include contaminant amounts within various spatial domains (e.g., the repository, the host rock, and the wider geosphere) and fluxes of contaminants at various points in the DGR system.

The long timescales under consideration mean that there are uncertainties about the way the DGR system will evolve. In addition to assessing alternative future evolutions through different scenarios, uncertainties were addressed through the adoption of conservative assumptions, the evaluation of variant deterministic cases within each scenario, and probabilistic calculations.

The results for the Normal Evolution Scenario indicate that the DGR system provides effective containment of the emplaced contaminants. Most radionuclides decay within the repository or the deep geosphere. The amount of contaminants reaching the surface is very small, such that the maximum calculated dose for the Normal Evolution Scenario is more than five orders of magnitude below the public dose criterion of 0.3 mSv/a for all calculation cases. In addition, maximum calculated concentrations in the biosphere are well below the criteria for protection of biota from radionuclides, and for protection of humans and biota from non-radioactive contaminants. The isolation afforded by the location and design of the DGR limits the likelihood of disruptive events potentially able to bypass the natural and engineered barriers to a small number of situations with very low probability. 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 associated risk criterion is met.