The disposal of Canada's nuclear fuel waste: the vault model for postclosure assessment

The concept for disposal of Canada's nuclear fuel waste involves emplacing the waste in a vault excavated at a depth of 500 to 1000 m in plutonic rock of thc Canadian Shield. The solid waste would be isolated from the biosphere by a multibarrier system consisting of engineered barriers, including corrosion-resistant containers and clay- and cement-based sealing materials, and the natural barrier provided by the massive geological formation. The technical feasibility of this concept, and its impact on the environment and human health, is being documented in an Environmental Impact Statement (EIS) that will be submitted for review under the federal Environmental Assessment and Review Process. The present report is one of nine EIS primary references. The report describes the vault model, which is used to calculate the time-dependent release of radioactive and non-radioactive contaminants from the engineered barrier system (vault) into the surrounding rock (geosphere). The model calculations presented are for a specific reference vault design that comprises used CANDU (CANada Deuterium Uranium) fuel bundles in Grade-2 titanium containers, which would be emplaced in boreholes in the floor of a mined excavation located at a depth of 500 m in plutonic rock. The containers would be surrounded by a compacted buffer material that is a mixture of 50 wt. % sand and 50 wt. % bentonite. Disposal rooms and tunnels would be sealed with a layer of backfill mixture composed of 25 % glacial lake clay and 75% crushed granite and an overlying layer of buffer material. The vault model is a computer code that calculates the failure times of titanium containers, the rate of release of radionuclides from used-fuel bundles into the groundwater that would flow into the failed containers, and the rate of transport of radionuclides from the fuel through the groundwater-saturated buffer and backfill materials and into the surrounding rock. The vault model uses distributed or probabilistic parameter values to account for both uncertainty and variability. In describing the model, particular emphasis is placed on the mechanistic understanding of the processes leading to radionuclide release and on explaining the basis for selection of the parameter values used in the model. The reliability of the vault model is discussed in terms of verification through independent calculations; the evaluation of research data and underlying models; the use of conservative assumptions, code intercomparisons and quality assurance; and the proposed 10 000-a time scale for quantitative risk assessment. For those situations where it was considered inappropriate to specifically include certain processes in the vault model (e.g., nuclear criticality, or gas generation), the justification for excluding the process and the results of supplementary analyses are provided. The information presented on the vault model indicates that it is a conservative model that is appropriate for describing the long-term performance of engineered barriers in a nuclear fuel waste disposal vault. The Canadian Nuclear Fuel Waste Management Program is funded jointly by AECL and Ontario Hydro under the auspices of the CANDU Owners Group. (author)