Safety philosophy at Ontario Power Generation used fuel dry storage facilities

Full text: Ontario Power Generation (OPG) operates the Pickering Waste Management Facility (PWMF) and Western Waste Management Facility (WWMF) where OPG has been storing 10-year or older used fuel in Dry Storage Containers (DSCs) since 1996 and 2003 respectively. OPG currently has 5300 tonnes of spent fuel in dry storage. A construction licence for a third facility, the Darlington Waste Management Facility (DWMF), was obtained in August 2004 and the facility is expected to start operations in late 2007. The used fuel dry storage safety philosophy embodies the defence-in-depth approach to keep radionuclide emissions and gamma radiation dose rates within the regulatory limits and As Low As Reasonable Achievable (ALARA). The defense-in-depth approach is accomplished with multiple barriers between the used fuel and the public during each stage of the used fuel dry storage process. Each barrier independently provides a measure of safety toward preventing the release of radioactive materials as well as effective neutron and gamma shielding: - the uranium dioxide matrix effectively contains the radionuclides present in the at-least ten- year-cooled used fuel (either under wet or dry storage conditions), except for the free fractional inventory of tritium (in vapour form) and krypton-85 (which is a gas); - the fuel cladding additionally contains the free fractional inventory of tritium and krypton- 85 that would otherwise be available for release; - a further layer of defence is provided during loading the DSCs by the water in the Irradiated Fuel Bay (with the exception of noble gases, the bay water traps fission products that may be released from the fuel cladding) and also provides neutron and gamma shielding; - the DSC Transfer Clamp with the elastomeric seal are designed to retain a slightly sub-atmospheric pressure in the DSC cavity prior to on-site transfer; - the DSCs are designed to prevent or reduce to an acceptably low level the escape of fission products in the unlikely event a fuel failure occurs under normal or accident conditions and to protect the fuel from extensive damage from missiles or falling objects during abnormal or accident conditions; - the reinforced concrete and steel construction of the DSCs provides neutron and gamma shielding throughout the used fuel dry storage process; - the processing and storage buildings also provide an additional barrier to further reduce gamma dose rates to the public during processing and storage of the DSCs. The focus is on the prevention of fuel damage during the entire used fuel dry storage operation, to ensure that fission products remain contained within the fuel elements. If an abnormal event or accident occurs, then the used fuel dry storage process provides sufficient mitigation and accommodation measures to ensure sufficient barriers remain intact so the radioactive releases remain below the Canadian Nuclear Safety Commission (CNSC) regulatory requirements. Safety assessment of each used fuel dry storage facility and the associated systems and operations is required to support each request for regulatory approval to construct and operate the facility. The objective of the safety assessment is to assess the dose consequences to the public and the workers under normal operation and postulated credible accident scenarios and to confirm that dose rates are below the regulatory dose limits for the workers and members of the public. When assessing malfunctions and accidents, postulated external and internal events are considered. Consideration is also given to the design basis accidents of the specific nuclear generating station that could affect the used fuel dry storage operations as the waste sites are in close proximity to the nuclear operating stations. The probability of occurrence for each initiating event considered is calculated and for those events deemed credible (i.e. frequency > 10-7 events per year), a bounding fuel failure consequence is predicted. Given the chemical characteristics of the radionuclides in used fuel, the design of the CANDU fuel and t he conditions inside the DSC, then in the event that a used fuel bundle should become damaged during used fuel dry storage operations, the only significant radionuclide species that are volatile are krypton-85 and tritium. Release of these radionuclides is considered in calculating public and worker doses. For the DWMF safety assessment, the worst case scenario assumes 30% fuel failure within a DSC as well as failure of the DSC transfer clamp seal allowing the release of 30% of the Krypton-85 and tritium available. The consequences to the public from this release were concluded to be less than 0.2% of the regulatory limit. (author)