STR-1051-1 3 rd International Structural Specialty Conference 3 ième conférence internationale spécialisée sur le génie des structures Edmonton, Alberta June 6-9, 2012 / 6 au 9 juin 2012 Beyond Design Basis Seismic Evaluation of Nuclear Power Plants at Bruce Site Amitabh Dar 1 , James D. Hanna 2 1 Technical Advisor, Bruce Power, 123 Front Street, Toronto, ON, Canada, M5J 2M2 Email: dara@mcmaster.ca 2 Senior Technical Specialist, Bruce Power, Tiverton, ON, Canada, N0G 2T0 E-mail: james.hanna@brucepower.com Abstract: Occurrences of earthquakes higher than the design basis at Nuclear Power Plants (NPPs) in the recent past have created a requirement for beyond design basis evaluations of the NPPs in Canada. The Bruce Site in Canada having two NPPs known as Bruce A and Bruce B, falls under the East North American zone (closer to the Atlantic Ocean in comparison to the Pacific) and hence is considered as an east coast site in the rest of the paper. The foundations of both the NPPs are supported by rock. Bruce A was constructed before Bruce B with limited seismic analysis whereas the Bruce B station was designed for the Design Basis Earthquake (DBE) based on the west coast earthquake records for soil sites. Bruce A was considered as a plant with no DBE and its seismic capacity was assessed in accordance with the Seismic Margin Assessment (SMA) methodology outlined in the Electric Power Research Institute report EPRI- NP-6041 (1991) for a seismic event known as the Review Level Earthquake (RLE), considered more severe than the DBE and very similar to the credible event represented by the East North American (ENA) spectrum for rock sites. The DBE contains inherent conservatism over the ENA spectrum over the frequency range of interest. This paper compares the SMA of Bruce A with the original design basis of Bruce B and explores the seismic margin of Bruce B over and above the credible event represented by the ENA spectrum on the basis of Bruce A evaluation. It also demonstrates the expected extra capacity of Bruce A beyond RLE based on the ductility factors outlined in EPRI-NP-6041 but practically not utilized in the SMA of Bruce A. 1. Introduction Some of the Nuclear Power Plants (NPPs) in eastern Canada were constructed with the DBE spectra based on the west coast spectrum given by Newmark, Blume and Kapur (1973) known as NBK spectrum whereas some others were constructed more than 30 years ago with very limited seismic design requirements without any DBE. In the recent past, some NPPs outside Canada have been struck by earthquakes having magnitudes much higher than the design basis creating a requirement to assess the seismic margin of all plants over and above the DBE. This calls for the seismic margin evaluation of the existing plants at Bruce site over and above the design basis. The generic response spectrum given in CSA N289.3 (1981 and 2010) is similar to the NBK spectrum, derived from the west coast earthquake records for soil sites and scaled down for the east coast. According to Atkinson and Elgohary (2007) the CSA N289.3 spectrum is rich in frequency content over low frequencies but lacks conservatism over the high frequency range. According to Boore and Atkinson (1992), many moderate earthquakes including the Saguenay Earthquake (1988) as a classic example in eastern Canada were found to be richer than the