Assessment of effects of structural response on plant risk. Preliminary results for Peach Bottom Atomic Power Station

The US Nuclear Regulatory Commission has been sponsoring tests at Los Alamos National Laboratory on the dynamic response of Seismic Category I reinforced concrete shear wall structures. As test results accumulated, it became clear that there was a significant difference between as-calculated and measured shear wall stiffnesses and frequencies, and that these differences existed both in static and dynamic tests. For very low level tests, measured frequencies were found to range between 50% and 80% of the computed values. During simulated earthquake tests, measured frequencies were found to further decrease as the earthquake level increased. The observed differences between calculated and measured stiffnesses and frequencies represents a potentially important issue relative to the seismic design and safety of nuclear power plants. Thus, this frequency difference issue has potentially important implications with respect to the safety of power plants during seismic events. In order to assess the importance of this frequency difference issue the US Nuclear Regulatory Commission has funded Sandia National Laboratories to re-evaluate several existing seismic PRAs by modeling and incorporating the effects of the frequency reductions. This report presents the results for the initial re-evaluation of the seismic risk at the Peach Bottom Atomic Power Station