Generation of In-Structure Response Spectra for Nuclear Power Plants Subjected to High-Frequency Ground Motion

The purpose of seismic qualification of Structures, Systems and Components (SSCs) in nuclear power plants is to ensure that their intended safety function will not be compromised during and after a postulated earthquake event. The seismic performance of the equipment is generally evaluated using In-Structure Response Spectra (ISRS) at equipment-support locations as an input motion. Traditionally, these ISRS are generated based on design ground spectra prescribed by either U.S. Nuclear Regulatory Commission Regulatory Guide 1.60 or other design spectral shapes, which normally consider the frequencies content up to 33Hz. However, it has been recently recognized that probabilistic hazard-based site specific ground motion response spectra (GMRS) for Central and Eastern United States (CEUS) hard rock sites contains significant energy in the high frequency range, far beyond 33Hz. Since the motion at equipment support locations is highly affected by the dynamic characteristics of the soil or rock surrounding the building foundations and those of the structure itself, the adequacy of dynamic modeling and analysis techniques for determining the ISRS is critical to seismic qualification of safety-related equipment.This paper provides examples on dynamic modeling and analysis techniques required to accurately capture the structural responses for purposes of calculating ISRS throughout the frequency range of interest, including the high frequency responses typically expected at the CEUS sites. The discussion includes the selection of finite element mesh size, and sensitivity analysis performed to demonstrate that the propagation of these high frequencies through the different levels of the structure is properly captured. Other analytical considerations, such as the selection of time step size, for conducting time-history analysis, are also presented.Copyright © 2012 by ASME