Seismic Fragility Analysis for Structures, Systems, and Components in Nuclear Power Plants

Abstract

Seismic fragility analysis has been widely used to evaluate seismic capacities of structures, systems, and components (SSCs) in nuclear power plants. In the seismic fragility analysis, a single ground motion parameter (GMP),such as peak ground acceleration (PGA), is chosen to characterize the Review Level Earthquake (RLE) and represent the seismic capacity of an SSC.However, due to the use of a single GMP, problems have been observed in engineering practice.

It is well known, from elastic structural dynamic analyses, structural responses under earthquake excitations depend primarily on spectral accelerations at its dominant natural frequencies. Choosing spectral accelerations at structural dominant natural frequencies as vector-valued GMPs (VGMPs) can more accurately characterize the input RLE and more precisely predict structural responses. The purpose of this study is to develop weighting seismic fragility analysis method that overcomes the problems in current seismic fragility analysis method. The proposed method mainly includes that 1. vector-valued probabilistic seismic hazard analysis (VPSHA) is performed to determine the weights of input ground response spectra (GRS); 2. seismic fragility analysis considering VGMPs method is proposed to calculate seismic fragility based on VGMPs; 3. weights of input GRS and seismic fragility are combined to obtain the weighting seismic fragility of an SSC. By using VGMPs, the proposed method resolves the problems in current seismic fragility analysis, thus it can obtain more accurate seismic capacities of safety-related SSCs. In addition, weighting seismic fragility curves and High Confidence and Low Probability of Failure (HCLPF) seismic capacities are represented by a single GMP such as PGA, hence they are readily incorporated into Seismic Probabilistic Risk Analysis and Seismic Margin Assessment (SMA).

Based on weighting seismic fragility analysis method, an improved SMA procedure is proposed. The procedure combines the use of weighting and current seismic fragility analysis methods, i.e., 1. weighting seismic fragility analysis is performed to determine HCLPF seismic capacities of “weak link” SSCs, and 2. current seismic fragility analysis is conducted to calculate HCLPF seismic capacities of less important SSCs. This ensures that more accurate plant seismic capacity is obtained, while computational cost is acceptable. The proposed SMA procedure can save redesign cost of “weak link” SSCs.

The proposed weighting seismic fragility analysis method is accurate and applicable, providing more accurate seismic capacity estimates of safety-related SSCs, thus saving redesign cost of “weak link” SSCs that do not satisfy seismic margin requirement.