Seismic Fragility Evaluation of Interface Pipes in Seismically Isolated NPPs by Using Scale Model Test

Seismic isolation system can be an effective alternative to protect the NPPs (Nuclear Power Plants) against to the strong seismic events. Therefore, some research activities to adopt the seismic isolation concept to the design of the next generation NPPs have been progressed for last few years in Korea. Nuclear structures, secondary systems and components must remain undamaged during and after the SSE (Safe Shutdown Earthquake) event. The seismic events will cause the high seismic response in the stiff structural systems and extremely high demands of deformation on the safety-related secondary systems like piping components. If seismic isolation devices are installed in nuclear power plant for seismic stability, safety against seismic load of power plant may be improved. But in some equipment, seismic risk may increase because displacement may become greater than before installation of seismic isolation device. Therefore, it is necessary to select the equipment in which seismic risk increases due to increase in displacement by the installation of seismic isolation device, and perform a research on seismic performance evaluation of equipment. In this study, one of the typical Korean NPPs assuming the application of seismic isolation devices, and one of the interface piping systems which introduced this NPP was used for seismic analysis. The numerical models include representations of seismic isolation devices. In order to validation of numerical piping system model and defining failure mode & limit states, quasi-static loading tests were conducted on the scale-modeled piping components before the analysis procedures. The fragility analysis was performed by using results of inelastic seismic response analysis. Inelastic seismic response analysis was carried out by using shell finite element model of piping system considering internal pressure. The implicit method was used for the direct integration time history analysis. Generally, PGA (Peak Ground Acceleration) was used for seismic intensity of fragility curve. However, in the case of the displacement sensitive system, lateral displacement could be an useful alternative measure for estimation of probability of failure. Thus in this paper, fragility curves were plotted based on maximum relative displacement.