Seismic fragility analysis of seismically isolated nuclear power plants piping system

• The critical points of a seismically isolated NPP piping system are identified.
• The simulation results are validated through a monotonic and cyclic test of the critical points.
• The conditional mean spectrum method is used to scale the selected records.
• The fragility curves of the NPP piping system are estimated.
• Computation of the fragility parameters is addressed.

Nuclear power plants are high risk facilities due to the possibility of sudden seismic events, because any possible failure could initiate catastrophic radioactive contamination. The seismic fragility analysis of NPPs and related equipments (such as piping systems) is a proven method to determine their performance against any possible earthquake. In this study the Brookhaven National laboratory benchmark model of a piping system was considered for the fragility analysis. A tensile test was conducted to define the material properties. An initial seismic analysis of the piping system is performed to indicate the critical sections of the piping system. Numerical analysis was validated through a monotonic and cyclic loading experiment of two identified critical points of the piping system. The tests were conducted at the Korea Construction Engineering Development (KOCED) Seismic Simulation Test Center, Pusan National University, Korea. Fragility curves were expressed for critical points of the system as a function of the spectral acceleration of the records and the maximum relative displacement. The standard deviation of the response and capacity were calculated using mathematical formulas, assuming that those follow a log-normal distribution. We determined that the fragility curve of a pipe elbow must be derived for both the opening and closing mode, regarding the difference between the capacities of the elbow on those modes. The high confidence of low probability of failure for the considered fragility functions in a straight section in any direction is comparatively greater than the corresponding elbow section.