Effect of nonlinear structural response on seismic fragility and risk of emergency power supply and distribution system in a nuclear power plant
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Nonlinear building responses were observed in some nuclear power plants (NPPs) under beyond-design level earthquakes. Seismic risk assessments on NPPs reported cases where the dominant risk-contributing ground acceleration range is around two times the extreme design earthquake level. In this range, the NPP structures can hardly maintain linear or equivalent linear. However, the widely adopted separation of variables method for computing equipment fragility does not take into account the impact of structural nonlinearity accurately. Using the emergency power supply and distribution system in a NPP as an example, this study investigated the effect of nonlinear structural response on the seismic fragilities of various equipment components as well as on the seismic risk of a critical system. A new method for fragility analysis is employed, where seismic demand is obtained from probabilistic nonlinear response analyses at multiple ground motion intensity levels. As the intensity increases, the structural damage propagates and alters the amplitude and frequency content of the floor response. Both testbased and analysis-based equipment fragility evaluations are demonstrated, with Latin hypercube sampling of the random variables. Considering structural nonlinearity does affect the equipment fragilities and consequently the system risk. Vertical floor excitation is found out to control the test-based fragilities.
Abstract
Nonlinear building responses were observed in some nuclear power plants (NPPs) under beyond-design level earthquakes. Seismic risk assessments on NPPs reported cases where the dominant risk-contributing ground acceleration range is around two times the extreme design earthquake level. In this range, the NPP structures can hardly maintain linear or equivalent linear. However, the widely adopted separation of variables method for computing equipment fragility does not take into account the impact of structural nonlinearity accurately. Using the emergency power supply and distribution system in a NPP as an example, this study investigated the effect of nonlinear structural response on the seismic fragilities of various equipment components as well as on the seismic risk of a critical system. A new method for fragility analysis is employed, where seismic demand is obtained from probabilistic nonlinear response analyses at multiple ground motion intensity levels. As the intensity increases, the structural damage propagates and alters the amplitude and frequency content of the floor response. Both testbased and analysis-based equipment fragility evaluations are demonstrated, with Latin hypercube sampling of the random variables. Considering structural nonlinearity does affect the equipment fragilities and consequently the system risk. Vertical floor excitation is found out to control the test-based fragilities.