The seismic fragility of base-isolated NPP buildings

The research work here described is devoted to the development and testing of a numerical procedure for the computation of seismic fragilities for equipment and structural components in Nuclear Power Plants (NPP). The proposed procedure for fragility computation makes use of the Response Surface (RS) methodology to model the influence of the random variables on the dynamic response. To account for stochastic loading the latter is computed by means of a simulation procedure. Given the RS, the Monte Carlo method is used to compute the failure probability. The procedure is here applied to the preliminary design of the NPP reactor building within the IRIS international project; a base isolation system based on the introduction of HDRB (High Damping Rubber Bearing) elements is considered, leading, on one hand, to a markedly non linear behavior. On the other hand, when supported by an isolation system, the building tends to behave as a rigid body, this allowing for a low-dimensional model for capturing the dynamic response under seismic excitation. The fragility analysis is performed on the basis of a tentative limit-state function relating horizontal and vertical loads applied to isolation devices.