SEISMIC ANALYSIS OF A NUCLEAR POWER PLANT

Abstract

In recent years, analysis of massive structures (which can generate threat to society) such as hyperbolic cooling towers, chimneys, reactor containment shells, etc., subjected to seismic excitation, has attracted a considerable attention, especially nuclear power plants (NPP’s). Design of such critical facilities require an accurate and precise evaluation of seismic demands, as any possible failure of such facilities can cause immense irreversible loss to life and property due to possible radiation hazards. The design philosophy of these massive structures, like NPP’s is quite different from the conventional structures. The ductility permitting inelastic deformations is considered in conventional structures with an acceptable damage criterion but the system should behave elastically during any seismic excitation for NPP’s. NPP (having a significant embedment depth) interacts with the surrounding soil mass during an earthquake excitation. This interaction evolves some important aspects, which must be considered in the dynamic analysis, such as (a) characteristics of ground motion and its frequency content, (b) soil-structure interaction (SSI) effect, (c) energy dissipating characteristics of structure and foundation. Among all of these, SSI is an important phenomenon, which affects the dynamic behavior of a nuclear reactor building under an earthquake excitation. Since, the conventional design methodology assumes that the structural foundation is rigidly attached to surrounding soil mass having no interaction between the structure and the surrounding soil, hence the effects of SSI phenomenon are ignored in the conventional design methodology. This leads to the conservative dynamic response of such massive structures. Therefore, the dynamics analysis of such critical and massive facilities requires the incorporation of SSI particularly in case of soft soil mass. Researchers, in the past, observed the effects of SSI particularly for NPP’s for various seismic events. But, the effects bi-directional earthquake excitation is not addressed in the past studies. To explore this case, an attempt has been made in the present study to investigate the effects of bi-directional earthquake excitation for a NPP. Previously researchers have carried out the study to obtain dynamic analysis of NPP’s using different mathematical modelling techniques, such as two-dimensional (2D), three-dimensional (3D) finite element (FE) modelling, lumped mass modelling, beam on non-linear Winkler foundation (BNWF) approach etc. In the present study, 3D FE modelling is used for the simulation and analysis of reactor building of NPP. To evaluate precise results, proper iv boundary conditions are simulated, which prevent effects of reflection and refraction from the boundaries. Numerical simulation of 3D modelling, damping characteristics, mesh size, selection of ground motion and suitable boundary conditions are the main modelling challenges.