Nuclear power plant (NPP) design is strictly dependent on the seismic hazards and safety aspects related to the external events of the site. Passive vibration isolators are the most simple and reliable means to protect sensitive equipment from environmental shocks and vibrations. This paper concerns the methodological approach to treat isolation applied to a near term deployment reactor and its internals structures in order to attain a suitable decrease of response spectra at each floor along the height of the structure. The aim of this evaluation is to determine the seismic resistance capability of as-built structures systems and components in the event of the considered Safe Shutdown earthquake (SSE). The use of anti-seismic techniques, such as seismic isolation (SI) and passive energy dissipation, seems able to ensure the full integrity and operability of important structures and systems even in very severe seismic conditions. Therefore the seismic dynamic loadings, propagated up to the main reactor system and components, may be reduced by using the developed base-isolation system (high flexibility for horizontal motions) that might combine suitable dampers with the isolating components to support reactor structures and building. To investigate and analyze the effects of the mentioned earthquake on the considered reactor internals, a deterministic methodological approach, based on the evaluation of the propagation of seismic waves along the structure, was used. To the purpose of this study a numerical assessment of dynamic structural response behaviour of the structures was accomplished by means of the finite element approach and setting up, as accurately as possible, a representative three-dimensional model of mentioned NPP structures. The obtained results in terms of response spectra (carried out from both cases of isolated and not isolated seismic analyses) were compared in order to highlight the isolation technique effectiveness.

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