RESPECTIVE ROLE OF THE VERTICAL AND HORIZONTAL COMPONENTS OF AN EARTHQUAKE EXCITATION FOR THE DETERMINATION OF FLOOR RESPONSE SPECTRA OF A BASE ISOLATED NUCLEAR STRUCTURE - APPLICATION TO GEN IV REACTORS

摘要

Several nuclear energy systems, identified as GEN IV technologies, such as Liquid Metal Fast Breeder Reactor (LMFBR) must resist high temperature transients and do operate at relatively low internal pressure levels. Therefore, most of the components constituting these reactors are thin compared to their PWR counterparts. This induces a specific vulnerability to strong seismic loadings. For this reason, seismic base isolation is considered a very promising option. Its main goal, for this application, would be to decrease the loading on the components and systems by reducing the floor response spectra at their anchorage points. Despite some historical practices, the floor response spectra of a seismically isolated structure can not be properly determined using stick models or multi-degree of freedom spring mass models. Such models would actually lead to an over-estimation of the base isolation efficiency in the horizontal direction. Indeed, the vertical excitation component of the earthquake does excite some modes with significant local horizontal accelerations. As a result, the horizontal floor response spectra do exhibits significant peaks at frequencies higher than the isolation frequency. The representation of these local modes does generally require 3D modeling. The opposite phenomenon is also observed: the horizontal excitation does excite some modes with significant local vertical accelerations. Given that the horizontal excitation is filtered by the base isolation system, the vertical floor response spectra can be significantly reduced. By changing the vertical stiffness of the base isolation system or adding some damping in this direction useful design tools are available in order to influence both the horizontal and the vertical floor response spectra.