Effects of Foundation Rotational Motion on the Non-Linear Response of a Base-Isolated Nuclear Power Plant subject to Earthquake Loading

Past research has shown that, for seismically isolated structures, rocking excitation has little effect on the peak value of shear deformations in the isolation devices and accelerations in the building structure. Recently, however, seismic isolation has been considered for the reactor buildings of almost all new generation nuclear power plants (NPPs). The safety issues related to NPPs require an assessment of previous results, considering that the seismic isolation needs to be designed in order to both guarantee high performance in case of “beyond-design” earthquakes and to ensure the continuous functionality of the structure. Thus, all factors affecting the isolators performance assume the utmost importance in risk identification and evaluation. The work, described in this paper, aims at the evaluation of the effects of the rotational components of the foundations motion, excited by earthquakes, on seismic isolation systems. The case study is the building housing a Generation 3+, medium- size nuclear reactor, equipped with HDRB devices. The rubber bearings are simulated in the non-linear field through a phenomenological model from the literature, implemented within a multipurpose finite element code (ABAQUS). The bearings model is non-linear and cyclic with respect to the horizontal displacements and bi-linear-elastic, with different stiffness in tension and compression, for the vertical displacements.