Rocking induced axial response and performance of lead‐rubber bearings in seismically isolated nuclear plants

Abstract

Seismic isolation using lead‐rubber (LR) bearings can be an effective earthquake mitigation technique to ensure the safety of Nuclear Power Plants (NPPs) against extreme earthquake events. Accurate estimation of the axial and rocking response of NPPs is critical to the design of the safety‐related secondary components, piping, and auxiliary system crossing the interface of the isolation system. The isolators along the periphery of the isolation interface are especially susceptible to buckling in compression and cavitation in tension due to the rocking motion of the structure. No study so far has looked into rocking response of base‐isolated NPP and its consequence on isolator's performance. This paper investigates the axial and rocking behavior of base‐isolated NPPs at a site of high seismic hazard. A numerical study is conducted on the different representations of NPP models with the varying height of the center of mass isolated using a range of isolation systems of different periods and characteristic strengths. The rocking responses are obtained using simplified and advanced isolator models, which consider the interaction between the axial and shear response. The results provide estimates of the rocking responses and their influence on the axial response of isolators. It was observed that the rocking response of a base‐isolated NPP increases with an increase in the isolation period and the characteristics strength of the isolation system. Rocking effects may amplify the axial forces in the lead‐rubber isolators by more than 100% and need to be considered for true performance assessment of isolators when the horizontal to vertical shaking intensity exceeds a ratio of 0.5.