Progress in Nuclear Energy xxx (xxxx) xxx Please cite this article as: Chunfeng Zhao, Progress in Nuclear Energy, https://doi.org/10.1016/j.pnucene.2019.103144 Available online 5 September 2019 0149-1970/© 2019 Elsevier Ltd. All rights reserved. Study on the dynamic behavior of isolated AP1000 NIB under mainshock-aftershock sequences Chunfeng Zhao a, * , Na Yu b, ** , Tao Peng a , Vincenzo Lippolis c , Aldo Corona c , Y.L. Mo d a Department of Civil Engineering, Hefei University of Technology, Hefei, 230009, China b Department of Economics, Hefei University, Hefei, 230601, China c Department of Engineering and Architecture, University of Parma, Parma, 43121, Italy d Department of Civil and Environmental Engineering, University of Houston, Houston, 77021, USA A R T I C L E INFO Keywords: AP1000 nuclear island building (NIB) Isolation system Mainshock-aftershock Floor response spectrum Dynamic analysis ABSTRACT A mainshock can trigger several following severe aftershocks, which may take a threat to the structure. AP1000 nuclear power plant buildings (NPPB) may subject to severe mainshock and aftershock sequences during its whole life cycle. This paper mainly focuses on the seismic behavior of isolated AP1000 NPPB under mainshock and aftershock sequences. A 3D fnite element model (FEM) of isolated AP1000 NPPB is established to perform the dynamic analysis. The effects of the mainshock-aftershock sequences on the maximum acceleration, peak displacement, acceleration response spectrum, and stress distribution are carried out using parametric analysis. The results indicate that the aftershock has a small impact on the seismic performance of the isolated nuclear island building (NIB) due to the isolation system. Mainshock and aftershock also infuence the foor response spectrum in horizontal directions and the stress state to some extent, but the effects are relatively small compared with the non-isolated NIB. 1. Introduction AP1000 NPP as a type of generation III þ nuclear power plant (NPP) with advanced passive safety features (APS) is developed by Westing- house Company and approved by the U.S. NRC (Zhao et al., 2017; Zhao and Yu, 2018). AP1000 NPP is advanced and low-cost due to its passive design philosophy for operation and maintenance(Wang et al., 2019; Zhao and Chen, 2014; Zhao et al., 2014, 2015). Although NPP has a series of advanced features and safety, it may also encounter a potential threat of earthquakes due to its uncertainty and randomness. Seismic or base isolation as an effective approach can reduce the dynamic response of the structure and keep the structure security, particularly for the NPP structure (Chen et al., 2014). A severe earthquake may trigger many strong aftershocks frequently, which take severe threats to the security of NPPs. In the past, several researchers have studied the dynamic response of NPPs under single earthquake excitations, which have made excellent progress in the security of NPPs. Zhao et al. investigated the dynamic response and the effectiveness of isolators in the NPP under SSE loading (Chen et al., 2014; Zhao and Chen, 2013). The results showed that the isolation system could effectively reduce the structural response and protect the structure more security. Huang et al. (Huang et al., 2010) evaluated the benefts of base isolation in providing the safety of NPP compared with a conventional NPP by using performance assessment method. Nakamura et al. used a type of nonlinear material to establish a 3D FEM of NPP and investigated the infuence of vertical ground motion on the structural lift. It indicated that the basement uplift was small under earthquake loads (Medel-Vera and Ji, 2015). Micheli et al. (2004) used a pure FEM to study the seismic behavior of an NPP with isolators under earthquake excitation. It was observed that the isolator could decrease the acceleration response drastically and make the NPP more safety (Micheli et al., 2004). Lo Frano adopted a numerical method to simulate the seismic isolation effects in EGIV NPP. The results showed that the isolation technology could reduce the seismic response the NPP compared with the non-isolated structure (Lo Frano and Forasassi, 2011). Xu et al. used SPH and FEM models to analyze the sloshing and oscillation of the water in the gravity water storage tank under different earthquakes. Jeltsov et al. (2018) used CFD and VOF methods to study the lead sloshing and seismic response of ELSY reactor under artifcial earthquake. Liu et al. (2015) used an equivalent mechanical model to study the dynamic * Corresponding author. . ** Corresponding author. E-mail addresses: zhaowindy@hfut.edu.cn (C. Zhao), yn2016@hfuu.edu.cn (N. Yu). Contents lists available at ScienceDirect Progress in Nuclear Energy journal homepage: http://www.elsevier.com/locate/pnucene https://doi.org/10.1016/j.pnucene.2019.103144 Received 29 May 2019; Received in revised form 11 August 2019; Accepted 27 August 2019