energies Article Seismic Performance of Piping Systems of Isolated Nuclear Power Plants Determined by Numerical Considerations Sungjin Chang 1,† , Bubgyu Jeon 1, * ,† , Shinyoung Kwag 2 , Daegi Hahm 3 and Seunghyun Eem 4, *   Citation: Chang, S.; Jeon, B.; Kwag, S.; Hahm, D.; Eem, S. Seismic Performance of Piping Systems of Isolated Nuclear Power Plants Determined by Numerical Considerations. Energies 2021, 14, 4028. https://doi.org/10.3390/ en14134028 Academic Editors: Min Kyu Kim, Jung Han Kim and Gilberto Mosqueda Received: 26 April 2021 Accepted: 28 June 2021 Published: 4 July 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Korea Construction and Transport Engineering Development Collaboratory Management Institute, Suwon 17058, Korea; sjchang@pusan.ac.kr 2 Department of Civil and Environmental Engineering, Hanbat National University, Daejeon 34158, Korea; skwag@hanbat.ac.kr 3 Mechanical and Structural Safety Research Division, Korea Atomic Energy Research Institute, Daejeon 37224, Korea; dhahm@kaeri.re.kr 4 Department of Convergence & Fusion System Engineering, Major in Plant System Engineering, Kyungpook National University, Sangju 41566, Korea * Correspondence: bkjeon79@pusan.ac.kr (B.J.); eemsh@knu.ac.kr (S.E.); Tel.: +82-51-510-8192 (B.J.); +82-54-530-1483 (S.E.) † Both are Senior Researcher. Abstract: The interest in the seismic performance of nuclear power plants has increased worldwide since the Fukushima Daiichi Nuclear Power Plant incident. In Korea, interest in the seismic safety of nuclear power plants has increased since the earthquake events in Gyeongju (2016) and Pohang (2017). In Korea, studies have been conducted to apply seismic isolation systems to ensure seismic safety while minimizing the design changes to nuclear power plants. Nuclear power plants with seismic isolation systems may have a higher seismic risk due to the failure of the piping system in the structure after a relatively large displacement. Therefore, it is essential to secure the seismic safety of pipes for the safe operation of nuclear power plants. The seismic safety of pipes is determined by seismic fragility analysis. Seismic fragility analysis requires many seismic response analyses because it is a statistical approach to various random variables. Typical numerical conditions affecting the seismic response analysis of pipes are the convergence conditions and mesh size in numerical analysis. This study examined the change in the seismic safety of piping according to the numerical conditions. The difference in the seismic response analysis results of the piping according to the mesh size was analyzed comparatively. In addition, the change in the seismic fragility curve of the piping according to the convergence conditions was investigated. Keywords: pipe analysis; seismic-response analysis; finite element method; cyclic loading tests; seismic fragility 1. Introduction Ensuring the seismic performance of nuclear power plants has attracted increased interest worldwide since the Fukushima nuclear incident. Recently, earthquakes with a magnitude higher than 5, such as the Gyeongju earthquake (2016) and the Pohang earthquake (2017), have raised interest in the seismic safety of the nuclear power plants in Korea. Nuclear power plants use various systems to produce and transport energy using nuclear reactors and use a large amount of piping to connect these systems and facilities. Accordingly, it is essential to ensure the seismic safety of these piping systems. According to various experiments and studies [1–5] conducted on nuclear piping, the primary cause of pipe damage due to earthquakes appeared to be low-frequency fatigue. The pipe elbows were vulnerable to seismic events. A shake table test was conducted to analyze the seismic behavior of nuclear power plant piping. A quantitative study was performed to define the actual failure (leakage) of the piping system [6–8]. Recently, studies Energies 2021, 14, 4028. https://doi.org/10.3390/en14134028 https://www.mdpi.com/journal/energies