Detailed Finite Element Modeling of US EPR™ Nuclear Island for Seismic SSI Analysis

The US EPR™ standard design currently under development by AREVA consists primarily of a nuclear island (NI) and several other significant structures outside of and in close proximity to the NI. The NI structures consist of the Reactor Building (RB), Fuel Building (FB), Safeguard Building 1 (SB1), Safeguard Building 2/3 (SB2/3), Safeguard Building 4 (SB4), and Reactor Building Internal Structures (RBIS) — all of which share a common foundation basemat. The Nuclear Island is embedded approximately 11.6 m below the ground surface. Seismic soil-structure interaction (SSI) analysis of nuclear power plants is often performed in the frequency domain using a lumped-mass stick and/or coarse finite element model of the structure. These models are designed to capture the global dynamic response of the system, the results of which provide the inertia forces that are used for foundation stability assessment as well as input to a static detailed finite element model of the structure for design. The in-structure response spectra is calculated from a separate dynamic analysis of detailed structural model on fixed base excited by the base motion developed from the SSI analysis or often by including single-degree-of-freedom (SDOF) oscillators representing the local response in the stick/coarse finite element SSI models. With recent advances in computer software and hardware technologies, it is now possible to perform SSI analysis of detailed structural models in the frequency domain. This paper presents the results of the seismic SSI analysis of the US EPR™ nuclear island using both a stick and detailed finite element representation of the structure. The soil profile corresponds to a medium stiff soil case used for the standard design. Because the EPR™ nuclear island is a complex, unsymmetric structure, the stick model consists of multiple interconnected sticks developed and calibrated against a detailed finite element model of the structure on a fixed base. Both models are analyzed using SASSI [1]. The results of the detailed finite element model in terms of maximum accelerations and response spectra, as well as total interstory forces and moments, are calculated and compared with those of the lumped-mass stick model.