Seismic Isolation of Nuclear Power Plants Using Sliding Bearings (NUREG/CR-7254)
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Manuscript Completed: February 2016
Date Published: May 2019
1IIT Gandhinagar, India; formerly graduate student at University at
Buffalo, State University of New York
2MCEER, University at Buffalo, State University of New York
212 Ketter Hall, Buffalo, NY 14260
Ramón L. Gascot Lozada, NRC Project Manager
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington DC 20555-0001
Nuclear power plants (NPPs) are designed for earthquake shaking with very long return periods. Seismic isolation is a viable strategy to protect NPP structures from extreme earthquake shaking because it filters a significant fraction of earthquake input energy. This study addesses the seismic isolation of NPP structures using sliding bearings, with a focus on the single concave Friction Pendulum™ (FP) bearing.
Friction at the sliding surface of an FP bearing changes continuously during an earthquake as a function of sliding velocity, axial pressure and temperature at the sliding surface. The temperature at the sliding surface, in turn, is a function of the histories of coefficient of friction, sliding velocity and axial pressure, and the travel path of the slider. A simple model to describe the complex interdependence of the coefficient of friction, axial pressure, sliding velocity and temperature at the sliding surface is proposed, and then verified and validated.
Seismic hazard for a seismically isolated nuclear power plant is defined in the United States using a uniform hazard response spectrum (UHRS) at mean annual frequencies of exceedance (MAFE) of 10-4 and 10-5. A key design parameter is the clearance to the stop (CHS), which is influenced substantially by the definition of the seismic hazard. Four alternate representations of seismic hazard are studied, which incorporate different variabilities and uncertainties. Response-history analyses performed on single FP-bearing isolation systems using ground motions consistent with the four representations at the two shaking levels indicate that the CHS is influenced primarily by whether the observed difference between the two horizontal components of ground motions in a given set is accounted for in the analyses.
The UHRS at the MAFE of 10-4 is increased by a design factor (≥ 1) for a conventional (fixed-base) nuclear structure to achieve a target annual frequency of unacceptable performance. Risk-oriented calculations are performed for eight sites across the United States to show that the factor is equal to 1.0 for seismically isolated NPPs, if the risk is dominated by horizontal earthquake shaking.
Response-history analyses using different models of a seismically isolated NPP structure are performed to understand the importance of the choice of friction model, model complexity and vertical ground motion for calculating horizontal displacement response across a wide range of sites and shaking intensities. A friction model for the single concave FP bearing should address heating. The pressure- and velocity-dependencies were not important for the models and sites studied. Isolation-system displacements can be computed using a macro model comprising a single FP bearing.
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