Since the dynamic response of an isolated structure depends on the combined characteristics of the ground motion, bearings, and structure, standard isolator prototype component tests may not by themselves be sufficient to assess seismic performance and verify the adequacy of numerical models used for computer simulation. Thus, when assessing the applicability of seismic isolation to nuclear power facilities, tests that simulate dynamic response under realistic excitations are desired. However, available shaking tables have limits on the size, strength, and weight of the specimens they can test. Shaking table tests of reduced-scale test specimens introduce uncertainties about the realism of test results and complicate validation of numerical models since the properties of isolation bearings are likely sensitive to scale and rate-of-loading effects. Even where a relatively small number of full-scale bearings might be used in tests of simplified shaking table specimens, significant technical and economic challenges must be addressed in order to capture the stress and deformation conditions that would occur in bearings supporting large nuclear power plants (NPPs) subjected to gravity and three-dimensional seismic excitations. The best option for testing such large specimens may be through the use of “hybrid dynamic simulation.”
Because laboratory facilities exist to test full-scale seismic isolation bearings under prescribed displacement or load protocols, and the supported structure is expected to remain essentially within the elastic range of response, hybrid simulation methods provide a unique opportunity to assess experimentally the dynamic behavior of base isolated NPPs using full-scale bearings. The adaptation of such a testing facility and the implementation of hybrid simulation using full-scale experimental bearings in a seismically isolated NPP model are described in this report.
The research program was able to confirm that hybrid simulation is indeed a viable and very promising testing method to experimentally assess the behavior of very large isolators in full- scale. It was confirmed that it is feasible and necessary to employ high-performance parallel computing analysis machines to perform hybrid simulations of large structures with several thousands of degrees of freedom, such as seismically isolated NPPs.
The primary objective of the research reported herein was to evaluate the dynamic response of an isolated NPP and assess key response parameters. A simplified, but realistic numerical model of an APR-1400 NPP designed by KEPCO Engineering and Construction (KEPCO E&C) was used as the basis of these studies. Two different types of isolation systems were considered: one based on lead plug rubber bearings, and the other based on friction bearings. Both have relatively high effective damping ratios. The isolation system was represented in the hybrid model by (a) a single test bearing representing all of the bearings supporting the plant, and (b) various combinations of numerically modeled and physically tested bearings. Ground motions used in the hybrid simulations were selected to represent situations that might be encountered in the design of NPPs in the U.S. or Europe for design-level events. Hybrid simulations were conducted considering one or two horizontal components of ground motion, as well as considering three components of excitation.
These hybrid simulations demonstrated the ability of the seismic isolation systems employed to perform well under design level conditions for the ground motions considered, and iv to protect the supported structure and components from the intense vibrations that might be expected in a fixed-base plant. The hybrid tests showed that adding a second horizontal component of motion tended to narrow hysteretic loops and somewhat increase bearing displacements. It is believed that this is related in large part to the greater energy dissipated (and temperature rise) that occurs during two-dimensional motions because of the larger distance traveled compared to the one-dimensional excitation case. The tests also revealed that both of the bearing types tested showed substantial verticalhorizontal coupling. While this behavior had negligible effect on bearing displacement demands, it had a major effect on floor response spectra. For both of the relatively high-damping bearings considered, significantly amplified horizontal spectral ordinates were observed near the vertical natural frequencies of the isolated plant. This amplification was somewhat greater for cases with two horizontal components of excitation, and for cases using the friction isolation bearing. It is concluded that it is essential to include vertical ground motion input to accurately predict horizontal floor response spectra near the plant’s fundamental vertical frequencies. Overturning moments had a negligible effect on the behavior of the isolators and resulted in relatively small increases in floor response spectra. For the friction-type isolators studied, the hybrid tests revealed some issues with breakaway and static frictions. True real-time hybrid simulations might alleviate such problems.
The study presented here investigated the behavior of seismically isolated NPPs for design-basis events. Future hybrid simulations should not only consider design-basis events but also investigate bearing behavior, moat wall impact, and possible bearing failure for beyond- design-level events. Additional development work that is needed to improve speed and performance of the hybrid test apparatus used in these tests is presented at the end of this report.
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