Seismic Isolation of Safety-Related Nuclear Structures

Seismic isolation is a viable method of protecting nuclear safety-related structures from the damaging effects of earthquake shaking. The soon-to-be completed 2013 edition of ASCE Standard 4 will include detailed provisions and commentary (Section 7.7) to enable the horizontal seismic isolation of nuclear facilities such as nuclear power reactors and waste storage facilities. Although the provisions and commentary focus on building-type structures, they can be applied, in principle, to other structures, systems, and components, including small modular reactors and safety-related systems such as diesel generators. The performance expectations associated with the provisions, and their integration with ASCE 43-05, are presented together with the design basis for the isolated superstructure and safety-related secondary systems, the isolators, the foundation and the umbilical lines that cross the isolation interface.

ASCE 43-05 presents two performance objectives for the design of nuclear structures: 1) 1% probability of unacceptable performance for 100% Design Basis Earthquake (DBE) shaking, and 2) 10% probability of unacceptable performance for 150% DBE shaking. To develop procedures for the analysis and design of baseisolated nuclear power plants (NPPs) to meet the intent of ASCE 43-05, we performed a series of nonlinear response-history analyses to study the impact of the variability in both earthquake ground motion and mechanical properties of isolation systems on the seismic responses of base-isolated NPPs. Computations were performed for three representative sites (rock and soil sites in the Central and Eastern United States and a rock site in the Western United States), three types of isolators (lead rubber, Friction Pendulum TM and low-damping rubber bearings), and realistic mechanical properties for the isolators. Estimates were made of the ratio of the 99%-ile (90%-ile) response of isolation systems computed using a distribution of spectral demands and distributions of isolator mechanical properties to the median response of isolation systems computed using best-estimate properties and 100% (150%) spectrum-compatible DBE ground motions. Only the results for the soil site in the Central and Eastern United States and LR and FP bearings are presented.

Seismic isolation is a viable strategy for protecting safety-related nuclear structures from the effects of moderate to severe earthquake shaking. Although seismic isolation has been deployed in nuclear structures in France and South Africa, it has not seen widespread use because of limited new build nuclear construction in the past 30 years and a lack of guidelines, codes and standards for the analysis, design and construction of isolation systems specific to nuclear structures. The funding by the United States Nuclear Regulatory Commission of a research project to the Lawrence Berkeley National Laboratory and MCEER/University at Buffalo facilitated the writing of a soon-to-be-published NUREG on seismic isolation. Funding of MCEER by the National Science Foundation led to research products that provide the technical basis for a new section in ASCE Standard 4 on the seismic isolation of safety-related nuclear facilities. The performance expectations identified in the NUREG and ASCE 4 for seismic isolation systems, and superstructures and substructures are described in the paper. Robust numerical models capable of capturing isolator behaviors under extreme loadings, which have been verified and validated following ASME protocols, and implemented in the open source code OpenSees, are introduced.

Nuclear Engineering and Design, 1991

Work on seismic isolation of nuclear and non-nuclear structures was started by ENEA in cooperation with ISMES in 1988. The first activity consisted of a proposal for guidelines for seismically isolated nuclear plants using high-damping, steel-laminated elastomer bearings. This is being performed in the framework of an agreement with General Electric Company. Furthermore, research and development (R&D) work has been defined and recently initiated to support development of the seismic isolation guidelines as well as that of qualification procedures for seismic isolation systems in general. The present R&D work includes static and dynamic experiments on single bearings, shake table tests with multi-axial simultaneous excitations on reduced-scale mockups of isolated structures supported by multiple bearings, and dynamic tests on large-scale isolated structures with on-site test techniques. It also includes the development and validation of finite-element nonlinear models of the single bearings, as well as those of simplified design tools for the analysis of the isolated structures' dynamic behavior. Extension of this work is foreseen in a wider national frame. 0029-5493/91/$03.50 © 1991 -Elsevier Science Publishers B.V. (North-Holland)

Earthquake Engineering & Structural Dynamics, 2010

Seismic or base isolation is a proven technology for reducing the effects of earthquake shaking on buildings, bridges and infrastructure. The benefit of base isolation has been presented in terms of reduced accelerations and drifts on superstructure components but never quantified in terms of either a percentage reduction in seismic loss (or percentage increase in safety) or the probability of an unacceptable performance. Herein, we quantify the benefits of base isolation in terms of increased safety (or smaller loss) by comparing the safety of a sample conventional and base-isolated nuclear power plant (NPP) located in the Eastern U.S. Scenario-and time-based assessments are performed using a new methodology. Three base isolation systems are considered, namely, (1) Friction Pendulum TM bearings, (2) lead-rubber bearings and (3) low-damping rubber bearings together with linear viscous dampers. Unacceptable performance is defined by the failure of key secondary systems because these systems represent much of the investment in a new build power plant and ensure the safe operation of the plant. For the scenario-based assessments, the probability of unacceptable performance is computed for an earthquake with a magnitude of 5.3 at a distance 7.5 km from the plant. For the time-based assessments, the annual frequency of unacceptable performance is computed considering all potential earthquakes that may occur. For both assessments, the implementation of base isolation reduces the probability of unacceptable performance by approximately four orders of magnitude for the same NPP superstructure and secondary systems. The increase in NPP construction cost associated with the installation of seismic isolators can be offset by substantially reducing the required seismic strength of secondary components and systems and potentially eliminating the need to seismically qualify many secondary components and systems. Analysis of plant systems and accident sequences Characterization of seismic hazard Structural response simulation Damage assessment of NPP components Risk computation Figure 1. The procedure of seismic performance assessment of NPPs.

Nuclear Technology, 1992

ENEA began work on seismic isolation in Italy in 1988 in cooperation with ISMES. Until now, work has been limited to horizontal systems and focused on high-damping steel-laminated elastomer bearings. Work consists of both the assessment of proposed design guidelines for isolated nuclear reactors (developed in collaboration with General Electric Company) and research and development (R&D) experimental and numerical studies, partly performed in support of the guideline development. Experiments include static and dynamic characterization of single bearings, analysis of a full-scale isolated structure and an actual building with in situ techniques, and shake table tests of scaled isolated structures. The main features of the guidelines document and R&D studies are described, and some initial measured data are presented.

Science and Technology of Nuclear Installations, 2020

The new Structural Seismic Isolation System (SSIS) intends to provide high safety for important structures such as nuclear power plants, offshore oil platforms, and high-rise buildings against near-fault and long-period earthquakes. The presented SSIS structure foot base and foundation contact surfaces have been designed as any curved surfaces (spherical, elliptical, etc.) depending on the earthquake-soil-superstructure parameters, and these contact surfaces have been separated by using elastomeric (lead core rubber or laminated rubber bearings with up to 4-second period) seismic isolation devices. It would allow providing inverse pendulum behavior to the structure. As a result of this behavior, the natural period of the structure will possess greater intervals which are larger than the predominant period of the majority of the possible earthquakes including near-fault zones. Consequently, the structure can maintain its serviceability after the occurrence of strong and long-period e...

Earthquake Engineering & Structural Dynamics, 2013

The American Society of Civil Engineers (ASCE) 43-05 presents two performance objectives for the design of nuclear structures, systems and components in nuclear facilities: (1) 1% probability of unacceptable performance for 100% design basis earthquake (DBE) shaking and (2) 10% probability of unacceptable performance for 150% DBE shaking. To aid in the revision of the ASCE 4-98 procedures for the analysis and design of base-isolated nuclear power plants and meet the intent of ASCE 43-05, a series of nonlinear response-history analyses was performed to study the impact of the variability in both earthquake ground motion and mechanical properties of isolation systems on the seismic responses of base-isolated nuclear power plants. Computations were performed for three representative sites (rock and soil sites in the Central and Eastern United States and a rock site in the Western United States) and three types of isolators (lead rubber, Friction Pendulum and low-damping rubber bearings) using realistic mechanical properties for the isolators. Estimates were made of (1) the ratio of the 99th percentile (90th percentile) response of isolation systems computed using a distribution of spectral demands and distributions of isolator mechanical properties to the median response of isolation systems computed using best-estimate properties and 100% (150%) spectrum-compatible DBE ground motions; (2) the number of sets of three-component ground motions to be used for response-history analysis to develop a reliable estimate of the median response of isolation systems. The results of this study provide the technical basis for the revision of ASCE Standard 4-98.

Seismic isolation has been implemented in many civil structures, including buildings, bridges, liquid natural gas tanks, and off shore oil platforms, both in the United States and other countries, to mitigate the damaging effects of earthquakes. Seismic isolation has also been implemented in nuclear structures in France and South Africa, but not yet in the United States, in either Department of Energy facilities or commercial nuclear power plants (NPPs). This is primarily due to a lack of guidelines, and codes and standards for the analysis, design and construction specific to seismically isolated nuclear structures. However, seismic isolation of nuclear structures has seen increased research interest in the recent years and the forthcoming version of the national consensus standard America Society of Civil Engineers (ASCE) Standard 4-16 (ASCE, 2016) "Seismic analysis of safety related nuclear structures", recently incorporated language and commentary (Chapter 12) for seismically isolating surface or near-surfacemounted nuclear facilities, including NPPs.