Seismic isolation in New Zealand

Bridges, buildings, and industrial equipment can be given increased protection from earthquake damage by limiting the earthquake attack through seismic isolation. A broad summary of the seismic responses of base-isolated structures is of considerable assistance for their preliminary design. Seismic isolation as already used in New Zealand consists of a flexible base or support combined with some form of energy-dissipating device, usually involving the hysteretic working of steel or lead. This paper presents examples of the New Zealand experience, where seismic isolation has been used for 42 bridges, 3 buildings, a tall chimney, and high-voltage capacitor banks.Additional seismic response factors, which may be important for nuclear power plants, are also discussed briefly.     

Evaluation methodology for seismic base isolation of nuclear equipments

An evaluation methodology for seismic base isolated nuclear equipments is proposed. The evaluation can be classified into two steps. In the first step, the seismic functional failure probability during the lifetime of equipment without base isolation devices is quantified in order to decide the applicability of the base isolated structure. The second step is comparative and calculates the ratio of the seismic failure frequency of the equipment without base isolation devices to that with them in order to evaluate the effectiveness of the base isolated structure. The sample evaluation considers the case of high voltage type emergency transformer with ceramic tubes.     

Damping characteristic identification for a nonlinear seismic isolation system

The steady-state response of structures to harmonic excitation is of both direct and indirect importance. Such a response is of obvious direct importance in problems which involve excitation from rotating machinery or other sources of steady harmonic excitation. It is also of indirect importance in problems involving transient excitation where knowledge of the harmonic response may be used in estimating and interpreting the transient structural response. For example, the “effective” natural frequencies, damping ratios and mode shapes identified from full-scale harmonic tests of structures are often used to interpret the transient non-linear response of these structures to earthquakes. The non-linearity is confined to the connection between the structure and the moving base. This system might be a highly idealized model for a reactor structure including non-linear seismic isolation effects.In this paper a phase resonance method is given for damping characteristic identification of the nonlinear device seismic isolation in the harmonic excitation case. The method allows the shape of the symmetric function of the system to be determined, when is not known. If F_d(x) is asymmetric then the elasticity characteristic should be known.The identification algorithm has been derived assuming that F_d(x) is given in an analytic form. The validity of the method was checked for some systems with strongly nonlinear damping characteristics.     

Seismic behavior of a free-standing core in a large LMFBR

This paper describes a study on the seismic analysis and qualification of an LMFBR core. A non-linear response analysis method with FINAS is validated by comparing its results to a set of existing experimental data. The method is then applied to assess the seismic response and safety capacity of some typical configurations of a large free-standing core, under various seismic inputs including a case of base isolation. Some discussions are made on the possibility of the free-standing core.     

ENEA activities on seismic isolation of nuclear and non-nuclear structures

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.     

Non-linear seismic response of base-isolated liquid storage tanks to bi-directional excitation

Seismic response of the liquid storage tanks isolated by lead-rubber bearings is investigated for bi-directional earthquake excitation (i.e. two horizontal components). The biaxial force-deformation behaviour of the bearings is considered as bi-linear modelled by coupled non-linear differential equations. The continuous liquid mass of the tank is modelled as lumped masses known as convective mass, impulsive mass and rigid mass. The corresponding stiffness associated with these lumped masses has been worked out depending upon the properties of the tank wall and liquid mass. Since the force-deformation behaviour of the bearings is non-linear, as a result, the seismic response is obtained by the Newmark's step-by-step method. The seismic responses of two types of the isolated tanks (i.e. slender and broad) are investigated under several recorded earthquake ground to study the effects of bi-directional interaction. Further, a parametric study is also carried out to study the effects of important system parameters on the effectiveness of seismic isolation for liquid storage tanks. The various important parameters considered are: (i) the period of isolation, (ii) the damping of isolation bearings and (iii) the yield strength level of the bearings. It has been observed that the seismic response of isolated tank is found to be insensitive to interaction effect of the bearing forces. Further, there exists an optimum value of isolation damping for which the base shear in the tank attains the minimum value. Therefore, increasing the bearing damping beyond a certain value may decrease the bearing and sloshing displacements but it may increase the base shear.     

Discussion of the paper: “Dynamic decoupling of secondary systems, by A.K. Gupta and J.M. Tembulkar, in: Nucl. Engrg. Des. 18 (1984) 359”

Technical aspects of seismic isolation systems show merit for their use in nuclear power plants. Less quantifiable non-technical aspects must be evaluated in the decision to employ a seismic isolation system.First, non-technical aspects are discussed. An historical and applications perspective is given, and it is suggested that the number of applications of seismic isolation systems is correlated with the amount of research activity in this area. For nuclear plants, it is suggested that application of seismic isolation systems is in part related to standardized plant designs in high seismic regions. Also, for nuclear plants, it is suggested that direct capital cost, enhanced seismic safety, regulatory licensing and unknown locations of nearby active faults are all factors which can weigh in favor and/or not in favor for seismic isolation application.Second, technical aspects are discussed. The technical results show that seismic isolation reduces building response, and reduces floor response spectra/equipment response. These results combine in application to reduce seismic risk and thus enhance safety for nuclear plants.     

Seismic isolation for nuclear power plants: Technical and non-technical aspects in decision making

Technical aspects of seismic isolation systems show merit for their use in nuclear power plants. Less quantifiable non-technical aspects must be evaluated in the decision to employ a seismic isolation system.First, non-technical aspects are discussed. An historical and applications perspective is given, and it is suggested that the number of applications of seismic isolation systems is correlated with the amount of research activity in this area. For nuclear plants, it is suggested that application of seismic isolation systems is in part related to standardized plant designs in high seismic regions. Also, for nuclear plants, it is suggested that direct capital cost, enhanced seismic safety, regulatory licensing and unknown locations of nearby active faults are all factors which can weigh in favor and/or not in favor for seismic isolation application.Second, technical aspects are discussed. The technical results show that seismic isolation reduces building response, and reduces floor response spectra/equipment response. These results combine in application to reduce seismic risk and thus enhance safety for nuclear plants.     

Seismic Isolation Design for JSFR

This paper describes the seismic design of Japan Sodium-Cooled Fast Reactor (JSFR), which includes the seismic condition, the seismic isolation system, and the seismic evaluation of the primary components. Since the design seismic loading is set out severely than ever since The Niigata-ken Chuetsu-oki Earthquake in 2007, an advanced seismic isolation system is aimed to reduce the seismic force loaded on the primary components of JSFR to be less than that of the previous seismic isolation system. The advanced seismic isolation system is developed by optimizing the performance based on the previous seismic isolation system considering the natural frequency of the primary components. The laminated rubber bearings thicker than the previous ones and oil dampers are adopted for the advanced seismic isolation system of SFR. The seismic evaluation of nuclear reactor components applying the advanced seismic isolation system is performed and its feasibility is confirmed.     

DEMT experimental and analytical studies on seismic isolation

This paper reviews the experimental and theoretical studies performed at CEA/DEMT related to the overall behavior of isolated structures. The experimental work consists of the seismic shaking-table tests of a concrete cylinder isolated by neoprene sliding pads, and the vibrational tests on the reaction mass of the TAMARIS seismic facility. The analytical work consists of the development of procedures for dynamic calculation methods: for soil—structure interaction where pads are placed between an upper raft and pedestals, for time-history calculations where sliding plates are used, and for fluid—structure interaction where coupled fluid and structure motions and sloshing modes are important.Finally, this paper comments on the consequences of seismic isolation for the analysis of fast-breeder reactor (FBR) vessels. The modes can no longer be considered independent (SRSS Method leads to important errors), and the sloshing increases.     

Nonlinear natural rubber bearings for seismic isolation

This paper summarizes the results of a ten-year joint research program on base isolation carried out by the Malaysian Rubber Producers' Research Association in England and the University of California at Berkeley.Special rubber bearings have been developed which protect buildings and their contents from earthquake damage without requiring additional mechanical devices to enhance damping or to avoid problems of wind movement. The lack of complexity in the system makes prediction of response more certain and reduces cost so that in many designs the additional protection will be achieved at a cost lower than that for conventional protection. The principles of design are explained and results given for the many experimental investigations of the system which have included measurements of the response of both the building and its internal equipment.Practical aspects of the use of the system are discussed with reference to a large structure at present being constructed on nonlinear natural rubber bearings in a highly seismic area.     

Soil-structure interaction of nuclear reactor structures considering through-soil coupling between adjacent structures

The theoretical problem concerning the influence of through-soil coupling between adjacent structures on the seismic loading of nuclear reactors has been investigated by considering a soil-structure interaction model in which several three-dimensional flexible structures are bonded to an elastic half-space. These structures, which are allowed to be either similar or dissimilar, are modeled as conventional discrete systems mounted on separate base slabs of close proximity. For the purpose of this study, it is assumed that the stiffness of any structure such as piping connecting the adjacent buildings is negligible.For purposes of comparison, the seismic responses of structural masses are determined both with and without the influence of nearby structures. Both transient and steady-state results are presented and discussed for some typical simplified two- and three-structure complexes. Emphasis is placed on the effects of through-soil coupling on the dynamic response of the system rather than actual magnitudes of response which have previously been treated for plants erected on a single base slab. The significant findings are that nuclear power plants can be designed to achieve a reduction in seismic loads due to interaction with neighboring structures. Conversely, improper plant design and layout may result in mutual reinforcement of resonances with increased loads.     

Seismic isolation of nuclear power plants — EDF's philosophy

The elastomer bearing pads used since 1963 as supports for prestressed concrete pressure vessels (PCPVs) was quickly chosen by Electricité de France (ED) to improve the capability of nuclear power plants (NPPs) to withstand strong earthquakes and to reduce the seismic loads on structures and equipment. The standardized units for 900 and 1300 MW(e) pressurizedwater reactor (PWR) plants have moderate seismic design loads of 0.2 and 0.15 g, respectively. These design loads were exceeded by the site dependent spectra of Cruas (France) and Koeberg (South Africa). To keep the plant design unchanged and to take the advantages of standardization, these units were put on laminated bearings with or without sliding plates. For the future French 1500 MW(e) fast breeder reactors (FBRs), which are more sensitive to seismic loads, the base isolation is considered by EDF at the beginning of the design, even for low ground motions of 0.1 g. The buildings are placed on laminated bearings while the reactor block is supported by springs and dampers. The isolated plant has identical costs as a conventional design such as SPX1 at Creys—Malville.     

Seismic isolation retrofitting of the Salt Lake City and County Building

The City and County Building, a massive unreinforced masonry structure completed in 1894, has been seismically retrofitted using base isolation. The isolation system consists of 443 lead-rubber isolators installed underneath the building on top of existing spread footings. The building is isolated from the surrounding ground by a perimeter moat wall, permitting lateral movement to take place during an earthquake.It is believed that this is the first historic structure in the world to be retrofitted against possible seismic damage using base isolation.     

Effect on non-linear soil-structure interaction due to base slab uplift on the seismic response of a high-temperature gas-cooled reactor (HTGR)

In high seismic regions it has often been the practice to use oversized base slabs for the major nuclear power plant structures in order to prevent, or at least minimize, the amount of dynamic base slab uplift which will result from the overturning moments developed during seismic ground motion. Two major reasons have been expressed as to why dynamic base slab uplift should be minimized: (1) As nuclear power plants are normally designed for seismic loadings based upon linear analysis, and since soil-structure interaction becomes nonlinear when only a portion of the base slab is in contact with the soil, linear elastic analysis may be unacceptable if base slab uplift occurs (as the resultant design loads may be incorrect), and (2) substantial uplift could cause excessive toe pressures in the supporting soil and significant impact forces when the slab recontacts the soil.The primary purpose of this paper is to evaluate the importance of the nonlinear soil-structure interaction effects resulting from substantial base slab uplift occurring during a seismic excitation. The structure considered for this investigation consisted of the containment building and prestressed concrete reactor vessel (PCRV) for a typical HTGR plant. A simplified dynamic mathematical model was utilized consisting of a conventional lumped mass structure with soil-structure interaction accounted for by translational and rotational springs whose properties are determined by elastic half space theory. Three different site soil conditions (a rock site, a moderately stiff soil, and a soft soil site) and two levels of horizontal ground motion (0.3 and 0.5 g earthquakes) were considered.Based upon the parametric cases analyzed in this investigation, it may be concluded that linear analysis (which ignores the nonlinear soil-structure interaction effects of base slab uplift) can be used to conservatively estimate the important behavior of the base slab even under conditions of substantial base slab uplift. For all cases investigated here, linear analysis resulted in higher base overturning moments, greater toe pressures, and greater heel uplift distances than nonlinear analyses. It may also be concluded that the nonlinear effect of uplift does not result in any significant lengthening of the fundamental period of the structure. Also, except in the short period region (period less than half of the fundamental period) only negligible differences exist between in-structure response spectra based on linear analysis and those based on nonlinear analysis.Finally, it may be concluded that for sites in which soil-structure interaction is not significant, as for the rock site, the peak structural response (shears and moments) at all locations above the base mat are not significantly influenced by the nonlinear effects of base slab uplift. However, for the two soil sites the peak shears and moments are, in a few instances, significantly different between linear and nonlinear analyses. As a result, linear analysis may be used to determine all structural response for rock sites even when there is substantial base slab uplift. However, for soil sites, nonlinear analyses are necessary if substantial base slab uplift occurs.     

Finite element modeling of the AP1000 nuclear island for seismic analyses at generic soil and rock sites

The AP1000 is a standard design developed by Westinghouse and its partners for an advanced nuclear power plant utilizing passive safety features. The design has been certified by the US Nuclear Regulatory Commission based on their review of seismic analyses at hard rock sites. The plant has five principal building structures: the nuclear island, the turbine building, the annex building, the diesel generator building and the radwaste building. The nuclear island consists of the containment building (the steel containment vessel and the containment internal structures), the shield building and the auxiliary building. These structures are founded on a common basemat and are collectively known as the nuclear island. This paper describes shell and stick finite element models used in fixed base dynamic analyses for the hard rock design certification using the general purpose finite element program ANSYS. It describes a coarser shell model developed for use in soil structure interaction (SSI) analyses. This model is developed in both ANSYS and the soil structure interaction (SSI) program SASSI. Results of the three types of models from ANSYS analyses are compared for a hard rock site. Results are also compared between the ANSYS and SASSI analyses for the same model.     

Seismic fragility analyses of nuclear power plant structures based on the recorded earthquake data in Korea

By nature, the seismic fragility analysis results will be considerably affected by the statistical data of design information and site-dependent ground motions. The engineering characteristics of small magnitude earthquake spectra recorded in the Korean peninsula during the last several years are analyzed in this paper. An improved method of seismic fragility analysis is evaluated by comparative analyses to verify its efficiency for practical application to nuclear power plant structures. The effects of the recorded earthquake on the seismic fragilities of Korean nuclear power plant structures are also evaluated from the comparative studies. Observing the obtained results, the proposed method is more efficient for the multi-modes structures. The case study results show that seismic fragility analysis based on the Newmark's spectra in Korea might over-estimate the seismic capacities of Korean facilities.     

Seismic protection technology for nuclear power plants: a systematic review

Seismic protection systems (SPS) have been developed and used successfully in conventional structures, but their applications in nuclear power plants (NPPs) are scarce. However, valuable research has been conducted worldwide to include SPS in nuclear engineering design. This study aims to provide a state-of-the-art review of SPS in nuclear engineering and to answer four significant research questions: (1) why are SPS not adopted in the nuclear industry and what issues have prevented their deployment? (2) what types of SPS are being considered in nuclear engineering research? (3) what are the strategies for location of SPS within NPPs? and (4) how may SPS provide improved structural performance and safety of NPPs under seismic actions? This review is conducted following the procedures of systematic reviews, where possible.

The issues concerning the use of SPS in NPPs are identified: cost, safety, licensing and scarcity of applications. NPPs demand full structural integrity and reactor's safe shutdown during earthquake actions. Therefore, horizontal isolation may be insufficient in active seismic zones and isolation in the vertical direction may be required. Based on the results in this review, it is likely that next generation reactors in seismic zones will include state-of-the-art SPS to achieve full standardised design.     

Alexisismon isolation engineering for nuclear power plants

This paper reviews the special requirements regarding efficiency, licensibility (reliability) and cost which should be met to achieve an optimum base isolated nuclear power plant design. It then describes the Alexisismon-2, patented isolation system developed by the author, underlines its original properties (linearity and separation of functions) and presents a conceptual design of its application to a nuclear power plant. The great reliability of the system components is demonstrated. The efficiency of the A-2 is found to be very high: a reduction factor of the base shear induced in the plant higher than 25 is achieved for all examined real accelerograms scaled to 1 g GPA. So the isolation components, the structural system of the plant, its equipment and systems can be easily designed to remain in the elastic range of stresses and strain even for seismic input with GPA higher than 2 g.