Base-isolated Nuclear Power Plants Research Articles

Dynamic soil-structure interaction analysis for base-isolated nuclear island building based on the method of external source wave motion

Soil-structure interaction (SSI) is crucial to the dynamic response of nuclear power plants (NPPs), and should be properly considered when analysing the seismic isolation performance of base-isolated NPPs. In this study, a three-dimensional numerical simulation program is developed for the time-domain dynamic analysis of base-isolated NPPs considering SSI effects. The seismic input is realised using the method of external source wave motion, and the nonlinear behavior of isolators is fully considered. Using this program, the SSI effects of the base-isolated nuclear island building under different sites are analysed and compared, including the acceleration, displacement, and floor response spectra. The numerical results show that the SSI effects change the spectral characteristics of the actual seismic input. When the shear wave velocity of the site is relatively large, the SSI effects can increase the dynamic response of base-isolated NPPs. Additionally, the isolation performance of the base-isolated NPPs is weakened when SSI effects are considered, and with a decrease in the shear wave velocity of the site, the isolation capacity is further reduced. The influence of the SSI effects should be fully considered in the base isolation design of NPPs.

Global method for nonlinear seismic analysis of embedded base-isolated nuclear island buildings in layered sites

Owing to the increasing scarcity of high-quality sites, the seismic analysis of base-isolated nuclear power plants (NPPs) under complex non-lithology sites has gained important engineering significance. In this study, a global method for nonlinear numerical analysis was developed for the seismic evaluation of base-isolated NPPs in complex layered sites. The seismic analysis of layered sites is realised using the equivalent linear method and one-dimensional wave propagation theory, the radiation damping of the infinite domain is modelled with a visco-elastic artificial boundary, and the nonlinear behaviour of the isolators is fully considered using a bilinear hysteretic model. Using this method, the influence of soil–structure interaction (SSI) and soil nonlinearity on the behaviour of base-isolated NPPs under the design basis earthquake and beyond design basis earthquake was analysed and compared, in terms of the floor response spectra (FRS) and inter-story displacement angle. In addition, the influence of embedment is also studied. The results show that in layered sites, the SSI significantly amplifies the response values for base-isolated NPPs, and as the ground motion input increases, the soil nonlinearity more effectively inhibits the response of base-isolated NPPs. In addition, embedding can significantly reduce the response, but it increases the FRS at some frequency domains. The isolation effect of base-isolated NPPs decreases significantly when SSI effects are considered, and the soil nonlinearity and embedment have little impact on the base isolation capacity. The method proposed in this study can be easily applied to study base-isolated NPPs in layered sites.

Seismic performance of advanced three-dimensional base-isolated nuclear structures in complex-layered sites

The application of three-dimensional (3D) base isolation for nuclear structures is advantageous. In this study, an advanced 3D base isolation technique was developed and the nonlinear seismic response of 3D base-isolated nuclear structures in complex layered sites was investigated using a global method. The study first designed a novel 3D isolation system, including horizontal and vertical isolation, based on the disc spring theory, which can flexibly adjust the structural stiffness and vertical bearing capacity. To verify the applicability of 3D base isolation, the dynamic behaviour of a 3D base-isolated nuclear island building in complex-layered sites subjected to beyond-design basis ground motion was investigated using a proposed high-precision simulation program of soil-structure interaction (SSI) based on the global method. The findings demonstrate that in complex-layered sites, the SSI increases the horizontal floor response spectra (FRS) but decreases the vertical FRS of 3D base-isolated nuclear power plants, and both the horizontal and vertical isolation rates remarkably decrease. The influence of the SSI becomes more evident as the intensity of the ground motion increases. The proposed 3D base isolation system exhibits excellent isolation effects. When the vertical stiffness is reduced, the vertical isolation effect becomes more significant. However, the adverse rocking motion of the island building should be effectively controlled. The research results support the practical application of 3D base isolation technology for nuclear structures located in complex-layered sites with high seismic intensities.

Global torsional response of seismically isolated nuclear plants and evaluation of codal provisions

A major challenge in the application of seismic isolation to nuclear structures is the torsional response of base-isolated Nuclear Power plants (NPPs), which amplifies the isolator displacement around the periphery of the isolation system. Additionally, degradation in the mechanical properties of the isolator due to the response associated with extreme ground shaking may also affect the torsional response. This paper presents a comprehensive study on the torsional response of a NPP seismically isolated using lead rubber bearings with a focus on the isolation properties and eccentricity in the isolation system. Nonlinear response history analysis is conducted with different NPP model representations, which are derived from a benchmark NPP model for a range of eccentricities with different time periods and characteristic strengths. It was observed that torsional response in base-isolated NPPs could be controlled by using a flexible isolation system of higher characteristic strength, and it may be important to include the strength degradation in lead rubber isolators to correctly estimate torsional amplification to isolator displacements. The torsional amplification predicted by the nonlinear response history analysis was also compared with the provisions of ASCE 7 for buildings, and recommendations are provided on their suitability for seismically isolated NPPs.

The Influence of Near- and Far-field Earthquakes on the Seismic Performance of Base-Isolated Nuclear Power Plant Structures

This study aims to investigate the influence of near- and far-field earthquakes on the seismic performance of base-isolated Nuclear Power Plant (NPP) structures. Two earthquake motion groups of near-field and far-field characteristics are selected for fragility evaluation analysis. A base-isolated advanced reactor power 1400 (APR-1400) is employed for numerical analysis. A set of fragility curves are derived for various limit states based on the maximum likelihood estimation. The limit states are defined in terms of Lead Rubber Bearing (LRB) deformation capacity. The numerical results reveal that the median maximum deformations of LRBs were smaller for far-field ground motions than for near-field motions. Also, the comparison of fragility curves demonstrates that the probability of failure of base-isolated NPP structures is higher for near-field ground motions than far-field motions. It is crucial to select earthquake ground motions with both near- and far-field motions for the seismic evaluation of NPP structures.

Seismic performance evaluation of base-isolated nuclear power plant reactor building considering aging effect of isolation device

ABSTRACT The base isolation (BI) device is widely used for reducing seismic energy transmitted to the upper part of the structures. The rubber property of lead rubber bearing (LRB) BI devices degrades with time and environmental conditions, affecting the controlling performance of BI devices. This study investigates the degradation of capacity of base-isolated nuclear power plant (BI-NPP) considering the aging of rubber material used in the LRB BI device. An NPP reactor building with BI devices is numerically modeled. To capture the nonlinear behavior of the BI device, a bilinear hysteretic model is considered. Assuming different ages, the nonlinear incremental dynamic analysis is conducted using a set of 25 earthquakes and the seismic outcomes are evaluated through fragility analysis. The seismic capacity is extracted from the fragility parameters in terms of High Confidence of Low Probability of Failure (HCLPF) capacity. The outcome shows that the capacity degraded remarkably up to 21% for 100 years of service life. Finally, the seismic capacity degradation model is proposed through the investigation, which will be helpful to evaluate the capacity of the BI-NPP structure for any instant of time.

Experimental study on the floor responses of a base-isolated frame structure via shaking table tests

Seismic isolation represents one of the innovative means to improve the seismic safety of nuclear power plants (NPPs) against strong earthquakes. Many difficulties remain, though, in applying seismic isolation systems to real NPP structures due to the variability of the material properties of seismic isolators and the nonlinear behavior characteristics of the base isolation system in spite of the elastic behavior of the superstructure. Specifically, there is a limit to estimate the dynamic properties of base isolation systems for numerical analysis. The in-structure response of a base isolated-structure, which is related to equipment safety installed on the floors, is more sensitive than the base shear of the superstructure to the nonlinear properties of the isolation system. In this study, the complex behavioral characteristics of a base-isolated elastic frame structure are experimentally investigated through shaking table tests. Variation in response according to the frequency contents of the input earthquake is investigated, and the earthquake responses of the base-isolated structure for each frequency range are analyzed. In addition, the nonlinear increment of the in-structure response spectra to input ground motion intensity is evaluated, along with the nonlinear behavior of the seismic isolators. The experimental results obtained in this study can help identify various behavioral characteristics of base-isolated structures, with the nonlinear properties examined here acting as important references for evaluating the seismic safety of base-isolated NPPs.

Evaluation of the Limit State of a Six-Inch Carbon Steel Pipe Elbow in Base-Isolated Nuclear Power Plants

The installation of base isolation systems in nuclear power plants can improve their safety from seismic loads. However, nuclear power plants with base isolation systems experience greater displacement as they handle seismic loads. The increase in relative displacement is caused by the installed base isolation systems, which increase the seismic risk of the interface piping system. It was found that the failure mode of the interface piping system was low-cycle fatigue failure accompanied by ratcheting, and the fittings (elbows and tees) failed due to the concentration of nonlinear behavior. Therefore, in this study, the limit state was defined as leakage, and an in-plane cyclic loading test was conducted in order to quantitatively express the failure criteria for the SCH40 6-inch carbon steel pipe elbow due to low-cycle fatigue failure. The leakage line and low-cycle fatigue curves of the SCH40 6-inch carbon steel pipe elbow were presented based on the test results. In addition, the limit state was quantitatively expressed using the damage index, based on the combination of ductility and energy dissipation. The average values of the damage index for the 6-inch pipe elbow calculated using the force−displacement (P–D) and moment−relative deformation angle (M–R) relationships were found to be 10.91 and 11.27, respectively.

Optimal Earthquake Intensity Measures for Probabilistic Seismic Demand Models of Base-Isolated Nuclear Power Plant Structures

The purpose of this study is to evaluate the optimal earthquake intensity measures (IMs) for probabilistic seismic demand models (PSDMs) of the base-isolated nuclear power plant (NPP) structures. The numerical model of NPP structures is developed using a lumped-mass stick model, in which a bilinear model is employed to simulate the force-displacement relations of base isolators. In this study, 20 different IMs are considered and 90 ground motion records are used to perform time-history analyses. The seismic engineering demand parameters (EDPs) are monitored in terms of maximum floor displacement (MFD), the maximum floor acceleration (MFA) of the structures, and maximum isolator displacement (MID). As a result, a set of PSDMs of the base-isolated structure is developed based on three EDPs (i.e., MFD, MFA, and MID) associated with 20 IMs. Four statistical parameters including the coefficient of determination, efficiency (i.e., standard deviation), practicality, and proficiency are then calculated to evaluate optimal IMs for seismic performances of the isolated NPP structures. The results reveal that the optimal IMs for PSDMs with respect to MFD and MID are velocity spectrum intensity, Housner intensity, peak ground velocity, and spectral velocity at the fundamental period. Meanwhile, peak ground acceleration, acceleration spectrum intensity, A95, effective peak acceleration, and sustained maximum acceleration are efficient IMs for PSDMs with respect to MFA of the base-isolated structures. On the other hand, cumulative absolute velocity is not recommended for determining the exceedance of the operating basis earthquake of base-isolated NPP structures.

A nonlinear soil-structure interaction analysis technique based on seismic isolation design response spectrum for seismically isolated nuclear structures with rigid basemat

Seismic Analysis of Safety-Related Nuclear Structures (ASCE 4-16) offers a two-step nonlinear soil-structure interaction analysis procedure for seismically base-isolated structures based on the seismic isolation design response spectrum. The two steps are (I) development of seismic isolation design response spectrum at the foundation level of an isolated structure using frequency domain soil–structure interaction (SSI) analysis for an equivalent linear SSI system and (II) nonlinear time domain analysis of the isolated structure using foundation input motions, which is consistent with the seismic isolation design response spectrum. In step II, the rocking component of the structure shall be considered when its effects are significant. However, the ASCE 4-16 does not present how to consider the rocking input. In this study, we propose a method of taking account the rocking component of a base-isolated structure with rigid basemat. Specifically, this paper presents a method of developing rocking input motion that is fully coupled with horizontal input motion at the base of the isolated structure. The method was verified by comparing numerical results using the rocking input for a base-isolated nuclear power plant with rigid basemat on a compliant soil medium with those of a refined nonlinear response history analysis.

Evaluation of the vibration response of third generation nuclear power plants with isolation technology under large commercial aircraft impact

With the development of nuclear power technology, third-generation nuclear power plants (NPPs) are being built in many places. Examples include the American passive AP1000 NPP, the French EPR NPP, and China's Hualong One NPP. These will become the main reactor type for future NPP construction. Base isolation is employed as the aseismic technology in NPPs, and its design standard has been standardized according to ASCE 4–16 and other related codes. Since the 9/11 terrorist incident, large commercial aircraft impact (AI), as a beyond design-basis event, has attracted increasing attention in nuclear research. In the case of a third-generation NPP with isolation technology, it is necessary to evaluate its safety and integrity against AI. In this study, the dynamic response of a base-isolated third-generation NPP subjected to impact by a large commercial aircraft, the Airbus A340-300, is investigated. The most typical design for a third-generation NPP is to build many buildings on the same base-mat. The presence of a flexible isolation layer renders the dynamic behavior of a base-isolated NPP different from that of a non-isolated NPP under AI loads. Five key factors, including the horizontal stiffness of isolation bearings, horizontal damping of isolation bearings, total weight and weight distribution of the NPP, and AI force and direction affect the vibration characteristics of NPPs. The horizontal stiffness of isolation bearings affects the constraint effect of the foundation on the superstructure, while the damping of isolation bearings affects the dissipation of impact energy. Meanwhile, the total weight and weight distribution of the NPP affect the vibration response of different regions, and the impact force affects the vibration intensity of the NPP.

Identifying significant earthquake intensity measures for evaluating seismic damage and fragility of nuclear power plant structures

Abstract Seismic design practices and seismic response analyses of civil structures and nuclear power plants (NPPs) have conventionally used the peak ground acceleration (PGA) or spectral acceleration (Sa) as an intensity measure (IM) of an earthquake. However, there are many other earthquake IMs that were proposed by various researchers. The aim of this study is to investigate the correlation between seismic responses of NPP components and 23 earthquake IMs and identify the best IMs for correlating with damage of NPP structures. Particularly, low- and high-frequency ground motion records are separately accounted in correlation analyses. An advanced power reactor NPP in Korea, APR1400, is selected for numerical analyses where containment and auxiliary buildings are modeled using SAP2000. Floor displacements and accelerations are monitored for the non- and base-isolated NPP structures while shear deformations of the base isolator are additionally monitored for the base-isolated NPP. A series of Pearson's correlation coefficients are calculated to recognize the correlation between each of the 23 earthquake IMs and responses of NPP structures. The numerical results demonstrate that there is a significant difference in the correlation between earthquake IMs and seismic responses of non-isolated NPP structures considering low- and high-frequency ground motion groups. Meanwhile, a trivial discrepancy of the correlation is observed in the case of the base-isolated NPP subjected to the two groups of ground motions. Moreover, a selection of PGA or Sa for seismic response analyses of NPP structures in the high-frequency seismic regions may not be the best option. Additionally, a set of fragility curves are thereafter developed for the base-isolated NPP based on the shear deformation of lead rubber bearing (LRB) with respect to the strongly correlated IMs. The results reveal that the probability of damage to the structure is higher for low-frequency earthquakes compared with that of high-frequency ground motions.

Effects of Significant Duration of Ground Motions on Seismic Responses of Base-Isolated Nuclear Power Plants

The purpose of this study is to investigate the effects of the significant duration of ground motions on responses of base-isolated nuclear power plants (NPPs). Two sets of ground motion records with short duration (SD) and long duration (LD) motions, scaled to match the target response spectrum, are used to perform time-history analyses. The reactor containment building in the Advanced Power Reactor 1400 (APR1400) NPP is numerically modeled using lumped-mass stick elements in SAP2000. Seismic responses of the base-isolated NPP are monitored in forms of lateral displacements, shear forces, floor response spectra of the containment building, and hysteretic energy of the lead rubber bearing (LRB). Fragility curves for different limit states, which are defined based on the shear deformation of the base isolator, are developed. The numerical results reveal that the average seismic responses of base-isolated NPP under SD and LD motion sets were shown to be mostly identical. For PGA larger than 0.4g, the mean deformation of LRB for LD motions was bigger than that for SD ones due to a higher hysteretic energy of LRB produced in LD shakings. Under LD motions, median parameters of fragility functions for three limit states were reduced by 12% to 15% compared to that due to SD motions. This clearly indicates that it is important to select ground motions with both SD and LD proportionally in the seismic evaluation of NPP structures.

Seismic Fragility of Base-Isolated Nuclear Power Plant Considering Effects of Near-Fault Ground Motions

Seismic Fragility of Base-Isolated Nuclear Power Plant Considering Effects of Near-Fault Ground Motions

Wavelet analysis of soil-structure interaction effects on seismic responses of base-isolated nuclear power plants

Seismic base isolation has been accepted as one of the most popular design procedures to protect important structures against earthquakes. However, due to lack of information and experimental data the application of base isolation is quite limited to nuclear power plant (NPP) industry. Moreover, the effects of inelastic behavior of soil beneath base-isolated NPP have raised questions to the effectiveness of isolation device. This study applies the wavelet analysis to investigate the effects of soil-structure interaction (SSI) on the seismic response of a base-isolated NPP structure. To evaluate the SSI effects, the NPP structure is modelled as a lumped mass stick model and combined with a soil model using the concept of cone models. The lead rubber bearing (LRB) base isolator is used to adopt the base isolation system. The shear wave velocity of soil is varied to reflect the real rock site conditions of structure. The comparison between seismic performance of isolated structure and non-isolated structure has drawn. The results show that the wavelet analysis proves to be an efficient tool to evaluate the SSI effects on the seismic response of base-isolated structure and the seismic performance of base-isolated NPP is not sensitive to the effects in this case.

Seismic fragility evaluation of the base-isolated nuclear power plant piping system using the failure criterion based on stress-strain

Abstract In the design criterion for the nuclear power plant piping system, the limit state of the piping against an earthquake is assumed to be plastic collapse. The failure of a common piping system, however, means the leakage caused by the cracks. Therefore, for the seismic fragility analysis of a nuclear power plant, a method capable of quantitatively expressing the failure of an actual piping system is required. In this study, it was conducted to propose a quantitative failure criterion for piping system, which is required for the seismic fragility analysis of nuclear power plants against critical accidents. The in-plane cyclic loading test was conducted to propose a quantitative failure criterion for steel pipe elbows in the nuclear power plant piping system. Nonlinear analysis was conducted using a finite element model, and the results were compared with the test results to verify the effectiveness of the finite element model. The collapse load point derived from the experiment and analysis results and the damage index based on the stress-strain relationship were defined as failure criteria, and seismic fragility analysis was conducted for the piping system of the BNL (Brookhaven National Laboratory) - NRC (Nuclear Regulatory Commission) benchmark model.

Dynamic assessment of the seismic isolation influence for various aircraft impact loads on the CPR1000 containment

Abstract An aircraft impact (AI) on a nuclear power plant (NPP) is considered to be a beyond-design-basis event that draws considerable attention in the nuclear field. As some NPPs have already adopted the seismic isolation technology, and there are relevant standards to guide the application of this technology in future NPPs, a new challenge is that nuclear power engineers have to determine a reasonable method for performing AI analysis of base-isolated NPPs. Hence, dynamic influences of the seismic isolation on the vibration and structural damage characteristics of the base-isolated CPR1000 containment are studied under various aircraft loads. Unlike the seismic case, the impact energy of AI is directly impacting on the superstructure. Under the coupled influence of the seismic isolation and the various AI load, the flexible isolation layer weakens the constraint function of the foundation on the superstructure, the results show that the seismic isolation bearings will produce a large horizontal deformation if the AI load is large enough, the acceleration response at the base-mat will also be significantly affected by the different horizontal stiffness of the isolation bearing. These concerns require consideration during the design of the seismic isolation system.

Response of Systems and Components in a Base-Isolated Nuclear Power Plant Building Impacted by a Large Commercial Aircraft

AbstractEarthquake shaking must be addressed in the design of a nuclear power plant (NPP). The impact of a large commercial airliner on a nuclear power plant building is considered a beyond-design-...

Seismic incidence on base-isolated nuclear power plants considering uni- and bi-directional ground motions

The effects of bi-directional ground motions of the input seismic on base-isolated Nuclear Power Plants are investigated by comparing to the responses under uni-directional ground motion. For this aim, an analytical model is used to calculate the incidence angle (IA) of ground motions that vary from 0 to 360 degrees, with the interval of 15 degrees, and to show their effects on diverse engineering demand parameters. The observed responses corresponding to the displacement, the rotation and the element force are determined. In addition, the critical angles of the structure under two components with respect to one component are obtained. It is found that the maximum responses of the base-isolated structure for individual records may be decreased at some angles while increased at other angles. In case of bi-directional ground motions, the effect of IA is less considerable than uni-directional ground motion. Moreover, the critical responses under bi-directional seismic excitations are more adequate than uni-directional one. The displacement and shear force responses from one component are less than corresponding values from two components.

Effect of the incoherent earthquake motion on responses of seismically isolated nuclear power plant structure

Base-isolated nuclear power plant (BI-NPP) structures are founded on expanded basemat as a flexible floating nuclear island, are still lacking the recommendation of the consideration of incoherent motion effect. The effect of incoherent earthquake motion on the seismic response of BI-NPP structure has been investigated herein. The incoherency of the ground motions is applied by using an isotropic frequency-dependent spatial correlation function to perform the conditional simulation of the reference design spectrum compatible ground motion in time domain. Time history analysis of two structural models with 486 and 5 equivalent lead plug rubber bearing (LRB) base-isolators have been done under uniform excitation and multiple point excitation. two different cases have been considered: 1) Incoherent motion generated for soft soil and 2) Incoherent motion generated for hard rock soil. The results show that the incoherent motions reduce acceleration and the lateral displacement responses and the reduction is noticeable at soft soil site and higher frequencies.