Seismic performance of base-isolated AP1000 shield building with consideration of fluid-structure interaction

Nuclear Engineering and Design 353 (2019) 110241

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Seismic performance of base-isolated AP1000 shield building with consideration of fluid-structure interaction

T

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Dayang Wanga,b, Yongshan Zhanga, Chengqing Wua,c, Guofeng Xuea, , Wencheng Huanga a

School of Civil Engineering, Guangzhou University, 510006, PR China Guangdong Engineering Research Centre for Metal Cladding and Roofing System (GDERC-MCRS), 510006, PR China c School of Civil and Environmental Engineering, University of Technology Sydney, NSW 2007, Australia b

A R T I C LE I N FO

A B S T R A C T

Keywords: Shield building Base isolation Water level FSI effect Seismic performance Finite element method

Seismically-induced FSI effect on dynamic responses of AP1000 shield building with base isolation is focused in this study using numerical simualtion. The numerical model is firstly validated by comparison with existing experimental results, which is capable of simulating dynamic behaviors of the water partially-filled shield building with seismic isolation using high damping rubber (HDR) bearings. The influences of FSI on seismic performance of the base-isolated and base-fixed AP1000 shield building with various water levels and on the corresponding isolation effectiveness are comparatively explored in details. Results show that base isolation can reduce the fundamental frequency of the shield building and make it close to water sloshing frequency. It is necessary to ensure an reasonable isolation design to keep the fundamental frequency away from the sloshing frequency and then to avoid the water resonance. Seismic isolation can offer a substantial benefit for the earthquake-resistant design of the shield building even filled with different levels of water, because the structural primary resonance is effectively transformed to the sub-resonance by application of base isolation. Dynamic responses of base-isolated models are influenced significantly by FSI and an optimal water level ratio of 0.8 is suggested to achieve excellent seismic performance for such structures.

1. Introduction Focus on nuclear energy industry has been stimulated greatly in many countries nowadays as extensive market demand on clean and reliable energy, such as America, France and China. Approximately 15% of the world’s annual electricity production each year is contributed by nuclear power plants (NPP), which reduces about 2.5 billion tons of CO2 emission (Meiswinkel et al., 2013). Unfortunately, around 20% of NPPs worldwide are established in seismic areas (Medel-Vera and Ji, 2016). Seismic resistance have always been the main consideration of the design and construction of NPPs. Although plentiful efforts and prominent achievements in the seismic engineering, disastrous damage for building structures frequently happens in earthquakes (Lo Frano and Forasassi, 2010). The damage of Fukushima DaiIchi NPP is a typical case in 3.11 East Japan earthquake, which causes great worry about seismic performance of nuclear structures (Seong et al., 2018). More efforts have to be made on protection of nuclear power safety associated with external natural and man-made disasters, especially for earthquakes (Forni et al., 2014). Seismic isolation, as a recognized seismic technology, has been

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successfully used in plentiful engineering applications of civil structures to mitigate earthquake-induced vibrations (Cancellara and De Angelis, 2017; Baratta and Corbi, 2004; Lewandowski et al., 2016). Seismic isolation can protect a building from being damaged by uncoupling the isolated building from external disturbances (Cancellara and De Angelis, 2017). Although there are lots of applications of base isolation in civil engineering, only two NPPs adopt this technology in practical design till now, namely Koeberg NPP in South Africa and Cruas NPP in France (Coladant, 1991). The reason of the limited application of base isolation in NPPs is due to lack of codes, guidelines and standards for analysis, design and construction of base isolation of nuclear structures (Whittaker and Kumar, 2014). Even so, research in this aspect has never stopped. Initial research of NPP with isolation technology can be traced back to the early 1980s (Bhatti et al., 1982), which investigate effectiveness of base isolation and damped interaction between a stream generator and its primary housing structure under earthquake excitation using experiment and theory methods. After this forward-looking start, studies on seismic protection involving NPP are greatly stimulated and valuable researches are contributed by scholars from various countries (Medel-Vera and Ji, 2015; Ambrosini et al., 2017; Lee et al.,

Corresponding author. E-mail addresses: [email protected] (D. Wang), [email protected] (Y. Zhang), [email protected] (G. Xue).

https://doi.org/10.1016/j.nucengdes.2019.110241 Received 23 June 2019; Received in revised form 30 July 2019; Accepted 31 July 2019 Available online 08 August 2019 0029-5493/ © 2019 Elsevier B.V. All rights reserved.

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engineering to control structural responses caused by external excitations like earthquakes or strong winds (Dall’Asta and Ragni, 2008; Dall’Asta and Ragni, 2006; Wang et al., 2015). One reason for the promising HDR bearing is that its elasto-plastic dissipating behavior is considered to be preferable as the filled rubber is a fading memory material, which means that permanent deformation will be avoided even after strong earthquakes. Another is that ideal energy dissipation can be generated with lower strain-rate sensitivity even under small external excitations. Furthermore, materials used in HDR bearing are best-known harmlessness to environments. Considering these advantages, HDR bearing is adopted in this study as the isolation bearing of AP1000 shield building.

2016; Zhang et al., 2001; Wang et al., 2018). Benefits of a base-isolated NPP located in eastern U.S. on rock site are discussed based on seismic probabilistic risk assessment procedure by comparison with a conventional nuclear structure (Huang et al., 2010). Effects of impacting loading on dynamic responses of seismically isolated NPPs are conducted and the corresponding forces are captured by a moat wall model to investigate variable parameter properties on the impact responses (Sarebanha et al., 2018). To improve performance of the isolation layer for nuclear structures, different isolation bearings were proposed, such as lead-rubber bearings (Buckle, 1985; Fujita, 1991), air spring bearings (Suhara et al., 2003; Shimada et al., 2005), elastomeric bearings (Kumar et al., 2014) and sliding bearings (Kumar et al., 2015). Experimental tests under various loading conditions along with hybrid numerical simulations (Schellenberg et al., 2015; Sarebanha, 2018) on these isolation bearings contribute positive data and insight to their practical applications and scientific researches in the base-isolated NPPs. In this study, the investigators assess the dynamic performance of base-isolated AP1000 nuclear shield building in the case of seismicallyinduced fluid–structure interaction (FSI) effect, aiming to reveal FSI effect on seismic responses of the base-isolated shield building. It is well known that a passive containment cooling water tank (PCCWT) with capacity of about 3000 tons of water is equipped on top of the shield building to provide cooling system for AP1000 in emergencies, which may obviously generate influence on vibration responses under external excitations (Lu et al., 2015). Studies on FSI effect of nuclear structure with water-filled tanks can be found with valuable conclusions (Song et al., 2017; Zhao et al., 2014; Bogaers et al., 2016; Zhao et al., 2016). However, these studies concentrate mainly on NPP with fixed foundation. A recent study on the seismically-caused FSI effect for ELSY (European lead-cooled system) reactor is investigated and the adverse resonance effect of isolation system is demonstrated because FSI induced hydrodynamic loads (Jeltsov et al., 2018). The seismically-induced FSI effect on dynamic responses of nuclear structures with base isolation is still an unsolved topic. For example, whether resonance of water sloshing in PCCWT can be aroused for the isolated NPP, and how about the isolation effectiveness considering FSI effect and influence of different water levels on the isolated NPP vibrations. These unsolved topic will be concentrated and therefore become the focus of this study. Results of this study is expected to highlight these topics to provide technical reference for NPP design.

2.1. Nonlinear behavior simulation and simplification of HDR bearing 2.1.1. HDR modelling In the initial phase of this study, a strict simulation with finite element method is performed to investigate the mechanical characteristics of HDR bearing. The corresponding numerical results are compared to existing experimental data (Markou and Manolis, 2016) to validate the finite element process. Geometrical characteristics of HDR bearing used in the experiments (Markou and Manolis, 2016) are listed in Table 1. Physical characteristics of HDR bearing are summarized in Table 2. ANSYS platform (Ansys release, 2011) is adopted to simulate the HDR bearing. Components of both steel and rubber are modeled by solid-185 element. The solid-185 element is capable of capturing material’s properties of plasticity and hyperelasticity. Nonlinear property of steel component is modeled by classical bilinear kinematic hardening plasticity algorithm, which employs Von Mises yield criterion together with the kinematic hardening rule. Fig. 2 shows the finite element modelling of HDR bearing. The rubber material of HDR bearing is assumed to be hyperelastic (Moslemi and Kianoush, 2016) and strain energy potentials, namely Ogden constitutive law considering Mullins effect, are adopted to describe the constitutive law of the bearings’ hyperelastic material (Ogden, 1984). Expression of Ogden constitutive can be written as: N

W=

N

μ

∑ αi (λ¯1αi + λ¯2αi + λ¯3αi − 3) + ∑ i=1

i

k=1

1 (J − 1)2k dk

(1)

where, W is the strain energy potential function, J is the ratio of deformed elastic volume over undeformed volume of material, λ¯1, λ¯2 and λ¯3 are the modified principal stretch ratios, μi , αi and dk are the material constants, which can be defined by experimental force–displacement data of rubber materials. N is the order of the Ogden potential function, which is considered to be more consistent with the exact solution if higher order of N is used. But paradoxically, numerical difficulty of fitting the material constants appears if N > 3 (Moslemi and Kianoush, 2016). The order of N is thus adopted as 3 in this study. Then, the material constants can be obtained by the least squares fit optimization methodology based on the test data (Markou and Manolis, 2016), which are μ1 = 532,883, α1 = 1.52, μ2 = -21.634, α2 = −1.72, μ3 = 188, α3 = 5.93. In addition, by assuming incompressible rubber material, the material constant dk can be defined as 0.

2. Seismic isolation design and FSI realization Seismic isolation is designed by a flexible isolation bearing layer between superstructure and foundation, which reduces earthquake energy transmission from the foundation to the superstructure and then decreases the dynamic response of the superstructure. There are various seismic isolation bearings, such as HDR bearing, lead-rubber bearing and friction pendulum bearing (Micheli et al., 2004). HDR bearing is generally made up of natural rubber with black carbon filler added, aiming to enhancing a wide range of desirable material properties, such as strength and damping capacities, as shown in Fig. 1. As an energy dissipation device, HDR bearing is promising for application in civil

Table 1 Geometrical characteristics of HDR bearing.

Fig. 1. Components of the high damping rubber (HDR) bearing. 2

Parameter

Value

Parameter

Value

External diameter Diameter of steel plates Thickness of steel plates

500 mm 490 mm 3 mm

188574.1 mm2 169 mm 12

Thickness of single rubber layer Total rubber thickness

8 mm

Cross section area Total height Number of rubber layers Primary shape factor Secondary shape factor

5.10

96 mm

15.31

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Table 2 Physical characteristics of HDR bearing. Steel component

Value

Rubber component

Value

Young’s modulus Yield stress Density Poisson’s ratio

205 GPa 275 MPa 7850 kg/m3 0.28

Initial shear modulus Density Poisson’s ratio NA

0.42 MPa 1150 kg/m3 0.4995 NA

2.1.2. Model verification and simplification Consistent cyclic loading conditions with the literature (Markou and Manolis, 2016), namely loading frequency of 0.5 Hz and vertical compressive stress of 6 MPa, are ensured to obtain comparable data. The force-displacement curves corresponding to experimental and numerical approaches are indicated in Fig. 3. As shown in Fig. 3, good agreement between the experimental and the current numerical results is achieved in all loading cycles with different deformations, indicating that the finite element procedure and material’s constitutive law employed in this study are feasible and credible of capturing mechanical behaviors of typical rubber bearings. It will be very time-consuming and even impossible to employ solid finite element model of HDR bearing in seismic analysis of nuclear structures as the mesh size of the solid model is too small, namely 3 mm of the steel plate and 8 mm of the rubber for the HDR bearing. Besides, it is well known that the isolation layer of nuclear structure is dominated by shear deformation and its bending deformation can be ignored. Thus, an idealized constitutive law, bilinear constitutive law representing HDR bearing, is further considered to simulate the mechanical characteristics of HDR bearings. To describe the bilinear constitutive law, three key parameters, elastic stiffness (ke), yield strength (Fy) and plastic stiffness (kp), should be firstly defined based on the experimental data or theoretical calculation with the following formulations (Naeim and Kelly, 1999):

keq =

Geq A hd

, kp = keq (1 − μ), ke = αkp

Fig. 3. Comparison of force-displacement curves between experiment and simulation.

experiment, the solid finite element model and the simplified bilinear constitutive law, is shown in Fig. 4, in which the three parameters are calculated in view of the corresponding experimental data based on the above equations. As indicated in the two figures, reasonable fitting can be observed between the results predicted by the simplified constitutive law and the experimental data, which means that the simplified constitutive law is capable of representing the mechanical behaviors of HDR bearing, which will then be used in the following discussions. 2.2. Application of HDR bearing in AP1000 shield building 2.2.1. Overview of AP1000 shield building AP1000 shield building is a key component with strict seismic requirement, as shown in Fig. 5. At the top of the shield building, water partially-filled PCCWT is equipped and the filled water can be drained by gravity from its container to guarantee 72-hour safety without operation for AP1000 steel containment vessel in emergencies. The shield building has an outer diameter of 44.2 m, an inner diameter of 43.288 m, a height of 83.37 m and a thickness of 912 mm. PCCWT has an out diameter of 27.13 m, an inner diameter of 10.668 m, a height of 11.8 m and a thickness of 600 mm. There’re certainly some internal equipments in the shield building, such as pipe and lifting systems, which are not considered in this computational model as it is very hard to take into account all the assembled complex internals at a time and some limitations also exist in CPU and memory space of a computer. As an alternative, these internals are simulated by mass and applied on the model (Zhao et al., 2014). The shield building is made of reinforced concrete with axial compressive strength of 29.6 MPa, Poisson ratio of 0.2, and ultimate compressive strain of 0.0035. Density of the filled water is 1000 kg/m3. Plastic damage constitutive model proposed by Popovics (1973) and Yip (1998) is used to describe mechanical behavior of the reinforced

(2)

Q = μkeq hd γ

(3)

where, keq , kp and ke are the equivalent shear stiffness, plastic stiffness and elastic stiffness respectively. Q is the characteristic strength. A and hd are the cross sectional area and rubber thickness of HDR bearing. α is a constant of the stiffness ratio before and after yielding, which is taken as about 10. γ is the shear strain. Geq and μ are the shear elastic modulus and ratio of the characteristic strength to the maximum hysteretic shear force, which can be expressed as follows when the shear strain are in the range of 10%~270% (Naeim and Kelly, 1999): 4 3 2 ⎧Geq (γ ) = 0.62(0.1364γ − 1.016γ + 2.903γ − 3.878γ + 2.855) 3 2 ⎨ μ (γ ) = 0.408(0.03421γ − 0.2083γ + 0.2711γ + 0.9028) ⎩

(4)

Comparison of the force-displacement curves, obtained from the

Fig. 2. Three dimensional finite element model of HDR bearing. 3

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Fig. 4. Comparison of force-displacement curves of HDR bearing.

bearing can be represented as a simplified model with the bilinear constitutive law. Thus, the HDR bearing is simulated by two-node link model in ANSYS platform, namely combin-40 element, which has capacity of simulating plasticity, large strain and shearing behavior with acceptable accuracy (Markou and Manolis, 2016). Vertical carrying capacity of HDR bearing is simulated by two-node linear spring element of combin-14. Fig. 6 shows the three dimensional finite element model and the corresponding HDR layout of the base-isolated AP1000 shield building. 2.3. FSI realization Fluid domain of the water filled shield building is simulated by eight-node isoparametric brick fluid element, which has four DOFs, namely three-direction translational displacements and one vertical pressure. Stiffness and mass matrices and all related components are based on 2 × 2 × 2 Gaussian integration points in the fluid domain. To perform coupling analysis between fluid and structure, FSI label is specified at the mesh interface between the fluid domain and the structural domain. The fluid elements located at the interface are defined to be FSI specification, namely FSI flag will be assigned at the interface to reflect FSI coupling. Instead, removing the FSI flag will obviously clear FSI specification. To perform this, all of the mesh nodes of both the fluid elements and structural elements located at the fluidstructure interfaces should be collected firstly and then are assigned with the FSI flag, which ensures that the boundary conditions can be defined reasonably at different interfaces surrounding the fluid domain. By performing such operations, FSI specification at the side and the bottom fluid-structure interfaces will be finally finished, indicating that the structural nodes will be coupled with corresponding fluid nodes. DOFs is further defined at the fluid-structure interfaces. DOFs of threedirection translational displacements are defined for the structural nodes contacting with the fluid nodes in the domain of structural model. DOFs of three-direction translational displacements and pressure are defined for the fluid nodes contacting with the structural nodes in the fluid model domain. In addition, at top face of the fluid, no FSI specification is required as this face is a free water surface. But atmospheric pressure is applied on this top surface. Seven numerical models of AP1000 shield building with different water levels in PCCWT are established. The water levels are 0 m (WLM1), 5.1 m (WLM2), 6.8 m (WLM3), 7.7 m (WLM4), 8.6 m (WLM5), 9.7 m (WLM6), and 10.7 m (WLM7) respectively. WLM1 means an empty tank with no water filled in PCCWT, and WLM7 means a full tank filled with water. Obviously, water level ratios of WLM1 ~ WLM7, defined by the ratio between the filled height of water and the whole clear height of PCCWT, are 0, 0.48, 0.64, 0.72, 0.80, 0.91 and 1.0 respectively. Fig. 7 shows the seven models of partially-filled AP1000 shield building.

Fig. 5. Overview of AP1000 shield building (Meiswinkel et al., 2013).

concrete. Damping ratio of 7% is defined according to AP1000 European DCD of Westinghouse (0000). Rock site is assumed without consideration of soil-structure interaction (Wang et al., 2018, GB50011-, 2016). ANSYS is adopted to perform the finite element analyses. Structural domain of the shield building is simulated by fournode quadrilateral shell element, which has bending and membrane properties with six DOFs, namely three-direction translational displacement and three-direction rotational displacement. Stiffness and mass matrices and all related components are based on 2 × 2 Gaussian integration points in the structural members. 2.2.2. Base isolation design In this study, HDR bearing with a diameter of 1000 mm, rubber height of 204 mm and shear modulus of 0.62 MPa at 100% shear strain is selected. The mechanical properties of HDR bearing can be calculated based on the formulations (1)–(4), namely equivalent horizontal stiffness of 2.32 kN/mm, elastic stiffness of 13.73 kN/mm, plastic stiffness of 1.37 kN/mm, characteristic strength of 198.72 kN, yield force of 550 kN, and the maximum deformation of 280 mm at 100% shear strain. Vertical stiffness of the selected HDR bearing is 2.2 × 106 kN/m. Totally 157 HDR bearings with the same mechanical properties are designed in the AP1000 shield building. Each of the HDR bearing undertakes an average vertical force of 5,107.28 kN, which is less than the ultimate bearing capacity of 9200 kN. As discussed previously, the mechanical performance of HDR 4

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Fig. 6. Seismic isolation model of AP1000 shield building.

3. Validation of finite element model

3.2. Validation on dynamic response analysis

3.1. Convergence analysis

To further validate the proposed finite element technique, an experimental and numerical investigation on fluid sloshing and FSI effect of AP1000 shield building (Lu et al., 2015) is conducted. The geometry dimensions of the shield building in the comparative literature are the same with those of this study. The water level in PCCWT is defined as 8.6 m with density of 1000 kg/m3. The same reinforced concrete is adopted with density of 2300 kg/m3, elastic module of 33.5 GPa and Poisson ratio of 0.2. Earthquake input of El Centro wave with PGA of 0.3 g is used to perform seismic analysis based on Newmark transient method. Damping ratio of 7% is accounted for structural damping. The computational results obtained from this study are compared with the comparative literature, as shown in Table 4. Good agreement can be observed in view of the natural frequency, top structural peak response and water sloshing response, which can be further approved by the time history response comparisons shown in Fig. 8. There are certainly some inevitable errors with the maximum error of 6.66% for the height of water sloshing at the inner wall. The reason for these errors may be synthesized factors, such as mesh processing, material constitutive

Convergence analysis is conducted by a model with four mesh sizes, 800 mm (Case A), 1000 mm (Case B), 1500 mm (Case C) and 2000 mm (Case D) respectively. The water level is set as 7.7 m for all the four Cases. Three-direction artificial earthquake with PGA of 0.3 g:0.3 g:0.2 g is adopted. Computation is carried out on the same computer with 16 Xeon E5580 3.2 GHz CPUs and 16 GB Infineon memory. Results are summed up in Table 3. It can be obtained that the computational time for Case A is 4.78 times to that of Case B, 11.29 times to Case C, and 16.78 times to Case D. Furthermore, errors of the top accelerations for Case B, Case C and Case D in Y-direction are 0.91%, 4.4%, 21.55% respectively by comparing with Case A. Therefore, mesh size of 1000 mm is adopted considering both resultant precision and computational time.

Fig. 7. Finite element models of AP1000 shield building with different water levels. 5

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Table 3 Mesh convergence analysis. Case

Case A Case B Case C Case D

Type

Value Value Error Value Error Value Error

Total number

Computer time (h)

Element

Node

78,277 30,194 NA 12,616 NA 7415 NA

105,818 47,702 NA 20,963 NA 12,706 NA

Basic period (s)

Peak acceleration 2

36.92 7.73 NA 3.27 NA 2.2 NA

0.2768 0.27627 0.19% 0.27579 0.36% 0.27286 1.42%

Peak displacement

X (m/s )

2

Y (m/s )

X (mm)

Y (mm)

8.85 8.74 1.24% 8.65 2.26% 7.64 13.67%

6.59 6.53 0.91% 6.3 4.40% 5.17 21.55%

22.28 22.04 1.08% 21.82 2.06% 18.81 15.57%

4.24 4.22 0.47% 4.22 0.47% 3.02 28.77%

Note: X and Y represent the horizontal and vertical directions. Error = ABS (Case B/C/D – Case A)/Case A × 100%. Peak acceleration and peak displacement refer to the dynamic responses of the top point.

base-isolated liquid storage tanks (Committee, 2006). For example, the impulsive frequencies are changed from 7.555 Hz to 9.875 Hz for WLM4, from 6.013 Hz to 7.116 Hz for WLM6, from 5.453 Hz to 6.504 Hz for WLM7. However, with increasing of the water levels, both the convective and impulsive modes show a gradual trend of change, namely increasing for the former and decreasing for the latter, indicating that the dynamic characteristics of both the base-isolated and base-fixed models can be influenced by water level. In addition, the isolation system is associated with a large displacement for the isolation layer and very small superstructure drift amplitude, which can be referred as typical rigid body motion. As presented in the table, the frequency of WLM1 is shifted from 2.885 Hz (base-fixed model) to 0.328 Hz (base-isolated model), which is similar to other six models, indicating slow motion of the shield building as base isolation. Further investigation on free vibration analysis is performed to discuss whether the mode characteristics of the partially-filled shield building have contributions to dynamic responses, especially for resonance vibration. It is well known that the fundamental frequency of the base-isolated model should be lower than that of the impulsive mode in the corresponding base-fixed model for an effective isolation design. Besides, the fundamental frequency of the base-isolated model must keep away from the fundamental convective frequency to avoid that excessive increase of the sloshing height due to resonance effect. It can be observed from Table 5 that the convective frequencies are not in close vicinity to the fundamental frequencies of the base-isolated models, indicating that no resonance sloshing can be aroused under the designed parameters of HDR bearings. For example, the fundamental frequencies for the convective and base-isolation modes are 0.154 Hz and 0.330 Hz for WLM2, 0.195 Hz and 0.332 Hz for WLM4, 0.208 Hz and 0.334 Hz for WLM6. Although the two frequencies are closer with the increasing water levels, the resonance effect can still be avoided effectively based on reasonable isolation design. Actually, the impulsive modes do not contribute much to the overall dynamic response or the resonance response of base-isolated structure (Moslemi and Kianoush, 2016; Committee, 2006), which means that there is basically no need to take into account the influence of the parameters that are relevant to the impulsive frequency, such as the stiffness of water tank and that of the supporting structure. Consequently, to avoid the resonance effect, only the parameters associated with the convective and isolation structural frequencies should be considered. The isolation structural

model and solving algorithm. But on the whole, the resultant comparison seems to be consistent with reasonable errors. 3.3. Validation on base isolation Design criteria (Kim and Lee, 1995) is used to assess the feasibility of the above designed base isolation of AP1000 shield building. Two main indexes, including fundamental frequency and maximum horizontal displacement, are used for consideration of judging the reasonability. Firstly, the fundamental frequency of the designed isolated shield building is 0.328 Hz, which is in the common range of 0.25 Hz1 Hz for base-isolated structures (Moslemi and Kianoush, 2016; Kim and Lee, 1995). In addition, if the designed isolated shield building is simplified as a single degree of freedom, its fundamental frequency can be calculated to be 0.335 Hz based on theory formulation of T = 2π M / K , where T is the isolated structural period, M is the mass of superstructure and K is the total horizontal equivalent stiffness of the isolation layer. Obviously, the frequency error between simulation and theory calculation is only 2.09%. Secondly, beyond design basis earthquake considering 1.5 times of PGA for the design basis earthquake is taken into account in artificial wave and the maximum horizontal displacements of the designed isolated shield building are 269 mm in X-direction and 243 mm in Y-direction, which are all less than the maximum design deformation of 280 mm for HDR bearing. It can then be concluded that the designed isolation layer parameters of the shield building are reasonable and feasible. 4. Results and discussion 4.1. Free vibration analysis Fundamental frequencies of the convective, impulsive and baseisolated models of AP1000 shield building as well as those of base-fixed models are shown in Table 5. The convective mode is associated with the free surface motion of the water in PCCWT. It can be found that the fundamental frequencies of the convective modes are close between the base-isolated and base-fixed models at any water levels, while the fundamental frequencies of the impulsive modes for the base-isolated models are higher than that of the base-fixed models at any water levels, which is consistent with the study on seismic performance of the Table 4 Result comparison between this study and the comparative literature. Item

This study Literature (Lu et al., 2015) Error

Natural frequency (Hz)

Top peak response 2

Height of water sloshing

Shield building

Water sloshing

Acceleration (m/s )

Displacement (mm)

Outer wall (mm)

Inner wall (mm)

3.72 3.89 4.37%

0.106 0.109 2.75%

14.65 14.66 0.07%

25.12 24.00 4.67%

291.56 286.13 1.90%

435.25 408.06 6.66%

Note: Error = ABS (This study – Literature)/Literature × 100%. 6

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Fig. 8. Comparison of time history curves of top responses between (Lu et al., 2015) and this study.

base-fixed models, as shown in Fig. 10. In this study, the X-direction and the Z-direction are defined as horizontal directions, while the Ydirection is defined as vertical direction. It can be found that the horizontal responses are greatly reduced for the base-isolated models. As presented in these figures, the top peak accelerations of WLM5 are reduced from 9.87 m/s2 to 3.6 m/s2 in X-direction in EW1, from 8.262 m/ s2 to 3.08 m/s2 in Z-direction in EW7, namely vibration control effectiveness of 63.5% and 62.7% respectively. The contrast results show clearly that period elongation effect (Moslemi and Kianoush, 2016) of the base-isolated models is achieved with respect to transient response by comparison with the base-fixed models. Besides, the time history curves appear slow and gentle vibration for the base-isolated models, while the base-fixed models appear fast and intense vibration. Relative horizontal displacements are investigated and shown in Fig. 11&12. It can be seen that the horizontal acceleration and displacement responses along structural height of the base-isolated models appear almost a straight line, as shown in Fig. 11. The horizontal acceleration and displacement responses of the base-fixed models increase with increasing height, namely from zero at the base point to the maximum value at the top point with linear increasing pattern, as shown in Fig. 12. These results demonstrate that the transient vibration behaviors of the base-isolated models show a rigid body motion, which means that classical translational characteristics can be used to describe the motion of the base-isolated systems (Kim and Lee, 1995). The horizontal responses of the base-isolated models are all less than those of the base-fixed models, and the response of the former mainly concentrates at the isolation layer, indicating that HDR bearings undertake most of the structural dynamic responses and therefore results in lateral translations of the superstructure. Obviously, reduction of the superstructure responses for the base-isolated models is at the expense of large deformation of the isolation layer, which therefore requires an optimal design of HDR bearings to achieve the lowest ground motion propagation to the superstructure and to ensure operation safety. It can be acquired that the average horizontal deformation of the isolation layer in seven earthquakes are 200 mm in X-direction and 175 mm in Zdirection respectively, which are less than the designed deformation of 280 mm for HDR bearings under 100% shear strain (Committee, 2006), namely reliable isolation design is guaranteed in this study. Investigation of FSI effect on dynamic responses of the base-isolated models is also a main research concentration of this study. Relationship between peak vibration response and water level in the horizontal directions are shown in Fig. 13. It can be found there’s significant influence of FSI on structural responses in both horizontal directions, indicating that FSI effect on seismic behavior of base-isolated shield building must be taken into account. On the whole, the peak responses appear to be higher for the models of WLM2 (water level ratio of 0.48), WLM3 (water level ratio of 0.64) and WLM4 (water level ratio of 0.72), which means that negative influence will be caused by FSI effect for the shield buildings with water level ratio of less than 0.72. After that, the peak responses appear a tendency of decreasing firstly and then rising, and reaching the minimum at the water level ratio of 0.8 (WLM5). In other words, FSI effect will play positive influence on seismic response

Table 5 Free vibration analysis results. Model

WLM1 WLM2 WLM3 WLM4 WLM5 WLM6 WLM7

Natural frequency of base-fixed model (Hz)

Natural frequency of base-isolated model (Hz)

Partially-filled water in PCCWT

Partially-filled water in PCCWT

Convective

Impulsive

NA 0.160 0.192 0.202 0.209 0.215 0.218

NA 12.121 8.576 7.555 6.771 6.013 5.453

Whole structure

3.885 3.742 3.665 3.620 3.572 3.510 3.452

Convective

Impulsive

NA 0.154 0.187 0.195 0.201 0.208 0.211

NA 14.543 9.798 9.875 8.024 7.116 6.504

Whole structure

0.328 0.330 0.331 0.332 0.333 0.334 0.335

frequency is mainly dominated by mass of the superstructure and the stiffness of the isolation layer. The convective frequency is mainly dominated by the geometry property of the storage tank, namely the free surface diameter of the tank and the fluid equivalent height with the same volume and the same free surface diameter (Chopra, 2000). As presented in the table, the convective frequency is gradually changed from 0.154 Hz (WLM2) to 0.211 Hz (WLM7) as variation of the equivalent height for the partially-filled water in the same PCCWT. As a result, it can be obtained that the resonance effect can be reasonably avoided by adjusting the dominated parameters of the bearing stiffness and the geometry. 4.2. Time history analysis 4.2.1. Earthquake inputs Six natural earthquakes chosen from PEER Database, Kern County (EW1), San Fernando 01 (EW2), San Fernando 02 (EW3), Imperial Valley 02 (EW4), Borrego (EW5) and San Fernando 03 (EW6), and one artificial earthquake (EW7) are adopted. Three-direction earthquakes are used as input of the computational model. The components are scaled in such a way that the peak ground acceleration (PGA) in two horizontal directions are set as 0.3 g and vertical PGA is 2/3 of the horizontal direction. The three-direction acceleration spectrums of the seven earthquakes and RG 1.60 design standard spectrum are compared in Fig. 9. It can be observed that the seven spectrums are close to the corresponding standard spectrums in both the horizontal and vertical directions, and the mean spectrums are more agreement with the standard spectrum. 4.2.2. Horizontal dynamic response To investigate the horizontal vibration response of the partiallyfilled shield building and to study the control effectiveness of the baseisolated system, typical time history comparison of the horizontal top acceleration and base shear responses for WLM2 and WLM5 are discussed and contrasted with the results obtained from the corresponding 7

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Fig. 9. Acceleration response spectrum comparison with damping ratio of 5%

Fig. 10. Time history comparison between base-isolated and base-fixed models.

Fig. 12. Dynamic responses along structural height for base-fixed model.

of the base-isolated models, which further reduces the horizontal deformation and as a result of improving the robustness of nuclear structural safety. As presented in Table 6, the average peak displacement response of WLM5 in X-direction is 206.2 mm, 17.05% reduced by comparison to the maximum response of WLM2 (248.6 mm), and 13.25% for that in Z-direction. Reasonable design of water level in PCCWT is therefore demonstrated to have great significance, even for AP1000 shield building that has been protected by base isolation. In addition, it is worth mentioning that discreteness can be found for the structural responses among different earthquakes. The reason is that spectral characteristics of different earthquakes have great discreteness, which causes differential of the structural responses. To make structural design more statistically significant and acceptable, the average value of multi-earthquake waves (normally, 7 waves) is widely suggested in structural design (Committee, 2006).

Fig. 11. Dynamic responses along structural height for base-isolated model.

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Fig. 13. Relationship between top peak structural response and water level. Table 6 Horizontal average deformation of HDR bearing in the seven earthquakes (Unit: mm). EW

EW1 EW2 EW3 EW4 EW5 EW6 EW7 Ave.

WLM1

WLM2

WLM3

WLM4

WLM5

WLM6

WLM7

X

Z

X

Z

X

Z

X

Z

X

Z

X

Z

X

Z

207.4 231.9 214.1 252.9 183.8 210.7 231.2 218.9

168.2 207.3 215.1 231.2 116.7 143.6 202.6 183.5

235.1 263.5 257.3 276.5 214.7 237.0 255.9 248.6

179.9 230.7 225.4 259.3 127.6 161.3 215.8 200.0

225.1 237.1 238.3 263.2 196.4 227.0 251.8 234.1

174.6 220.0 225.3 245.0 121.2 158.9 211.1 193.7

231.4 240.8 248.7 267.7 201.0 227.2 252.2 238.4

174.9 224.8 228.5 250.6 123.1 165.0 212.9 197.1

207.3 206.0 202.9 231.2 170.3 205.3 220.4 206.2

146.7 198.2 202.5 218.1 108.9 149.3 190.7 173.5

216.0 212.2 215.5 243.9 184.7 225.3 242.2 220.0

160.8 205.0 227.0 226.0 111.9 157.6 202.6 184.4

223.3 215.0 225.7 248.4 189.0 227.7 245.6 225.0

163.3 209.5 230.6 231.2 113.8 164.1 206.1 188.4

with reasonable water level are all less than those of the model with empty PCCWT. This phenomenon of mitigating structural vibration may be explained by the reason that the partially-filled PCCWT with reasonable water level can act as an excellent absorbing device (Zhao et al., 2016), which is capable of dissipating seismic propagating energy to improve structural safety. It can be found that all of the top spectrums in Fig. 14 generate the first wave-crest with smaller response value at the frequency band of [0.3 Hz, 0.4 Hz]. As is known from Fig. 15, the magnitude of the earthquake inputs is small and continuous in the period range of [2.5 s, 3.33 s], which corresponds to the above frequency band of [0.3 Hz, 0.4 Hz]. Under this condition, no wave-crest of the dynamic response can be aroused normally by earthquake in this frequency band. But actually, as shown in Table 5, the fundamental frequencies of the baseisolated models are all in the same range of [0.3 Hz, 0.4 Hz], which implies that structural resonance will be caused when subjected to seismic excitation of this frequency band and therefore generates the first wave-crest. However, although the resonance is aroused for all the base-isolated models at this frequency band, the dynamic responses

4.2.3. Horizontal response spectrum Top acceleration response spectrum, established by Fourier transform, is adopted to explore FSI effect on vibration responses of the baseisolated models. The horizontal response spectrums of the top point of the seven base-isolated models are shown in Fig. 14. It can be obtained that WLM5 has the lowest peak spectral acceleration, followed by the models of WLM6, WLM7 and WLM1, and finally the models of WLM2, WLM3 and WLM4. For example, the spectral accelerations of WLM5, WLM6 and WLM7 in EW1 in X-direction are 14.09 m/s2, 15.58 m/s2, 15.91 m/s2, which are 80.86%, 89.4% and 91.3% of the maximum spectral acceleration (17.43 m/s2) of WLM2, as shown in Fig. 14(a). Correspondingly, the spectral accelerations in Z-direction are 14.42 m/ s2, 15.86 m/s2, 16.16 m/s2 for WLM5, WLM6 and WLM7, which are 80.87%, 89% and 90.66% of the maximum spectral acceleration (17.82 m/s2) of WLM2, as shown in Fig. 14(b). Similar phenomenon can also be detected in Fig. 14(c)-(f) for other earthquake excitation cases. These results highlight the fact that the base-isolated models with reasonable water level can play positive effect of mitigating the structural vibration. The structural responses of the base-isolated models 9

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Fig. 14. Response spectrum at top point of the shield building with different water levels.

caused by forced vibration caused by earthquake excitations. This is because the seismic energy is concentrated in the same frequency band of [0.1 s, 1 s], as shown in Fig. 15. Nevertheless, even though the larger and largest wave-crests with relatively greater structural response are caused by the forced vibration for the base-isolated models in this frequency band, which is only about 1/9 of that of the base-fixed models. As presented in Fig. 16, the peak spectral accelerations for the base-isolated and base-fixed models are 12.98 m/s2, 119.38 m/s2 respectively for WLM2, and 10.67 m/s2, 92.46 m/s2 respectively for WLM5, meaning that the responses of the former are only 1/9.2, 1/8.67 of the latter respectively. There are two factors of causing such a large structural response for the base-fixed models. On the one hand, the earthquake energy is primarily concentrated in the frequency band of [1 Hz, 10 Hz] as shown in Fig. 15, which clearly causes larger structural vibration. On the other hand, the fundamental periods of all the base-

resulted from the resonance are still smaller. This is because that the seismic excitation frequency band arousing the above resonance is not the main frequency band of an earthquake involving huge seismic energy. In other words, the earthquake energy involved in the frequency band of [0.3 Hz, 0.4 Hz] is too small to produce larger structural resonance responses, even though a small resonance is caused. The small resonance is defined as sub-resonance in this study, indicating gentle structural response represented by a small wave-crest in the spectrum curve. This phenomenon reveals great advantage of isolation system, meaning that the fundamental frequency of base-isolated structure is ensured to be far away from the main frequency band of an earthquake, so as to avoid seismically-induced excessive vibrations. Furthermore, also shown in Fig. 14, the larger and largest wavecrests of the spectrum acceleration of the base-isolated models are in the frequency band of [1 Hz, 10 Hz], which, as a matter of fact, is 10

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Fig. 15. Normalized earthquake input spectrum.

Fig. 16. Comparison of top response spectrum between base-isolated and base-fixed model.

fixed models, the average 3.635 Hz (0.275 s) as shown in Table 5, are found to be included in the earthquake main frequency band involving huge seismic energy, which doubtlessly generates violent resonance and then amplifies structural responses. This violent resonance, defined as primary resonance in this study, indicates strong structural dynamic response represented by greater wave-crests in the spectrum curve. It is obvious that the base-fixed models easily arouses the primary resonance as its fundamental frequency is included by the main frequency band of seismic excitations. As a result, to achieve an excellent seismic performance, base isolation technology is undoubtedly a good option.

respectively. It can be found that response reduction is obtained to a certain extent for the base-isolated models. However, combining with the discussion in the previous section, it can be observed obviously that more reduction is acquired by the horizontal isolation when compared to vertical isolation effect. This may be two aspects: one is horizontal additional damping effect resulting from hysteretic energy dissipation of HDR bearings, and another is the flexibility of HDR bearings in horizontal isolation case compared to the case of vertical isolation. As shown in Fig. 18, it can be found that the vertical displacement generates at both the HDR bearing and the superstructure for the baseisolated models. The displacements are 0 mm at the bottom of the basefixed models, while about 1 mm can be found at the bottom of the baseisolated models. The 1 mm is the axial compression displacement of HDR bearings. With increasing of the structural height, the vertical displacements appear also increasing for both the base-fixed and baseisolated models, which is the structural deformation due to material compressive strain. It can also be observed from Fig. 18 that the change

4.2.4. Vertical dynamic response Further discussion on vertical dynamic responses is carried out for the base-isolated and base-fixed models with variable water levels. Fig. 17 shows top acceleration responses of WLM5 in EW1 and EW7. Specific control effect of the peak acceleration and displacement along structural height for different models are shown in Table 7 and Table 8

Fig. 17. Time history comparison between base-isolated and base-fixed models. 11

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Table 7 Control effectiveness of the peak acceleration (Unit: m/s2). Height (m)

81.16 69.87 60.70 50.48 39.49 29.53 19.67 9.63

WLM2

WLM4

WLM6

WLM7

BFIX

BISO

δ(%)

BFIX

BISO

δ(%)

BFIX

BISO

δ(%)

BFIX

BISO

δ(%)

13.16 12.80 7.69 6.67 5.59 4.43 3.07 1.52

10.92 10.73 6.49 5.88 5.30 4.70 4.02 3.24

17.05% 16.20% 15.63% 11.87% 5.22% −6.14% −30.86% −112.6%

12.01 11.76 6.48 5.57 4.64 3.66 2.54 1.26

9.84 9.67 5.69 5.18 4.66 4.12 3.52 2.83

18.08% 17.77% 12.19% 6.97% −0.34% −12.57% −38.36% −124.6%

11.27 11.04 6.64 5.70 4.80 3.78 2.62 1.32

8.43 8.29 5.03 4.63 4.22 3.76 3.24 2.63

25.19% 24.91% 24.20% 18.77% 12.08% 0.53% −23.85% −99.24%

11.14 10.94 6.78 5.82 4.92 3.88 2.69 1.36

8.29 8.17 4.97 4.56 4.14 3.69 3.17 2.56

25.56% 25.35% 26.70% 21.65% 15.85% 4.80% −17.93% −88.79%

Note: BFIX represents base-fixed model. BISO represents base-isolated model. δ represents vibration control effect induced by base isolation, δ = (BFIX-BISO)/ BFIX × 100%. Table 8 Control effectiveness of the peak displacement (Unit: mmm). Height (m)

81.16 69.87 60.70 50.48 39.49 29.53 19.67 9.63

WLM2

WLM4

WLM6

WLM7

BFIX

BISO

δ(%)

BFIX

BISO

δ(%)

BFIX

BISO

δ(%)

BFIX

BISO

δ(%)

6.24 6.14 6.82 6.30 5.62 4.71 3.49 1.90

4.21 4.16 2.76 2.53 2.30 2.05 1.75 1.41

32.53% 32.25% 59.53% 59.84% 59.07% 56.48% 49.86% 25.79%

5.84 5.75 6.38 5.86 5.21 4.36 3.23 1.75

4.49 4.43 2.89 2.65 2.4 2.14 1.83 1.46

23.12% 22.96% 54.70% 54.78% 53.93% 50.92% 43.34% 16.57%

6.72 6.62 6.89 6.31 5.59 4.66 3.43 1.86

4.69 4.62 2.95 2.68 2.42 2.15 1.83 1.47

30.21% 30.21% 57.18% 57.53% 56.71% 53.86% 46.65% 20.97%

7.02 6.92 7.31 6.68 5.91 4.92 3.62 1.96

4.9 4.84 3.04 2.77 2.50 2.22 1.89 1.5

30.20% 30.06% 58.41% 58.53% 57.70% 54.88% 47.79% 23.47%

Fig. 18. Vertical responses along structural height for base-isolated and base-fixed models.

Fig. 19. Relationships between peak response and water level ratio for the reference points at different heights (Point A: 1.80 m, Point B: 9.63 m, Point C: 19.68 m, Point D: 29.53 m, Point E: 39.49 m, Point F: 50.48 m, Point G: 60.7 m, Point H: 69.87 m, Point I: 81.16 m).

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responses and the optimal water level ratio of 0.8 is suggested for the base-isolated shield building. Base isolation with optimal water level provides substantial benefit of improving structural seismic robustness, namely the vibration control effect is over 60% for the horizontal acceleration and over 20% for the vertical acceleration. 4. Primary resonance is aroused for the partially-filled base-fixed model as its frequency band ([3 Hz, 4 Hz]) is included in the main frequency band ([1 Hz, 10 Hz]) of earthquakes involving concentrated seismic energy, which, however, is avoided by the partially-filled base-isolated model with frequency band of [0.3 Hz, 0.4 Hz]. Although the sub-resonance is generated for the base-isolated models, the structural responses are gentle with only about 1/ 9 of that of the base-fixed models.

tendency of the vertical vibration responses along the height of the base-isolated models is different from that of the base-fixed models. The dynamic responses of the base-isolated models are larger than those of the base-fixed models at the lower part, while less than those of the base-fixed models at the upper part. For example, as shown in Table 7, the control effect of the acceleration response for WLM2 with isolation is −30.86% at the height of 19.67 m, and 17.05% at the height of 81.16 m. Similar phenomena can be found in the models of WLM4, WLM6 and WLM7. This phenomenon indicates that the vertical relative motion of the base-isolated models is restrained effectively by comparison to the base-fixed models. The restraint of the vertical relative motion shows a predominant advantage for vertical isolation, especially for the nuclear power plant as the piping and equipment system failure that may be experienced because of excessive vertical vibration. Consequently, it can be summarized that vertical isolation strategy can be better alternatives if horizontal structural movements are not permitted for existing nuclear structural design. Instead, lateral isolation strategy will be suggested to achieve more effective seismic control in case that larger lateral displacement is not main concern. Relationship between the water level ratio and the vertical peak response in different structural heights is shown in Fig. 19. It can be found that the relationship shows a curve change pattern between the peak responses and the water levels. The curves become more remarkable with the height increasing, indicating that the upper structural responses are influenced by FSI effect more than those of the lower. In addition, with the increase of water level ratio, both the vertical acceleration and displacement show a trend of growing firstly, then declining and finally rising, which is the same with that of the horizontal dynamic response as discussed in the last section. The minimum vertical dynamic response is at the water level ratio of 0.8 (WLM5). Therefore, it can be concluded that water level plays a key role of affecting structural responses of the base-isolated models according to the integrated investigation on horizontal and vertical vibration responses. Reasonable water level in PCCWT contributes most to the reduction of structural vibration and improves the seismic performance of the base-isolated shield building in external earthquakes. Water level ratio of 0.8 shows the optimal value. Besides, it is worth noting that the relationships at Points I and H in vertical direction are very close and different from those of other points, as shown in Fig. 19. This is because the vertical stiffness of the top shield building, where Points I and H located, has great reduction by comparison to other parts, which causes strong vertical synchronous vibration for the top part in earthquakes.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This project is supported by the National Natural Science Foundation of China (51778162, 51878191). References ACI Committee 350.3-06, 2006. Seismic design of liquid-containing concrete structures (ACI 350.3-06) and commentary (ACI 350.3R-06). American Concrete Institute, Farmington Hills (MI, USA). Ambrosini, D., Codina, R.H., Curadelli, O., Martínez, C.A., 2017. Structural of the CAREM-25 nuclear power plant subjected to the design basis accident and seismic loads. Ann. Nucl. Energy 108, 42–56. Ansys release 14.0., 2011. ANSYS-CFX: CFX Introduction, CFX Reference Guide, CFX Tutorials, CFX-Pre User's Guide, CFX-Solver Manager User’s Guide, CFX-Solver Modeling Guide, CFX-Solver Theory Guide. Ansys inc., USA. Baratta, A., Corbi, I., 2004. Optimal design of base-isolators in multi-storey buildings. Comput. Struct. 82, 2199–2209. Bhatti, M.A., Ciampi, V., Kelly, J.M., Pister, K.S., 1982. An earthquake isolation system for steam generators in nuclear power plants. Nucl. Eng. Des. 73, 229–252. Bogaers, A.E.J., Kok, S., Reddy, B.D., Franz, T., 2016. An evaluation of quasi-Newton methods for application to FSI problems involving free surface flow and solid body contact. Comput. Struct. 173, 71–83. Buckle, I.G., 1985. New Zealand seismic base isolation concepts and their application to nuclear engineering. Nucl. Eng. Des. 84, 313–326. Cancellara, D., De Angelis, F., 2017. Assessment and dynamic nonlinear analysis of different base isolation systems for a multi-storey RC building irregular in plan. Comput. Struct. 180, 74–88. Chopra, A.K., 2000. Dynamics of Structures: Theory and Applications to Earthquake Engineering. Prentice-Hall, pp. second ed.. Coladant, Ch., 1991. Seismic isolation of nuclear power plants-EDF’s philosophy. Nucl. Eng. Des. 127, 243–251. Dall’Asta, A., Ragni, L., 2006. Experimental tests and analytical model of high damping rubber dissipating devices. Eng. Struct. 28, 1874–1884. Dall’Asta, A., Ragni, L., 2008. Nonlinear behavior of dynamic systems with high damping rubber devices. Eng. Struct. 30, 3610–3618. Forni, M., Poggianti, A., Scipinotti, R., Dusi, A., Manzoni, E., 2014. Seismic isolation of lead-cooled reactors: The European project siler. Nucl. Eng. Technol. 46, 595–604. Fujita, T., 1991. Seismic isolation rubber bearings for nuclear facilities. Nucl. Eng. Des. 127, 379–391. GB50011-2010, 2016. Code for Seismic Design of Buildings. China Construction Industry Press, Beijing. Huang, Y.N., Whittaker, A.S., Luco, N., 2010. Seismic performance assessment of base isolated safety-related nuclear structures. Earthquake Eng. Struct. Dyn. 39, 1421–1442. Jeltsov, M., Villanueva, W., Kudinov, P., 2018. Seismic sloshing effects in lead-cooled fast reactors. Nucl. Eng. Des. 332, 99–110. Kim, N.S., Lee, D.G., 1995. Pseudodynamic test for evaluation of seismic performance of base-isolated liquid storage tanks. Eng. Struct. 17, 198–208. Kumar, M., Whittaker, A.S., Constantinou, M.C., 2014. An advanced numerical model of elastomeric seismic isolation bearings. Earthquake Eng. Struct. Dyn. 43, 1955–1974. Kumar, M., Whittaker, A.S., Constantinou, M.C., 2015. Characterizing friction in sliding isolation bearings. Earthquake Eng. Struct. Dyn. 44, 1409–1425. Lee, J.H., Kim, J.H., Kim, J.K., 2016. Perfectly matched discrete layers for three-dimensional nonlinear soil-structure interaction analysis. Comput. Struct. 165, 34–47. Lewandowski, R., Denning, R., Aldemir, T., Zhang, J.S., 2016. Implementation of condition-dependent probabilistic risk assessment using surveillance data on passive

5. Conclusions The complex seismic performance of the base-isolated shield building is investigated with a rigorous finite element method capable of accounting for FSI effect and nonlinear response of isolation system. The following conclusions can be drawn based on discussion on the results: 1. The convective fundamental frequencies show less difference between the base-isolated and base-fixed models, while the impulsive frequencies for base-isolated models are higher than that of the base-fixed models under certain water level. Both the convective and impulsive frequencies changes with increasing of the water level, namely increasing for the former and decreasing for the latter. 2. To avoid resonance effect of water sloshing, reasonable isolation design must be ensured as the fundamental frequency is usually prolonged by base isolation, which easily approaches to the convective frequency of the filled water. It can be found that the convective frequency is changed from 0.154 Hz (WLM2) to 0.211 Hz (WLM7), which is much closer to the structural fundamental frequency of 0.335 Hz (WLM7). 3. FSI effect has prominent influence on the structural dynamic 13

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D. Wang, et al.

status of multiunit risk assessment in nuclear power plants. Nucl. Eng. Technol. 50, 319–326. Shimada, T., Suhara, J., Takahashi, K., 2005. Study on 3-dimensional base isolation system applying to new type power plant reactor. Part 2. In: Proceedings of 18th International Conference on Structural Mechanics in Reactor Technology (SMiRT 18), Beijing, China. Song, C.N., Li, X.J., Zhou, G.L., Wei, C., 2017. Research on FSI effect and simplified method of PCS water tank of nuclear island building under earthquake. Prog. Nucl. Energy 100, 48–59. Suhara, J., Tamura, T., Ohta, K., Okada, Y., Moro, S., 2003. Research on 3-D base isolation system applied to new power reactor 3-D seismic isolation device with rolling seal type air spring. Part 1. In: Proceedings of 17th International Conference on Structural Mechanics in Reactor Technology (SMiRT 17), Prague, Czech Republic. Wang, D.Y., Tse, K.T., Zhou, Y., Li, Q.X., 2015. Structural performance and cost analysis of wind-induced vibration control schemes for a real super-tall building. Struct. Infrastruct. Eng. 11, 990–1011. Wang, D.Y., Wu, C.Q., Zhang, Y.S., Ding, Z.X., Chen, W.R., 2018. Elastic-plastic behavior of AP1000 nuclear island structure under mainshock-aftershock sequences. Ann. Nucl. Energy 123, 1–17. Wang, D.Y., Zhuang, C.L., Zhang, Y.S., 2018. Seismic response characteristics of baseisolated AP1000 unclear shield building subjected to beyond-design basis earthquake shaking. Nucl. Eng. Technol. 50, 170–181. Westinghouse. The AP1000 European DCD, UK AP1000 Safety, Security, and Environment Report. Chapter 3, pp. 51–53. Whittaker, A.S., Kumar, M., 2014. Seismic isolation of nuclear power plants. Nucl. Eng. Technol. 46, 569–580. Yip, W.K., 1998. Generic form of stress-strain equations for concrete. Cem. Concr. Res. 28, 499–508. Zhang, R.J., Wang, W.Q., Hou, S.H., Chan, C.K., 2001. Seismic analysis of nuclear reactor core. Comput. Struct. 79, 1395–1404. Zhao, C.F., Chen, J.Y., Xu, Q., 2014. FSI effect and seismic performance evaluation of water tank of AP1000 subjected to earthquake loading. Nucl. Eng. Des. 280, 372–388. Zhao, C.F., Chen, J.Y., Xu, Q., Wang, J.F., Wang, B., 2016. Investigation on sloshing and vibration mitigation of water storage tank of Ap1000. Ann. Nucl. Energy 90, 331–342.

components. Ann. Nucl. Energy 87, 696–706. Lo Frano, R., Forasassi, G., 2010. Isolation systems influence in the seismic loading propagation analysis applied to an innovative near term reactor. Nucl. Eng. Des. 240, 3539–3549. Lu, D.G., Liu, Y., Zeng, X.J., 2015. Experimental and numerical study of dynamic response of elevated water tank of AP1000 PCCWST considering FSI effect. Ann. Nucl. Energy 81, 73–83. Markou, A.A., Manolis, G.D., 2016. Mechanical models for shear behavior in high damping rubber bearings. Soil Dyn. Earthquake Eng. 90, 221–226. Medel-Vera, C., Ji, T.J., 2015. Seismic protection technology for nuclear power plants: a systematic review. J. Nucl. Sci. Technol. 52, 607–632. Medel-Vera, C., Ji, T.J., 2016. Seismic risk control of nuclear power plants using seismic protection systems in stable continental regions: The UK case. Nucl. Eng. Design 307, 377–391. Meiswinkel, R., Meyer, J., Schnell, J., 2013. Design and Construction of Nuclear Power Plants. Ernst & Sohn, Berlin, Germany. Micheli, I., Cardini, S., Colaiuda, A., Turroni, P., 2004. Investigation upon the dynamic structural response of a nuclear plant on aseismic isolating devices. Nucl. Eng. Des. 228, 319–343. Moslemi, M., Kianoush, M.R., 2016. Application of seismic isolation technique to partially filled conical elevated tanks. Eng. Struct. 127, 663–675. Naeim, F., Kelly, J.M., 1999. Design of Seismic Isolated Structures from Theory to Practice. John Wiley & Sons. Ogden, R.W., 1984. Nonlinear Elastic Deformations. Dover Publications Inc. Popovics, S., 1973. A numerical approach to the complete stress-strain curve of concrete. Cem. Concr. Res. 3, 583–599. Sarebanha, A., 2018. Experimental and Numerical Simulation of Seismically Isolated Critical Facilities under Extreme Seismic Loading. University of California, San Diego Doctor of Philosophy. Sarebanha, A., Mosqueda, G., Kim, M.K., Kim, J.H., 2018. Seismic response of base isolated nuclear power plants considering impact to moat walls. Nucl. Eng. Des. 328, 58–72. Schellenberg, A.H., Sarebanha, A., Schoettler, M.J., Mosqueda, G., Benzoni, G., Mahin, S. A., 2015. Hybrid Simulation of Seismic Isolation Systems Applied to an APR-1400 Nuclear Power Plant. PEER Report No. 2015/05. Seong, C.K., Heo, G.Y., Baek, S.J., Yoon, J.W., Kim, M.C., 2018. Analysis of the technical

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