Y. JIE
Shanghai Nuclear Engineering & Research Institute Xuhui District/ Shanghai, China
Email: yangjie@snerdi.com.cn L. SHAOPING
Shanghai Nuclear Engineering & Research Institute Xuhui District/ Shanghai, China
Abstract
As we know, Japanese KashiwazakiKariha and Fukushima NPPs have experienced strong earthquake beyond design basis, requirement for enhancing the condition of design basis ground motion and the seismic isolation technology are getting more and more attention worldwide. The paper presents the research on the seismic isolation with locking device of the CAP1400 nuclear island (NI), which can increase the seismic capacity of CAP1400 units from 0.3g to 0.6g (0.6g in horizontal and 0.4g in vertical), in the condition of keeping the superstructures of CAP1400 NI standard design unchanged. A series of nonlinear time-history analysis are performed to predict the maximum displacement and acceleration of the isolation layer, the maximum stress of the isolation units, and the floor response spectra of each story of the superstructure in the design basis earthquake of 0.6g, considering the realistic mechanical properties and the layout of the isolator. At the same time, a shaking table test of a reducedscale model ofthe base-isolated nuclear structures is introduced in the paper. The dynamic characteristic was examined, together with the vibration acceleration and displacement under different seismic intensities.
1. INTRODUCTION
Base isolated structure system is a passive control system, which reduces the response of a structure to horizontal ground motion
In order to confirm the isolation effect of the CAP1400 Nuclear Island structure, a series of nonlinear time-history analysiswere performedand shaking table tests of a reduced-scale model were accomplished. The dynamic characteristics of the isolated model structure for design and beyond-design basis earthquake shaking were tested, including the horizontal peak accelerations, displacement of the superstructure, and the force and the hysteretic curve of the isolated bearings.
The results of this study provide the technical basis for the base-isolated design of CAP1400.
2. BASE ISOLATED DESIGN OF CAP1400 NUCLEAR ISLAND STRUCTURE
CAP1400 Nuclear Island (NI) structure consists of steel containment vessel, containment internal structure, shield building, and auxiliary building. These buildings are founded on a commonbasemat. The size of the basemat is 91.4m×43.5m, and the height of NI structure is 75.5m. The total weight of the NI structure is 20.5×104t.
The target of base-isolated design of CAP1400 NPP is making the seismic design benchmark of safe shut earthquake (SSE) to 0.6g in horizontal direction instead of the original 0.3g. Adopting seismic isolation solution can increase the safety of the superstructure and the devices inside the superstructure with the benefit of realizing the standard design, which can enlarge the adaptability of the nuclear plant site.
The isolated bearings are laid under the basemat, and the spacing is 3m~4m. The type of the isolated bearing is lead rubberbearing, and the total amount is 450. Lead rubber bearings typically use natural rubber as their elastomeric material. Lead is an ideal material because it has high horizontal stiffness before yielding and then behaves perfectly plastic after yielding. It can forever nearly recover its original mechanical properties following inelastic action such as that imposed by an earthquake.
In order to avoid the breakage of component (e.g., pipes, cable trays, cable ducts and conduits) that crosses the isolation interface under small earthquakes, lock devices are added to the rubber bearing. And these devices will not affect the function of isolation system under big earthquake.
Fig. 1 shows the layout of the isolation unit.
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3. FINITE ELEMENT METHOD ANALYSIS OF BASE-ISOLATED DESIGN OF CAP1400 NPP The analysis model of the nuclear island stick model is composed of auxiliary and shield building, containment internal structure, steel containment vessel and reactor coolant loop, see Fig. 2. The hysteresis behaviour of isolation unit in horizontal direction is modelled by element type Combin40.
A series of nonlinear time-history analysis are performed to predict the maximum displacement and acceleration of the isolation layer, the maximum stress of the isolation units, and the floor response spectra of each story of the superstructure in the earthquake 0.6g, considering the realistic mechanical properties and the layout of the isolator.
4. SHAKING TABLE TEST RESULT OF BASE-ISOLATED DESIGN OF CAP1400 NPP
A reduced-scale earthquake simulation of base-isolated nuclear structures on a shaking table was performed, which provided realistic data to improve and validate current modelling approaches (see Fig. 3).
The study was primarily focused on the response of superstructure and the isolation unit. The test results of a reduced-scale nuclear island model tested on a shaking table were compared with three-dimensionalfinite element simulation results.
FIG. 2. Analysis model of base-isolated Nuclear Island.
FIG. 3. Reduced-scaleshaking table test.
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Acceleration response
Fig. 4 illustrates the benefits of seismic isolation using an acceleration distribution plot in isolation layer and superstructure, in which node 1 represents lower raft, and nodes 2~5 represent superstructure of different elevations. From the plot, we find the acceleration of superstructure in horizontal direction reduce significantly.
FIG. 4. Acceleration distribution plot in isolation layer and superstructure.
The acceleration time histories of analysis and test results are compared in Fig. 5. According to the comparison of acceleration results under different load conditions, the test results match the analysis results very well, especially under the unidirectional loading condition.
Hysteretic behaviour of isolation layer
From the test, the shear force-displacement hysteretic curves of the isolated layer were obtained. The hysteretic curves of test results are compared with the numerical simulation in Fig. 6.
FIG. 6. Hysteretic curve under biaxial loading condition.
From the results, it can be seen that the shape of the hysteresis curve is more stable, and closer to the numerical simulation results under unidirectional loading condition. And under the multi direction loading, the measured hysteresis curves show a more complex shape, but the overall trend is consistent with the numerical simulation results.
FIG. 5. Acceleration time histories of analysis and test results.
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Fig. 7 illustrates the benefits of seismic isolation using an acceleration response spectrum. The red line is the spectra of standard design in 0.3g earthquake, and the blue line is the spectra of base-isolated design in 0.6g earthquake.
FIG. 7. Comparison of the Spectra of the isolation layer in horizontal direction between the base isolated model and fixed base model.
From the comparison results above, the floor response spectra in horizontal direction reduce significantly in the frequency range that larger than 1Hz, but amplify in the frequency range that less than 1Hz.
Although modern isolation systems substantially decouple the superstructure from horizontal ground shaking, none mitigates response to vertical ground motion.
The amplification of the response spectra in the low frequency range of horizontal direction and in the vertical direction has been evaluated. They have little contribution to the response of the components and systems contained in the superstructure.
5. CONCLUSION
Finite element analysis and shaking table tests were performed for the isolation and non-isolation models of CAP1400 nuclear island structure. By the comparison with the test and analysis results, the following conclusions are obtained:
The natural frequency of the isolated structure is about 20% of the non-isolated structure. Base isolation technology extends the natural period of the structure, and increases the damping to reduce the input of earthquake of the superstructure. So the floor response spectra in horizontal direction reduce significantly in the frequency range that larger than 1Hz, but amplify in the frequency range that less than 1Hz.
Under the action of 0.6g earthquake, the frequency of the base-isolated structure is reduced by 2%, which demonstrates that the structure is only slightly damaged. But the frequency of the non-isolated structure is decreased by 30%, which demonstrates that the structure is degenerated into the elastic plastic state.
The peak acceleration response of the isolation layeris much smaller than the acceleration of the non-isolated structure’s basemat, which indicates that the acceleration of the superstructure has been effectively controlled, and the earthquake response of the structure reduces significantly through the base isolation technology.
Under the action of different levels of seismic wave, the hysteretic curve is full, which shows that it has good energy dissipation capacity.
The calculation and analysis of the isolated structure are in good agreement with the experimental results, which shows the analysis result could be used for the design of CAP1400.
ACKNOWLEDGEMENTS
Funding for this study was provided by National Science and Technology Major Project.Additional support for testing of the isolation system at shaking table was provided by China Academy of Building Research. The isolators and connection plates were provided by Liuzhou OVM Machinery Corporation. The authors are grateful to all sponsors for making this possible.
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REFERENCES
[1] HALL, J.F., Problems encountered from the use (or misuse) of Rayleigh damping. Earthquake Engineering and Structural Dynamics, 35 5 (2006) 525.
[2] ERDURAN, E., Evaluation of Rayleigh damping and its influence on engineering demand parameter estimates.
Earthquake Engineering and Structural Dynamics, 41 14 (2012) 1905.
[3] RYAN, K.L., POLANCO, J., Problems with Rayleigh damping in base-isolated buildings, J. Struct. Eng., 134 11 (2008) 1780.
[4] PANT, D.R., WIJEYEWICKREMA, A.C., ELGAWADY, M.A., Appropriate viscous damping for nonlinear time-history analysis of base-isolated reinforced concrete buildings, Earthquake Engineering and Structural Dynamics, 42 (2013) 2321.
[5] WANG, T., WANG, F., DING, L.T., Theoretical and experimental study on three-dimensional base-isolated nuclear power plant, China Civil Engin. J. 45 1 (2012) 238．
[6] ZHOU, F.L., Engineering structure vibration control, Seismological Press, Beijing (1997).
[7] LI, D.M., Analysis of a nuclear power plant containment isolation seismic response, Harbin Engineering uIliversity Harbin (2007).
[8] FRANO, R.L., FORASASSI, G., Isolation systems’ influence in the seismic loading propagation analysis applied to innovative near term reactor, Nucl. Engin. Design，240 10 (2010) 3539.
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