Study on Seismic Safety of the Small-bore Piping and Support System

Transcription

1 23 Study on Seismic Safety of the Small-bore Piping and Support System KUNIHIKO SATO *1 MASATSUGU MONDE *2 DAISAKU HIRAYAMA *3 TAKUYA OGO *4 In Japan, according to the revised Regulatory Guide for Aseismic Design of Nuclear Power Reactor Facilities, September 2006, criteria of design basis earthquakes of Nuclear Power Reactor Facilities become more severe. Electric power companies were requested by the government to recheck (back-check) the seismic design of their nuclear power plants. Since seismic safety is one of the major key issues of nuclear power plant safety, it has been demonstrated that the nuclear piping and support system possesses large safety margins by various durability test reports for piping in ultimate conditions. Though knowledge of the safety margin has been accumulated from these reports, there are few seismic margin tests that both the piping and support structures show inelastic behavior in extremely high seismic excitation levels. In order to obtain the influences of inelastic behavior of the support structures to the whole piping system response when both piping and support structures show inelastic behavior, we examined seismic proving tests using E-Defense, which is the largest shaking table in the world and owned by the National Research Institute for Earth Science and Disaster Prevention. This paper introduces major results of the seismic shaking tests of the piping and support system. 1. Introduction Since the seismic design of the pipes and supports in nuclear power plants is based on the design yielding points, it has been considered that the piping and support system capacity has a large margin that has been demonstrated by old studies. Yet, still some technical uncertainties remain concerning the phenomenon when both piping and support structures show inelastic behavior in extremely high seismic excitation levels. The study is to comprehend the vibration characteristics (including inelastic properties) of the complete piping and support system under immense earthquake conditions, and verify the seismic margin. In addition it is part of the recheck (back-check) process for nuclear plant pipes. Pipes with a bore size of 4 and 2 inches, which are commonly used in nuclear power plants, and their corresponding supports were used in the tests. This article reports the vibration test results of the 4-inch pipes using E-Defense. Vibration tests of 2-inch pipes were executed using the shaking table in our Takasago Research Laboratory 1, and it has been verified that they have sufficient seismic margin against design-basis earthquakes. 2. Test The support element test was conducted to obtain load-displacement characteristics, and seismic proving tests to obtain the inelastic response of the piping and support system vibration test aiming at examining the vibration characteristics of the relevant system by using a full-scale test model containing all elements such as pipes, supports, and fixtures. *1 Deputy General Manager, Nuclear Energy Systems Engineering Center, Nuclear Energy Systems Headquarters *2 Takasago Research & Development Center, Technical Headquarters *3 Nuclear Plant Maintenance Engineering Department, Kobe Shipyard & Machinery Works *4 Nuclear Energy Systems Engineering Center, Nuclear Energy Systems Headquarters

2 2.1 Support element test The support element tests were designed to obtain the relationship between force and displacement at piping, when the seismic force loaded on piping support equipment consists of a U-bolt, support element, base plate and anchorage on a concrete base by confirming the behavior of the equipment until failure is attained. Figure 1 shows the outline drawing of a typical test specimen used in the element test. Figure 2 shows the simulation analysis model. Figure 3 shows load-displacement characteristics of support element test and simulation analysis. With respect to linear and second rigidities, the simulation analysis results are similar to the test results. The analysis model does not include concrete fixtures, but the end of anchor bolt is fixed. 24 Figure 1 support test model Figure 2 FEM model of piping support The model imitates the welding part. Figure 3 Load-displacement characteristics 2.2 Piping and support system vibration test (1) Test model The model is a piping and support system with a piping bore of full-scale 100A, Sch40, which has elbows, a tee. Table 1 shows specification of test model. Figure 4 shows the piping and support system model. (2) Seismic wave Since the input excitation wave should cover the major mode of typical buildings of nuclear power plants in Japan. It was difficult to create the seismic wave of the wide-band target spectrum. So, we designed three successive waves. Input level of the seismic wave was controlled according to the purpose of test cases. Figures 5 and 6 show the basic excitation waves and response spectra, respectively.

3 (3) Test cases Major test cases for each piping and support system are presented in Table 2. Test case 3, weights were added to the pipes to input a big seismic force that exceeds the vibration capacity of the shaking table. Test case 4, weights were added to the pipes and removal of five main supports to make the pipes and support system more sensitive to vibration, and applied vibration at the level to damage the pipes. 25 Table 1 specification of test model Element Quantity Material Remarks Total length: Approx. 24 m Pipe 1 set Elbow: 9 ea. 4B carbon steel Tee: 1 ea. Nozzle: 3 places Internal fluid Support - Water Internal pressure: 1.4 MPa 16 ea. Cantilever type Expansion Anchor (with U-bolts) (including embedded type) Total Base mat size weight Approx. 80 t - (10.4 m 6.4 m) Figure 4 schematic view of test model Figure 5 excitation waves Figure 6 Response spectra of the excitation waves (horizontal direction) Table 2 test case for piping and support system Test case Input level Test model Remarks Case 1 S 2 Test model A Applied vibration at the design earthquake level of the test model Case 2 9 S 2 Test model A Amplified the acceleration of the S 2 wave by 9 times. Case 3 α S 2 Test model B Amplified the response of the piping and support system with additional weights. The shaking waveform was adjusted (with the time axis) to the natural frequency of the piping system. Case 4 β S 2 Test model C Removed five main supports (supports 4,5,7,8,9) to make the pipes and support system more sensitive to vibration, and applied vibration at a level to damage the pipes. *S 2 : Design seismic wave of the test model 3. Vibration Test Results for the Piping and Support System (1) Outline of test results Table 3 lists the natural frequencies of the each test model sets. Figure 7 shows the vibration modes of these test model sets. The tee-branch portion is excited exclusively in the shaking direction on the first mode. The upper location (support 15) is dominant in the vertical direction on the second mode.

4 Test case 1 is adjusted to the design acceleration (target: 1.47 m/s 2 ) which is input. Test case 2 is adjusted to a level about 9 times as large as the design acceleration (target: 13.8 m/s 2 ), which substantially exceeds the response acceleration (maximum: 6.8 m/s 2 ) observed at the upper end of the foundations of TEPCO s Kashiwazaki-kariwa Nuclear Power Station during the Niigata Chuetsu-Oki Earthquake. Test case 3 has the additional weights to enlarge pipe displacement. Test case 4 has additional weights and support removal to generate larger pipe displacement. These test cases did not leak internal liquid from the piping, and the piping and the support system held the seismic margin more than 9 times as large as the design seismic wave. 26 Table 3 Natural frequencies of respective piping and support system Natural frequency(hz) 1st 2nd 3rd 4th Remarks Test model A Original test model Test model B Amplified the response of the piping and support system with additional weights. Test model C Five supports were removed. Figure 7 Vibration modes of piping systems (test model A) (2) Analysis and evaluation The test model did not break as a result of the test, because of the damping ratio generated higher than design damping ratio. The damping ratio of the piping and support system increases at the shaking level because of the increase mainly due to the wear of the U-bolt and the plastic deformation of support members. These vibration tests proved that the magnitude of the damping effect was an important element in assuming the ultimate state of pipes and supports and therefore, we conducted analyses on the relationship between response displacement and damping of the pipes based on test results. For test case 4 where the maximum response displacement was measured, we evaluated the inelastic behaviors of the pipe elbow based on the simulation analysis. (a) Response displacement and damping of pipes Table 4 shows the maximum response displacements summarized for each test case. Figure 8 shows the relationship between maximum displacement and damping. It has been proved that the damping ratio is enlarged with an increase in the vibration level, and a maximum damping ratio of about 9% is observed in test model A. The damping ratio is estimated by the half power method from the transmission function, which is derived from the horizontal acceleration to the vibration table, by the Auto Regression method with the duration of the largest response level. Test case Table 4 Maximum response displacement of the piping system (tee and elbow A are those shown in Figure 9) Max. acceleration of the wave Max. displacement of piping and support system (at tee) m/s mm m/s mm m/s mm Support displacement (ductility ratio) No. 7 No. 8 No. 9 support support support 1.6 mm 1.3 mm 1.6 mm (0.12) (0.1) (0.12) 15.9 mm 9.1 mm 15.4 mm (1.2) (0.68) (1.16) 50.3 mm (3.78) 31.8 mm (2.39) 46.6 mm (3.5) Maximum Strain range of elbow A Notes 0.01% Elastic range 0.04% 0.17% m/s mm %

5 27 Figure 8 maximum response displacement and the damping ratio (b) Inelastic behavior of the elbow The relations between the strain amplitude at the flank of the elbow outer surface and displacement amplitude of the piping system are obtained in Figure 9 for Elbow A by the analysis. The maximum displacement of the tee at the test case 4 is about 240 mm, which was measured by displacement sensor. Figure 10 show the maximum circumferential strain range of Elbow A is about 1.3 %. Figures 11 show the simulation result. The circumferential strain of the elbow generated under 240-mm displacement of the tee agrees with the test result of 1.3%. This means that we have succeeded in accurately obtaining the status of fatigue of the pipe elbow. Figure 10 Local strain waveform at elbow A (test case 4) Figure 9 Pipe elbow simulation analysis model Figure 11 Relation between tee displacement and local strain of elbow A

6 28 4. Conclusion The results of the study are summarized as follows: (1) By loading a repetitive force to the support element, we succeeded in verifying the behaviors and actual yield strength of the piping and support system during the period until the system lost the support function. (2) In the vibration test in which immense earthquake conditions were simulated, we succeeded in verifying the seismic safety of the small-bore piping and support system. (3) In the post-test simulation analysis and a later comparison between the test and simulation results, the simulation model excellently reproduced the inelastic behaviors actually observed. This result proves that the analytical method used is valid for simulating inelastic behaviors. Acknowledgment This study was entrusted to us by the Kansai Electric Power Co., Inc., Kyushu Electric Power Co., Inc., Shikoku Electric Power Co., Inc., Hokkaido Electric Power Co., Inc., and Japan Atomic Power Company. We received various advice and assistance from these companies, for which we would like to express our appreciation. References 1. E. Shirai et al., Inelastic test of the small bore piping and support system : Part 1, ASME PVP, E. Shirai et al., Inelastic test of the small bore piping and support system : Part 2, ASME PVP, 2008