Seismic safety is one of the major key issues of nuclear power plant safety in Japan. It is demonstrated that nuclear piping possesses large safety margins in the various piping ultimate test reports. But it is appeared that there still remain some technical uncertainties about the phenomenon when both piping and supports show inelastic behavior in the extremely high seismic excitation level. In order to obtain the influence of the inelastic behavior of the support to the whole piping system response, and the subsequent interaction when both piping and supports show inelastic behavior, the following two tests have been started. • Support element test: Load-displacement characteristics of the support system including U-bolt, support itself and concrete anchorage are obtained by the forced displacement test. • Seismic proving test of piping system: The small bore piping and support system consisted of three dimensional piping, supports, U-bolts, and concrete anchorages will be excited on the table by the extremely higher seismic level. This paper introduces the major results of seismic proving test of piping and support system. The support element test results is presented in the paper of part 2, and the simulation analyses of these tests are summarized in the paper of part 3 [1, 2].
In the Application for Construction Plan License after the Great East Japan Earthquake, it was needed to revalidate the damping ratio to apply 3% for seismic analyses of Reactor Coolant Loop (RCL) with two-point-support Steam Generators (SGs) which was normally 0.5% or 1% in the past Applications. For the revalidation, vibration tests of SGs were carried out at Unit 2 and Unit 3 in Mihama Power Station of the Kansai Electric Power Co., Inc. In the test at Mihama Unit 2, SG top was hit horizontally by the pendulum type hammering device. As a result, in the hot leg (HL) direction, 9% damping ratio has been obtained. In the test at Mihama Unit 3, electro-hydraulic actuators were installed at the top of reinforced concrete wall surrounding SG and SG upper manhole was excited. In the excitation test, frequency response curves were obtained by changing the frequency stepwise in sinusoidal wave at constant amplitude. The damping ratio has been confirmed as more than 3%, specified in JEAG 4601-1991 as standard value, in the HL perpendicular direction which provided smaller damping ratio compared to the HL direction. Dissipation energy of snubber was measured and it has been confirmed that snubbers themselves do not contribute damping effect for small SG displacement like tests in Mihama Unit 2 and Unit 3. Large dissipation energy of snubbers would be expected in earthquake. It has been realized that conservative large responses are computed in RCL seismic analysis if the damping ratios obtained are used.
Ensuring a complete integrity of steam generators’ tubes during earthquakes is critical. So far, excitation tests have been conducted for various square array tube bundles in order to grasp vibration behavior during seismic excitation. A seismic analysis method using a simple model has been developed and subsequently verified by comparing the results against those of the above mentioned tests. In this study, seismic excitation tests were carried out for a U-shaped tube bundle with a triangular close-packed array using a full-scale test specimen as the triangular tube array with thinner anti-vibration bars (AVBs) has different vibration characteristics compared with a squared array one. Natural frequencies, modes and stress distribution in the tube bundle, obtained during the experiment, were applied in order to verify the seismic analysis method through the use of a linear analysis model with all tubes and AVBs connected to each other by linear springs as well as with a nonlinear analysis model with gap elements to simulate contact and sliding between all tubes and AVBs. The vibration displacement of the top of the tube bundle calculated by the linear model were in good agreement with the test result. Stress distribution in the tube array calculated using the nonlinear model were in good agreement with the test result. It has been concluded that the linear and nonlinear analysis models are applicable to seismic analysis including for the case of a U-shaped triangular tube array in a steam generator.
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