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.
Securing an adequate seismic margin has been important in safety reviews regarding the seismic design of equipment and piping systems in nuclear power plants, and there exists an increasing need for a more exact method for evaluating seismic margins. To this end, it is reasonable to take into account the reduction of seismic responses resulting from elastoplastic deformation. The authors, therefore, launched a research program to develop an approach to seismic design that uses elastoplastic dynamic analysis for equipment and piping systems. The allowable limit is one of the essential parameters, especially for our approach of using elastoplastic analysis, and was focused on in the program. We studied this approach by utilizing the conventional allowable limit and other potential limits such as the ductility factor. The applicability of the proposed approach was investigated by comparison with the conventional design method. For the investigation, nonlinear time-history analyses producing elastoplastic responses were conducted, and the results were compared with those of the conventional elastic analysis to quantify the response reduction leading to the seismic margin. For the comparison, the authors used three models that simulated a cantilever beam, tank, and core shroud. In this paper, the beam was constructed and applied to the analysis herein. In the next report, the authors will discuss the applicability of the three models. The cantilever beam is the simplest structure among the three models, and it might be useful for obtaining suggestive results from the analysis. The discussion on the beam, therefore, was conducted prior to the other two models, and, in addition, the sensitivity of model parameters such as yielding stress and secant stiffness will be examined in a parametric study using the model. In this paper, we outline the research program and present a scheme for developing the design approach of using elastoplastic analysis. Moreover, calculated analysis results for the cantilever beam are partly reported, and the applicability of the design approach of using elastoplastic analysis is discussed.
Securing adequate seismic safety margins has been important in safety reviews regarding the seismic design of equipment and piping systems in nuclear power plants, and there exists an increasing need for a more exact method for evaluating these margins. To this end, it is reasonable to take into account the reduction of seismic responses resulting from inelastic deformation. The authors studied this approach utilizing an elastic allowable limit in existing standard. The applicability of the proposed evaluation method was investigated by comparison with the conventional evaluation method. The proposed method consists of an inelastic dynamic analysis and an elastic-static analysis. The elastic-static analysis uses a load obtained from the inelastic dynamic analysis. For the investigation, the result obtained from the proposed method was compared with that obtained from the conventional elastic analysis to quantify the reduction in responses leading to seismic safety margins. For the comparison, the authors constructed three models that simulate a cantilever-type beam, four-legged tank, and core shroud and applied them to the analysis herein, and the applicability of our method was discussed for these models. In this paper, we present three topics. First, we present a scheme for developing the design approach of using inelastic analysis. Second, we report a sensitivity study of model parameters, such as yielding stress and second stiffness, done by analyzing the cantilever-type beam for the proposed method. Finally, we report the application of the method to the four-legged tank and core shroud.
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