Application of Limit Analysis to Seismic Evaluation of Steam Generator Tube Bundles - Part 1: Seismic Test and Nonlinear Analysis Model With Refined Modeling of Tube and Tube Support Plate Contact

Ensuring the structural integrity of Steam Generator (SG) U-bend tube arrays during out-of-plane earthquakes is crucial. The current primary stress evaluation method requires that tubes with the highest stress satisfy the primary stress criterion. However, even if one tube exceeded the criterion, the tube arrays is not supposed to easily collapse due to mechanism by which the tube shares the load between surrounding tubes through Anti-Vibration Bars (AVBs). Therefore, the current evaluation method seems to be conservative, and the rationalization of the design code is desired considering the increasing trend of design earthquake level. Therefore, the primary stress criterion of tube arrays was investigated through a limit analysis using a nonlinear model which can precisely reproduce the load sharing mechanism of tubes by modeling all tubes and AVBs. In this study, seismic tests using full-scale mock-up of square arrayed SG tubes were conducted to validate the nonlinear model. Previous studies showed that high stresses induced by deformation under seismic excitation occurs near the upper Tube Support Plate (TSP). As the out-of-plane response of the tube bundle increases, a change in contact occurs, resulting in the tube bundle only in contact with the upper surface of the TSP to also enter into contact with the lower surface of the TSP. Due to this mechanism, the stress near the TSP is affected by the contact state and a non-linearity occurs in which the bending strain per response increases. In order to accurately predict the stress, a nonlinear analysis model which can consider the change in contact state at the upper and lower surfaces of the upper TSP was developed. Moreover, the contact stiffness of tubes and TSP used in the model was obtained from the single tube analysis considering the elastic-plastic deformation. In the seismic tests, it was observed that, as the out-of-plane displacement of tube bundle increases, the bending strain per out-of-plane displacement increases in the high stress region thereof. The same tendency was also observed in the pseudo-static nonlinear analysis carried out on the nonlinear model developed to simulate the out-of-plane first mode. When the out-of-plane deformation of the tube bundle were the same as those of the test, the strain and the distribution thereof were well consistent between the analysis and the test. Furthermore, the relationship between the input acceleration of the pseudo-static analysis and the response displacement of the nonlinear model was well consistent with the relationship between the acceleration and the displacement of the out-of-plane first mode obtained from the response spectrum method using the equivalent linear model. In conclusion, the improved nonlinear model can be applied to the limit analysis.