Development of the Buckling Evaluation Method for Large Scale Vessel by the Testing of Gr. 91 Vessel Subjected to Horizontal and Cyclic Vertical Loading

Abstract

Abstract The current buckling design standard of Class 1 vessels for fast reactors (FRs) power plant in Japan, “Design and Construction for Nuclear Power Plants, Division 2 Fast Reactors” by the Japan Society of Mechanical Engineers, focuses primarily on plastic buckling of austenitic stainless steel vessel. For next-generation FRs, the higher-yield material, especially ASME Grade 91 steel, plans to be applied to the vessels such as steam generators in addition to austenitic stainless steel. Seismic isolation system is also being devised in the next plant to meet the design seismic load in Japan. To accommodate these conditions, the standard buckling strength equations in Japan were proposed in previous study, which were modified by considering elasto–plastic buckling of vessel. The modified equations consisted of elasto–plastic axial compression, bending, shear buckling, and their interactions, as well as ASME/BPVC CASE N-284, considering the reduction of buckling strength by cyclic larger vertical load with long–period lower horizontal load under the horizontal seismic isolation plant design. The applicability of the modified buckling equations to high-yield material was confirmed by a series of buckling tests under the monotonic load to Grade 91 steel vessels with an initial imperfection mode in previous study. In this study, the applicability of the proposed modified buckling equations was confirmed through a series of buckling tests under cyclic loading considering horizontal seismic isolation, or with the circumferential wrinkle shape corresponding to elephant’s foot buckling (EFB) mode as the conservative initial imperfection shape. These tests were simulated by elasto–plastic buckling analyses considering stress–strain relationship and imperfections in the test vessels with high accuracy, and these test and analysis results showed the applicability of the modified equations. The effect of initial imperfection shape of large vessels manufactured by multiple plates welding on the buckling load was evaluated via a series of analyses. In these analyses, the superimposition of global imperfection shape due to accumulation of misalignment of plates in the buckling mode imperfection, and the local imperfection and residual stress by welding had minimal effect on the buckling load. These buckling test and analysis results confirmed the contribution of the factors to the buckling strength such as initial imperfection and loading, and showed that the modified equations are applicable to the vessels in FRs power plant.