The current buckling design code 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 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 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 were proposed in the previous study, which were modified by considering elastic-plastic buckling of vessel. The modified equations consisted of elastic-plastic axial compression, bending, shear buckling, and their interactions, as well as ASME/BPVC Code 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 the various yield stress materials applied in FRs under the severe imperfection was confirmed by a series of buckling tests to Grade 91 steel and austenitic stainless vessels mainly with a circumferential initial imperfection corresponding to elephant’s foot buckling (EFB) mode under monotonic compressive load accompanied with constant horizontal load. However, the effect of cyclic axial load was confirmed for only a Grade 91 steel vessel with longitudinal wrinkles imperfection resembling the shear buckling mode. The test result showed larger buckling strength compared to the vessels with the circumferential initial imperfection, and slight reduction of buckling strength due to elastic buckling by applying high yield stress material.

In this study, we confirmed the applicability of the proposed modified equations considering the reduction of buckling strength due to cyclic axial load with constant horizontal load corresponding to seismic isolation design calculated by the analyses, through a series of buckling test of vessels with the circumferential initial imperfection made of Grade 91 steel (high yield stress) and austenitic stainless steel (low yield stress) subject to the cyclic axial loading. The experimental result has confirmed the conservatism of the modified equations to Grade 91 steel and austenitic stainless steel vessels with circumferential wrinkle shape corresponding to the EFB mode as a significant initial imperfection, under the dominant cyclic axial compressive load accompanied with horizontal load. The tests have also been simulated by elastic-plastic buckling analyses considering stress-strain relationship and imperfections in the test vessels. The result of the test and its analysis have showed that the modified equations are applicable to the vessels in FRs power plant.

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