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 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 to vessels of Grade 91 steel under cyclic loading considering horizontal seismic isolation, or with the circumferential wrinkle shape corresponding to elephant’s foot buckling (EFB) mode in the previous study.

In this study, the applicability of the proposed modified buckling equations has been confirmed through the test of austenitic steel, which has lower yield stress than Grade 91 steel, with imperfections under the load condition corresponding to seismic isolation design. The experimental result has confirmed the conservativeness of the modified equations under the dominant axial compressive load accompanied with horizontal load, and the austenitic steel vessel which has the circumferential wrinkle shape corresponding to the EFB mode as a significant initial imperfection. The test has also been simulated by elasto-plastic buckling analyses considering stress-strain relationship and imperfections in the test vessels. The result of the test and its analysis have showed the applicability of the modified equations.

Moreover, a series of verification analyses conducted for the modified equations with 3D buckling analysis models under the conditions in FRs power plant. These verification analysis results confirmed the contribution of the factors to the buckling strength such as initial imperfection and material, and showed that the modified equations are applicable to the vessels in FRs power plant.

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