In order to predict the flow induced vibration levels of SUPERPHENIX reactor internal structures, finite element calculation and full scale tests have been carried out. The study concerns the big "thin shell type" internal vessels and correspondent crossings. 3D calculations have been processed using a substructuration method, where each substructure had an axisymetric geometry. Results showed high modal densities. Transfer functions of the structure have been obtained during full scale tests. They showed also a big density of close and coupled resonances. Then two methods were used for modal identification of test results with limited equipment. In the first method, modal parameters were extracted using a single degree of freedom model. Mode shapes showed coupling between free internal shell axisymetric modes. In the second, transfer function Fourier series Decomposition was accomplished in order to recognize free shells azimuthal modes and influence of crossings. Comparison showed good agreement between experimental and calculation results.
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