Due to a new design, a multi-hole orifice was arranged on the Spent Fuel Pit Cooling system (SFPC) of a French nuclear power plant. Serious pipe vibrations and a high level of noise were generated during the qualification testing. On-site measurements showed that a pure tone appeared at 640 Hz for a precise value of the mean flow. Furthermore, when the multi-hole orifice was replaced by another one with a different set of holes, the pure tone frequency was shifted to 520 Hz. The Strouhal number of the orifice holes is close to unity in the high-vibration regime. Besides, the acoustic boundary conditions appear highly reflecting. A fully-coupled finite element analysis of the piping system shows that several natural modes in the 500–700 Hz range exhibit simultaneously a shell deformation of the orifice, a pattern of plane acoustic waves upstream the orifice and a shell deformation of the piping system. The phenomenon is explained as a lock-in between vortex shedding inside the multi-hole orifice and the acoustic cavity, enhancing shell deformation of the pipe.

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