Flow-induced vibration (FIV) is a constant concern in nuclear power plants. Demand for better thermal performance challenges the mechanical and flow characteristics of fuel designs. In the hypothetical case of a loss of coolant (LOCA) event in a reactor, the hydrodynamic pressure would increase significantly across the baffle plates. PWRs include safety features such as Loss-of-Coolant-Accident (LOCA) holes and slots in the core periphery baffles surrounding the fuel assemblies to release the pressure build up during a LOCA event. Accordingly, these fuel assemblies are subjected to combined axial and jet cross-flow at certain axial locations along their spans due to their proximity to the LOCA holes.

The jet flow could induce vibrations for fuel assemblies located near LOCA holes, which might lead to grid-to-rod fretting and thus potential fuel failure. Research on circular jet induced vibrations of rod bundles is limited. Thus, it is required to investigate the dynamical behavior of rod bundle subjected to jet flow to define the critical velocity at which the fuel rods may undergo instability.

This article presents an experimental study of jet flow induced vibrations for a 6 × 6 closely packed normal square rod bundle with a pitch-to-diameter ratio of 1.32 simulating the actual PWR fuel rod dimensions. A specialized test apparatus was designed to investigate the stability effect of jet centerline offset from array centerline (jet eccentricity). From the test results the instability threshold of the rod bundle subjected to jet cross-flow is determined. The results show that the rod array vibration is affected by the jet eccentricity. Two excitation mechanisms are identified. The first is an apparent lock-in type mechanism that maybe related to shear layer or jet oscillation. The second, more important excitation, is an apparent fluidelastic instability induced by the jet flow.

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