Flow-induced vibration (FIV) in nuclear power plants is a constant concern as the demand for better thermal performance and efficiencies challenges the mechanical, flow and irradiation exposure characteristics of fuel designs. By design certain PWRs incorporate safety features such as Loss-of-Coolant-Accident (LOCA) holes and slots in the core periphery baffles surrounding the fuel assemblies, providing release from pressure build up in the event of a LOCA. As a result, at certain axial locations for this plant design, fuel rods are subjected not only to axial flow but also to transverse flow conditions due to their proximity to the LOCA holes and baffle slots.  

Aging of PWR reactor designs can produce dimensional changes and subsequent core baffle plate separation resulting in localized cross flow conditions that challenge the flow stability of peripheral fuel rods making them susceptible to higher vibration levels. In extreme cases, the fuel rods may undergo (intermittent) fluid-elastic instability that may cause excessive and damaging oscillations. Flow periodicities, induced by shear layer oscillations and vortex shedding, may also cause strong resonance vibrations responsible for grid-to-rod fretting wear of the fuel rod cladding.  

While the flow-induced vibration of rod bundles subjected to uniform transverse (cross) flow is well understood, little work has been done on the case where rod bundles are subjected to a highly, localized (jet) cross-flow. The flow complexity, due to its axisymmetric shear layer, highlights the difficulty of the resulting flow-induced vibration problem.

The work presented here investigates the fundamental mechanisms underlying rod bundle excitation due to a circular jet cross-flow. A specialized test apparatus to investigate the stability behavior of a rod bundle has been designed and built. The jet-flow induced vibrational behavior is investigated for a flexibly supported 6x6 square lattice rod bundle having a spacing ratio P/D=1.32. Additionally, the stability effects of jet parameters, including jet location relative to the rod bundle and eccentricity between the centerlines of jet and rod, are studied. Test results analysis leads to identification of the instability threshold of rod bundle subjected to jet cross flow.

The test results provide a reference basis for the development of a basic quasi-steady model for the prediction of jet-induced fluidelastic instability.

The quasi-steady approach is based on the model developed by Price & Paidoussis in the early eighties. In their work, the authors introduce a time delay parameter to account for the phase lag between tube motion and flow-induced (fluid-elastic) forces. The proposed model accounts for jet-induced shear via a shear parameter. Model results are compared with the case of a rod bundle subjected to uniform cross-flow.


Fluidelastic Stability Behavior of a Rod Bundle Subjected Jet Cross-Flow

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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