Jet cross-flow induced vibrations in rod bundles. Part I: Experimental apparatus and stream-wise vibration results

Abstract

Jet cross-flow induced vibration of fuel assemblies has recently been observed in some reactors that have fail-safe features for pressure release in the event of a loss of coolant accident (LOCA). The generated pressure in the barrel–baffle region during LOCA is mitigated by discharging the flow through LOCA holes and slots normally to the primary axial coolant flow inside the core. However, the jet flow injected through the LOCA holes and slots can induce vibration in the fuel assembly near the baffle before being mixed with the axial flow, and subsequent it can cause grid-to-rod-fretting (GTRF). In this paper, experimental investigations were performed to characterize the jet cross-flow induced vibration of fuel rods by performing fluidelastic instability (FEI) tests on flexible rod bundles. The effect of the stream-wise which is called ”stand-off distance”, is studied by the changing number of rows in the tested bundle. Furthermore, an offset between the centerlines of the jet and the bundle (jet eccentricity) which means the transverse direction on the jet centerline, is changed to show its effect on rod bundle vibration. An experimental test facility was designed to give the capability to study different jet eccentricities to examine their stability effects. The stream-wise results show that varying jet eccentricity excited the rod bundle with different mechanisms. The rods vibrations occurred with the centered jet with rod bundle case had a lock-in/synchronization phenomenon, where the resonant peak is observed. While, the excitation mechanism is switched to turbulence-induced vibrations by moving the jet flow by 0.25 of pitch. However, the aligned jet-rod case showed a fluidelastic instability phenomenon at relatively high jet velocity. On the other hand, the two resonant peaks are obtained when the rod bundle moved away from the jet flow (i.e larger stand-off distance).