Jet cross-flow induced vibrations in rod bundles. Part II: Transverse vibration results and excitation mechanisms

Abstract

Jet cross-flow in fuel assemblies has recently led to grid-to-rod fretting (GTRF) in reactors with fail-safe features, for the event of a loss of coolant accident (LOCA), including LOCA holes and slots. The findings in Part I of the paper showed that the stream-wise response behavior of the rod bundle was strongly dependent on the jet eccentricity and stand-off distance. These two parameters were defined to identify the location of the rod bundle centerline with respect to the jet centerline.The experimental investigation was extended to identify the direction in which the instability is most critical. The present Part II of the paper discusses the effect of both jet eccentricity and stand-off distance on the stability of transverse rod bundle vibrations and the underlying excitation mechanisms. This is achieved by adjusting the flexible rods to vibrate in the direction normal to the jet flow. The results show that jet cross-flow causes fluidelastic instability in the transverse direction. The rod bundle vibration and instability depends strongly on the jet eccentricity. The transverse vibrations are more dominant than those in the stream-wise direction. The critical velocity decreases with increasing stand-off distance and then reverses. This stand-off distance behavior in the transverse direction is opposite of that found for stream-wise vibrations.