Vibration of fuel bundles subjected to combined axial and localized cross-flow

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Nuclear fuel assemblies are exposed to primarily high speed axial flow during operation. Under axial flow conditions, the fuel assembly undergoes random vibration induced by the flow turbulence. The resulting turbulence excitation is well understood and has been accounted for at the design stage of the fuel assemblies thus ensuring fuel mechanical integrity. For certain design and/or operating conditions, the fuel assemblies can be exposed to more complex flow conditions which have led to significantly higher vibrations and in some cases fuel rod fretting wear and failure. Most serious is the case where the fuel assembly is subjected to (partial) cross-flow transverse to the longitudinal axis of the fuel assembly. Transverse flow may, for instance, result from core baffle plate separation in older plants. The result is strong localized cross-flow particularly for fuel rods in the bundle periphery. The grid-rod fretting wear induced by the localized cross-flow is of significant concern as plants continue to operate beyond their 40 year threshold. For some plants specific design features may also introduce cross-flow in the fuel bundles. To mitigate extreme pressures in the case of a Loss of Coolant Accident (LOCA), strategically located LOCA holes or slots have been incorporated in the reactor core, which introduces a high speed jet flow transverse to the fuel rods. The present project addresses the problem of fuel bundle vibration induced by combined axial/transverse flow. The primary goal of the project is to develop an understanding of the fuel bundle dynamics for transverse jet flows of circular cross-section as well as planar jets through slots normal to the fuel bundle. More broadly, the project aims to uncover the fundamental mechanisms underlying fuel vibration and instabilities for these complex axial/transverse flows. The importance of the expected fundamental results goes beyond the nuclear field. High speed complex flows are encountered in a wide range of industrial settings ranging from energy generation to chemical industries. The research results are therefore expected to significantly contribute to Canadian industries in this wider range of applications. ************