Investigation of Fluidelastic Instability of Fuel Rod Bundles Subjected to Combined Axial Flow and Jet Crossflow

Abstract Flow-induced vibration is a major concern in the nuclear industry. The combined axial flow and jet crossflow has been found to induce fluidelastic instabilities (FEI) within the rod bundle, thus increasing the risk for fretting wear of the fuel rods. This paper presents an experimental work aiming to improve our understanding of the dynamic behavior of two rod bundles subjected to combined axial flow and localized jet crossflow. Two different bundle geometries were experimentally investigated. The first was a 6 × 5 flexible single-span bundle and the second one was a 7 × 5 reduced scale nuclear fuel assembly mockup. Two working flow conditions, pure axial flow and jet in transverse flow (JITF) are studied. The experiments show that the bundle is stable under pure axial flow as expected. Then, for the single-span the response of the array under JITF in two eccentricity scenarios is tested for different axial velocities. The results show that the fluidelastic instability (FEI) threshold could occur whenever the velocity ratio (VR=Vjet/Vaxial) is above 1.25. The second part of the work is the design, fabrication and tests on the multispan array. Characterization tests are performed to identify the modal parameters of the rod bundle. Two working flow conditions, pure axial flow and JITF are studied. The experiments show that the rod bundle is stable under pure axial flow. The response of the array under JITF in three eccentricity scenarios is tested for two axial velocities. The results show that the rod bundle is stable under the jet velocity tested for all cases.