Investigation of fluidelastic instability of fuel rod bundles subjected to combined axial flow and jet crossflow

Flow-induced vibration is a major concern in the nuclear industry. The combined axial flow and jet crossflow was found to induce fluidelastic instabilities (FEI) within the rod bundle and thus induce fretting wear of the fuel rods. This paper presents an experimental work to understand the dynamic behavior of two rod bundles subject axial flow and localized jet crossflow. A 6×5 flexible single-span bundle and a 7×5 flexible multi-span with spacer grids are tested to simulate a part of a fuel assembly under combined flow. 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-sapn the response of the array under JITF in two eccentricity scenario is tested for different axial velocities. The results show that the FEI threshold could occur whenever the velocity ratio $(VR=VjetVaxial)$ is above 1.25. The second part of the work is the design, fabrication and tests on the multi-span 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 scenario is tested for two axial velocities. The results show that the rod bundle is stable under the jet velocity tested for all cases.