12th International Symposium on Symbiotic Nuclear Power ( ISSNP-2026)

Flow-induced vibration of control rods is a critical factor affecting the structural safety and operational stability of reactor internal structures. In this paper, a self-programmed solution is developed for the vibration response calculation of control rods. Based on the fluid forces obtained from Fluent simulation, the program converts the surface pressure into nodal fluid forces, solves the natural frequencies and modal functions under given boundary conditions, and employs the modal superposition method to solve the modal coordinates at each time step, through which the nodal displacements are finally reconstructed. The vibration characteristics of a single rod under cross flow are analyzed with the reduced velocity gradually increasing from 0. The results show that the single rod exhibits periodic cross-flow vibration caused by vortex shedding. As the reduced velocity increases, the vibration mode transitions from the first to the fourth order, and the maximum RMS amplitude and maximum amplitude first increase and then decrease within each mode region, reaching peaks at Ur=3.0and Ur=6.0, and decreasing as the mode order increases. For the 7-rod bundle at Ur=3.0, the maximum axial amplitude of both the central rod and the peripheral rod occurs at the axial midpoint. The central rod vibrates more in the Y direction, while the peripheral rod vibrates more in the X direction. The peripheral rod exhibits a larger overall amplitude than the central rod due to non-uniform fluid forces and higher turbulence intensity at the edge and corner subchannels.
 

control rods,flow-induced vibration,numerical simulation