Analysis of Unsteady Fluid Induced Vibration Forces on a Nuclear Fuel Rod Bundle in Turbulent Axial Flow

At a nuclear power plants a set of fuel rods held together by spacer grids form a fuel bundle. The coolant flow passing around the rods and the resulting flow turbulence, especially around the spacer grids, exerts transverse, vibrational and unsteady forces on the rods. The periodic tensions generated by these vibrations will result in fatigue and reduced mechanical strength of the rods as well as erosion corrosion of the rod clad. In this study, these forces are analyzed in terms of magnitude, amplitude and intensity of oscillations. To identify the effect of mixing vanes on the spacer grids, simulations were performed for spacer grids both with/without vanes. The obtained results are in good agreement with experimental results. Results showed a 17% increase in pressure drop due to the presence of mixing vanes. The forces exerted to the spacer in the downstream are 10% less than the forces in upstream. Analysis of the fluctuations of the forces applied to the fuel rod bundle showed the amplitude of the vibrations is relatively high at frequencies below 300 Hz.