Experimental Study On Flow-induced Vibration Characteristics Of Control Rod Assemblies In Fast Reactors

During the operation of a fast neutron reactor,the control rod assembly is an essential device for regulating reactor power and stability.As the control rod assembly operates continuously in the reactor,fluid scouring can cause vibration in the components.Due to the small gap between the control rod at the top of the assembly and the sleeve at the bottom,even slight vibrations can cause them to collide,leading to wear,fatigue,and potential structural failure of the assembly,ultimately affecting the safe operation of the reactor.Therefore,it is necessary to study the flow-induced vibration characteristics of control rod assemblies to provide a basis for improving the safety and stability of reactor operation.The problem of flow-induced vibration in fast reactor control rod assemblies involves complex flow channels and coupled vibrations of structures,and their flowinduced vibration characteristics are not yet clear.Some simplified scaled model experiments have been conducted abroad,but these cannot guarantee similarity with the prototype structure,fluid,and fluid-solid coupling processes,resulting in differences between experimental results and the original prototype.In actual operation,the vibration amplitude and power density of the control rod assembly are affected by factors such as flow rate and solid boundary conditions,and the influence of these parameters on the component amplitude and power density is not yet clear.In this study,a full-scale flow-induced vibration test bench is constructed to simulate the actual component constraints and rod position heights in the operation of a real reactor,and modal experiments and flow-induced vibration tests are conducted on the components.The modal experiments use the Single-Input Multi-Output(SIMO)method,obtaining component natural frequencies and vibration modes,which serve as a reference for the preparation and analysis of flow-induced vibration experiments.Flow-induced vibration experiments collect dynamic response data of components at different rod positions and flow rates,investigating the effects of control rod positions and flow rate parameters on vibration amplitude and power density.By examining the distribution of excitation energy in the frequency spectrum,reasonable inferences can be made about the vibration situation.Finally,the safety of the components is evaluated based on the experimental results.The analysis of vibration experimental data at different control rod positions(600mm,800mm.1200mm)shows that the amplitude and rod position have a nonlinear relationship due to the increase in flow velocity and different coupling effects at different rod positions.When the rod position is 800mm.the component vibration is the largest.and as the flow rate increases.the flow properties change to vortex resonance,with the most significant power density at the first-order frequency.When the rod position is 600mm,the amplitude is second,and at 1200mm,the amplitude is the smallest,with flow properties characterized as turbulence-induced excitation;the power density is more evenly distributed in the frequency range for these two rod positions.In conclusion,after evaluation,it is determined that the control rod and sleeve will not collide,and the maximum stress is within the allowable stress range for material cyclic alternating loads,so the component will not experience fatigue damage during long-term operation.This study pro vides a crucial basis for evaluating the safety of fast reactor control rod assemblies regarding flow-induced vibration.