Transient flow characteristics analysis on the step-up process of control rod hydraulic drive system

•The step-up transient flow is the key physical process of Control rod hydraulic drive system (CRHDS).•The transient flow performance experiments of CRHDS are completed.•The step-up transient flow model is established, and the results are in good agreement with the experimental data.•The variations of key parameters and the mechanisms with different driving pressures at high temperature are analyzed. Control rod hydraulic drive system (CRHDS) has been invented for 200 MW nuclear heating reactor (NHR-200). Water hammer occurs and adversely affects the operation of CRHDS. The step-up transient flow model was built, and the mechanism of the flow process was analyzed. The transient flow performance experiments were performed and verified the theoretical model. Next, the model was applied to high temperature working conditions. The findings suggest that there are three water hammer phenomena in the step-up process, and the corresponding pressures decay quickly. The peak pressure of the second water hammer is the largest. In addition, the hydraulic cylinder acts as both a water hammer source and a fluid energy damper in the transient flow process. What’s more, the variation laws of key parameters under different driving pressure conditions were obtained and studied. The research results provide a basis for the vibration reduction design and operation monitoring of CRHDS.