Transient flow characteristic analysis of the step-down process of the control rod hydraulic drive system

[Objective] Based on 5-MW hydraulic drive technology and commercial pressurized water reactor magnetic drive technology, Tsinghua University has developed a control rod hydraulic drive system (CRHDS). CRHDS is a new type of built-in control rod drive technology primarily utilized in integrated reactors, such as the 200-MW nuclear heating reactor (NHR200). CRHDS uses three hydraulic cylinders to drive two sets of latch assemblies to move in a predefined sequence and achieve the control rod step-up, step-down, and scram functions. Water hammer occurs during the operation of CRHDS. It can trigger a large fluctuation of fluid pressure, causing vibration in the driving system and equipment, interfering with the instruments, and endangering the safety of the system. Therefore, the transient flow process needs to be analyzed through experimental and theoretical studies.[Methods] Theoretical modeling and experiments were performed at the system level. The transient flow mechanism of CRHDS was illustrated, and the key characteristic parameters were analyzed with driving pressure at high temperature. Most importantly, the composition and principle of CRHDS are described. The structure of the hydraulic cylinder was described in detail because it was a key component of the theoretical model. Combined with the structure of CRHDS, a full-scale transient flow performance test rig was built, and the experiments were completed. Based on the displacement and hydraulic cylinder pressure test results, the transient flow process of CRHDS was analyzed at different stages. Based on the mechanism analysis, a step-down transient flow model incorporating the trend and water hammer models was innovatively established. The trend model comprised the fluid continuity equation, fluid momentum equation, leakage flow relationship, and dynamic and kinematic equations. The trend model was solved using the finite difference method. The water hammer model was solved using the method of characteristics. The boundary conditions included a hydraulic cylinder, straight junction, integrated valve, and test vessel. The step-down transient flow model results were verified using experimental data. In the NHR200, CRHDS operates under high temperature and pressure conditions. Finally, the step-down transient flow model of CRHDS was applied to the high-temperature condition, and the fluid physical properties were changed accordingly. The driving pressure was set at 800, 850, and 950 kPa. Variations in key parameters with driving pressure are explained.[Results] (1) The step-down transient flow model of CRHDS comprises the trend and water hammer models. The trend model represents the overall change in hydraulic cylinder pressure, while the water hammer model illustrates the water hammer phenomenon in the system. The step-down transient flow process is analyzed by superimposing the solution results of these two models. (2) Combined with the motion of the inner cylinder, the step-down process can be divided into pre-step, step-down, and post-step stages. Pressure decreases rapidly in the pre-step and post-step stages but slowly in the step-down stage. (3) The rapid movement of the hydraulic cylinder and the sudden change in the leakage flowrate cause the water hammer phenomenon, and the water hammer pressure decays rapidly. The hydraulic cylinder acts as a water hammer source and fluid energy damper during the entire transient flow process. (4) The variation rules of the key parameters of CRHDS at high temperature are obtained. As the driving pressure increases, the pre-step stage duration increases, the step-down time decreases, and the average step-down velocity and water hammer pressure amplitude increase.[Conclusions] The research results illustrate the transient flow mechanism of the step-down process of CRHDS, guide the design of vibration reduction, and provide a basis for the operation monitoring of CRHDS. © 2024 Tsinghua University. All rights reserved.