Hydraulic control rod drive mechanism concept for passive in-core cooling system (PINCs) in fully passive advanced nuclear power plant

Abstract

Safety systems in nuclear power plants have become an important research topic owing to the significant impact of nuclear disasters. Herein, a passive in-core cooling system is proposed to delay the time of fuel melting and to enhance the safety of nuclear power plants during station blackout accidents. The passive in-core cooling system consist of a hydraulic control rod drive mechanism and a hybrid control rod assembly that combines the heat pipes with the control rods. To achieve the unique features of its system, hydraulic control rod drive mechanism is designed using hybrid control rods located inside the reactor vessel. The study deals with the test of a 4-finger hydraulic-rod control system as one of the components of the passive in-core cooling system. The experimental study focuses on the force-induced system based on the pressure differential between the grooved cylinders, which controls the elevation of the upper cylinder, i.e., the hybrid control rod assembly. As a result, the stable inlet flow and position were found at each step, yielding a well predicted holding flow, in accordance to the force balance equation. In withdrawal steps, reduced inlet flow was not observed, but reduced pressure was observed due to the narrow gap between the cylinders during the lift delay condition. Drop test results yield good agreement with those predicted by the simple model, resulting in percentage errors within±15%. In the heat removal test, the time taken to reach the maximum pool temperature was increased by approximately 2.5 times by using the refrigerant as the working fluid of the hybrid control rod. The test shows the behavioral features of the hydraulic control rod drive mechanism in conjunction with the hybrid control rod, and the possibility of the in-core application of the hydraulic drive system to PINCs.