Experimental Study on Core Flow Instability Caused by Vortex Flow in Reactor Lower Plenum

Strong turbulent mixing of coolant in each loop of the PWR occurs in the lower plenum, and a large number of vertex are generated in the lower plenum, which can lead to random fluctuation of the fuel assembly inlet flow and trigger transient flow instability in the core, thus affecting the reactor thermal safety, structural safety or heat transfer performance. In this paper, the flow characteristics of the fuel assembly area in the reactor are studied, and the flow pulsation data of the fuel assembly inlet with and without flow distribution skirt in the lower plenum are obtained by hydraulic test under various operating conditions. The experimental results show that the lower plenum flow distribution skirt has obvious effect on restraining vortex, and the core flow pulsation is obviously reduced under the action of vortex breaking rectification. With the decrease of the number of operating loops, the symmetry of the flow field in the lower plenum decreases, the vertex flow increases, and the flow pulsation in the core increases obviously. The vortex in the lower chamber will also adversely affect the uniformity of flow distribution at the inlet of the core, and the flow distribution factor corresponding to the area with large flow pulsation is obviously small.