CFD simulation on the flow characteristics in the PWR lower plenum with different internal structures

The detailed thermal-hydraulic phenomena inside the lower plenum of Reactor Pressure Vessel (RPV) in Pressurized Water Reactor (PWR) affect the mass flow rate distributions at core inlet significantly, leading to an impact on the operation safety of nuclear reactors. From the engineering point of view, a homogeneous flow rate distribution at the core inlet is expected. Actually, series of design concepts of lower plenum internal structures were proposed for different PWRs in the world. In this paper, the three-dimensional CFD simulations with three types of internal structures in the lower plenum, TYPE 1 (Vortex suppression plate with the secondary supports), TYPE 2 (Flow distribution skirt board with square coolant holes), TYPE 3 (Ellipsoid perforated drums and supports), were performed. The influence of different internal structures on the flow characteristics in the lower plenum were achieved and analyzed. The three types of internal structures were adapted into the same experimental facility “BORA”, which is a 1/5th scale 4-loops mock-up model, to realize the single variable criterion and obtain a uniform standard during the comparison. Results indicate that the mass flow rates in central fuel assemblies are much higher than that in peripheral fuel assemblies at core inlets with TYPE 1 and TYPE 2 internal structures, while the core inlet mass flow rate distribution with TYPE 3 internal structure is more homogeneous. This research has high reference value for the future advanced PWR lower plenum internal structures optimization and design.