Visualization and control of vortical flow in the lower plenum of the Westinghouse AP600 reactor

The initial design for the Westinghouse AP600 reactor called for a reactor vessel lower plenum relatively free from obstructions. There was some concern that this design could result in vortical flow in the plenum. To research this hypothesis, a 1:9 scale model of the reactor vessel was built from clear acrylic plastic, with accurate geometric reproduction of the cold legs, downcomer, lower plenum, and core support plate. Because of the difficulties of reproducing actual reactor flow conditions, air was used as the working fluid, with fluid velocity maintained as the similarity parameter. The flow field in the lower plenum was visualized using a horizontally positioned tuft grid. Schlieren optical techniques were also used to locally visualize injected helium. With no obstructions present in the lower plenum, two strong counterrotating vertical-axis vortices were observed. These vortices were positioned around a circle of constant radius and extended from the core support plate to the plenum surface. The vortices moved azimuthally about this circle, sometimes joining but seldom dissipating. To prevent vortical flow, a vortex suppression device designed by Westinghouse was placed in the plenum perpendicular to the vortex axis. Repetition of the tests showed that all signs of organized vortical flow had disappeared. This result held under all flow conditions studied, including cases of gross flow imbalance between cold legs. From these results it was concluded that vortical flow could develop in the AP600 lower plenum if the plenum were free from obstruction, but that vortex formation could be effectively controlled by placing an appropriate suppression device in the plenum.