Formation of vortices in the lower plenum of existing nuclear power reactors has been hypothesized to cause observed localized coolant starvation, which results in a loss of thermal margin. A 1:9 scale model was built to study vortex formation and suppression in the Westinghouse AP600 advanced reactor design. Geometric similarity was maintained in the regions upstream of the reactor core. Air was used as the working fluid, and a wide range of model velocities were tested. Flow fields in the plenum were visualized with smoke injection and a tuft grid. Twin counterrotating vortices were observed. It is thought that these vortices were induced by viscous shear. The vortices were tested for sensitivity to overall reactor flow, imbalances in the individual coolant loop flows, and position of alignment keyways. Suppression of these vortices was achieved with a passive device placed in the lower plenum. The effect of this device at different axial elevations was studied.