Knowles, PhilipThe purpose of this thesis is to examine effects of multiple plumes in a Pressurized Water Reactor downcomer under scaled Pressurized Thermal Shock conditions, then assess the flow patterns and mixing compared to a single plume. Most computational and experimental studies have been performed using only single plumes; the findings of multiple plume experiments indicate that plume interaction significantly changes the flow pattern in the downcomer. A globally-induced recirculation region was created by the collective interaction of the multiple plumes in the UMD experiments, which does not occur for a single plume under similar conditions. From the evolving concentration field measurements and entrainment theory, it's argued that two merged plumes experience lower entrainment rates than a single plume. This implies the possibility of a higher thermal stress on the downcomer wall than would be inferred from single plume tests. Flow visualization was employed to examine the differences in plume behavior.en-USThesisEngineering, MechanicalPressurized Water ReactorPressurized Thermal ShockexperimentLIFplume