Study of impact of the AP1000™ upper internals design on fuel performance

The Westinghouse AP1000™ ^[1] reactor is a new advanced passive reactor design. Four AP1000 reactors are being built in China, and numerous reactors are being planned for the United States. One aspect of the AP1000 reactor design is the reduction in the number of major components and simplification in manufacturing. One design change relative to current Westinghouse reactors of similar size is the reduction in the number of reactor vessel outlet nozzles / hot legs leaving the upper plenum. The AP1000 reactor has two nozzles / hot legs, while current Westinghouse reactors of similar size have three nozzles / hot legs. With regard to fuel performance, this design difference creates a different flow field in the AP1000 upper plenum. The upper plenum is the region above the core that the core flow exits into. The flow entering the upper plenum must turn 90° toward either outlet nozzle, flow laterally through the upper plenum around support structures, and exit through one of the two outlet nozzles. While the flow in the top of the core is mostly axial, there is some lateral flow component as the core flow reacts to the flow field and pressure distribution in the upper plenum. The pressure distribution in the upper plenum varies laterally depending upon various factors including the proximity to the outlet nozzles. To determine how the lateral flow in the top of the AP1000 core compares to current Westinghouse reactors, a computational fluid dynamics (CFD) model of the flow in the upper portion of the AP1000 reactor vessel was created. The purpose of this model is to capture the top region of the core, the upper plenum, the reactor vessel outlet nozzles, and a portion of the hot legs. Due to geometric symmetry, the computational domain was reduced to a quarter (from the top view) that includes 1/4 of the core, 1/4 of the upper plenum, and 1/2 of an outlet nozzle. The details of the developed CFD model will be discussed in the paper. Results from this model include predicted velocity fields and pressure distributions throughout the model domain. From these results, comparisons of AP1000 flow versus current Westinghouse plants will be presented. Field performance information from current Westinghouse plants will be shown to demonstrate an experience base of acceptable core lateral flows. From this experience base and the CFD results for AP1000 lateral flows, acceptability of the AP1000 upper plenum design on the fuel performance of the AP1000 fuel design will be demonstrated.