This paper aims to study the pressure distribution and flow patterns in the top fuel region of the AP1000™ reactor using CFD. This study is being performed as part of a CFD evaluation of the flow in the top fuel and upper plenum regions of a PWR reactor vessel. The flow patterns, including cross flows in the top fuel region, are inter-related with the flow distribution and pressure forces in the reactor vessel upper plenum region. Before detailed computations of the flow in the whole top fuel and upper plenum region are performed, conducting local computations for segments of the domain can provide information about physical aspects of the flow as well as mesh sensitivities. The domain of interest in this paper is the top fuel region including the upper part of the fuel assembly (top grid, fuel rods, top nozzle), upper core plate, and core component hold-down device. The commercial CFD computer code STAR-CCM+ is employed to generate the computational mesh, to solve the Reynolds-averaged Navier-Stokes equations for incompressible flow with a Realizable k-epsilon turbulence model, and to post-process the results. The complicated geometry of the top fuel region needs to be simplified so that the mesh size for the CFD model of the whole upper plenum and top fuel region does not exceed current software and hardware capabilities. In this study, several different trimmed meshes have been generated to study the effects of the geometries of the hold-down device and the lateral flows. Mesh sensitivity studies have been conducted for each individual part, i.e., the top grid, top nozzle, upper core plate, and hold-down device, in order to determine the proper geometrical simplifications. Pressure drop measurement data are compared with the predicted CFD results and act as a guideline for the mesh selection. These studies support the applicability of the geometrically simplified models and chosen mesh size for the CFD model of the full upper plenum and top fuel regions.

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