The increase of power outputs enables us to decrease a generation cost such as over thirty nuclear power plants which have adapted an up-rating method in the United States. To success it, it is necessary to evaluate detail thermal hydraulics behavior with high accuracies due to the severe use of internal core structures. The evaluation of coolant flow at a lower plenum of an advanced boiling water reactor (ABWR) is very important because there are a lot of structures such as control rod guide tubes (CRGT) and the core support beams on the fuel assemblies. The coolant flow direction changes from downward to upward with three-dimensional complicated flow in the lower plenum. The simulation results by a CFD (Computational Fluid Dynamics) code can predict such complicated flow in the lower plenum. It is necessary to compare the simulation results with the actual flow in wide range of high Reynolds numbers. And it is required to establish the database of flow structure in lower plenum of ABWR experimentally for the benchmark of CFD code. In the constructed model of the lower plenum of ABWR, we measured velocity profiles by LDV (Laser Doppler Velocimetry) and PIV (Particle Image Velocimetry) techniques with a high speed video camera. The turbulent flow structure of lower plenum of ABWR was evaluated experimentally. In the range of Reynolds number from 103 to 104, the velocity at the center of the test section was faster than the velocity near the wall. The intensity of turbulent increased when the Reynolds number was higher. The velocity profiles in downstream showed the tendency to be flat in the core support beam.

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