One of the strategies of cost reduction of nuclear power generation is the increase of power outputs. Especially, in order to achieve performance upgrade of Advanced Boiling Water Reactor (ABWR), it is extremely important to evaluate coolant flow in the lower plenum of ABWR. With the plenty construction in the lower plenum, it is thought that the flow structure is complicated. Moreover, according to the previous studies, there is strong evidence that vortexes arise around side entry orifice when coolant flows in there. Such complicated flow may affect the pressure loss (differential pressure in the lower plenum) and the coolant flow distribution to each core fuel assemblies, and consequently it would influence advancement of fuel economics. Although the simulation results by a CFD code can predict such complicated flow in the lower plenum, the accuracy of simulation data are not enough. Hence, the present study is focusing on the establishment of the benchmark of CFD code by using the visualization method in the lower plenum of ABWR. The objective of the present study is to investigate correlation between the structure of vortexes and complicated flow in upstream of core support beam, and the effect of such fluid behavior to the differential pressure. In the constructed model of the lower plenum of ABWR, velocity profiles were measured by LDV (Laser Doppler Velocimetry) and PIV (Particle Image Velocimetry) techniques. And differential pressure of constructed model is measured by differential pressure instrument. Each measurement was worked out in the range of Reynolds number from 103 to 104. It was found from the LDV measurement that the velocity at the center of the test section was faster than that near the wall in upstream. In downstream, the velocity profiles showed the tendency to be flat in the core support beam. Vortexes were observed around side entry orifice by PIV measurement. Concerning differential pressure, it is necessary to examine correlation between complicated flow structure and differential pressure. Thus in the present study, the differential pressure distribution of constructed model is experimentally investigated.

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