Effect of complicated coolant flow behavior in the ABWR lower plenum on flow distribution to the core

•A 1/10th model of the lower plenum was constructed to obtain the benchmark data.•Complicated flow behaviors were measured using LDV and PIV.•Flow distribution in the lower plenum became non-uniform.•Flow distribution became uniform at the core inlet through the side entry orifices.•The vortices were observed around the side entry orifices.