Experimental visualization of flow structure inside subchannels of a 4 × 6 rod-bundle

•The PRIUS-I test focused on the crossflow and mixing phenomena between the adjacent subchannels.•PRIUS-I test facility can simulate the uniform and non-uniform inlet flow conditions to validate the effect in crossflow between the subchannels.•Flow visualization has been performed using a MIR-PIV technique.•Detailed information for the two-dimensional movement of single-phase flow has been quantified and compared with CFD analysis results.