Pressurized water reactors using Direct Vessel Injection (DVI) incorporate deflector structures that affect coolant flow and mixing. This study examines the annular downcomer and lower plenum flow in a 1:6 scale HPR1000 model with and without the DVI deflector. Particle Image Velocimetry (PIV) measured velocity distributions under varied loop numbers and inlet Reynolds numbers. Without the deflector, unstable low-velocity regions form and cause flow fluctuations. With the deflector, these zones are disrupted, stabilizing flow and reducing mixing in the lower plenum. Upstream velocities remain lower than downstream, with larger velocity differences and low-velocity regions as loop numbers increase. Velocities correlate positively with Reynolds numbers, while the DVI deflector diminishes downstream velocity sensitivity to Reynolds number changes. These findings clarify the DVI deflector’s impact on flow characteristics and provide experimental data for optimizing passive safety systems in advanced pressurized water reactors.