At an unhealed 4 × 4 rod bundle air/water test facility, optical investigations of the entrainment behavior and countercurrent flow experiments were performed under a large variety of test conditions: flow duct geometry; internals (tie plate, bundle length, number of grid spacers, rod diameter); type of injection (different nozzles, porous sinter metal) different mass flux for air and water. In addition, several flooding models were compared with experimental data. It was found that the type of injection has only a weak influence, whereas the geometric conditions upstream of the narrowest flow area (presence of bundle and grid spacer) have an important effect on the flooding behavior. In addition, a comparison of the applicability of different flooding models shows that only the models based on dimensionless numbers expressed by superficial velocities show a good agreement with experimental data.