In this article we present the results of six experiments identified as HP5 series, performed in the large-scale PANDA (Passive Nachzerfallswärmeabfuhr und Druck-Abbau Testanlage) thermal-hydraulics facility within the OECD/NEA HYMERES (Hydrogen Mitigation Experiments for Reactor Safety) project. These experiments focus on thermal stratification and mixing induced by steam injection in a water pool through a vertical sparger with horizontal holes. The pool stratification was created at low steam flow rate and mixing was achieved by either: steam injection at higher flow rate or a combined effect of injection low steam flow rate and horizontal water jet. The phenomena investigated are relevant for the safety of light water reactor, normal operation and accident scenarios during which steam is directly vented and condensed in a large water pool. We find that the initial pool temperature plays an important role in the thermocline formation. Higher initial pool temperature leads to the formation of a thermocline at lower elevations farther away from the sparger and with a larger density difference across the thermocline. The flow field in the pool is affected by initial pool temperature as indicated by Particle Image Velocimetry (PIV). Higher steam flow rate during the second phase showed the effect of steam momentum on mixing. It was found that for the range of parameters considered in the experiments with water injection the density difference between the pool water and the water injected by the nozzle has a greater influence on the mixing time compared with the momentum of the water jet.