Water pools are used in BWRs and some advanced PWR as heat sink to condense steam generated in the reactor pressure vessel during a postulated accident, and therefore to mitigate the pressure build-up in the containment. One of the main concerns associated with the BWR suppression pool in general is the reduction of heat sink capability in the case of thermal stratification. In this paper, we present the experimental results of a PANDA experiment identified as HP5_5 performed to study the thermal evolution in a large water pool induced by steam and steam-helium (used to simulate hydrogen) released from a multi-hole sparger submerged in the water pool. The HP5_5 test was characterized by four venting phases. In phase 1and phase 3 steam was released in the pool through the sparger. In phase 2 and phase 4, steam and helium were released in the pool through the sparger. The release of helium enhanced the mixing and temperature homogenization dramatically. The higher helium mass flow rate corresponds to a shorter mixing time and for the specific initial and boundary conditions of the HP5_5 experiment, is almost proportional. The temperature maps at the end of the mixing phases 2 and 3 show that the entire pool is mixed.