This article presents the experimental results and the phenomenological analyses of the H2P4 tests performed in the PANDA facility, within the OECD/NEA HYMERES project. H2P4 series is characterized by two phases: (i) the steam injection in the water pool with development of thermally stratified layer and pressurization of the system, and (ii) water spray injection above the pool to depressurize the gas space. This article analyzes two tests that are characterized by different sparger submergence levels. The results indicate that the sparger submergence affects the pool's thermal behavior, and the pressurization history. A decrease in submergence height leads to a faster pressurization of the vessel. An analysis of the experimental data is carried out using mass and energy balances for the liquid and gas volumes. This analysis contributes to the understanding of the pool phenomena and suggests that mass and energy are lost through vaporization at the liquid–gas interface and a significant amount of steam is condensed at the walls.