Hydrogen leaking inside a nuclear power plant poses a critical nuclear safety issue, and accurate prediction of the dispersion process of hydrogen has become an important topic of research. In the present study, temporal evolution of hydrogen dispersion in a test facility (called PANDA) for nuclear safety is analyzed using high-fidelity simulation techniques such as large eddy simulation (LES), high-order discretization methods, immersed boundary (IB) method, etc. An important topic in this study is how a turbulent buoyant jet of a released gas-air mixture interacts with a stratified layer formed near the ceiling of a containment. In this study, the interaction of jet penetration with the stratified layer is characterized as the "slow erosion process". The height of the jet penetration is limited by light gas at the top due to the negative buoyant effect. From a viewpoint of the safety of the containment, the present study shows that by a continuous release of hydrogen, the flammable region expands significantly over time. However, the change of the detonable region over time is much smaller compared to that of the flammable region. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.