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a b s t r a c tHydrogen may be released into the containment atmosphere of a nuclear power plant during a severe accident. Locally, high hydrogen concentrations may be reached that can possibly cause fast deflagration or even detonation and put the integrity of the containment at risk. The distribution and mixing of hydrogen is, therefore, an important safety issue for nuclear power plants.Computational fluid dynamics (CFD) codes can be applied to predict the hydrogen distribution in the containment within the course of a hypothetical severe accident and get an estimate of the local hydrogen concentration in the various zones of the containment. In this way the risk associated with the hydrogen safety issue can be determined, and safety related measurements and procedures could be assessed. In order to further validate the CFD containment model of NRG in the context of hydrogen distribution in the containment of a nuclear power plant, the HM-2 test performed in the German THAI (thermalhydraulics, hydrogen, aerosols and iodine) facility is selected. In the first phase of the HM-2 test a stratified hydrogen-rich light gas layer was established in the upper part of the THAI containment. In the second phase steam was injected at a lower position. This induced a rising plume that gradually dissolved the stratified hydrogen-rich layer from below. Phenomena that are expected in severe accidents, like natural convection, turbulent mixing, condensation, heat transfer and distribution in different compartments, are simulated in this hypothetical severe accident scenario.The hydrogen distribution and associated physical phenomena monitored during the HM-2 test are predicted well by the CFD containment model. Sensitivity analyses demonstrated that a mesh resolution of 45 mm in the bulk and 15 mm near the walls is sufficiently small to adequately model the hydrogen distribution and dissolution processes in the THAI HM-2 test. These analyses also showed that wall functions could be applied. Sensitivity analyses on the effect of the turbulence model and turbulence settings revealed that it is important to take the effect of buoyancy on the turbulent kinetic energy into account. When this effect of buoyancy is included, the results of the standard k-ε turbulence model and SST k-ω turbulence model are similar and agree well with experiment. The outcome of these sensitivity analyses can be used as input for setting up the guidelines on the application of CFD for containment issues.