Application of a CFD based containment model to different large-scale hydrogen distribution experiments

Abstract

In the event of core degradation during a severe accident in water-cooled nuclear power plants (NPPs), large amounts of hydrogen are generated that may be released into the reactor containment. As the hydrogen mixes with the air in the containment, it can form a flammable mixture. Upon ignition it can damage relevant safety systems and put the integrity of the containment at risk. Despite the installation of mitigation measures, it has been recognized that the temporary existence of combustible or explosive gas clouds cannot be fully excluded during certain postulated accident scenarios. The distribution of hydrogen in the containment and mitigation of the risk are, therefore, important safety issues for NPPs.Complementary to lumped parameter code modelling, Computational Fluid Dynamics (CFD) modelling is needed for the detailed assessment of the hydrogen risk in the containment and for the optimal design of hydrogen mitigation systems in order to reduce this risk as far as possible. The CFD model applied by NRG makes use of the well-developed basic features of the commercial CFD package ANSYS-FLUENT. This general purpose CFD package is complemented with specific user-defined sub-models required to capture the relevant thermal-hydraulic phenomena in the containment during a severe accident as well as the effect of mitigation measures. Based on sensitivity analyses and parameter studies performed on the THAI HM2 test, quality guidelines are developed by NRG for the practical application of this CFD containment model.In order to confirm the general applicability of the containment model and model settings, more experimental tests have been analyzed. In this paper, the CFD containment model of NRG is further validated in the context of hydrogen distribution with experiments from the TOSQAN, THAI and PANDA facility. The selected experimental tests cover different processes and conditions typical for a severe accident. The CFD based containment model shows an overall good agreement with the experiments.