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Hydrogen is a combustible gas that can be formed during a severe accident in nuclear power plants (NPPs) by degradation of the core. Safe operation of a nuclear plant requires adequate control of the hydrogen risk. Despite the installation of hydrogen mitigation measures, it has been recognized that the temporary existence of flammable gas clouds cannot be fully excluded during certain postulated accident scenarios.
Complementary to lumped parameter code modeling, Computational Fluid Dynamics (CFD) modeling 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 for the assessment of the hydrogen risk 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 such as passive autocatalytic recombiners (PARs). PARs were installed in most Pressurized Water Reactors (PWRs) after the TMI accident to reduce the hydrogen concentration during severe accidents. At the catalytic plates inside the PARs, the hydrogen is converted into steam. The exothermal recombination reaction generates a buoyant flow through and out of the PAR. The hot exhaust gas coming from the PAR mixes the containment atmosphere.
The primary goal of the present study is to further develop, test and optimize the PAR modeling of NRG in the FLUENT CFD code, keeping in mind that it should be applicable to full reactor scale in the end. The ultimate goal is to validate the improved PAR modeling strategy by applying it to a large-scale experiment performed in the THAI facility (60m3).