Modelling of nonhomogeneous atmosphere in nuclear power plant containmentKrpan, Rok (Avtor) Kljenak, Ivo (Mentor) nuclear safetycomputational fluid dynamicsturbulence modellingatmosphere homogenizationvertical injectionturbulent Schmidt numberturbulent Prandtl numberDuring a severe accident in a light water reactor nuclear power plant, hydrogen may be generated, and its combustion could threaten the integrity of the nuclear power plant containment, which could lead to release of radioactive material into the environment. The study of hydrogen distribution in the containment is important to predict the occurrence of regions with flammable mixture, in order to effectively install hydrogen mitigation systems. A theoretical model to describe the homogenization of the layered atmosphere due to the vertical injection in an enclosure was developed. In linear two-equation eddy viscosity turbulence models, constant values of turbulent Schmidt and Prandtl numbers are usually used. It was shown that constant values fail to reproduce some phenomena observed in experiments. For the purposes of this work, firstly, a local Froude number is introduced, which can be directly used in computational fluid dynamics calculations. Secondly, a model that prescribes turbulent Schmidt and Prandtl numbers dynamically, which uses local Froude number and vertical velocity, is proposed. Non-essential parts of the experimental facilities, which do not contribute much to the mixing process, were identified according to experimental results. Accordingly, axisymmetric two-dimensional and three-dimensional numerical models of cylindrical vessels were developed, and the effects of numerical domain reduction were studied. Since OpenFOAM is a multipurpose computational fluid dynamics code, it was first adapted to simulate the considered cases. An additional term accounting for enthalpy diffusion due to compositional changes was implemented into the energy equation, and a term describing the molecular diffusion had to be added to the convection-diffusion equation for mean mass fraction. The developed physical model for simulation of mixing in nonhomogeneous atmosphere in large enclosures was verified and validated on fourteen different experiments performed in three experimental facilities. The comparison with experimental results showed that the selected modelling approach is able to correctly predict the helium distribution and temperature values. Furthermore, with the proposed model the successful modelling of breaking-up of atmosphere stratification, induced by vertical injection, was extended to isothermal conditions with high mass inflow.20222022-12-10 08:15:02Doktorsko delo/naloga143263sl