CFD4NRS, Garching, Munich, 5-7 Sept. 2006 ON THE IMPORTANCE OF VALIDATION WHEN USING COMMERCIAL CFD CODES IN NUCLEAR REACTOR SAFETY Heinz Wilkening a , Daniele Baraldi a and Matthias Heitsch b a Institute for Energy, DG Joint Research Centre, European Commission, Petten, The Netherlands b Gesellschaft für Anlagen- und Reaktorsicherheit GRS (mbH), Köln, Germany Abstract In this validation work two turbulence models (k-ε and SST model) and two grids (a finer hybrid grid and a tetrahedral coarser grid) are considered in order to model helium release and dispersion. Simulation results are compared against an experiment of jet release phenomena in the Battelle Model Containment facility (BMC), a multi-compartment facility with a total volume of about 560 m 3 . In the selected test, HYJET Jx7, helium was released into the containment at a speed of 42 m/s over a time of 200 s. Although the k-ε model is the most commonly used turbulence model in most CFD applications, it does not provide the most accurate predictions for this application. Alternatively the SST turbulence model has been employed giving more accurate results. This investigation provides a confirmation that the validation of commercial CFD codes is always required in order to select the more suitable physical models and computational grids for each specific application. 1 Introduction CFD (Computational Fluid Dynamics) simulations are playing an increasingly important role in safety analysis for nuclear applications. This trend is driven by the fact that experiments, especially at large scale, are very costly and often not feasible. Moreover the ever-increasing speed of today’s high performance computer is making possible to perform CFD simulations of realistic scenarios. Initially the CFD codes used in nuclear reactor safety were mostly in-house research codes developed for a dedicated purpose. Similarly, at JRC we have developed our own CFD code REACFLOW for simulations of hydrogen explosions during a severe nuclear accident. In more recent times users have also the choice to use commercial multi-purpose CFD codes. These codes have a wide range of application especially in the non-nuclear field, such as aerodynamics, turbo-machinery, automotive and so on. Typically commercial codes have been validated by the suppliers. Nevertheless when a user applies a commercial CFD code to a specific problem, he/she has to make several choices in the generation of the computational mesh and in the selection of the physical models. One challenging problem in CFD analysis is the modelling of turbulent flows. Up to now there is no universal turbulence model that is suitable for all applications. Therefore commercial CFD codes offer a wide range of turbulence models to choose from. The choice should depend only on the physics of the problem. Nevertheless the user should be aware that his choice has several implications e.g. also on the computational grid to be used. 525