• Finite Element Method
• Hydrogen Mixing

The containment of a nuclear reactor is the last barrier that avoid the release of radioactive material; for this reason any excessive load that could jeopardize its integrity must be avoided.

In case of a severe accident, the heat up of the core and its degradation lead to the formation of hydrogen following cladding oxidation. Then, hydrogen could migrate into the containment and accumulate in the upper region, where it could exceed the concentration's limit for flame propagation. As result hydrogen explosions could occur threatening containment integrity. For this reason present containments are inertized or incude many engineering safety features preventing hydrogen accumulation.
These features consist in mixing devices for homogenizing the atmosphere and hydrogen recombiners.
Natural circulation could also enhance the mixing through ascending and descending flows.
Thanks to the recent improvements in computer capabilities, in last decades CFD codes capable of modeling 3D containment facilities have been developed with the aim of simulating containment scenarios and proving the safety of nuclear power plants.

The aim of this work consists in understanding and solving some issues related to the modeling of gas transport and mixing inside a closed volume. Cast3M finite element CFD code has been adopted to perform all the simulations.
In particular we have focused the attention on simple problems related to pure diffusion processes, where the fluid is expected to be at rest. CFD computations have shown some discrepancies in results since non-physical velocities appear in the computational domain altering the status of fluid. These `spurious velocities' seem to be linked to the use of an unstructured mesh coupled with a numerical method, that does not rigorously ensure incompressibility constrains.
Due to the formation of a non-real flow, CFD transient simulations could be affected by transport phenomena that enhance the mixing inside the volume. As result, computed scenarios could present non-conservative situations in which hydrogen does not reach flammability limits.

To decrease the magnitude of these spurious velocities the Helmholtz-Hodge decomposition method has been adopted. Thanks to this method real improvements have been achieved in reducing the influence of numerical errors in the simulations.

Eventually the pure diffusion process of chemical species has been coupled with a simple turbulence model. The aim is to achieve a wider comprehension on the phenomena that are responsible for the fastening of the mixing process. At the end of this analysis, a more suitable relation for the mixing-length, that allows us to better model diffusion processes, has been evaluated.