Computational thermo-fluid dynamics is a powerful and well-established tool adopted in the development and analysis of components of nuclear reactors. To get accurate results a valid computational model has to be developed, starting from a suitable computational mesh. In this thesis, we present the development of a mesh for the simulation of one component of the ESFR (European Sodium Fast Reactor), done together with ENEA. The REDAN is a steel liner that divides the hot and cold pools of the reactor, and it is difficult to model in thermal-hydraulic system codes due to its open geometry. The thesis explores two different approaches to the construction of a suitable geometry and mesh for the hot pool of the reactor, delimited by the REDAN. In the first case, the sections of the geometry that show high regularity are meshed with a structured approach, while the others are kept unstructured. In the second one, all surfaces are meshed with the unstructured algorithm. A comparison of OpenFOAM simulations done with the two different meshes shows that the first approach leads to better results with a comparable computational cost of the simulation.