In the effort to reduce greenhouse gas emissions, nuclear energy stands out as a low-carbon solution able to supply constant and dispatchable energy in a world shifting towards a global electrification. The fuel assembly is a key component of a nuclear reactor, enabling the controlled fission process that generates heat. Operating under extreme temperature and pressure conditions, its thermohydraulic behavior is an important field of study, as it directly influences the fuel assembly’s ability to ensure safe heat removal, structural integrity, and overall reactor performance. During my master’s thesis, I had the opportunity to work for six months with the Thermohydraulic team of the FUEL Design department from the company Framatome responsible for thermohydraulic study of the fuel assembly. This report presents a Computational Fluid Dynamics (CFD) study aimed at analyzing fluid mixing behavior under conditions representative of nuclear applications. Significant time was spent setting up the simulation to replicate as accurately as possible the temperature field at the outlet of the fuel assembly. A comparison between the CFD simulation and experimental temperature measurements was carried out to assess the accuracy of the simulated results. Finally, sensitivity studies were conducted on the wall treatment of the boundary layer to evaluate its impact on the outlet temperature field.