In the present work, a numerical study of countercurrent flow limitation (CCFL) with air-water as working fluids was carried out using Computational Fluid Dynamics (CFD). The CFD code Star-CCM+ was used, presenting itself a robust and reliable tool for this type of problem. The used geometry was a scaled (1:14) PWR hot leg, with small deviations from the experimental facility built in the Centro de Desenvolvimento da Tecnologia Nuclear (CDTN). The aim of this work was to study numerically the behavior of the system in a potential Loss of Coolant Accident (LOCA) coming from the cold leg of the reactor, which would cause an interruption in the core cooling capabilities. Experimental data from the CDTN experimental facility were used as input for Star-CCM+, which imply in a fixed water mass flow and raising air mass flow. The raising in air mass flow was performed by giving ramp increments through simulation time, providing numerical stability and reproducing the experimental conditions. A numerical tactic was employed regarding the air density variation in the experiment. As a result, the degree of freedom of the simulation were decreased, leading to an enhancement in the numerical stability. As a first assessment, only one mesh was used with a fixed time step, allowing the evaluation of the numerical stability using global Convective Courant Number. The numerical result was compared with experimental ones, showing consistency with the actual physical behavior of this kind of system.

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In the century past, numerical thermo -hydraulic experiments with nozzles, perforated plates, spacers grids, and other parts of the nuclear reactors were carried out to enhance the heat transfer, increase the safety margin and general performance in nuclear reactors. With the recent computational advances is required a high resolution turbulence model for validation of the numerical spatial simulation experiments. Currently, the prototyping is used as way and allows cheaper experiments. The objective of this paper aims to carry out an analysis of the influence of manufacturing parameters inherent to the prototyping processes applied in pressure drop analysis of prototyping parts. The results provide data for conducting CFD simulation models. Methodology: Six reference acrylic perforated plates were produced in a CNC router with different patterns of geometric precision depending on the machining parameters. The pressure drop caused by the plates was measured on the test section, the plates have angle's edge 90º. For each condition, a CFD simulation was carried out using a single hole, since the result can be extended to the entire perforated plate. One plate with the same geometric dimension was produced in polylactic acid ( PLA), a thermoplastic aliphatic polyester, through 3D printing. The pressure differential was measured and compared with those of the acrylic plates and simulations obtained during the first step. Conclusion: Parts printed in 3D printing have an edge with a small radius inherent to the process. A small difference on diameter has a greater influence on pressure drop than geometric deviations with the same order. The effect of roughness on parts, especially printed ones, has little influence on the final pressure drop results. There are many variables that affect the production of 3D printed parts, therefore, to get high resolution is a challenging task.