[en] This work presents experimental results of a circuit when operation in natural circulation. These results allow to analyze the behavior of an emergency core cooling system when a bypass line that connects the hot source with the cold source is opened. This work also reports the hydraulic characterization of the experimental loop, given geometric and hydraulic data including experimental friction factors specific to this circuit. It was observed that, to a fixed thermal power, when the bypass line is opened, the heater outlet temperature increases. This temperature increase is due to the decrease in the flow rate through the heater. The heat exchanger flow rate is subjected to a small increase. This flow rate is the sum of the bypass line and heater mass flow rates. This work also shows that the vertical position of the connection of the bypass line in the hot-leg determines the flow direction in the bypass line. If the bypass line connection is in the lowest position, the flow is from the cold to the hot-leg. If the bypass connection is in the highers position, the flow is from the hot to the cold-leg. A numerical model used to evaluate friction factors and heat transfer coefficients influence was developed. It was used to confirm the possibility of precise experiments simulation. The conservation equations are solved using 'Engineering Equation Solver' a thermal hydraulics analysis tool. The model was adjusted with natural circulation experimental data and was tested with results of natural circulation without bypass line. (author)