This master thesis focuses on the conceptual design of an experimental facility to study the passive heat removal by natural circulation, with focus on the decay heat removal from a nuclear reactor. This study includes the description of the natural circulation phenomena, a literature analysis on the passive heat removal systems already in use in nuclear reactors and the experimental natural circulation facilities already built and operated worldwide, the developments of a simplified steady-state theoretical model to be applied for the preliminary design of an experimental facility named HEATREM (HEAT REMoval) in the Energy Department of Politecnico of Turin, and finally the development of a nodalization of the facility and prediction of its performance by the ATHLET system code (Analysis of THermal-hydraulics of LEaks and Transients).