Natural circulation has many engineering application and one of them is the decay heat removal from the core of a nuclear power plant. Passive safety systems based on natural circulation are used to accomplish safety function without requiring any active power source or moving mechanical parts. There are two main components in these kind of systems: a large pool (the heat sink) and an heat exchanger (the heat source). The heat removal process involves a complex interaction of mass and heat transfer, characterized by single and two-phase flow. Density gradient induces the fluid's motion inside the pool, where a non uniform temperature distribution can be detected, as a consequence of the thermal stratification.