Abstract: In this paper, the instability phenomenon of a conceptual passive containment cooling system, which is designed to mitigate the consequences after the release of mass and energy during such scenarios as loss of coolant accident (LOCA) or main steam line break (MSLB), is numerically studied. The system is composed of series of heat exchangers, long connecting pipes with relative large diameter, valves and a water tank, which is located at the top of the system and served as the final heat sink. The simulation results show that the two phase instability may occur when the fluid temperature reaches to some value, even if the fluid remains single phase at the exit of the heat exchanger. This type of two phase flow instability is initiated only by the flashing in the riser. The water tank needs to be cooled or refilled with cooler water due to the possible consequences of the instability phenomenon.

Keywords: instability; passive system; open loop; flashing