Abstract: The system according to the invention comprises a first room which accommodates a reactor plant and an active-type sprinkler means to condense steam released by the ebullient coolant after an accident. A loss of coolant may occur at any point of the reactor plant; that notwithstanding, the rising pressure in said first room drives the air out of that room through holes provided in the walls of the room in immediate proximity to the floor and spaced at a maximum distance from one another over the perimeter of the walls. The air is forced into an intermediate chamber wherefrom it is directed to a second room through channels and a basin-type condenser arranged in a second room. The length of the channels is selected so as to form a water seal to prevent the backflow of air driven from the first room to the second room and thus produce rarefaction in the first room.