A combined coolant storage tank and sump located within the containment structure which surrounds and isolates the nuclear reactor. The combined tank and sump provides a means for storing a supply of emergency coolant for use in cooling the nuclear core of the reactor in the event of a loss of coolant accident in the primary coolant circulating loop and also provides a means for accumulating any coolant fluid discharged into the containment atmosphere. Conduit means flow couple the combined tank and sump and the nuclear reactor, and drive means are provided for driving the emergency coolant from the combined tank and sump into the nuclear reactor in the event of an emergency requiring injection of the coolant into the nuclear reactor. Also the combined tank and sump provides a means for relieving the pressure in the secondary side of the heat exchanger and in the pressurizer.