15.27. Engineered safety features rely on gravity or stored energy for core cooling and decay heat removal. In the event of a core overheating accident leading to excessive system pressure, relief valves automatically release the steam-reactor coolant mixture to a suppression pool similar to that in present BWRs. Nitrogen is used to inert the containment to reduce the potential for a hydrogen explosion. The system, now at low pressure, is then flooded by gravity with water stored in an elevated pool. Since there are no large pipes attached to the vessel at or below the core level, the core should remain covered for all design basis events. The consequences of a severe accident would be mitigated by gravity-driven dry well flooding.

15.28. Decay heat removal while the system is at full pressure is ac­complished by nonpassive pumps and heat exchangers. However, after blowdown, when the reactor vessel is isolated from the turbine-condenser, decay heat is removed by three passive condensation loops. Following a loss-of-coolant accident, the steam-air-nitrogen mixture in the upper dry — well flows to three condensers which are submerged in a separate elevated water pool which supplies the cooling. Condensate is drained to the gravity — driven injection pool and noncondensable gases are discharged below the surface of the suppression pool. Steam formed in the condenser pool as a result of the 30-MW(th) heat-transfer capacity is vented to the atmosphere. The pool has a capacity to provide 3 days of condenser cooling before water replenishment, which, of course, would normally be accomplished automatically.

15.29. A passive natural circulation air system is used to serve the con­trol room area in the event of station blackout. Since the various core cooling features are also passive, the safety grade emergency diesel gen­erators necessary for present systems can be eliminated.