AP600 Passive Containment Cooling System (PCS) provides the safety-related ultimate heat sink for the plant

As demonstrated in computer analysis and tests, the PCS is able to effectively cool the containment following an accident such that the design pressure is not exceeded and the pressure is rapidly reduced. The steel containment vessel itself provides the heat transfer surface that removes heat from inside the containment and rejects it to the atmosphere. Heat is removed from the containment vessel by a natural circulation flow of air that cannot be isolated.

In the unlikely event of an accident, the air cooling is supplemented by water evaporation on the outside of the containment shell. The water is drained by gravity from a tank located on top of the containment shield building. Two normally closed, fail-open valves are opened to initiate the water drain. The water tank is sized for three days of operation, after which time the tank is expected to be refilled to maintain the low containment pressure achieved after the accident. If the water is not resupplied after three days, the containment pressure will increase, but the peak is calculated to reach only 90 percent of design pressure after about two weeks.