Passive Core Cooling System - Residual Heat Removal

The AP600 has a passive residual heat removal (PRHR) subsystem that protects the plant against transients that upset the normal steam generator feedwater and steam systems. Westinghouse analysis results, using NRC-approved codes, has shown that the PRHR subsystem satisfies the NRC safety criteria for loss of feedwater, feedwater line breaks, and steam line breaks with a single failure. Anticipated transients without reactor trip have also been analyzed and shown to result in peak RCS pressures of about 2900 psig, well within NRC criteria.

The Passive RHR Heat Exchanger

The Passive RHR heat exchanger consists of a 100 percent capacity bank of tubes connected to the reactor coolant system (RCS) in a natural circulation loop. The loop is normally isolated from the RCS by valves that are normally closed, but fail open if power is lost. The heat exchanger tubes are located in the in-containment refueling water storage tank (IRWST). This location places the Passive RHR heat exchanger above the RCS loop so that hot water leaving the RCS hot leg will rise to the top of the PRHR HX where it is cooled. The difference in temperature between the hot inlet water and the cold outlet water drives the natural circulation loop. If the reactor coolant pumps are running, they boost the PRHR HX flow.

The In-containment Refueling Water Storage Tank (IRWST)

The IRWST provides the heat sink for the passive RHR heat exchanger. The IRWST water volume is sufficient to absorb decay heat for about 2 hours before the water would start to boil. After that, steam would be generated and enter the containment. This steam would condense on the steel containment vessel and then drain back into the IRWST.