Passive residual heat removal (PRHR) heat exchangers are incorporated into several advanced PWR designs. Their primary function is to provide extended periods of core decay heat removal by transferring heat using a single-phase liquid natural circulation loop as shown in Figure 6. The PRHR heat exchanger loop is normally pressurized and ready for service. Single-phase liquid flow is actuated by opening the isolation valve at the bottom of the PRHR heat exchanger. The PRHR system design is optimized for single-phase (contrary to isolation condenser which is optimized for boiling and condensation) liquid heat transfer. It is particularly useful in mitigating the station blackout scenario. In general it eliminates the need for ‘bleed and feed’ operations for plant cool-down. This is a Category D passive safety system.

This CRP does not deal with natural draft air flow cooling of tubes. Figure 5 is added for completeness.

REACTOR

VESSEL

FIG. 6. Core decay heat removal using a water-cooled passive residual heat removal heat exchanger loop.