With respect to thermal hydraulic phenomena, normal full-power operation is typical of most pressurized water reactor (PWR) systems. Under shutdown cooling conditions, a key feature of the AP600 and AP1000 designs is that it uses core decay heat to drive the core cooling process by natural circulation. In fact, the AP600/AP1000 designs use core decay heat to drive the following six natural circulation processes:

• Primary system natural circulation (2 x 4 loop)

• PRHR loop circulation (1 loop)

• CMT loop circulation (2 loops)

• Lower containment sump recirculation (2 loops)

• Containment internal circulation (steam)

• Containment external circulation (air)

Figure V-3 presents a schematic that describes the connections of the primary system passive safety systems.

AP600 Passive Core С00І11Ш System

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FIG. V-3. passive safety systems used in the AP600/AP1000 designs.

The AP600/AP1000 passive safety systems consist of:

• A passive residual heat removal (PRHR) system

• Two core make-up tanks (CMTs)

• A four stage automatic depressurization system (ADS)

• Two accumulator tanks (ACC)

• An in-containment refueling water storage tank (IRWST)

• A lower containment sump (CS)

• Passive containment cooling system (PCS)