15.17. The passive features center on the operation of the safety injec­tion systems and provisions for removing core decay heat. Passive systems are defined as those which are self-contained or self-supported. To perform their safety functions, they rely on gravity, as in natural circulation cooling, or stored energy, such as that in compressed gases. Emergency electrical needs must be met by battery power rather than by standby diesel gen­erators as is current practice. Valves to initiate safety system operation should be check valves or those activated by stored energy. Passive systems are expected to perform their safety functions for up to 72 hours after the initiating event independent of operator action and off-site power. Thus, should there be an emergency shutdown, decay heat must be passively removed from the core. Also, coolant must be replaced if there is a loss — of-coolant accident.

15.18. In the AP600, the safety injection systems are integrated with the reactor coolant makeup and residual heat removal systems. Decay heat is removed by a natural circulation system utilizing a heat exchanger sub­merged in a large refueling water tank within the containment as shown in Fig. 15.2. The tank volume is sufficient to absorb decay heat for about 2 hours, after which the water would start to boil. Then, the generated steam would condense on the inside of the containment and drain back into the refueling tank. Gravity-driven injection or pressurized accumu­lators replace water lost in the coolant system. As shown in Fig. 15.2, two core makeup tanks containing borated water at full cooling system oper­ating pressure are available to inject water by gravity in the event of a small-break LOCA. For larger breaks, two accumulators pressurized to 4.93 MPa(a) (715 psia) by nitrogen are available for injection. Finally, additional water can be injected by gravity from the in-containment re­fueling water storage tank provided that the cooling system pressure has first been reduced to about 1.72 MPa(a) (25 psia). This passive safety injection system eliminates the need for various pumps and support com­ponents such as diesel generators, thus significantly reducing the number of plant components required.

15.19. Passive cooling is also provided for the containment. Figure 15.2 shows an inner steel containment vessel which serves to transfer heat to outside air moving by natural circulation between the steel shell and the outer concrete shield. Additional cooling is provided by water stored in a tank at the top of the concrete shield. This water is allowed to evaporate

on the outside of the steel vessel. This water tank is sized for 3 days of operation, after which refilling is expected. However, if the water supply is not replenished for some reason, the containment pressure will rise to a maximum in about 2 weeks, but will then reach only 90 percent of the design pressure.