Some designs utilize the reactor cavity and other lower containment compartments as a reservoir of coolant for core cooling in the event of a break in the primary system. As such, water lost from the reactor system is collected in the containment sump. Eventually the reactor is completely immersed in water and the isolation valves are opened. Decay heat removal occurs by boiling in the core. The steam generated in the core travels upward through an automatic depressurization system (ADS) valve that vents directly into containment. The density difference established in the situation depicted in Figure 8 between the core region and the pool produces a natural circulation flow that draws water up through the sump screen into the reactor vessel and is adequate in removing the decay heat. In some design cases, natural circulation inside the reactor vessel may be sufficient to remove decay heat without the need of ADS operation. This is a Category D passive safety system.

Annexes I through XX present descriptions of how different variations of these systems work in combination in various advanced water-cooled NPP designs to provide core cooling after a reactor scram.

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