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How do passive safety systems use gravity to cool newer nuclear reactor designs?

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Labeled diagram showing the process of a nuclear power plant with a pressurized-water reactor. Water circulates through the reactor core, heats to produce steam in a steam generator, which drives a turbine connected to an electric generator. Steam is then cooled in a condenser using water from a cooling tower, which releases nonradioactive water vapor. Control rods, pumps, and transmission lines are also indicated. — nuclear power Schematic diagram of a nuclear power plant using a pressurized-water reactor.

Newer Generation III nuclear reactor designs, such as the Westinghouse AP1000, utilize passive safety systems to maintain cooling without the need for human intervention or electrical power. Unlike older "active" systems that rely on pumps and electricity, these designs leverage gravity to circulate cooling water.12

In the AP1000 design, for example, reservoirs located within the containment structure feed water to the reactor core via gravity. As this water absorbs decay heat, it boils and rises as steam to the top of the containment structure, where it condenses and flows back into the cooling system. The heat from this condensation is then transferred to the containment structure itself, which is cooled by water flowing by gravity from an external tank positioned atop the structure. This process allows the reactor to dissipate heat into the atmosphere through evaporation on the exterior of the containment, ensuring safety even during a complete station blackout. By reducing the reliance on complex mechanical components like pumps and valves, these passive designs enhance safety while potentially lowering construction costs.12