just linking back to the IEEE Spectrum piece again because the AP1000 safety features were described so well.
Passive safety features will shut down this reactor without any power, pumps, or people
How it works:
The AP1000 core is similar to standard PWR cores: The fuel produces heat that turns water into steam, which drives a turbine.
The reactor’s designers have significantly improved the safety features on an otherwise standard PWR. Whereas conventional reactors rely on motor-powered valves and water pumps to deal with accidents, the amended AP1000 design has safety systems that rely on airflow, pressure changes, and gravity. For example, if a coolant pipe breaks, both the pressure and temperature rise inside the containment vessel . Those changes trigger a water-flooding emergency system [2, 3, 4]. The water inside the sealed containment vessel heats up and turns into steam. The steam rises to the top, where the steel shell has been cooled by air circulating around the vessel. Thus cooled, the steam condenses back into water. This cycle reduces the pressure and temperature, and the nuclear chain reaction ends. Unlike other pressurized-water reactors, the AP1000 needs no safety features beyond the passive ones.
Or try this promo on Youtube.