 Power reactors of this general type continue to produce heat from radioactive decay products even after the main reaction is shut down, so it is necessary to remove this heat to avoid meltdown of the reactor core.In the AP1000, Westinghouse's Passive Core Cooling System uses a tank of water situated above the reactor.When the passive cooling system is activated, the water flows by gravity to the top of the reactor where it evaporates to remove heat.