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Why did the loss of electrical power at Fukushima make cooling the reactor cores difficult?

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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.

The loss of electrical power at the Fukushima Daiichi plant made cooling the reactor cores difficult because it disabled the critical systems required to remove decay heat.12

While the reactors were successfully shut down following the 2011 earthquake, they continued to generate residual heat that required active cooling. Nuclear power plants rely on emergency power systems—typically consisting of diesel generators and backup batteries—to power the pumps, valves, and instruments necessary to circulate coolant and manage this heat.312

At Fukushima, the tsunami flooded and damaged the backup generators, and the subsequent prolonged power outage eventually drained the emergency batteries. Without electrical power, the plant lost the ability to operate its emergency core-cooling systems, leading to a buildup of heat that caused the fuel rods to overheat and partially melt down. This highlights the vital importance of maintaining reliable power to support safety systems under all conditions, as the failure of these systems can lead to catastrophic outcomes.1342