There, half the reactor core melted in the first two hours of the accident. The melting stopped when an operator turned on emergency cooling, which had mistakenly been turned off.

The cooling problems at the Japanese plant raised fears of a repeat of 1979's Three Mile Island accident, the most serious in the history of the U.S. nuclear power industry.

The typical Component Cooling System is used during all phase of plant operation, including shutdown and post-accident.