Three Mile Island: How America’s worst nuclear accident unfolded via USA Today

Ramon Padilla, Janet Loehrke, and George Petras, USA TODAY

On March 28, 1979, Unit 2 at the Three Mile Island Nuclear Power Plant near Middletown, Pennsylvania, experienced a partial meltdown in what became known as the nation’s worst commercial nuclear accident. 

The incident sparked national protests, prompted more stringent safety standards for nuclear plants across the country, and sapped the industry’s momentum for decades.

Nuclear energy is making a comeback of sorts given ambitious goals by environmental groups and Democratic presidential candidates seeking carbon-free alternatives to fossil fuels. But the stigma of Three Mile Island still looms large more than four decades after the accident.

At noon on Friday, TMI’s remaining reactor will generate its last kilowatt of energy and close down.


The valve should close as pressure falls to normal levels, but it becomes stuck open, allowing coolant water from the reactor to escape. This loss of coolant would ultimately lead to the partial meltdown. Operators do not know water is being lost. They believe the relief valve is shut since a “close” command was sent to the device. However, there is no way to tell if the valve is actually closed.

High temperatures at that valve provided a clue that water was escaping there, but operators had been operating for weeks with a small leak at that valve and became accustomed to the abnormal high temperature. ¥


Unit 2 was closed permanently. The plant’s other nuclear reactor, Unit 1, returned to service in October 1985. It was licensed to operate until 2034, but is being closed due to financial losses.

The accident forced drastic changes in nuclear power plant operations and safety measures.

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3 Responses to Three Mile Island: How America’s worst nuclear accident unfolded via USA Today

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