Reactor overheats at Three Mile Island - Mar 28, 1979 - HISTORY.com

The most serious nuclear accident in United States history takes place at the Three Mile Island plant near Harrisburg, Pennsylvania, on this day in 1979, when one of the reactors overheats. Fortunately, a catastrophic meltdown was averted and there were no deaths or direct injuries from the accident.

The Three Mile Island plant had begun operations just months earlier on December 28, 1978. Very shortly after operations began, problems arose. It was 3:58 a.m. on March 28 when a pump that directed steam to the plant’s electric turbines stopped working, causing a water circulation pump to break down. Without the water, the temperature of the reactor rose dramatically and a relief valve opened to stop the pressure from building to dangerous levels. Unfortunately, the valve then would not close.

The plant operators, with no experience in emergencies, made key errors. Another valve was opened to allow water from the nuclear system into a waste tank. But this water ruptured the tank and radioactive water flooded into the reactor. Even worse, an operator shut off the automatic core-cooling system. The result of all these events and mistakes was that radioactive steam poured out of the plant. Additionally, radioactive water had to be released into the Susquehanna River. However, area authorities were not notified of these events until nearly three hours later.

Even when news of the accident was released, it was downplayed. But within days, radiation levels were elevated over a four-county area. Furthermore, the plant’s operators were still trying to get the situation under control. Pennsylvania Governor Richard Thornburgh directed that pregnant women and small children be evacuated from the area. Finally, on March 31, plant workers were able to address the problems and ended the threat of a meltdown. The area was deemed safe on April 9.

A subsequent investigation and report blamed a combination of human error and faulty design for the accident. In March 1984, the Metropolitan Edison Company pleaded guilty to knowingly using inaccurate test methods at the plant before the incident. The Three Mile Island accident also exposed the lack of an appropriate evacuation plan for the area.

In the years following this accident, there has been an ongoing controversy over whether the increased radiation released at Three Mile Island led to an increase of cancer and infant mortality in the surrounding areas.