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Three Mile Island
In the predawn hours of March 28, 1979, a pressure valve suddenly malfunctioned at the Three Mile Island nuclear power plant near Harrisburg, Pennsylvania. The accident started at the plant's Unit 2 reactor when a small valve failed to close, causing cooling water to drain from the nuclear core.

The core quickly began to overheat. Confronted by baffling and contradictory information, plant operators shut off the emergency water system that would have cooled the core. By early morning, Wednesday, March 28, the exposed part of the core was beginning to cook as temperatures in the reactor reached 4,300 degrees Fahrenheit--dangerously close to meltdown.

But when contaminated water leaked into an adjoining building and started to release radioactive gases inside the plant, Three Mile Island's supervisor declared the first general emergency ever to arise at a nuclear power plant in the United States. On March 30, pregnant women and school-age children were advised to leave the area. This announcement unleashed a wave of panic as residents tossed a few belongings into their cars and sped off. More than 140,000 would eventually flee.

Friday also brought a new, more terrifying revelation: a hydrogen bubble had formed above the reactor core. The bubble could prevent cooling and eventually lead to a meltdown. That afternoon, scientists finally determined that the hydrogen bubble posed no immediate threat and that the reactor core had stabilized. Three years after the accident, a robotic camera was lowered into the Unit 2 core, providing the first look at what had happened.

Fifty percent of the core was destroyed or molten, and something on the order of 20 tons of uranium found its way to the bottom head of the pressure vessel. Since the Three Mile Island incident, not a single new nuclear power plant has been started in the United States. Three Mile Island accident,

Three Mile Island accident: Three Mile Island [Credit: John S. Zeedick/Getty Images]accident in 1979 at the Three Mile Island nuclear power station that was the most serious in the history of the American nuclear power industry. The Three Mile Island power station was named after the island on which it was situated in the Susquehanna River near Harrisburg, Pa. At 4:00 am on March 28, an automatically operated valve in the Unit 2 reactor mistakenly closed, shutting off the water supply to the main feedwater system (the system that transfers heat from the water circulating in the reactor core).

This caused the reactor core to shut down automatically, but a series of equipment and instrument malfunctions, human errors in operating procedures, and mistaken decisions in the ensuing hours led to a serious loss of water coolant from the reactor core. As a result, the core was partially exposed, and the zirconium cladding of its fuel reacted with the surrounding superheated steam to form a large accumulation of hydrogen gas, some of which escaped from the core into the containment vessel of the reactor building. Very little of this and other radioactive gases escaped into the atmosphere, and they did not constitute a threat to the health of the surrounding population. In the following days, adequate coolant water circulation in the core was restored.

The accident at Three Mile Island, though minuscule in its health consequences, had widespread and profound effects on the American nuclear power industry. It resulted in the immediate (though temporary) closing of seven operating reactors like those at Three Mile Island. A moratorium on the licensing of all new reactors was also temporarily imposed, and the whole process of approval for new plants by the Nuclear Regulatory Commission was significantly slowed for years after the accident. No new reactors were ordered by utility companies in the United States from 1979 through the mid-1980s.

The accident increased public fears about the safety of nuclear reactors and strengthened public opposition to the construction of new plants. The unharmed Unit 1 reactor at Three Mile Island did not resume operation until 1985. The cleanup of Unit 2 continued until 1990; damage to the unit was so severe, however (52 percent of the core melted down), that it remained unusable.