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Meltdown at Three Mile Island

By Patrick Mondout

At four in the morning on March 28, 1979, a malfunction in the cooling system at the Three Mile Island Nuclear Generating Station led to the most serious commercial nuclear accident in US history and paved the way for reforms in the way nuclear power plants are operated and regulated. It also made Americans question the safety of nuclear power and helped make The China Syndrome - which had been released three weeks earlier - one of the biggest movies of the year.

About Three Mile Island

The Three Mile Island (TMI) Nuclear Generating station is located on 814 acres on an island in the Susquehanna River some 10 miles southeast of Harrisburg, Pennsylvania near some farmland. There are four separate generators at TMI and it was #2 that failed (it has been closed since then).

The Accident

There was nothing unusual about the early morning of March 28, 1979 at the Three Mile Nuclear Generating station. The weather was cold but not unusually so. But during routine maintenance, an automatically operated valve in the Unit 2 reactor closed when it should not have most likely due to either a mechanical or electrical failure. This shut off the water supply to the system that cools down the reactor core and prevented the steam generators from removing heat. Automated systems then shut down the reactor core. That should have been the end of the accident, but it was not.

A misreading by one of the engineers on duty compounded with a series of equipment and instrument malfunctions led to a dangerous loss of water coolant from the reactor core. As a result, the reactor core was partially exposed, which led to some radioactive gases escaping into the containment section of the reactor building. Though some of this radiation was released into the surrounding area, no immediate deaths or injuries occurred.


Stop Met-Ed sign in Middletown, PA, 1979. Metropolitan-Edison was the power company utilizing the power generated at TMI.


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Media Circus

Reporters descending on the scene the next day were welcomed by sirens which were warning residents that radiation was being released, residents going the opposite direction with their belongings, and a proclamation by then-governor Richard Thornburgh urging pregnant women and those with small children to leave the area and calling for the closure of more than 20 local schools.

At the same time the Nuclear Regulatory Commission was hedging its bets by saying there might come a time when everyone had to be evacuated. For a time it seemed the environmentalist had been right all along about nuclear power. President Carter visited the area a few days later to assure the nation that the area was safe.


According to the U.S. Nuclear Regulatory Commission's independent Rogovin Commission Report, we were a mere half an hour away from an irreversible meltdown as described in The China Syndrome. In fact, over 90% of the reactor core was damaged, 52% had melted down, and the containment building in which the reactor is located as well as several other locations around the plant were contaminated. In the end officials were able to restore enough coolant to the reactor core to prevent a complete meltdown and the #2 reactor at TMI was shut down permanently. The #1 reactor was also shut down and did not resume operation until 1985.


The Nuclear Regulatory Commission (NRC) issued bulletins to all plants operating Babcock and Wilcox equipment (which is what is used at TMI) and many were temporarily shutdown. The clean-up at the #2 reactor took over a decade to complete. It would be six years before a power company again had the courage to ask for permission to build a nuclear power plant or even to add a reactor to an existing one.

The National Institutes of Health released a study about the effects on the population around Three Mile Island in 1997. The study was carried out by Professor Steven Wing and colleagues at the University of North Carolina at Chapel Hill. The study revealed that exposure to radiation after the 1979 nuclear accident at Three Mile Island may have increased cancer among some Pennsylvanians downwind of the plant. The data behind these conclusions were published in the Feb 24 1997 issue of Environmental Health Perspectives, the journal of the National Institute of Environmental Health Sciences.

The new study involved re-analyzing data from a 1990 report that concluded the nationís worst civilian nuclear accident was not responsible for excess cancers because radiation exposures were too low. However, the new analysis takes a contradictory position. Dr. Wing comments:

"Several hundred people at the time of the accident reported nausea, vomiting, hair loss and skin rashes, and a number said their pets died or had symptoms of radiation exposure. We figured that if that were possible, we ought to look at [the data] again. After adjusting for pre-accident cancer incidence, we found a striking increase in cancers downwind from Three Mile Island... I would be the first to say that our study doesnít prove by itself that there were high-level radiation exposures, but it is part of a body of evidence that is consistent with high exposures."


Protestors at an anti-nuke rally at Pennsylvania's state Capitol building April 9, 1979


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In 1996, U.S. District Court Judge Sylvia Rambo dismissed more than 2,000 damage claims filed against the power plant by nearby residents. Dr. Wing complained, "After she threw out the evidence that people had been injured by the accident, including part of our work, then she ruled that there wasnít enough to proceed with the case."

In Judge Rambo's ruling, she writes: "The record presently before the court does not support the fundamental assumption made by Dr. Wing -- that doses were significantly higher than originally estimated. In the absence of this assumption, Dr. Wing himself admits that he would be unable to make a causal interpretation based upon his findings. Because Plaintiffs have presented no evidence in support of this assumption, the court finds the Wing cancer incidence study does nothing to assist Plaintiffs in creating a material factual dispute or meeting their burden of proof."

For a more technical analysis of this incident, see the NRC's overview.


Mike Gray, Ira Rosen, The Warning: Accident at Three Mile Island, W.W. Norton & Company, 2003.
Wilborn Hampton, Meltdown: A Race Against Nuclear Disaster at Three Mile Island: A Reporter's Story, Candlewick 2001.
Bonnie A. Osif, Anthony J. Baratta, Thomas W. Conkling, TMI 25 Years Later: The Three Mile Island Nuclear Power Plant Accident and Its Impact, Pennsylvania State University Press, 2004.
J. Samuel. Walker, Three Mile Island : A Nuclear Crisis in Historical Perspective, University of California Press, 2004.
M.S. Wood, Suzanne M. Shultz, Three Mile Island: A Selectively Annotated Bibliography, Greenwood Press, 1988.
Voices from Three Mile Island: The People Speak Out, Crossing Press, 1980.
Report of the President's Commission on the Accident at Three Mile Island.



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TMI Nuclear Power Plant opened in April of 1974 and has a license which does not expire until April of 2019. It produced 6.2 billion kWh of electricity in 1999.


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