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Three Mile Island

    On March 28, 1979 at around 4:00 a.m. a reactor near Middletown, Pennsylvania approached near catastrophe.  Construction on the Three Mile Island Unit 2 reactor had been completed only months prior and the reactor had only been operating for three months before the TMI-2 plant suffered a severe core meltdown.  In the morning of March 28 the main feed water pumps stopped running, it was never determined whether this was a mechanical or electrical fire but regardless the failure of the pump prevented the steam generators from removing heat from the core.  As a part of a fail safe procedure, first the steam turbine and then the reactor itself automatically shut itself down.  This caused the pressure in the reactor to increase triggering the opening of a relief valve.  Unfortunately the relief valve did not close automatically as it was supposed to but the control panel readings failed to notify operators of this malfunction.  This resulted in the mass expulsion of coolant water from the core causing the reactor to overheat.  The nuclear fuel overheated to the point at which the zirconium casing of the nuclear fuel burst and the fuel pellets themselves began to melt.  During the cleanup of the disaster it was discovered that over half of the core had melted in these early stages of the catastrophe.  By some estimates if the core had continued melting for another thirty minutes the containment building would have been breached.  Luckily, the melting of the core did not trigger a rupture in the containment building that would have released enormous amounts of radiation into the environment.[1]

            By later that night the instruments at the TMI-2 plant appeared to indicate that the reactor had been sufficiently cooled but the next morning a large release of radiation from the plant triggered new worries.  At this point the governor of Pennsylvania, Richard L. Thornburgh, approached the Nuclear Regulatory Commission about the possible necessity of evacuating citizens from surrounding areas.  The decision was made that it would be best if the citizens most vulnerable to radiation, pregnant women and young children, within a five-mile radius of the plant evacuate the area.  This evacuation affected nearly 3500 women and children and another 20,000 fled their homes as well.  The evacuation order was not lifted or schools opened until April 9, ten days after the original order was released.  That same day a large hydrogen bubble was discovered in the dome of the pressure vessel and this caused yet more worries.  The fear was that the hydrogen bubble might ignite and burn or explode causing a break in the pressure vessel.  Fortunately by Sunday, April 1 workers had been able to diminish the size of the bubble and the core had cooled allowing for a full shutdown of the TMI-2 reactor.[2]

            After a preliminary review of the accident, the Nuclear Regulatory Commission fined Metropolitan Edison, the operator of Three Mile Island, $155,000.[3]  The cleanup began in July of 1980 with the venting of enormous amounts of radiation for the reactor building in preparation for working crews.  On November 7, 1983 after more data on the disaster was able to be collected, Metropolitan Edison was indicted for falsifying data concerning the leak rates of the TMI-2 reactor and for destroying accident reports that revealed potential hazards before the accident.  The other reactor at the site Three Mile Island Unit 1 had been shut down at the time of the meltdown for refueling and due to the accident TMI-1 had its license temporarily suspended until a full examination of both it and TMI-2 could be conducted.  In 1985 the Nuclear Regulatory Commission voted to resume operations with the TMI-1 and it was back online by October of that year.[4]

            TMI-2 was deemed beyond repair and was permanently shut down after the accident.  The cleanup was not completed until 1993 and the facility still remains as a long-term monitored storage facility until the TMI-1 license expires at which time both reactors will be fully decommissioned with all waste and contaminated material moved offsite.  The fourteen years of cleanup resulted in costs of over $1 billion to bring the plant to a level where it was deemed safe.[5]  The effects of this disaster do not end with the cleanup bill, indeed the effects of this meltdown on the environment and human health could be argued to be worse and more costly than any cleanup.  Nearly two million people in the surrounding area were exposed to measurable doses of radiation from the plant and while concrete studies have yet to link the radiation exposure to any serious health issues, the specter still remains for every one of those people.[6]  For months after the accident many concerned scientists and citizens scoured  the area gathering thousands of samples of the air, vegetation, soil, milk from cows, and other produce in an effort to monitor and understand the full scope of the environmental damage caused by the reactor at Three Mile Island.  Studies since have determined that the majority of radiation remained contained within the facility but enough was released to leave levels high enough to cause concern among many of the citizens living near the nuclear power plant.

External Links:

Chronicle of the Disaster
NRC Release on TMI

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[1] “Fact Sheet on the Accident at Three Mile Island.”  Accessed April 3, 2006 at

[2] “A Nuclear Nightmare in Pennsylvania.”  Accessed April 3, 2006 at

[3] Ibid

[4]“Fact Sheet on the Accident at Three Mile Island.”  Accessed April 3, 2006 at

[5] “A Nuclear Nightmare in Pennsylvania.”  Accessed April 3, 2006 at

[6] “Fact Sheet on the Accident at Three Mile Island.”  Accessed April 3, 2006 at
[7] Image borrowed from
[8] Image borrowed from longterm/tmi/gallery/photo3.htm
[9] Image borrowed from /national/longterm/tmi/gallery/photo7.htm

Three Mile Island
Three Mile Island, April 11, 1979 [7]


Workers being checked for radiation exposure. [8]


Evacuated citizens at a shelter in Hershey, 
Pennsylvania [9]

Last updated:  5/2/2006


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