The accident in reactor unit 2 happened at 4 a.m. on March 28, 1979. It began as a minor malfunction in the secondary cooling circuit, which in turn caused the temperature in the primary coolant to rise. This made the reactor shut down automatically. Within few seconds a relief valve opened as it was supposed to, letting out some of the heat. But it did not close again, thereby causing the draining of the reactor.

At this point a series of errors began. The operators thought that the valve had closed, because their instruments showed them that a “close” signal had been sent to the valve. As the valve was open and letting out cooling water, high-pressure pumps automatically started pumping water into the reactor’s pressuriser, raising the water level in it. From the increased water level in the pressuriser, operators thought that the whole reactor system was filled with water and they reduced the flow of cooling water.

This made the temperature rise in the reactor system and the cooling pumps began to vibrate due to the heat and the steam. Operators then shut down the pumps, which in turn caused a forced cooling of the system. With the boiling away of the cooling water, the reactor’s fuel core became even hotter and let out radioactive material into the cooling water. Even though operators tried to isolate the radioactive material with compressors, there was a leak of radioactive gas into the environment.

A hydrogen burn was detected in the reactor and in the following days a “hydrogen bubble” formed as a result of the chemical reactions in the reactor. This caused great anxiety and speculations about the risk of an explosion. The bubble was removed and the accident did not cause any severe damage though it did have a great impact on the public. The US officials considered evacuating one million people and children and pregnant women were advised to evacuate the area.

The conclusion of the U.S. Nuclear Regulatory Commission's independent Rogovin Commission said that the reactor had come within 30 minutes of an irreversible nuclear meltdown.

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Links and references:

United States President's Commission on the Accident at Three Mile Island, and J.G. Kemeny, 1979. Report of the President's Commission on the Accident at Three Mile Island, Washington DC, The Commission.

Information on the accident from the World Nuclear Association:
http://www.world-nuclear.org/info/inf36.htm

Account of the accident from the Three Mile Island Citizens' Monitoring Network:
http://www.tmi-cmn.org/

TMI-2 Clean-up Highlights Program:
http://www.libraries.psu.edu/tmi/tmimov.htm

Inside TMI:
http://kd4dcy.net/tmi/

Washington Post coverage 20 years later:
http://www.washingtonpost.com/wp-srv/national/long...

Fact Sheet from the U.S. Nuclear Regulatory Commission:
http://www.nrc.gov/reading-rm/doc-collections/fact...