Three Mile Island Accident and Human Error

Joey Alfieri
March 13, 2016

Submitted as coursework for PH241, Stanford University, Winter 2016


Fig. 1: Three Mile Island Nuclear Power Station. (Source: Wikimedia Commons)

The Three Mile Island Nuclear Power Station, seen in Fig. 1, is located on an island in the Susquehanna River near Harrisburg, Pennsylvania. It was commissioned on September 2, 1974. The station is composed of two separate units, TMI-1 and TMI-2. They are 30 stories high. The water of the river is used to cool the intense heat produced by 150 tons of Uranium, which are located at the base of the two units. The event that occurred in 1979, caused by two workers who made a mistake cleaning valves with air pressure hoses, resulted in much change for nuclear safety in the world. [1]

The Three Mile Island Accident

At 4:00 AM, March 28, 1979, one of the automatically operated valves in unit 2 closed due to a malfunction and therefore caused a shut off of the supply of water to the primary feedwater system. This caused the temperature in unit 2 to rise until the reactor core was completely melted. Consequently, radioactive gas was released into the environment. [2]

The factor of human error played a big role in the situation getting out of hand quickly. Workers in the control room faced a situation that they had not prepared for in any of their training, and were forced to make decisions on the spot. Additionally, there were flaws in the control panel that caused the workers to be unaware that a valve had been left open. A rise in temperature in the reactor caused by human error led to the reactor to go into automatic shut down for approximately one second. It took them hours to realize the pilot-operated relief valve (PORV) had not closed as it should have ten seconds after it first opened, which caused a critical amount of reactor coolant water to leak, and in turn high pressure injection pumps automatically pushed water into the reactor system to compensate for the lost pressure. [3]

Finally, at 6:22 AM, over two hours after the incident began, operators closed the valve connecting the relief valve and the pressurizer. This prevented coolant water from leaking out further, but the super hot steam that was created blocked water from reaching the cooling system. [3]

Aftermath/Public Health Concerns

This event shed a lot of negative light toward the subject of nuclear energy. The public was kept in the dark for much of the crisis and not properly informed of the magnitude of the situation. Dick Thornberg, the Governor of Pennsylvania at the time, was contacted and told "everything is under control -there is no danger to public health and safety," when in reality the situation was worsening. At that time, radioactivity was so high at the top floor of the building that a person would die within 20 seconds of being in the room. Radiation was detected leaking into the atmosphere at 11:00 AM. [2]

Jack Herbine, an engineer, was the spokesperson for the media. He fails to give the media the full story of what is happening, and does not inform them of radiation leaking into the air where they are performing the interviews. This furthers the public's confusion even more. Essentially, the public thought the problem was just a small valve that got stuck. It was not until 8:00 AM the following morning that Governor Thornberg announced that pregnant women and preschoolers evacuate the area within a 5 mile radius indefinitely. [2]

© Joey Alfieri. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.


[1] E. Foner and J. A. Garrity, The Reader's Companion to American History (Houghton Mifflin, 1991).

[2] "Backgrounder on the Three Mile Island Accident," U.S. Nuclear Regulatory Commission, February 2013.

[3] "Backgrounder: Three Mile Island Accident," United States Nuclear Regulatory Commission.