The Accident at Three Mile Island

Sylvie Sherman
February 18, 2016

Submitted as coursework for PH241, Stanford University, Winter 2016


Fig. 1: Fig. 1: Basic Schematic of Three Mile Island Reactor 2. [3] (Courtesy of the Nuclear Regulatory Commission)

The global energy crisis has gained more and more public attention. Resources upon which we currently rely are running low and greenhouse gas emissions are running high, which has led scientists and the public to investigate new energy sources.

One of these sources is nuclear energy. Nuclear power is contained in atoms, which can be harnessed to generate electricity through a series of chemical reactions that occur in a nuclear reactor. The nuclear energy debate has gained traction in the past few decades as many countries, such as India, China, and Britain, have begun to use nuclear energy on a large scale. However, the American public has been hesitant to embrace the alternative energy source, because the advantages of nuclear energy (low greenhouse gas byproducts and significant energy production) are accompanied by serious dangers. One of such dangers is the risk of a nuclear reactor accident, such as the one that occurred at Three Mile Island in 1979.

The Three Mile Island accident took place in Middleton, Pennsylvania on the morning of March 28, 1979, during the early stages of America's nuclear energy development. [1]

The problem was caused by mechanical failure and human error, both of which are twofold.

"It was the first step in a nuclear nightmare, as far as we know at this hour, no worse than that. But a government official said that a breakdown in an atomic power plant in Pennsylvania today is probably the worst nuclear accident to date . . ." - Walter Cronkite. [2]

The Accident

The first mechanical mishap took place when the Three Mile Island nuclear plant's pump system, which was responsible for supplying water to cool the reactor, became clogged and stopped working. [1] Due to the increased temperature, pressure in the primary reactor chamber increased to about 100 PSI above normal. [2] This elevated pressure triggered the automatic opening of the reactor's pilot-operated relief valve (PORV), whose function was to relieve pressure inside the chamber. This brings us to the second mechanical error, which occurred in the PORV. [3]

After decreasing the pressure, the valve should have closed automatically. [1] However, the second mechanical failure took place when, once the pressure normalized, the PORV did not close automatically. As a result, almost a third of the reactor's cooling water drained out. [3]

This brings us to the human-related error that contributed to the Three Mile Island accident.

Following the failure of the feedwater pumps, two emergency feedwater systems were supposed to be activated automatically. However, human operators at the facility did not realize that the valves allowing these pumps to circulate water were closed due to routine maintenance. This was the first human error. [3]

The second human error occurred in the control room. A light indicating that the electrical power keeping the PORV open had turned off, which operators misinterpreted as a signal that the PORV had closed. However, the valve had actually failed, and was still open. This caused a loss of coolant, as a significant amount of steam and water was being released. [1] Had the operators realized this, they would have been able to close a backup, which would have prevented the escape of the cooling water from the primary reactor chamber. [3] Because there was a lack of coolant, the temperature in the reactor rose to about 2400 K. This resulted in the core's partial meltdown, and the production of a hydrogen gas bubble that created a public concern and media frenzy regarding the potential explosion of the bubble, which they thought would release radioactive material into the surrounding area. [3]

Experts alter determined that a combustion reaction was not of concern, due to the lack of oxygen in the chamber. [3]

Following the Three Mile Island Accident, no more nuclear power plants were built in the United States. [3]


Due to breakdowns in electrical and mechanical equipment, as well as the actions of human operators, a complication at the Three Mile Island nuclear plant led to one of the worst nuclear accidents in history, and shed a negative light on the future of nuclear energy in America. These mechanical failures and human errors resulted in zero deaths or injuries, but caused a public fear of nuclear energy that lasts to this day. [3]

© Sylvie Sherman. 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] C. Hopkins, "Three Mile Island," Physics 241, Stanford University, Winter 2015.

[2] M. Stencel, "A Nuclear Nightmare in Pennsylvania," Washington Post, 27 Mar 99..

[3] C. Lyons, "Three Mile Island," Physics 240, Stanford University, Fall 2010.