Three Mile Island

Chris Lyons
November 28, 2010

Submitted as coursework for Physics 240, Stanford University, Fall 2010

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

The energy industry of the United States is approaching a vital crossroads. With hydrocarbon resources running low, and concern about greenhouse gas emissions at an all-time high, many believe it is time to revisit nuclear energy. It's clean, it's relatively abundant, and the technology is well understood. Why are people so afraid of it?

At 4:00 am on the morning of March 28th 1979, pumps supplying cooling water to the nuclear reactor at a facility at Pennsylvania's Three Mile Island failed, sparking the episode that would ultimately result in the demise of the American nuclear energy industry. The confusion and controversy surrounding the ensuing events would cause critics of nuclear energy to argue that the accident clearly shows the danger associated with nuclear power plants, while many simultaneously argue that the event highlights the success of the reactor's safety systems. [1] The Report of the President's Commission on Three Mile Island presents its assessment of the accident and provides recommendations for ensuring safer practices in the future.

The primary mechanical failures that occurred at Three Mile Island were two fold. Firstly, the pump system supplying feedwater to cool the reactor failed, causing the pressure inside the primary reactor chamber to rise to 100 psi above normal due to increased temperatures (Fig. 1). [1,2] The increased pressure then led to the automatic opening of the reactor's pilot-operated relief valve (PORV), allowing pressure inside the reactor chamber to decrease, however the second mechanical failure occurred when the PORV did not close automatically once the pressure was brought back to normal, allowing almost one third of the cooling water located in the reactor chamber to drain out. [2,3]

The primary human failures that made the situation worse were also two fold. First, after the failure of the feedwater pumps, two emergency feedwater systems were automatically activated, but facility operators failed to realize that valves allowing these pumps to circulate water to the reactor chamber were closed due to routine maintenance. [2] Secondly, a light in the control room indicated that the electrical power that kept the PORV open had turned off, and facility operators misinterpreted this as a signal that the PORV had closed. If they had realized the mechanical failure of the valve, they would have been able to swiftly close a backup, preventing the essential cooling water from escaping the primary reactor chamber for a period of 2 hours and 22 minutes. [2,3]

The elevated temperature in the reactor (up to ~2400 K) caused by the lack of coolant resulted in partial meltdown of the core, and reaction of the zirconium containing fuel rod claddings with the water in the reactor to produce the infamous hydrogen gas bubble that created intense media frenzy. [2,3] The concern was that this gas bubble could potential explode, resulting in breach of the containment and release of radioactive debris to the surrounding populated area. However, experts determined three days later that this was of little concern due to the lack of oxygen within the chamber, precluding a combustion reaction. [2]

The Report of the President's Commission on Three Mile Island concludes that the primary issue surrounding the accident (and likely the nuclear energy industry as a whole) was the "mindset" that as long as the equipment was safe, the employment of operating staff could only result in improved safety, which clearly proved to be incorrect. They suggest that development of "A comprehensive system is required in which equipment and human beings are treated with equal importance." [2] They go as far as to say that the control room layout was simply too confusing for workers to operate comfortably in during abnormal or emergency situations.

The ensuing media confusion and lack of reliable information caused widespread panic regarding the accident. Two minor mechanical failures, made worse by gross human errors, which resulted in zero deaths or injuries, completely decimated the American nuclear energy industry. No more nuclear power plants were built in the United States following the Three Mile Island incident and none of the reactors that began construction after 1974 were finished. [4] However, today we are on the verge of renaissance. The Nuclear Regulatory Committee is currently lobbying the Obama administration for loans to help pay for the construction of a new round of 1000 megawatt reactors. [4] With the lessons learned from the Three Mile Island catastrophe and improved technologies and training today, we might witness the rebirth of the American nuclear energy industry soon.

© Chris Lyons. 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] M. Stencel, "A Nuclear Nightmare in Pennsylvania," Washington Post, 27 Mar 99.

[2] J. G. Kemeny et al., "Report of the President's Commission on the Accident at Three Mile Island," U.S. Government Printing Office, 0-303-300, October 1979.

[3] "Backgrounder on the Three Mile Island Accident," Office of Public Affairs, United States Nuclear Regulatory Commission, 11 Aug 09.

[4] P. Behr, "Three Mile Island Still Haunts U.S. Nuclear Power Industry," New York Times, 27 Mar 09.