The Three Mile Island Accident and its Effects on American Nuclear Power Use

Sean O'Bannon
March 20, 2018

Submitted as coursework for PH241, Stanford University, Winter 2018


Fig. 1: An anti-nuclear protest following the Three Mile Island accident, Harrisburg, 1979. (Source: Wikimedia Commons)

The Three Mile Island Accident refers to a partial meltdown of a nuclear power plant near Middletown, Pa. on March 28, 1979. [1] This accident fueled anti-nuclear groups and drove existing concerns about the effects of nuclear radiation home. Occurring seven years before Chernobyl, this accident precipitated public outcry and a levelling-off of nuclear power plant construction in the United States.


Prior to the partial meltdown, a machine failure prevented the flow of water from reaching the steam generators that remove heat from the reactor core. Without proper coolant flow, the reactor shut down, and pressure in the system began to increase. A pressure relief valve opened to release this pressure, but somehow remained stuck open when pressure fell to proper levels. However, instruments in the control room indicated that this valve had properly closed. [1]

Thus, when the reactor became active again, coolant began to flow out of this pipe. Operators, confused as to why the reactor appeared to be overheated, took steps that ultimately made the situation worse. In this stage, half of the nuclear core melted down. However, the reactor's containment building remained intact, so almost all the accident's radioactive material remained contained. [1] However, over the course of the incident, approximately 2.5 million curies of radioactive noble gases and 15 curies of radioiodines were indeed released. [2]

Official Communications

Initially, federal and state authorities were informed that small radioactive samples were detected near Middletown, but did not realize that the core had partially melted. Authorities appeared to have control of the radiation by the evening of March 28, but subsequent releases of gas from the container were accompanied with significant radiation. Concerned, then governor of Pennsylvania Richard L. Thornburgh initiated a voluntary evacuation, recommending that pregnant women and small children leave the area. [3]

Additionally, as the media continued to report on the situation, reports of a gas bubble of hydrogen that could explode scared more people into leaving. [4] The delayed manner in which this news was reported added fury to the mistrust of Metropolitan Edison, the owner of the power plant.

Public Outcry

Fig. 2: Graph of Global Nuclear Power Use from 1955 to 2005. [5] (Source: Wikimedia Commons)

After the accident, different agencies sought to change the way nuclear power and its accidents were handled. For instance, The Pennsylvania House of Representatives conducted an audit on evacuation procedures. The United States Nuclear Regulatory Commission (NRC) increased regulations and oversight and bumped up management and oversight of plants. Additionally, the NRC made stricter the process to receive nuclear power licenses. The net effect of these changes were longer construction times and skyrocketing costs for new nuclear power plants in the US. In the free market, this resulted in fewer being built. [3]

Anti-Nuclear groups were emboldened, holding events that disparaged the safety risks of nuclear power. In Harrisburg, near Middletown, protests were staged soon after the accident, as seen in Fig. 1.

Effect on Nuclear Power Systems

The Three Mile Incident occurred at a unique time in US Energy policy. During the 1970s, more than one hundred nuclear power plants were ordered, partially in response to the 1973 oil crisis. However, after the TMI accident, more than 51 of these were cancelled. [3]

Global Nuclear Power Use

Looking at Fig. 2, we see that global nuclear power use showed no signs of levelling-off until the TMI accident. Indeed, Additional reactors under construction was increasing rapidly even compared to active reactors. While this chart can only show correlation, not causation, it is likely that outcry from the TMI accident led to pullback on the nuclear front.

Cleanup and Health Effects

The cleanup of TMI cost approximately $1 billion dollars. [3] Studies on health effects have found no causal link between the TMI accident and increased rates of cancer or death. The NRC, in addition to several other federal agencies, conducted investigations and found that on average, individuals were exposed to 1 millirem above the average background dose. In perspective, a chest X-ray is approximately 6 millirem above the average background dose. Investigations by Columbia and the University of Pittsburg have found negligible health effects on the environment or human health. [1]

© Sean O'Bannon. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. 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] "Three Mile Island Accident," U.S. Nuclear Regulation Commission, Feburuary 2013.

[2] M. Rogovin and G. T. Framton, Jr., "Three Mile Island: A Report to the Commissioners and to the Public, Volume 1," U.S. Nuclear Regulatory Commission Special Inquiry Group, January 1980

[3] C. Peterson, "A Decade Later, TMI's Legacy Is Mistrust," Washington Post, 28 Mar 89.

[4] S. Sherman, "The Accident at Three Mile Island," Physics 241, Stanford University, Winter 2016

[5] "Nuclear Power Reactors in the World," International Atomic Energy Agency, Reference Data Series No. 2, April 2006.