On this day in medical history: Accident at Three Mile Island

By Paul Basilio, MDLinx
Published March 26, 2018

Key Takeaways

Early in the morning on Wednesday, March 28, 1979, a stuck valve and failures in a non-nuclear secondary system set into motion a chain of events that would lead to a partial core meltdown and the most significant accident in a commercial nuclear power plant in the United States.

While initial reports concluded that the amount of released radiation had negligible effects on physical health of the surrounding residents,1 new studies have shown a possible connection to radiation-related cancers in the area.

The cause

Reactor number 2 at Three Mile Island Nuclear Generating Station (TMI-2) in Middletown, PA, had only been operational for 3 months before the accident. Either a mechanical or electrical failure prevented the water pumps from properly removing heat from the reactor core, causing the turbine generator and the reactor to shut down. As the pressure began to increase, a valve opened, but did not close. This allowed coolant to drain from the valve.

The instruments in the control room, however, showed that the valve was closed. This led to a series of decisions that made the problem worse. Even though the core had melt down, TMI-2’s containment building remained intact, so consequences outside of the plant were believed to be minimal.1

After the incident, President Jimmy Carter created the President’s Commission on the Accident at Three Mile Island, to be headed by John Kemeny, president of Dartmouth College, Hanover, NH. The Kemeny Report stated, “The equipment [at TMI] was sufficiently good that, except for human failures, the major accident at Three Mile Island would have been a minor incident.”2

In fact, the report stated, a similar sequence of events occurred at the Davis-Besse Nuclear Power Plant in Oak Harbor, OH 18 months earlier. However, the operators at that plant identified the failure after 20 minutes, as opposed to 80 minutes at TMI. The Ohio plant was also operating at 9% power, whereas TMI-2 was operating at 97%.

The effect

Several studies conducted in the years after the accident concluded that the risk of long-term adverse health effects was low or negligible.

According to the Nuclear Regulatory Commission, approximately 2 million people in the area surrounding TMI-2 were exposed to an average radiation dose of about 1 millirem above the typical background dose. Radiation exposure from a chest radiograph is approximately 6 millirem, and the natural radioactive background dose for that area of Pennsylvania is approximately 100 to 125 millirem per year.1

As time passed, however, longer-term studies have shown there may be some connection between the accident and incidences of thyroid cancer. In 2013, Levin et al published data on thyroid cancer incidence in the counties surrounding TMI-2.3 The investigators found that thyroid cancer rates in York, Lancaster, Adams, and Chester counties were greater than expected from 1990 through 2009.

Although the authors stated that the findings supported a link between chronic low-level radiation exposure and thyroid cancer, they concluded that a direct correlation to the TMI-2 incident was unable to be made due to other potential confounders.

In 2017, researchers at the Pennsylvania State University College of Medicine, Hershey, PA, came the closest yet to pinning cases of thyroid cancer on the radiation exposure from TMI-2.4

The authors examined tumor samples from people with thyroid cancer who lived in the areas around the power plant at the time of the accident and who remained there.

David Goldenberg, MD, FACS, professor of surgery at the Milton S. Hershey Medical Center in Hershey, PA, and colleagues identified 44 patients who were treated at the Hershey Center for papillary thyroid cancer between 1974 and 2014. The patients were then divided into at-risk and control groups.

The at-risk group were those who developed cancer between 1984 and 1996, consistent with known latency periods of radiation-induced thyroid cancer, and who also lived in at-risk geographical areas based on reported weather patterns at the time of the accident.

Patients who developed cancer outside the expected latency period were placed in the control group.

The researchers found an increase in the genetic mutation caused by exposure to low-dose radiation in the at-risk group, and a decrease in the incidence of sporadic thyroid cancer, identified by a specific genetic mutation known as BRAF. The BRAF mutation is typically not present in radiation-induced types of thyroid cancer.

These observations are consistent with other radiation-exposed populations.

“While no single marker can determine whether an individual tumor is radiation-induced, these data support the possibility that radiation released from TMI altered the molecular profile of thyroid cancers in the population surrounding the plant,” Dr. Goldenberg said.

Although the population size is small, this study is considered to be the strongest correlation between the incident at TMI-2 and cancer in the surrounding population.


  1. Nuclear Regulatory Commission. Backgrounder on the Three Mile Island Accident. https://www.nrc.gov/reading-rm/doc-collections/fact-sheets/3mile-isle.html. Accessed March 23, 2018.
  2. Elliot JF. The Kemeny Report on the Accident at Three Mile Island. Ecology L Q. 1980;810. Available at: http://scholarship.law.berkeley.edu/elq/vol8/iss4/11. Accessed March 23, 2018.
  3. Levin RJ, et al. Incidence of thyroid cancer surrounding Three Mile Island nuclear facility: the 30-year follow-up. Laryngoscope. 2013;123(8):2064-2071.
  4. Goldenberg D, et al. Altered molecular profile in thyroid cancers from patients affected by the Three Mile Island nuclear accident. Laryngoscope. 2017;127 Suppl 3:S1-S9.
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