On March 28, 2019, we will mark the fortieth anniversary of the Three Mile Island accident, which was then and still remains the most serious accident by far at a nuclear power station in the United States. The plant, located on the Susquehanna River near Harrisburg, Pennsylvania, suffered a severe loss of coolant that, researchers later discovered, caused a meltdown of the reactor core and irreparable damage to the facility. It also produced high anxiety among federal and state government officials, and, with good reason, among the population of the region for five acutely tense days after the accident occurred.
Despite the enormous destruction that the accident caused to the plant, it did not allow the escape of hazardous amounts of the most dangerous forms of radiation to the environment. The safety systems designed to protect the public were tested as never before, but in the face of a massive meltdown, they held. Unlike the later nuclear power accidents at Chernobyl (1986) and Fukushima (2011), the Three Mile Island crisis did not turn into a disaster. According to extensive measurements taken by the U.S. Nuclear Regulatory Commission, other federal agencies, the state of Pennsylvania, and the utility that operated the plant, it did not seriously contaminate the surrounding area with radioactive isotopes that are most likely to cause cancer and other illnesses. Although the accident released a large volume of noble gases, which do not combine with bodily tissue, it discharged only tiny quantities of iodine-131 and no strontium-90 or cesium-137.
A team of researchers from the Graduate School of Public Health at the University of Pittsburgh, headed by Evelyn O. Talbott, reported in 2002 that they found no increase in rates of “radiosensitive” cancer mortality attributable to Three Mile Island among a cohort of 32,115 people who lived within a five-mile radius of the plant between 1979 and 1982. The study included members of the cohort through 1998, a period long enough for slow-developing cancers to show up. The richness of the data used in this project made it the gold standard of Three Mile Island epidemiology.
But it was not convincing to everyone who lived in the area of the plant. One study that claimed, on much weaker evidence, that the accident had elevated the incidence of leukemia and lung cancer was embraced by some residents. In a British Broadcasting Corporation radio program on Three Mile Island that aired in 2006, Stephen R. Reed, who served as mayor of Harrisburg for many years, contended that the levels of radiation that escaped the plant during the accident were orders of magnitude higher than federal or state authorities had reported. He did not explain to BBC listeners, however, how those agencies and several commissions that investigated the accident had managed to conceal such high levels of radiation, which, after all, cannot be hidden, or why they would agree to engage in such a huge and dishonest cover-up.
A study of thyroid cancer published in 2017 added fresh fuel to suspicions of a coverup about the effects of radiation from Three Mile Island. David Goldenberg, a surgeon and researcher at the Penn State College of Medicine, found thyroid cancer in 44 people who had lived in the area of the plant at the time of the accident. He was careful to point out that his work did not show that releases from the plant caused the illnesses, only that there was a “possible correlation.” Nevertheless, some residents viewed the findings as a further indication that they had not been told the truth about the consequences of the accident.
Although the Three Mile Island meltdown occurred forty years ago, the controversy over its effects endures.
Samuel Walker is the author of Three Mile Island: A Nuclear Crisis in Historical Perspective (UC Press, 2004).
1. No injuries, deaths or direct health effects were caused by the accident
Following the event, detailed studies of the accident’s radiological consequences were conducted by the Nuclear Regulatory Commission (NRC), the U.S. Department of Energy (DOE), the Environmental Protection Agency, the Department of Health, Education and Welfare (now Health and Human Services), and the Commonwealth of Pennsylvania.
Experts determined that the approximately 2 million people in the nearby area during the accident were exposed to small amounts of radiation. The estimated average radiation dose was about 1 millirem above the area’s natural background of about 100-125 millirem per year. To put this into further context, exposure from a chest X-ray is about 6 millirem. The accident’s exposure had no detectable health effects on the plant workers or surrounding public.
2. Unit 2 experienced equipment and instrumentation malfunctions
On March 28th, 1979, equipment failures and a stuck open relief valve prevented the removal of heat from the Three Mile Island Unit 2 reactor’s core–– an essential function that prevents reactor damage. The instrumentation incorrectly indicated to plant operators that the valve had reclosed as expected once proper pressure levels were reached. Unaware that the plant was experiencing a loss of coolant accident, operators took actions that ultimately exacerbated the issue. The malfunction and operator error resulted in inadequate cooling water circulation to the reactor core causing it to overheat and suffer a partial meltdown. Consequently, a small amount of radioactive material was released.
3. No adverse effects to the surrounding environment
Months after the incident, concerns were raised about possible adverse effects from the radiation exposure on the people, animals, and plants in the areas near Three Mile Island. Various government agencies monitoring the area collected thousands of environmental samples of air, water, milk, vegetation, soil, and foodstuffs. It was determined that very low levels could be attributed to the accident and that the radioactive release had negligible effects on the physical health of the individuals or the environment.
4. NRC Implemented Enhanced Safety & Training
Thorough analysis of the accident’s events led to widespread changes across the U.S. commercial nuclear industry. The NRC implemented new, more stringent regulations related to improved training, emergency response planning, as well as upgrades to plant design and equipment requirements. Operators are now fully trained to understand the entire physics of the system. This knowledge makes them more equipped to use their operating knowledge and plant procedures to work through and manage unusual accident scenarios.
The Institute of Nuclear Power Operations was established in 1979 by the nuclear industry to provide a unified industry approach to plant management, training, and operation to further enhance nuclear safety. The NRC expanded its international activities to share important technical upgrades with other countries.
DOE currently collaborates with our international partners to better understand nuclear incidents around the world like the Fukushima Daiichi Nuclear accident. The analysis and lessons learned from these events support the continued safe, secure, and peaceful use of nuclear energy.
5. Three Mile Island’s Unit 1 continued operation until 2019
Three Mile Island had two nuclear reactors–– Unit 1, which began operation in 1974, and Unit 2, which began operation in 1978. Unit 2 shutdown following its partial meltdown in 1979. The fuel was removed and, along with the damaged reactor core debris, shipped to Idaho National Laboratory. The unit was placed in long term, monitored storage in 1993 to await decommissioning at the same time as Unit 1.
Three Mile Island’s Unit 1 continued operation for 40 years before shutting down in the fall of 2019. The reactor had a generating capacity of more than 800 megawatts of carbon-free electricity and at its peak employed 675 people.