What do these charts say about the safety of nuclear reactors?

How safe is nuclear power? No question has generated more debate and disagreement in the world of energy.

There is safety in an absolute sense: how likely is it that in the day-to-day operation of a nuclear power plant, or in the event of an extreme event, will a release of radiation pose a threat to human health or natural systems? There is safety in a relative sense: what is the risk to human health and natural systems from a kWh generated by a nuclear power plant compared to a kWh generated from fossil fuels, solar, wind, hydropower, biomass, or geothermal energy? This article focuses on the first question. As such, it informs only one aspect of the health and safety discussion.

Many discussions about the safety of nuclear power focus on the “Big 3:” Three Mile Island (TMI) (1979), Chernobyl (1986), and Fukushima (2011). Unit 2 of the TMI facility,  located on the Susquehanna River just south of Harrisburg, Pennsylvania, experienced a partial core meltdown on March 28, 1979. The cause was a combination of mechanical equipment failures, poor system design, and human errors. The health effects of subsequent radiation release are widely—but not universally—accepted to be extremely small. Several federal and state agencies concluded that the 2 million people around TMI at the time of the accident received a radiation dose equal to 1/6th that a chest X-ray, or about 1/100th of the average annual natural background radiation in the area.1 Follow-up epidemiological studies found no causal link between the radiation release from TMI and rates of cancer in the area.2

The Chernobyl nuclear power station was situated 130 kilometers north of Kiev, Ukraine. On April 26, 1986, a  poorly designed test caused an explosion and fire that destroyed a reactor and released airborne radioactive contamination for at least nine days. The most damaging contaminants were short-lived iodine-131 and the long-lived cesium-137 radioactive isotopes. At least 150,000 square kilometers in Belarus, Russia, and Ukraine were highly contaminated, and 30 square kilometers around the plant were designated as a completely uninhabitable “exclusion zone.” Winds carried radioactive fallout over much of the northern hemisphere via wind and storm patterns, although at extremely low concentrations from a public health perspective.

The initial explosion killed two workers. Another 134 plant staff and emergency workers suffered acute radiation syndrome (ARS) due to high doses of radiation;  28 of those people later died from ARS. In 2018, the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR ) released a retrospective study of the excess cancer deaths that stemmed from the inhalation and ingestion of radionuclides generated by the explosion and nuclear fire.3 The biggest long-term impact is a significant increase in thyroid cancer in the exposed population, especially children and adolescents who ingested fresh milk contaminated with Iodine-131. UNSCEAR concluded that about 20,000 thyroid cancers occurred from 1991−2015 in males and females under 18 in 1986. About 5,000 of those cancers were due to radiation exposure from Chernobyl.

The Great East Japan Earthquake of magnitude 9.0 on March 11, 2011 generated a tsunami that swamped the Fukushima Daiichi nuclear power facility in Ōkuma, Fukushima, Japan. The cores in three of the four reactors at the site melted in the first three days. Radiation released into the atmosphere prompted the evacuation of about 165,000 people in a 30-kilometer radius of the facility. Large quantities of water contaminated with radioactive isotopes either leaked or were deliberately released into the Pacific Ocean during and after the disaster. More than 2,200 evacuees died due to stress and exhaustion from relocation, illness resulting from hospital closures, and suicides.4 A series of studies by UNSCEAR and the World Health Organization concluded that any health risks from radiation to workers and to the public are very small and likely to be undetectable by epidemiological studies.5 In 2018, the Japanese government acknowledged that one worker died from cancer after being exposed to radiation from the Fukushima site.6

Long-term studies of people who have lived through the TMI, Chernobyl, and Fukushima disaster accidents point to significant social and mental health impacts.  Living through a nuclear disaster is associated with higher levels of post-traumatic stress disorder, depression, and anxiety. Decontamination workers, those living in the closest proximity to the reactor, and evacuees experience higher rates of mental health problems after a nuclear disaster.7


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One can reasonably expand the criteria for concern to any event that includes damage to the reactor core.8 Including the Big 3, there have been 10 such events (the three core melts at Fukushima are counted as one event).9 At the end of 2021, there were 449 operating nuclear reactors in the world, and the industry had accumulated more than 19,000 reactor-years of experience (1 reactor operating for 1 year = 1 reactor-year).  How does one position the 10 core-melt events in this history?  Simple division yields an observed frequency of 1 core melt accident every 1900 reactor-years, a frequency that would make many people uncomfortable given that we currently accumulate reactor-years at about 450 per year.

But there are several reasons why the risk of any type of core damage is significantly lower than that. First, most of the events occurred in the industry’s early years and many occurred at experimental research reactors. This means that the frequency is highly dependent upon the chosen time frame. Second, the technology, operation, and regulation of commercial nuclear plants have dramatically evolved over time through via learning-by-doing and other changes wrought by Three Mile Island, Chernobyl, and other events. From 1992 to 2005, the frequency of any type of core damage to reactors in the U.S. dropped by a factor of five.10  In the early 2000s, estimates of core damage frequency ranged from  2 to 5 x 10-5 per reactor-year. In other words, one core damage incident every 20,000 to 50,000 reactors.11

The Fukushima disaster dramatically altered this discussion. One quantitative assessment concluded that Fukushima increased the nuclear risk for the near future to the same extent that safety improvements had decreased the risk over the prior 30 years. Other assessments revealed that, despite all the safety advances, the Fukushima disaster shared a common element with many previous disasters: a cascade of industrial, regulatory, and engineering failures that led to severe core damage.12

Where does this leave us?  The human element is a wildcard that cannot be zeroed out by engineering.  But climate change has shifted the debate because nuclear power is already the world’s single largest low-carbon source of electricity. Today’s nuclear reactors undoubtedly are more reliable than their predecessors. There are several promising new reactor designs such as the “microreactor” which typically is smaller than 10 megawatts. Nuclear also is among the most reliable sources of electricity: the global capacity factor for nuclear power is about 80%.

Every incident at a nuclear power plant is used by opponents to advance their case, a recent example being the leak of water containing tritium from the Monticello Nuclear Generating Plant in Minnesota.13 Proponents counter by arguing that taken as a whole, the nuclear supply chain poses a lower risk to human health compared to fossil fuels and some renewables.  Beyond health and safety, all energy systems have different social, economic, and security benefits and costs. People will continue to assign different weights to these factors.  

1 U.S. Nuclear Regulatory Commission. “Backgrounder on the Three Mile Island Accident.” June 2018. Link.

2 Han, Yueh-Ying, Ada O. Youk, Howell Sasser, and Evelyn O. Talbott. “Cancer Incidence among Residents of the Three Mile Island Accident Area: 1982–1995.” Environmental Research 111, no. 8 (November 1, 2011): 1230–35.Link.

3 United Nations Scientific Committee on the Effects of Atomic Radiation. “Assessment of Radiation Effects from the Chernobyl Nuclear Reactor Accident,” 2017.Link.

4 Hasegawa, A., T. Ohira, M. Maeda, S. Yasumura, and K. Tanigawa. “Emergency Responses and Health Consequences after the Fukushima Accident; Evacuation and Relocation.” Clinical Oncology, Fukushima – Five Years On, 28, no. 4 (April 1, 2016): 237–44,

5 United Nations Scientific Committee on the Effects of Atomic Radiation, “Levels and effects of radiation exposure due to the accident at the Fukushima Daiichi Nuclear Power Station: implications of information published since the UNSCEAR 2013 Report,” 2021, Link.

6 Reuters, “Japan acknowledges first radiation death among Fukushima workers,” September 5, 2018, Link.

7 Longmuir, Caley, and Vincent I O Agyapong. “Social and Mental Health Impact of Nuclear Disaster in Survivors: A Narrative Review.” Behavioral sciences (Basel, Switzerland) vol. 11,8 113. 23 Aug. 2021, doi:10.3390/bs11080113

8 Escobar Rangel, Lina, and François Lévêque. “How Fukushima Dai-Ichi Core Meltdown Changed the Probability of Nuclear Accidents?” Safety Science 64 (April 1, 2014): 90–98.

 9 Cochran, Thomas B., “Fukushima Nuclear Disaster and Its Implications for U.S. Nuclear Power Reactors,” Joint Hearings of the Subcommittee on Clean Air and Nuclear Safety and the Committee on Environment and Public Works United States Senate Washington, D.C., April 12, 2011, Link.

10 Gaertner, John, Ken Canavan, and Doug True. “Safety and Operational Benefits of Risk-Informed Initiatives.” EPRI White Paper. Electric Power Research Institute, 2008,Link

11 Leurs, B.A., R.C.N Wit, G.A. Harder, A. Koomen, F.H.J. Kiliaan, and G. Schmidt. “Environmentally Harmful Support Measures in EU Member States.” CE Delft, 2003.Link; Gaertner et al., op. cit.

12 Synolakis, Costas, and Utku Kânoğlu. “The Fukushima Accident Was Preventable.” Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 373, no. 2053 (October 28, 2015): 20140379.

13 AP News, “Radioactive water leaks at Minn. nuclear plant for 2nd time,” March 24, 2023, Link.


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