On that day, a combination of malfunctions and human error unleashed radioactive gases into the environment around the Three Mile Island nuclear power plant in Pennsylvania.
The plant's owner, Exelon Corporation, closed Three Mile Island on September 20, 2019, due to financial struggles.
Correction: May 19, 2023 — An earlier version of this story misstated Three Mile Island as the worst nuclear disaster in US history. However, it's come to our awareness that the Church Rock Nuclear Disaster was equally devastating. Therefore, we've corrected this post to describe Three Mile Island as one of the worst nuclear disasters in US history, but not the worst.
Early on March 28, 1979, a combination of electrical and mechanical malfunctions, as well as human error, unleashed dangerous radioactive gases into the environment around the Three Mile Island nuclear power plant in Pennsylvania. It wound up being one of the worst nuclear disasters in US history.
The Three Mile Island Nuclear Generating Station on March 30, 1979.
The island sits just outside of Harrisburg, Pennsylvania.
Google Maps/Business Insider
Three Mile Island was owned by utility company Metropolitan Edison at the time of the accident. The facility had two units, one of which was still operational before last year. The other has been shut and sealed since March 28, 1979.
A view of the Three Mile Island nuclear power plant on March 22, 1999.
Holly Garnish, who lived in the nearest house to the Three Mile Island plant at the time of the accident, told the Washington Post that it all started with a loud roar that "shook the windows, the whole house."
A sign marks the Three Mile Island nuclear power plant on March 15, 2011.
The pipe that pumped water into the second reactor, known as TMI-2, to prevent it from overheating stopped working. That caused the reactor core to boil.
Piping inside a reactor at the Three Mile Island plant on April 3, 1979.
President's Commission on the Accident at Three Mile Island/U.S. National Archives and Records Administration/Wikimedia Commons/Public Domain
Unaware of what had caused a problem, plant workers inadvertently made the situation worse by stopping further water flow to the reactor. So it kept overheating.
Reactor 1 (not the reactor that experienced a meltdown) at Three Mile Island on the day of the accident.
About half of the reactor core melted from the overheated nuclear fuel, and about 20 tons of radioactive uranium poured out of the reactor core. It covered the steel floor and nearly burned through it.
An aerial view of Three Mile Island on April 2, 1979.
Michael Abramson/The LIFE Images Collection via Getty Images/Getty Images
But the Three Mile Island nuclear crisis was a partial meltdown, not a full meltdown. So it could have been far worse.
Bettmann/Contributor/Getty Images
Other nuclear disasters, like Chernobyl in 1986 and Fukushima in 2011, are considered full meltdowns because the overheating caused the containment structures housing the reactors to split open. Those events released a much larger amount of radioactive material.
An aerial view of the Chernobyl nuclear power plant, the site of the world's worst nuclear accident, in April 1986, days after the explosion.
Fortunately, nobody died because of the Three Mile Island accident. Officials declared a state of emergency, but no official evacuation process had been established for this kind of scenario.
Evacuees in Civil Defense shelter at a local sports arena after the accident at the Three Mile Island reactor on April 2, 1979.
Michael Abramson/The LIFE Images Collection via Getty Images/Getty Images
Because of the lack of emergency planning, residents were unsure how to react and panicked.
A sign announces the closing of the observation center for the Three Mile Island nuclear plant in Pennsylvania, following an accident on March 28, 1979.
"It was like something out of a horror movie," Christine Layman, who lived about 7 miles from the plant at the time of the incident, told The Philadelphia Inquirer. "No one knew what was going on."
Gwen Majette, 4, of Middletown, Pennsylvania, sleeps on her mother's lap as Willie Majette reads the morning headlines about the evacuation from the 5-mile area around the Three Mile Island Nuclear Plant on March 31, 1979.
Two days after the accident, officials feared that the hydrogen bubble would burst, which could have led more radioactive material to leak into the surrounding environment.
An NRC inspector looks at meters in the control room of TMI-2 on April 3, 1979.
President's Commission on the Accident at Three Mile Island/U.S. National Archives and Records Administration/Wikimedia Commons/Public Domain
In total, about 2 million people within a 50-mile radius were exposed to small amounts of radiation because of the Three Mile Island nuclear accident. According to the US Nuclear Regulatory Commission (NRC), the radiation exposure equaled that of a chest X-ray.
A man measures radioactivity with a Geiger counter after the Three Mile Island nuclear accident.
The process of cleaning up the reactor took 14 years and cost an estimated $1 billion. The reactor was damaged beyond repair and was sealed shut with concrete following the accident. Its neighboring reactor remained operational.
Three Mile Island personnel clean up the contaminated building in 1979.
Report of The President's Commission on the Accident at Three Mile Island: The Need for Change: The Legacy of TMI/Wikimedia Commons/Public Domain
In an eerie coincidence, a film called "The China Syndrome" about a fictional nuclear power plant disaster came out just 12 days before the Three Mile Island accident. The movie starred Jane Fonda and Michael Douglas.
Jane Fonda and Michael Douglas on the set of "The China Syndrome."
In the film, one character even says the fictional nuclear accident has the potential to "render an area the size of Pennsylvania uninhabitable" (though the disaster in the film takes place in California).
Actress Jane Fonda and her husband, political activist Tom Hayden, are seen during a press conference outside the Three Mile Island nuclear plant on September 24, 1979.
The movie's title, "The China Syndrome," refers to the idea that melted radioactive material could travel from the US all the way through the Earth’s core to China.
A protester under the marquee of a theatre advertising the film 'The China Syndrome,' on March 29, 1979.
Nuclear-industry insiders scoffed at the premise at the time, saying it was almost impossible for reactors to overheat and experience such a nuclear meltdown.
A protester under the marquee of a theatre advertising the film 'The China Syndrome,' on March 29, 1979.
But during the Three Mile Island crisis, reactor staff really did fear that the nuclear fuel could melt through the containment structure and seep into the ground (though not through the entire globe).
The Three Mile Island nuclear power plant is seen on March 15, 201.
The nuclear-energy industry, including the NRC, says the radiation in 1979 had no serious health consequences. The accident’s "small radioactive releases had no detectable health effects on plant workers or the public," the NRC website says.
Radiation levels are checked on a group of workers at the Three Mile Island Nuclear Power plant on March 28, 1979.
Rolls Press/Popperfoto via Getty Images/Getty Images
However, a 2017 study at Penn State Medical Center found a possible link between the 1979 nuclear accident at Three Mile Island and cases of thyroid cancer in nearby parts of Pennsylvania.
Tom Fox of the Department of Energy tests water samples along the Susquehanna River for radioactivity on April 3, 1979.
The researchers behind that study relied on the same research method that was used after the Chernobyl nuclear disaster: They studied patients with thyroid cancer to see how many of them had a genetic mutation that makes them more prone to non-radiation-induced thyroid cancer.
A man fishes in the Susquehanna River in front of the Three Mile Island plant on May 30, 2017.
Their results suggested that while 83% of thyroid-cancer patients in a control group had this mutation, only 53% of thyroid-cancer patients who'd lived in at-risk locations at the time of the nuclear accident had the mutation.
President's Commission on the Accident at Three Mile Island/U.S. National Archives and Records Administration/Wikimedia Commons/Public Domain
Regardless of the health effects, the accident certainly had an impact on the anti-nuclear movement in the US, which coalesced in response to the nuclear arms race of the Cold War.
Following the accident, regulatory changes were made for nuclear-plant operations in the US to mandate improved safety controls and emergency response plans.
A plant official, President and Mrs. Carter, Governor Thornburgh, and NRC's Harold Denton at crippled Three Mile Island nuclear plant.
Dirck Halstead/The LIFE Images Collection via Getty Images/Getty Images
Because of the new rules, the process of designing and building new nuclear power plants became longer and more costly. Since the Three Mile Island accident, no new nuclear plants have been built in the US.
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