8 Facts About the Three Mile Island Nuclear Accident

A scene from Netflix's 'Meltdown: Three Mile Island' (2022).
A scene from Netflix's 'Meltdown: Three Mile Island' (2022). / Netflix © 2022

In 1979, the United States came the closest it’s ever been to a commercial nuclear catastrophe.

A malfunctioning valve led to a partial meltdown in one of the two reactors at the nuclear power plant on Three Mile Island, a small island located in Lake Frederic, a reservoir in the Susquehanna River, just about 10 miles downriver from Pennsylvania’s capital of Harrisburg. Fortunately, catastrophe was avoided.

The Three Mile Island accident has been overshadowed by more severe nuclear meltdowns at Chernobyl in 1986 and Fukushima in 2011, but the effects of Three Mile Island have reverberated for decades since, from changes in U.S. energy policies to popular culture. Now, a new four-part docuseries about the incident, Meltdown: Three Mile Island, will premiere on Netflix on May 4, 2022.

With that in mind, here are some important facts to know about the Three Mile Island accident.

1. There were two nuclear reactors on Three Mile Island. 

Construction began on Three Mile Island's first nuclear reactor, TMI-1, on May 18, 1968—a full decade after America's first commercial nuclear plant, the Shippingport Atomic Power Station, opened in western Pennsylvania, just about 250 miles west of Three Mile Island. Construction on Three Mile Island's second nuclear reactor, TMI-2, began more than a year later, on November 1, 1969.

TMI-1 went online in 1974; TMI-2 began operating in 1978. Both units were pressurized water reactors, which rely on feedwater pumps to send pressurized water to a steam generator, which then acts as the reactor's primary coolant. From the very beginning, the TMI-2 reactor, where the accident would shortly occur, was problematic. It was subject to unscheduled shutdowns due to leaks, and following the accident it was revealed that managers had deliberately falsified data about the reactor in order to keep the plant operational.

2. The disaster unfolded in a matter of seconds. 

At approximately 4 a.m. on March 28, 1979, a relatively minor malfunction occurred in the non-nuclear part of the plant when a feedwater pump failed to send water to the steam generator, which prompted both the turbine generator and the TMI-2 reactor itself to automatically shut down as a built-in safety precaution—a process that occurred in about one second.

To reduce the pressure and heat building within the reactor, a valve was opened to vent steam, then closed once the temperature within the reactor returned to a safe level. Except the valve never did close, despite the instrumentation in the control room reporting otherwise. As a result, according to the United States Nuclear Regulatory Commission (USNRC), "the plant staff was unaware that cooling water in the form of steam was pouring out of the stuck-open valve. As alarms rang and warning lights flashed, the operators did not realize that the plant was experiencing a loss-of-coolant accident."

More water was pumped into the reactor, but because of the malfunctioning instrumentation, nobody knew the valve was still open, meaning the water was going right back out. So the reactor continued to heat up and, with no coolant available, the core was exposed, causing major damage.

3. Later that day, radiation was vented into the air. 

The Three Mile Island Nuclear Generating Station
The Three Mile Island Nuclear Generating Station / Wally McNamee/GettyImages

Contrary to popular belief, properly functioning nuclear power plants don't produce a lot of radiation (coal-based power plants actually produce about three times as much). According to the USNRC, "most of an operating nuclear power plant's direct radiation is blocked by the plant's steel and concrete structures. The remainder dissipates in an area of controlled, uninhabited space around the plant, ensuring that it does not affect any member of the public."

Potentially 2 million people were exposed to excess radiation as a result of the Three Mile Island accident. While children and pregnant women were encouraged to evacuate, all told, an estimated 140,000 people left the area of their own volition. An 18-year study of those who lived within 5 miles of the plant—about 150,000 people in total—found no significant increase in mortality attributable to the Three Mile Island accident, though it did find an increase in the relative risk of certain cancers.

4. The Three Mile Island accident occurred less than two months into the term of newly elected Pennsylvania Governor Dick Thornburgh. 

Thornburgh, a Pittsburgh native who’d unsuccessfully run for the U.S. House, served as U.S. Attorney for Western Pennsylvania, and gained local renown for prosecuting organized crime figures. He was elected governor in 1978, and had been in office for just 72 days when the crisis hit. He and then-President Jimmy Carter toured the facility, and his steady hand—he notably did not issue a general evacuation, and he himself stayed in central Pennsylvania—led him to be called the hero of the crisis. Following two terms as governor, Thornburgh was appointed U.S. Attorney General by Ronald Reagan in 1988, and continued to serve in this role under George H.W. Bush until 1991. Thornburgh, who had a distinguished career in public service, passed away at age 88 on December 31, 2020.

5. The China Syndrome, a movie that eerily foreshadowed the disaster, premiered less than two weeks before the Three Mile Island accident.

The China Syndrome—which took its title taken from the idea (perhaps a little hyperbolic) that a full meltdown would lead the nuclear core to melt through its containment and burrow through the ground all the way to China—starred Jane Fonda and Michael Douglas as a television news crew who are present during a core accident, and features Jack Lemmon as the nuclear plant’s shift supervisor. The New York Times quoted Westinghouse executive John Taylor, who called the film “an overall character assassination of an entire industry.”

6. There were no human casualties.

No casualties were reported from the incident at Three Mile Island—but it effectively killed any growth in the nuclear power industry. In the 1960s and 1970s, nuclear power was the leading source of non-carbon-emitting energy, and building nuclear power plants was a major source of revenue and innovation for Westinghouse. But after Three Mile Island, power plants under construction faced new roadblocks, and orders for new ones dried up. Plans stopped for 39 plants that were being developed. The world belonged to coal and gas—and all their attendant emissions.

7. The disaster had a major effect on Bruce Springsteen.

Among the artists inspired to action by the incident was Bruce Springsteen, who lived less than 100 miles from Three Mile Island. He wrote a song called “Roulette” about the incident and the ongoing debate at the time about whether the benefits of nuclear energy outweighed the potential environmental and health risks. The song, which says that "I've got a house full of things that I can't touch," opens with the lyrics:

"We left the toys out in the yard
I took my wife and kids and I left my home unguarded
We packed what we could into the car
No one here knows how it started ..."

Though Springsteen originally recorded the song in 1979, it remained generally unreleased (although available “through the miracle of bootlegging,” as the Boss himself once said) until his Tracks box set in 1998. He also joined Musicians United for Safe Energy (MUSE), and performed at their famed “No Nukes” concert. His version of the Detroit Medley was released on the No Nukes album, but Springsteen released his full concert set in 2021.

8. The TMI-2 reactor never operated again. 

A scientist tracks radiation at Three Mile Island.
A scientist tracks radiation at Three Mile Island. / Roger Ressmeyer/GettyImages

The damage done in 1979 was too difficult to recover from. It took 12 years to fully decontaminate the reactor, which became a morbid tourist attraction, much like Chernobyl. TMI-1 continued operating until the power plant's closure on September 20, 2019, due more to economic issues than fears over nuclear power generation.