A Chernobyl-Level Catastrophe Almost Erupted at This Nuclear Plant in Pennsylvania 1979

Three Mile Island, Pennsylvania

Three Mile Island a river sandbar, sits in the middle of the Susquehanna River near Harrisburg, Pennsylvania.

Construction of the nuclear power station began during the 1973 oil crisis when America desperately needed new energy sources.

Metropolitan Edison, the power company running the plant, promoted nuclear energy as clean, affordable and reliable during a time of high oil prices and gas shortages.

Engineers finished Unit 1, an 880-megawatt pressurized water reactor, in 1974. Unit 2, a larger 959-megawatt reactor, started running in December 1978, just three months before disaster struck.

When the Cooling System Failed

At 4:00 a.m. on March 28, 1979, Unit 2 ran at 97% power. Without warning, the main water pumps stopped working in the non-nuclear section of the plant.

This triggered automatic shutdown systems. The reactor immediately stopped the nuclear reaction, but leftover heat kept building up.

A pressure valve opened to handle rising pressure but didn’t close afterward. Control room gauges wrongly showed the valve had closed.

Core temperatures climbed above 4,000 degrees, nearing the 5,000-degree point for complete meltdown. Cooling water leaked through the stuck valve while operators had no idea about the problem.

Hours of Confusion in the Control Room

More than 100 alarms went off in the first minutes, overwhelming operators who had never trained for this situation. The control panel showed opposite readings.

While pressure dropped, suggesting a leak, water level gauges showed enough cooling water remained. This impossible mix confused operators who lacked training for this specific problem.

At 4:20 a.m., operators made a critical mistake by cutting emergency cooling water flow, thinking the system had too much water. In truth, the core was losing coolant fast.

The danger grew by the minute as gauges failed to show what was actually happening inside the reactor.

The Partial Core Meltdown

For 98 terrifying minutes, the top of the reactor core sat exposed without cooling water. Temperatures reached near 5,000°F, causing fuel rod covering to melt.

The metal covering around uranium fuel mixed with steam, creating dangerous hydrogen gas. This reaction released more heat, speeding up the damage.

Later tests showed nearly half the core melted. About 19 tons of molten radioactive material flowed into the bottom of the reactor vessel.

The reactor came within one hour of a complete meltdown, which could have cracked the containment building and released huge amounts of radiation into the environment.

When Officials Downplayed the Danger

At 8:25 a.m., plant operators declared a site emergency. By 9:15 a.m., they upgraded to a general emergency, the most serious level.

Despite this, a Metropolitan Edison spokesman told reporters, “No radiation has been detected off plant grounds.” In reality, monitoring equipment had already picked up higher radiation levels nearby.

Nuclear Regulatory Commission officials Joseph Hendrie and Victor Gilinsky first described the situation as “a cause for concern but not alarm,” playing down how serious it was.

Pennsylvania Lieutenant Governor William Scranton first told the public everything was “under control” but hours later admitted the situation was “more complex than the company first led us to believe.”

The Dangerous Hydrogen Bubble Forms

Engineers later found a hydrogen bubble about 1,000 cubic feet in size containing 28,000 liters of explosive gas had formed at the top of the reactor.

Unknown to operators at the time, a hydrogen burn or small explosion had actually happened on March 28 but went unnoticed during the chaos. The sound seemed like a door closing.

This risk led to serious talks about wider evacuations as experts worked to figure out the situation.

Governor Orders Limited Evacuation

At 12:30 p.m. on March 30, two days after the accident began, Pennsylvania Governor Dick Thornburgh appeared on television. He told pregnant women and preschool-age children within five miles to leave.

No mandatory evacuation order came for everyone else, creating confusion about the actual danger level. Schools closed and residents were told to stay indoors.

About 144,000 people fled the area, roughly 22% of the 663,500 people living within a 20-mile radius. Traffic filled highways leading away from Harrisburg.

A later survey found 98% of evacuees returned within three weeks as the immediate crisis passed.

President Carter Visits the Plant

President Jimmy Carter arrived on April 1, 1979, the fourth day of the crisis. As a nuclear engineer who had helped take apart a damaged Canadian reactor during his Navy service, Carter understood the technical details.

Carter wore yellow protective shoe covers during his tour of the facility. His wife Rosalynn, Governor Thornburgh, and Nuclear Regulatory Commission officials came with him.

The president checked the control room where operators kept managing the damaged reactor. Television news showed images of the presidential visit across the country.

Carter’s high-profile tour aimed to restore public trust and show that authorities had the situation under control.

The Hydrogen Bubble Goes Away

By April 1, engineers achieved what they called a “dramatic decrease” in the hydrogen bubble’s size. They installed a hydrogen burner wrapped in lead bricks to safely remove the gas.

The regular cooling system slowly mixed hydrogen with coolant, then released it like champagne bubbles in another building away from the reactor.

By April 3, officials announced the bubble had disappeared completely, removing the explosion risk that had caused so much worry.

On April 27, operators got natural water circulation working, allowing the nuclear fuel to cool through natural water movement rather than mechanical pumping.

The Billion-Dollar Cleanup

The massive cleanup operation began in August 1979 but moved carefully. Workers couldn’t even remove the reactor vessel head until July 1984, finally allowing access to the badly damaged core.

Over 14 years, cleanup crews removed nearly 100 tons of damaged nuclear fuel from Unit 2.

The operation created more than 2.8 million gallons of contaminated water that needed processing and eventual evaporation.

Total cleanup costs reached about $1 billion, equal to around $2 billion today. Unit 2 remained too damaged to ever work again.

In February 1991, the American National Society of Professional Engineers named the TMI-2 Cleanup Program one of the top engineering achievements in the United States completed during 1990.

Legacy for Nuclear Power Industry

The accident stopped nuclear industry growth in the United States, and companies canceled 52 planned nuclear reactors between 1980 and 1984.

President Carter created the President’s Commission on the Accident at Three Mile Island in April 1979. Dartmouth College President John G. Kemeny led the investigation, which shared its final report on October 31, 1979.

The nuclear industry formed the Institute of Nuclear Power Operations to improve safety standards and operator training at all plants.

No new nuclear reactors were ordered in the United States from 1979 through the mid-1980s as public trust in nuclear energy fell apart.

Three Mile Island Today

Three Mile Island stands near Middletown, Pennsylvania, alongside the Susquehanna River. The plant’s distinctive cooling towers rise prominently on the horizon, visible from Highway 441.

Unit 1, which wasn’t damaged in the accident, operated safely until 2019 when Constellation Energy shut it down due to money challenges from cheaper natural gas.

In September 2024, Constellation announced plans to spend $1.6 billion to restart Unit 1 by 2028. A 20-year power purchase agreement with Microsoft will use the plant’s 835 megawatts to power data centers.

Once restarted, the plant will produce about 7 million megawatt-hours of electricity yearly, enough to power about 700,000 homes.

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