Three Mile Island accident: 40 years later

Beginning around 4 a.m. on March 28, 1979, a series of technical and human errors led to a partial meltdown inside the Unit 2 reactor at the Three Mile Island nuclear power plant near Harrisburg, Pennsylvania.

Four decades later, the event remains the most serious accident in U.S. commercial nuclear power plant operating history.

What happened

An combination of malfunctioning equipment, plant design and human error led to a loss of cooling water in Three Mile Island’s Unit 2 reactor, which had only been online for a few months.

The reactor core was exposed for hours.

Radioactive steam and waste water were released from the plant.

In addition to the problems inside the nuclear power plant, communication issues caused further concern. People had a hard time trying to parse technical and contradictory information they were hearing from Metropolitan Edison.

“The big problem was there was no communication,” said Dick Hoxworth, who worked for WGAL-TV at the time. “Metropolitan Edison treated public relations as a necessary nuisance. They didn’t really feel that public relations was something that you needed. Therefore, there was nobody there who could really communicate well–who could communicate what was going on. And the fact is that they tried to downplay what was going on.”

More: Listen to Hoxworth and WHP Radio’s Dave Sollenberger describe covering the incident


U.S. President Jimmy Carter shown April 1, 1979 in control room of nuclear plant, of the Three Mile Island nuclear plant in Middletown, Pa. Standing with Carter from left: Harold Denton, Director of the U.S. Nuclear Agency; PA. Gov. Dick Thornburgh; an unidentified control room employee. (AP)

Gov. Dick Thornburgh was also frustrated by the contradictory and confusing information. On Friday, March 30, two days after the accident, he spoke with President Jimmy Carter and asked for an expert to come to Harrisburg. The president dispatched Harold Denton, director of nuclear reactor regulation for the Nuclear Regulatory Commission, to help analyze the situation at Three Mile Island and communicate with government officials.

As more information came out about what happened Wednesday morning, and as mistrust of Metropolitan Edison increased, the governor ordered schools within the immediate area to close, and advised pregnant women and preschool-aged children to leave the area.

“There was nothing unusual, or a basis for that decision. It was sort of a compromise between doing nothing and doing a full-scale evacuation,” Denton said in 2004, adding that in hindsight, the only time evacuation should have been ordered was Wednesday morning when the core was melting–but no government officials knew that at the time.

Regardless, an estimated 80,000 people fled the region.

Also on March 30, a new problem became apparent: a gas bubble in the containment building. The bubble contained highly flammable hydrogen. The fear was if the bubble were to explode, it would cause a meltdown and radiation would spread for miles around. The bubble dissipated over the next several days.

WITF’s Tim Lambert takes you through the first few days of the crisis in this three-part podcast:

The aftermath

The events at Three Mile Island became known far and wide. Network news updated viewers and listeners across the country in the days that followed that fateful morning. It also inspired hundreds of songs, television programs, films and games. Even four decades later, references to Three Mile Island still surface in pop culture.

Tim Lambert / WITF

Protesters gather for a vigil outside the north gate of the Three Mile Island nuclear power plant in Londonderry Township, Dauphin County.

The accident fueled the anti-nuclear movement and essentially put a moratorium on construction of new nuclear plants across the United States. A group of anti-nuclear activists have continued to call attention to the accident and the unknowns surrounding it. For the 40th anniversary, just like every other year, a group held a vigil on public property just a few feet from the Three Mile Island entrance.

The confusion surrounding the 1979 accident left residents unsure of whether to stay or leave. Now, schools and institutions near Three Mile Island have evacuation plans, should the need ever arise.

Randy Padfield, acting director of Pennsylvania’s Emergency Management Agency, says there is also more information now about how a radiation release would spread, so the agency would be better informed and able to handle an emergency situation at the plant.

Photo courtesy of the the Historical Society of Dauphin County

In March 1979, Middletown Borough was home to approximately 11,000 residents. It was estimated at that time that 20 percent of the population packed up and temporarily left the area due to the uncertainty of what might transpire at Three Mile Island.


Cleanup of the Unit 2 reactor took 14 years and cost about $1 billion. Workers had to remove radioactive fuel and water, and encase the damaged reactor in concrete.

It was unsafe for humans to be in certain areas of the Unit 2 reactor after the partial meltdown. A young team from Carnegie Mellon University in Pittsburgh designed robots to assist with the cleanup. Prior to this use, robots were primarily made for repetitive motions in manufacturing.

Most of the waste from that cleanup was shipped to the Hanford nuclear production complex in Washington state, but the site banned off-site waste in the early 2000s. Federal officials want to establish a permanent repository for the nation’s radioactive waste, but with nowhere else to put it yet, nuclear reactors store their own waste on-site.

Did it affect health?

Martin Boutros / PennLive

Dr. David Goldenberg talks about his 2016 study, which showed that people who were near TMI during the accident and developed thyroid cancer share a common genetic marker.

The Nuclear Regulatory Commission concluded that the 2 million people who lived near Three Mile Island received an average dose of 1 millirem over the course of those few days. That’s far less than an x-ray, and equal to the amount of radiation a person would receive on a single coast-to-coast flight.

A handful of scientific studies have not found a link between the accident and negative health. Still, many people in central Pennsylvania blame Three Mile Island for their illnesses.

And in 2017, a small study from the Penn State College of Medicine found a correlation between the accident and thyroid cancer in the region.

A new nuclear dilemma

The current debate centers on whether governments should prop up the nuclear industry as a way to address climate change.

Nuclear power plants provide about 20 percent of the nation’s electricity, but many reactors are old and unprofitable. Three Mile Island is scheduled to close in September 2019—15 years before its operating license expires.

In the last few years, a handful of states have helped ailing nuclear power plants with billions in subsidies by broadening the definition of clean energy to include nuclear, which does not produce carbon emissions. Days before the 40th anniversary of the Three Mile Island accident, Rep. Thomas Mehaffie (R- Dauphin County) introduced HB 11. The bill aims to amend Pennsylvania’s Alternative Energy Portfolio Standard, a 2004 law requiring electric utilities to buy parts of their power from certain clean and alternative energy sources, like wind and solar. The bill faces major opposition from across the ideological spectrum.

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Energy & Environment

'I remember TMI': An oral history of the 1979 Three Mile Island accident as told by people who lived nearby