Three Mile Island accident

The Three Mile Island accident was a partial nuclear meltdown that happened at the Three Mile Island Nuclear Generating Station near Harrisburg, Pennsylvania. It began early in the morning on March 28, 1979, when equipment failed and caused problems with cooling the reactor. This led to a release of some radioactive material into the air, though scientists later said it did not harm anyone’s health.

This accident became the most serious in the history of nuclear power plant operations in the United States. It worried many people and made governments create new safety rules for nuclear power. After the accident, cleaning up the damaged reactor took many years and cost a lot of money.

Even though some studies looked for changes in health near the plant, it was hard to say for sure if the accident caused any illnesses. The other reactor at the same place, called TMI-1, kept operating for many years before closing down, but plans exist to use it again in the future to help power big Microsoft computers.

Accident

Background

In the hours before the accident, the TMI-2 reactor was running at 97% power while the TMI-1 reactor was shut down for refueling. The accident began at 4:00 a.m. on March 28, 1979, in TMI-2's secondary loop.

The accident started when operators tried to fix a blockage in one of the reactor's filters. This blockage was usually easy to fix, but this time, the usual method did not work. While trying to clear the blockage, a small amount of water leaked into an air line. This caused important pumps to turn off around 4:00 a.m., leading to a turbine trip.

Reactor overheating and malfunction of relief valve

With the pumps off, the reactor began to overheat. A safety valve, called the pilot-operated relief valve, opened to release steam. However, this valve got stuck open, and the operators did not realize it. They thought the valve was closed because a light on the control panel went out, but this light did not show the actual position of the valve.

Because the valve was stuck open, coolant water kept leaking out. The operators did not understand this at first and turned off important cooling pumps, thinking they were overfilling the system.

Depressurization of primary reactor cooling system

With coolant leaking out, the water level in a part of the reactor began to rise, even though pressure was falling. This confused the operators, who were not trained for this situation. They turned off emergency cooling pumps because they were worried about overfilling the system.

Partial meltdown and further release of radioactive substances

About 80 minutes after the accident began, a large bubble of steam formed in the reactor. This stopped water from circulating, and the reactor core began to overheat. By around 6:00 a.m., the top of the reactor core was exposed, and the intense heat caused a reaction that damaged the fuel rods and released radioactive materials.

Around 6:00 a.m., a new team noticed the high temperatures and tried to stop the leaking coolant, but a lot of coolant had already leaked out. It was not until 6:45 a.m. that radiation alarms went off, showing that radiation levels were very high.

Main article: Three Mile Island accident Main articles: Nuclear power accidents by country, List of nuclear and radiological accidents and incidents Further information: Effects of nuclear accidents

Emergency declaration and immediate aftermath

At 6:56 a.m., a plant supervisor declared a site area emergency. Soon after, the station manager announced a general emergency. Metropolitan Edison informed the Pennsylvania Emergency Management Agency, which then contacted state and local agencies, including Pennsylvania Governor Richard L. Thornburgh and Lieutenant Governor William Scranton III.

Officials were unsure about the amount of radiation released. Some said there was a small release, while others said there was none. Instruments showed some radiation had been released, but it was not high enough to harm people’s health.

Workers took manual readings to check the reactor, and after several hours, they added new water to cool it down. The next day, workers needed to take a special sample to make sure the reactor could not restart. Two workers carefully took the sample while monitoring radiation levels.

A few days later, a bubble of hydrogen gas was found inside the reactor. There was concern that it might explode, but tests showed there was no oxygen to support an explosion. Steps were taken to reduce the hydrogen, and it became smaller over the next day. Over the following week, steam and hydrogen were removed from the reactor.

Kemeny Commission

When the accident happened at Three Mile Island, many groups wanted to find out what went wrong. One important group was called the President’s Commission on the Accident at Three Mile Island. It was created by President Jimmy Carter and led by John G. Kemeny, the president of Dartmouth College.

This group studied what happened and found that some radioactive gases and a small amount of a material that could affect health were released into the air. They also learned that part of the reactor’s fuel had melted, but the container holding the fuel stayed strong and kept most of the dangerous materials inside. The commission pointed out many problems with how the plant was run and said that big changes were needed to make sure such accidents don’t happen again.

Mitigation policies

On March 28, 1979, a serious accident happened at the Three Mile Island nuclear power plant. Leaders told people to stay inside and close schools. They asked pregnant women and young children to leave the area for safety.

The accident changed how people thought about nuclear power. Building new nuclear plants slowed down, and some plans were canceled. It took many years and a lot of money to clean up the damaged reactor. Scientists studied whether the accident caused health problems, but found no clear proof of long-term effects.

The accident led to many protests against nuclear power. People worried about safety and wanted better rules to protect everyone.

21st century status

After the accident at TMI-2, the Nuclear Regulatory Commission stopped TMI-1 from operating. In 1982, nearby communities voted to retire Unit 1 permanently, but in 1985, the Nuclear Regulatory Commission allowed it to restart.

Over the years, several companies owned and operated the plant. In 2019, TMI Unit 1 permanently shut down due to financial issues and began the process of being taken apart.

In 2020, a new company, TMI-2 Solutions, took over the cleanup of TMI-2. By May 2023, they had cleaned up 99% of the nuclear fuel and planned to finish the cleanup by 2052.

In September 2024, Constellation Energy announced plans to restart the Three Mile Island nuclear plant to provide power for Microsoft’s data centers. They hope to restart Unit 1 in 2027, if approved.

Timeline

Documentaries

Meltdown: Three Mile Island is a four-part docuseries released by Netflix on May 4, 2022. The documentary tells the story of the accident, the debates it caused, and its lasting impact. It features Rick Parks, a nuclear engineer who spoke out against Bechtel; Lake Barrett, an energy expert who led the cleanup; Eric Epstein, chair of a group watching over nuclear safety; Michio Kaku, a physicist who questions nuclear power; and local residents affected by the event.

Radioactive: The Women of Three Mile Island is a 2023 documentary about the accident.

In popular culture

The movie The China Syndrome came out just before the Three Mile Island event. In the film, a reporter investigates a problem at a nuclear power plant. This movie created some debate about nuclear power.

Later, the film X-Men Origins: Wolverine used Three Mile Island as a special prison in its story. The movie showed the 1979 event as part of a fight between superhero characters.