Three Mile Island: Timeline of Events

Three Mile Island Nuclear Power Plant
Three Mile Island Nuclear Power Plant

4 a.m. March 28, 1979

The plant experiences a failure in the secondary, non-nuclear section of the plant. Pumps stop running, the turbine, then the reactor automatically shut down. Valves fail and the core reactor overheats. Alarms ring and lights flash. A core meltdown begins.

7:45 a.m. March 28

Federal and state authorities are concerned about the small release of radioactive gases that are measured off-site. The Nuclear Regulatory Commission’s regional office in King of Prussia is notified.

8 a.m. March 28

NRC headquarters in Washington, D.C. is alerted and the NRC Operations Center in Bethesda, Md., is activated. The regional office promptly dispatches the first team of inspectors to the site and other agencies, such as the Department of Energy and the Environmental Protection Agency, also mobilized their response teams.

9:15 a.m. March 28

The White House is notified.

11:00 a.m. March 28

All non-essential personnel are ordered off the plant’s premises.


Helicopters hired by TMI’s owner, GPU Nuclear, and the Department of Energy sample radioactivity in the atmosphere above the plant. A team from the Brookhaven National Laboratory is also sent to assist in radiation monitoring.


The core appears to be adequately cooled and the reactor appears stable.

Friday, March 30

New concerns arise. A significant release of radiation from the plant’s auxiliary building, performed to relieve pressure on the primary system and avoid curtailing the flow of coolant to the core, causes confusion and consternation. Gov. Dick Thornburgh, consults with the NRC about evacuating area residents. He and NRC Chairman Joseph Hendrie agree to remove those citizens most vulnerable to radiation, and Thornburgh advises pregnant women and pre-school-age children within a five-mile radius of the plant to leave the area.

Late Friday/early Saturday, March 31

The presence of a large hydrogen bubble in the dome of the pressure vessel–the container that holds the reactor core–causes new worries. Officials fear the hydrogen bubble may burn or explode and rupture the pressure vessel. In that event, the core would fall into the containment building and perhaps cause a breach of containment.

April 1

Experts determine the hydrogen bubble–which has now reduced in size–could not burn or explode because of the absence of oxygen in the pressure vessel.

Two weeks later

Pres. Jimmy Carter appoints a 12-member commission to investigate what happened and the possible impact on the health and safety of the public and plant personnel. Meanwhile, the NRC forms its own group to study the accident.

October 1979

The commission’s report recommends that the industry develop a standard of excellence and cites a need for agency-accredited training institutions for nuclear plant operators and immediate supervision of operations.

July 1980

Approximately 43,000 curies of krypton are vented from the reactor building and the first manned entry into the building since the accident takes place.

November 1980

An Advisory Panel for the Decontamination of TMI-2 composed of citizens, scientists, and state and local officials, holds its first meeting in Harrisburg.


A host of epidemiological studies begin to study the health effects of the accident. Experts conclude that the amount of radiation released into the atmosphere was too small to result in discernible direct health effects to the population in the vicinity of the plant.


A study finds no measurable impact on infant mortality wtihin a 10-mile radius of the plant, compared with infant mortality rates for the state from 1977-1979.

July 1984

The reactor vessel head (top) is removed.

October 1985

Defueling begins.


A study finds no significant difference in cancer mortality within a 20-mile radius of the plant during the 5 years preceding the accident and the 5 years following it. A study of four communities downwind from the plant finds no significant abnormalities in either cancer mortality or cancer incidence among residents considered to be at high risk.

July 1986

The off-site shipment of reactor core debris begins.

August 1988

GPU submits a request for a proposal to amend the TMI-2 license to a “possession-only” license and to allow the facility to enter long-term monitoring storage.


A study finds no connection between radiation or psychological stress and failed and complicated pregnancies, such as fetal and neonatal mortalities and other problems.


A study found no significant abnormalities in cancer mortality or incidence among residents of selected communities near the plant.

January 1990

Defueling is completed.

July 1990

GPU submits its funding plan for placing $229 million in escrow for radiological decommissioning of the plant.

January 1991

The evaporation of accident-generated water begins.

April 1991

NRC publishes a notice of opportunity for a hearing on GPU’s request for a license amendment.


Two studies show no increase in cancer incidence among people who lived near the plant in 1979. Although the studies found no increase in cancer, they did find evidence of psychological stress lasting in some cases for 5-6 years.

February 1992

NRC issues a safety evaluation report and granted the license amendment.

August 1993

The processing of 2.23 million gallons of accident-generated water is completed.

September 1993

The NRC issues a possession-only license.

September 1993

The Advisory Panel for Decontamination of TMI-2 holds its last meeting.

December 1993

Post-Defueling Monitoring Storage begins.


FirstEnergy acquires TMI-2 from GPU and contracts the monitoring to Exelon, the current owner and operator of TMI-1. The companies plan to keep the TMI-2 facility in long-term, monitored storage until the operating license for the TMI-1 plant expires, at which time both plants will be decommissioned.


Researchers at the University of Pittsburgh’s Graduate School of Public Health release a 20-year follow-up study that concludes there was no significant increase in overall deaths from cancer.

December 2003

After a series of legal challenges a federal appeals court dismisses the consolidated cases of 2,000 plaintiffs who sought damages against GPU. The court says the plaintiffs failed to present evidence that they had received a radiation dose large enough to possibly cause health effects.


The TMI-2 reactor is permanently shut down and defueled, with the reactor coolant system drained, the radioactive water decontaminated and evaporated, radioactive waste shipped off-site to an appropriate disposal site, reactor fuel and core debris shipped off-site to a Department of Energy facility, and the remainder of the site is monitored.

Source: U.S. Nuclear Regulatory Commission and the Nuclear Energy Institute