A part of the nuclear core was damaged, radiation was released, and thousands of residents were evacuated from the area. The cause was a loss-of-coolant accident due to a combination of human error, design deficiencies, and component failures. The Three Mile Island accident resulted in increased public fear of nuclear accidents and led to permanent and sweeping changes in Governmental Nuclear Regulatory Commission (NRC) regulations for the safety of nuclear power production.

Facts about Three Mile Island Accident
The following fact sheet contains interesting facts and information on Three Mile Island Accident.

The Three Mile Island accident began at 4.37 a.m. on Wednesday, March 28, 1979 and involved a partial nuclear meltdown in reactor number 2 of the Three Mile Island Nuclear Generating Station.

The cause of the Three Mile Island accident was a 'loss-of-coolant' accident due to a mechanical or electrical failure that prevented the main feedwater pumps from sending water to the steam generators that remove heat from the reactor core.

Without the proper water flow, the nuclear fuel overheated and about half of the core melted during the early stages of the accident.

During the accident part of the nuclear core was damaged, radiation was released, and thousands of residents were evacuated from the area.

The Three Mile Island accident, a nuclear core meltdown, was due to a combination of human error, design deficiencies, and component failures.

The Three Mile Island Nuclear Generating Station began operations in 1974, located on Three Mile Island in the Susquehanna River, south of Harrisburg, in Dauphin County, Pennsylvania.

The Three Mile Island nuclear power plant was built by General Public Utilities Corporation (GPU) and operated by Metropolitan Edison Company (Met-Ed), a subsidiary of the GPU Energy division. Its operating license was issued on February 8, 1978.

Three Mile Island Nuclear Generating Station (TMI) has two separate units, referred to as TMI-1 and TMI-2. Following the accident the reactor core of TMI-2 was removed from the site, but the site has not been decommissioned.

The Atomic Energy Act of 1954 provided for both the development and the regulation of the uses of nuclear materials and facilities in the United States. The Energy Reorganization Act of 1974 replaced the Atomic Energy Act of 1954 and created the Nuclear Regulatory Commission (NRC).

Three Mile Island is so named because its location is three miles downriver from Middletown, a borough in Dauphin County, Pennsylvania.

Dauphin County, Pennsylvania covers a total of 558 sq mi (1,445 km2). Cities within 25 miles of the Three Mile Island Nuclear Generating Station include Harrisburg, Pennsylvania's state capital (12 miles to the city center), York (13 miles to the city center), and Lancaster (24 miles to the city center).

In 1974 the population within a 25-mile radius of the Three Mile Island nuclear power plant was over 663,500 people.

The events leading up to the Three Mile Island accident began at 4.37 a.m. on March 28, 1979 when a minor malfunction in the non-nuclear part of TMI-2 occured triggering a series of automated responses to relieve pressure in the nuclear reactor's coolant system.

The relief valve failed to automatically close when the pressure dropped and operators in the Control room misread the situation and mistakenly believed that coolant was being pumped into the system.

An automated emergency cooling system turned off and the relief valve remained open for over two hours as vital reactor coolant leaked out. There was no instrument to show how much water covered the nuclear core.

High radiation levels began to register in several areas of the plant as nearly two-thirds of the 12-foot-high nuclear core stood uncovered with coolant. A partial meltdown of the fuel bundles occurred.

As warning lights flashed and alarm bells rang the operators did not realize that the nuclear plant was experiencing a loss-of-coolant accident. TMI-2 suffered a severe core meltdown, the most dangerous kind of nuclear power accident.

Just before 7 a.m. a site emergency was declared, a  procedural requirement when an event occurred which threatened "an uncontrolled release of radioactivity to the immediate environment."

State and local authorities and the state police and nearby counties were alerted. A general emergency was then declared and news of the Three Mile Island accident began to be broadcasted on the radio and T.V.

At about midday the U.S. Department of Energy began its first helicopter flight to monitor radiation levels but thankfully the consequences outside the nuclear power plant were minimal, but this was unknown at the time. Fears escalated and people began to make a hasty departure of their homes.

Unlike the Chernobyl disaster that occurred on 26 April 1986 at the Chernobyl Nuclear Power Plant in the Ukraine, the TMI-2 containment building remained intact, containing almost all of the accident’s radioactive material.

At about 11.00am Governor Thornburgh received a telephone call from President Carter, who assured him that expert Harold Denton would be sent to assess the situation. President Carter also promised that a special communications system would be set up to link Three Mile Island nuclear power plant, the Governor's office, the White House, and the Nuclear Regulatory Commission (NRC).

NRC chairman Joseph Hendrie advised the Governor of Pennsylvania, Richard "Dick" Thornburgh, on the evacuation "of pregnant women and pre-school age children...within a five-mile radius of the Three Mile Island facility."

On Friday March 30, 1979 the evacuation zone was extended to a 20-mile radius on Friday March 30. Within days, 140,000 people had left the area.

The great concern about a potential hydrogen explosion inside the TMI-2 reactor was raised on Saturday, March 31, 1979 and government offices were inundated with telephone calls from concerned and confused citizens. Residents in the area were told to stay indoors with windows closed and ventilating systems off.

President Carter announced his plans to visit Three Mile Island the following day. On April 1 President Carter toured Three Mile Island and addressed residents in the Middletown community building.

On April 3 Denton announced that the hydrogen bubble had been eliminated and on April 4 Governor Thornburgh, appeared on the NBC "Today" show and announced that the "threat of any immediate catastrophe is over." The small radioactive releases from the accident had no detectable health effects on plant workers or the public

More than a million gallons of radioactive water remained inside the containment building and a massive decontamination clean-up program began. The accident's radioactive waste was shipped off-site to an appropriate disposal area.

The President's Commission on the Accident at Three Mile Island, created by Jimmy Carter in April 1979 mounting investigations into the crisis.

In the months following the Three Mile Island accident, although questions were raised about possible adverse effects from radiation on human, animal, and plant life in the Three Mile Island area, none could be directly correlated to the accident. The TMI-2 reactor was permanently shut down and all its fuel was removed. The reactor coolant system was fully drained and the radioactive water decontaminated and evaporated.

Today, the TMI-2 reactor is kept in monitored storage until