Fukushima nuclear accident, Japan

Three Mile Island Nuclear Accident, United States

Three Mile Island nuclear accident, United States

The Three Mile Island nuclear accident is an accident that took place at the Three Mile Island nuclear power plant (USA) on March 28, 1979. The plant's TMI-2 nuclear reactor suffered a partial core meltdown, a year after unit commissioning.

The Chernobyl ‘Bridge of Death’
The Chernobyl ‘Bridge of Death’

At the time, it was the worst nuclear accident in US history . At the international level it had only been surpassed by the Kyshtym disaster (Soviet Union). However, seven years later it was widely surpassed by the Chernobyl accident, which together with the Fukushima accident are the two most serious nuclear disasters in the history of nuclear power.

Curiously, the accident was declared a few days after the release of the film "The China Syndrome" starring Jane Fonda in which an accident occurred at a nuclear plant.

Three Mile Island Power Plant Features

The Three Mile Island plant is located about 10 miles from Harrisburg in the state of Pennsylvania, in the United States. It is a nuclear power plant with two pressurized water nuclear reactors (PWR) built by Babcock and Wilcox:

  • The TMI-1 that came into operation on April 19, 1974 with an installed capacity of 786 MW.

  • The TMI-2 entered service in December 1978 with an installed capacity of 900 MW.

What Were the Causes of the Three Mile Island Accident?

The cause of the Three Mile Island accident was a failure in the cooling of the primary circuit of the plant.

Around 4 in the morning, the circuit in charge of supplying water to the steam turbines was disconnected, which caused the cooling circuit of the primary circuit to stop working.

The overheating produced in the reactor core led to an increase in pressure in the primary circuit. This increase led to the introduction of control rods designed to automatically shut down the nuclear reactor.

Supplemental water was pumped through the emergency cooling circuit. However, the valves that controlled the passage to the steam generator were blocked for a few moments. The responsible engineer disconnected the corresponding control automatism and mixed up various measuring instruments.

Due to these errors, the contaminated water came out flooding the containment building that surrounds the reactor. In this way, gases with high levels of radiation were released into the atmosphere (xenon and krypton). In addition, large amounts of water came out, with a low level of radioactive contamination, which ended up in the river.

Aftermath of the Three Mile Island Nuclear Accident

When 6 years later it was possible to enter the affected compound of Three Mile Island, an introduced camera was able to show that a part of the nuclear fuel had melted.

Thirty thousand people, living in the vicinity of the nuclear power plant, distributed within a radius of 8 km, were exposed to certain levels of radioactivity, although the effects of the radiation were very small.

According to data from the Nuclear Regulatory Commission (NRC), it was estimated that the effective equivalent dose until April 7 was 3,300 people. This represents an increase of 1.5% in the annual equivalent dose received in the area by natural radiation, which is 1 millisievert ( mSv).

The Three Mile Island nuclear power plant accident was classified as level 5 on the International Nuclear Event Scale ( INES Scale).

This accident motivated the future improvement of the safety of nuclear power plants. To this end, corrective measures were defined that have been included in all countries with nuclear facilities, in addition to the development of training programs and training for facility personnel.

What Were the Health Effects?

The nuclear industry conducted several studies on the effects on the health of the population. In these reports, it concluded that no short-term or long-term damage to people was demonstrated.

However, Greenpeace conducted parallel studies that maintained that cancer and leukemia cases increased markedly in the area near the nuclear plant.

The Decommissioning of the Three Mile Island Plant

Following the Three Mile Island nuclear accident, it took 13 years of work to complete the recovery of Unit 2. Work began in August 1979 and ended in December 1993, 3 years after the end of the drafted area recovery plan. in 1979.

In the final phase, it was estimated that the nuclear fuel to be withdrawn was more than 100 tons with an expenditure of approximately 975 million dollars. The fuel was withdrawn and fully treated from October 1985 to April 1990.

The remaining part of the site is actively monitored as no further changes are anticipated until at least 2034, the year in which the final closure of the other unit that is still in operation is planned.

There are currently no significant operations on the site, only surveillance and maintenance: the decommissioning of unit 1 was initially planned for 2014 at the end of forty years of activity, but already in 2009 the NRC in agreement with the operator of the plant, decided postpone to 2034, 55 years after the accident of Unit 2, to be able to carry it out simultaneously with that of the second (2036).


Published: April 9, 2010
Last review: November 4, 2021