Fukushima nuclear accident, Japan

Chernobyl nuclear accident, Soviet Union

Three Mile Island's Nuclear Accident

Three Mile Island's Nuclear Accident

The central Three Mile Island is located about 16 km from Harrisburg in the state of Pennsylvania, in the United States. It is a nuclear power plant with two nuclear pressurized water reactors.

The nuclear accident at the Three Mile Island nuclear power plant occurred on March 28, 1979, one year after the commissioning of unit 2. At around 4 am, the water supply circuit was disconnected. steam turbines which caused the cooling circuit of the primary circuit to stop working.

The overheating produced in the reactor core resulted in an increase in the pressure in the primary circuit, causing the introduction of control rods intended to automatically stop 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 confused various measuring instruments.

Due to these errors, the contaminated water left flooding the containment building that surrounds the reactor. In this way, radioactive gases 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.

Consequences of the Three Mil Island nuclear accident

When 6 years later it was possible to enter the affected area of ​​Three Thousand Island, an introduced chamber could show that 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 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 accident at the Three Mile Island nuclear power plant was classified as level 5 on the International Nuclear Event Scale ( INES Scale).

This accident motivated the future improvement of the safety of the nuclear power plants, defining corrective measures that have been included in all countries with nuclear facilities, in addition to the development of training and training programs for the installation's personnel.

The dismantling of the Three Mile Island plant

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

In the final phase, estimated nuclear fuel and corium for more than 100 tons at an expense of approximately $ 975 million were withdrawn and fully treated from October 1985 to April 1990. The remaining part of the site is actively monitored since no additional changes are foreseen until at least 2034, the year in which the final closure of the other unit that is still in operation is planned.

Currently there are no significant operations on the site, only monitoring and maintenance: the dismantling of unit 1 was initially planned for 2014 at the end of forty years of activity, but already in 2009 the NRC agreed with the operator of the plant, decided postpone it to 2034, 55 years after the accident of Unit 2, to be able to do it simultaneously with that of the second one (2036).

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Last review: May 2, 2019