An initial test carried out in 1982 indicated that the excitation voltage of the turbine-generator was insufficient; it did not maintain the desired magnetic field after the turbine trip.
The test was repeated in 1984 but again proved unsuccessful.
In 1985, a test was conducted a third time but also yielded negative results.
The test procedure was to be run again in 1986 and scheduled to take place during a maintenance shutdown of reactor No.4.
The flow exceeded the allowed limit at 01:19, triggering an alarm of low steam pressure in the steam separators. At the same time, the extra water flow lowered the overall core temperature and reduced the existing steam voids in the core and the steam separators. Since water absorbs neutrons better than steam, the neutron flux decreased and reduced the reactor power. The crew responded by turning off two of the circulation pumps to reduce feedwater flow in an effort to increase steam pressure, and by removing more manual control rods to maintain power.
At 01:23:40, as recorded by the SKALA centralized control system, a scram (emergency shutdown) of the reactor was initiated as the experiment was wrapping up.
The generators were to have completely picked up the MCPs' power needs by 01:23:43. In the interim, the power for the MCPs was to be supplied by the turbine generator as it coasted down. As the momentum of the turbine generator decreased, so did the power it produced for the pumps. The water flow rate decreased, leading to increased formation of steam voids in the coolant flowing up through the fuel pressure tubes.
A commission was established later in the day to investigate the accident. It was headed by Valery Legasov, First Deputy Director of the Kurchatov Institute of Atomic Energy, and included leading nuclear specialist Evgeny Velikhov, hydro-meteorologist Yuri Izrael, radiologist Leonid Ilyin, and others. They flew to Boryspil International Airport and arrived at the power plant on the evening of 26 April.
On 27 April 1986, at 14:00 each apartment block will be able to have a bus at its disposal, supervised by the police and the city officials. It is highly advisable to take your documents, some vital personal belongings and a certain amount of food, just in case, with you. All the houses will be guarded by the police during the evacuation period. Comrades, leaving your residences temporarily please make sure you have turned off the lights, electrical equipment, and water and shut the windows. Please keep calm and orderly in the process of this short-term evacuation.
By 15:00, 53,000 people were evacuated to various villages of the Kyiv region.
On the morning of 28 April, workers at the Forsmark Nuclear Power Plant (approximately 1,100 km (680 mi) from the Chernobyl site) were found to have radioactive particles on their clothes.
The fire inside reactor No. 4 continued to burn until 10 May 1986; it is possible that well over half of the graphite burned out. The fires were extinguished but many firefighters received high doses of radiation.
Lieutenant Volodymyr Pravyk died on 11 May 1986 of acute radiation sickness. He was the First on the scene was a Chernobyl Power Station.
This explanation effectively placed the blame on the power plant operators. The UKAEA INSAG-1 report followed shortly afterward in September 1986, and on the whole also supported this view, based also on the information provided in discussions with the Soviet experts at the Vienna review meeting.
By December 1986, a large concrete sarcophagus had been erected to seal off the reactor and its contents. The greater urban decontamination liquidators similarly first washed buildings.
in December 1986, with the help of a remote camera, they discovered an intensely radioactive mass more than two meters wide in the basement of Unit Four, which they called "the elephant's foot" for its wrinkled appearance The mass was composed of melted sand, concrete, and a large amount of nuclear fuel that had escaped from the reactor. The concrete beneath the reactor was steaming hot and was breached by now-solidified lava and spectacular unknown crystalline forms termed Chernobyl site. It was concluded that there was no further risk of explosion.
The United Kingdom restricted the movement of sheep from upland areas when radioactive cesium-137 fell across parts of Northern Ireland, Wales, Scotland, and northern England. the movement of a total of 4,225,000 sheep was restricted across a total of 9,700 farms, to prevent contaminated meat from entering the human food chain.
To reduce the spread of radioactive contamination from the wreckage and protect it from weathering, the protective Chernobyl Nuclear Power Plant sarcophagus was built by December 1986.
In September 1987, the I.A.E.A. held an Advisory Group Meeting at the Curie Institute in Paris on the medical handling of the skin lesions relating to the acute deaths. The only known, causal deaths from the accident involved workers in the plant and firefighters.
In 1987, Soviet medical teams conducted some 16,000 whole-body count examinations on inhabitants in otherwise comparatively lightly contaminated regions with good prospects for recovery. This was to determine the effect of banning local food and using only food imports on the internal body burden of radionuclides in inhabitants.
Worldwide, an estimated excess of about 150,000 elective abortions may have been performed on otherwise healthy pregnancies out of fears of radiation from Chernobyl, according to Robert Baker and ultimately a 1987 article published by Linda E. Ketchum in the Journal of Nuclear Medicine which mentions.
Detailed reports on the release of radioisotopes from the site were published in 1989.
In October 1991, a fire broke out in the turbine building of reactor No. 2 the authorities subsequently declared the reactor damaged beyond repair, and it was taken offline.
The Chernobyl Trust Fund was created in 1991 by the United Nations to help victims of the Chernobyl accident. It is administered by the United Nations Office for the Coordination of Humanitarian Affairs, which also manages strategy formulation, resources mobilization, and advocacy efforts.
The primary design cause of the accident, as determined by INSAG-7, was a major deficiency in safety features, in particular, the "positive scram" effect due to the control rods' graphite tips that actually initially increased reactivity when control rods entered the core to reduce reactivity. There was also an overly positive void coefficient of the reactor, whereby steam-generated voids in the fuel cooling channels would increase reactivity because neutron absorption was reduced, resulting in more steam generation, and thereby more voids; a regenerative process. To avoid such conditions, it was necessary for the operators to track the value of the reactor operational reactivity margin (ORM) but this value was not readily available to the operators.
In 1994, thirty-one deaths were directly attributed to the accident, all among the reactor staff and emergency workers.
Reactor No. 1 was decommissioned in November 1996 as part of a deal between the Ukrainian government and international organizations such as the IAEA to end operations at the plant.
In Russian Registry from 1991 to 1998 that suggested that "of 61,000 Russian workers exposed to an average dose of 107 mSv about [five percent] of all fatalities that occurred may have been due to radiation exposure".
On 15 December 2000, then-President Leonid Kuchma personally turned off reactor No. 3 in an official ceremony, shutting down the entire site.
In 2004, the UN collaborative, Chernobyl Forum, revealed thyroid cancer among children to be one of the main health impacts from the Chernobyl accident. This is due to the ingestion of contaminated dairy products, along with the inhalation of the short-lived, highly radioactive isotope, Iodine-131. In that publication, more than 4,000 cases of childhood thyroid cancer were reported.
According to the United Nations Scientific Committee on the Effects of Atomic Radiation, up to the year 2005, an excess of more than 6,000 cases of thyroid cancer had been reported. That is, over the estimated pre-accident baseline thyroid cancer rate, more than 6,000 casual cases of thyroid cancer have been reported in children and adolescents exposed at the time of the accident, a number that is expected to increase.
The Norwegian Agricultural Authority reported that in 2009 a total of 18,000 livestock in Norway required uncontaminated feed for a period before slaughter, to ensure that their meat had an activity below the government permitted value of cesium per kilogram deemed suitable for human consumption.
In 2011 Ukraine opened up the sealed zone around the Chernobyl reactor to tourists who wish to learn more about the tragedy.
As of 2016, 187 locals had returned and were living permanently in the zone.
In 2018, Ukraine spent five to seven percent of its national budget on recovery activities related to the Chernobyl disaster.