MOSCOW, April 26. /TASS/. April 26, 2021 marks 35 years since the Chernobyl nuclear power plant disaster (near the town of Pripyat, the Kiev Region of the Ukrainian Soviet Socialist Republic). It went down in history as the worst-ever nuclear power plant catastrophe.

In the early hours of April 26, 1986, the Chernobyl nuclear power plant’s fourth reactor was used for an experiment, in which the reactor’s emergency cooling system was turned off intentionally. The Unit 4 reactor was to be shut down for routine maintenance on 25 April, 1986. It was decided to take advantage of this shutdown to determine whether, in the event of a loss of station power, the slowing turbine could provide enough electrical power to operate the main core cooling water circulating pumps, until the diesel emergency power supply became operative. The aim of this test was to determine whether cooling of the core could continue to be ensured in the event of a loss of power. The attempt to shut the reactor down safely failed. At 01:23 Moscow time the reactor exploded, causing a fire. The emergency was the worst-ever disaster in the nuclear power industry: the reactor’s core was totally destroyed, part of the building collapsed and a heavy discharge of radioactive materials into the environment followed.

Intensive fire lasted for ten days, during which time the overall escape of radioactive materials into the environment reached some 14 exabecquerels (380 million curies). Radioactive contamination affected more than 200,000 square kilometers, 70% of this area lying in Ukraine, Belarus and Russia. Eventually, contamination spots were identified in Arctic areas of the Soviet Union, Norway, Finland and Sweden.

Soviet authorities’ official reaction

The first brief official report of the emergency was released by TASS on April 28, 1986. Former Soviet Communist Party leader Mikhail Gorbachev told BBC in an interview on 2006 that festive May 1 street processions in Kiev and other cities were not canceled, because the country’s leadership lacked the full picture of the incident and feared that mass panic might be triggered. On May 14, 1986 Gorbachev appeared on television with an address to disclose the real scale of the disaster.

Elimination of effects

The power plant was suspended immediately after the explosion. The reactor’s core with burning graphite rods was sealed by a mixture of boron carbide, lead, and dolomite, dropped from helicopters, and, after the active phase of the breakdown was over, by latex, rubber and other dust absorbers (by the end of June 1986 about 11,400 tonnes of dry and liquid materials was spent).

On April 27, the city of Pripyat (47,500 people) was evacuated. During May 1986 about 116,000 people were resettled from 188 communities within the thirty-kilometer exclusion zone around Chernobyl.

Over a period of five months (July-August 1986) the disabled reactor was encased in concrete. The shelter, occasionally referred to as sarcophagus, was 50 meters tall and measured 200 meters by 200 meters in area. The escapes of radioactive elements stopped. The shelter contains no less than 95% of irradiated nuclear fuel from the ruined reactor, including 180 tonnes of uranium-235.

Probe into the causes

The Soviet Union’s special panel of inquiry blamed the disaster on the power plant’s management and operators on duty. The International Nuclear Safety Group (INSAG), created by the International Atomic Energy Agency, in its report agreed with the findings of the Soviet inquiry.

On July 29, 1987 Russia’s Supreme Court finished hearings on the criminal case against the nuclear power plant’s former chiefs. The station’s former director Viktor Bryukhanov, former chief engineer and his deputy Anatoly Dyatlov were sentenced to ten-year prison terms; former chief of the shift on duty Boris Rogozhkin, to five years; former chief of the reactor room Alexei Kovalenko, to three years, and former state inspector of the Soviet Union’s nuclear power industry watchdog Yuri Laushkin, to two years. In 1989-1991 all convicts were released ahead of schedule.

In 1991, a special panel of the Soviet Union’s nuclear power industry watchdog arrived at the conclusion that what might have been a less serious incident developed into a disaster due to unsatisfactory design of the reactor RBMK-1000. In 1993, the INSAG released an additional report based on newly-discovered evidence and reports by Soviet specialists. It mentioned the RBMK-1000 reactor’s safety discrepancy, mistakes by personnel and general lack of safety culture in nuclear affairs at the state and local levels.

Medical effects of the disaster

As follows from a report by experts of the United Nations Scientific Committee on the Effects of Atomic Radiation, about 600 of the NPP’s personnel were heavily exposed to radiation, including 134 people who experienced particularly large doses; 28 died of radiation sickness over several months after the incident.

In 1986-1990 more than 530,000 people from all over the Soviet Union, who participated in the effort to eliminate the effects of Chernobyl suffered strong exposure: 120 millisieverts on the average. All these people are faced with potential risks of delayed effects (such as cancer and other diseases). Their health is thoroughly monitored.

Chernobyl NPP’s further operation

After decontamination three Chernobyl reactors were recommissioned: N. 1 on October 1, 1986, N. 2 on November 1986 and N. 3 on December 4.

After the Soviet Union’s breakup, the Chernobyl nuclear power plant became part of independent Ukraine’s energy system.

In 2000-2015, fuel was removed from the reactors that were gradually shut down, and put in store. The last operating reactor, N. 3, was shut down on December 15, 2000. Power generation at Chernobyl was stopped altogether.

Originally, the concrete shelter over the fourth reactor was not designed as a permanent one. It was not airtight. On November 14-29, 2016 another protective arch-shaped cover called the New Safe Confinement was created over the ruined building of the fourth reactor. The 36,000-tonne arch is 109 meters tall, 162 meters long and 257 meters wide. It is expected that the NSC will guarantee safety for 100 years to come. The construction project was financed by the European Bank for Reconstruction and Development. Donations were provided by more than 40 countries, including Russia.

Current condition and plans for cleaning the exclusion zone

In April 2015, the Chernobyl nuclear power plant officially entered the phase of withdrawal from operation. The technical personnel began preparations for mothballing the rectors and the most contaminated equipment. Specialists say the work to mothball the reactors will continue till 2028. Then there will follow a pause from 2028 to 2045, until the reactors’ radiation is down to acceptable levels. The reactors are to be dismantled and the ruins of the fourth reactor decontaminated in 2045-2064. The half-life of plutonium-239, the last radioactive element in the exclusion zone is 24,000 years.