Chernobyl disaster

The Chernobyl nuclear disaster occurred at 01:23 on 26 April 1986 in reactor unit 4 of the Chernobyl nuclear power plant near the Ukrainian town of Prypyat, founded in 1970. It was the first event to be classified in the highest category of catastrophic accident (INES 7) on the seven-level international nuclear event rating scale. This is not to be confused with a GAU, i.e. the technical design-basis accident of a nuclear facility.

During a simulation of a complete power failure, carried out under the direction of Anatoly Dyatlov and started on April 25, 1986, an uncontrolled power increase occurred due to serious violations of safety regulations and the design characteristics of the graphite-moderated nuclear reactor of the RBMK-1000 type, which led to the explosion of the reactor and the fire of the graphite used as a moderator at 1:23 a.m. on April 26. Within the first ten days after the explosion, radioactivity of several trillion becquerels was released into the earth's atmosphere. The radioactive substances thus released into the atmosphere, including the isotopes ^137Caesium with a half-life (HWL) of around 30 years and ^131Iodine (HWL: 8 days), contaminated mainly the region northeast of Chernobyl as a result of radioactive precipitation, as well as many countries in Europe through wind transport. After the disaster, so-called liquidators started decontamination of the most affected areas. Under the direction of the Kurchatov Institute, a temporary protective shell (Russian объект "Укрытие", object "Ukrytije") consisting of reinforced concrete was erected by November 1986, usually referred to as a "sarcophagus".

There has been controversy for years about the long-term health effects worldwide, especially those due to an increased effective dose compared to natural radiation exposure. In a report prepared jointly with the United Nations and the International Atomic Energy Agency, the WHO considers a total of about 4,000 deaths worldwide possible, mainly from cancer. According to this report, there were less than 50 deaths directly attributed to the catastrophe, mostly as a result of acute radiation sickness. The indirect and statistically determined death toll, on the other hand, is estimated to be much higher. In a report from 2016, the IPPNW statistically links hundreds of thousands of deaths to the nuclear disaster. According to the report, millions of people are suffering from late effects on their health. The UNSCEAR report published in 2008 concluded that at that time a total of 43 deaths were attributable to the nuclear accident.

The most significant effect observed in the heavily contaminated areas around Chernobyl was the increased incidence of thyroid cancer, a form of cancer with a very good prognosis. This increased incidence could have been prevented by the government of the time with simple medical means, a so-called iodine blockade.

In a public statement in 1986, the then General Secretary of the Central Committee of the CPSU, Mikhail Gorbachev, described Western coverage of the accident, which allegedly resulted in thousands of deaths, as "rampant anti-Soviet agitation" and called in the speech for international cooperation in the peaceful use of nuclear energy.

In 2006, Gorbachev wrote in a book that Chernobyl, perhaps even more than his perestroika, was the real cause of the collapse of the Soviet Union.

Reaction and countermeasures

After Prypyat was evacuated on April 27, 1986, the next evacuation step covered all inhabitants from a radius of 10 km around the reactor by May 3. On 4 May 1986, an area 30 km around the reactor was evacuated, affecting a further 116,000 people. In the following years, another 210,000 inhabitants were resettled. Meanwhile, the exclusion zone is 4300 km², which corresponds to a circle with a radius of 37 km.

Initially, the reaction to the Chernobyl accident was characterized by an underestimation of the situation and by disinformation. Even the morning after the explosion, the Soviet government was only informed about a fire in the nuclear power plant, not about an explosion. It was only when the civil defence in Prypyat measured dangerously high radiation levels during the day and reported them to Moscow that party leader Mikhail Gorbachev convened a crisis committee and sent experts to the site of the accident. Although there was talk of an incident in Prypyat, only instructions were given to take iodine tablets and close windows and doors. The town is less than 5 km from the nuclear power plant, where most of the residents also worked. While the evacuation of the town of Prypyat began 30 hours after the explosion in reactor unit 4, with the help of 1,000 buses, the experts settled there - initially without any protective measures against radioactive exposure. Only when two days after the explosion increased radioactivity was measured at a nuclear power plant in Sweden and scientists found out that the radiation came from outside, they asked in Moscow whether causes were known there.

Now the first steps were taken to cool the damaged, glowing reactor block and to prevent further problems from arising. To reduce the radiation, helicopters, some of which had been withdrawn from Afghanistan, threw sand and boric acid into the reactor block from a height of 200 metres. But this had no effect and the temperature rose. It was then decided to switch to lead. The extinguishing water that had collected under the reactor threatened to come into contact with the molten corium of fuel rods, graphite and concrete, which could have led to a steam explosion.

It was decided to pump out the extinguishing water with the help of the fire brigade from Prypyat. It was also decided, with the help of miners hastily transferred to Chernobyl, to dig a 150-metre-long access from the third reactor block under the fourth and to excavate a chamber there with a volume of 4500 m³ in order to install a complex cooling system. In the end, this chamber was filled with concrete, because they wanted to prevent the radiation from contaminating the groundwater around the reactor, which supplied the entire Ukraine. As further measures, they demolished all the small villages around Prypyat and tried to kill most of the animals.

Although the evacuation around the power plant continued until a 30 km zone was finally cleared, the population in the surrounding areas was still not informed of the danger, as the intention was to avoid mass panic. During the May Day celebrations, a particularly large number of people were outside without being informed of the danger. Internationally, however, the incident has since been publicized. On 5 May, Hans Blix, Director of the IAEA, visited Chernobyl at the invitation of Gorbachev and inspected the damaged reactor during a helicopter flight. At a press conference in Moscow, Blix and Soviet officials publicly announced an international conference on the Chernobyl incident in Vienna, at which the Soviet Union would provide all available information. On May 14, Gorbachev addressed the people in a televised speech, getting them in the mood to deal with the aftermath of the accident.

Shortly afterwards, a large number of helpers were transported to Chernobyl to seal the damaged reactor and to clean up the heavily contaminated area around the power plant. The so-called liquidators, who worked under the supreme command of General Nikolai Tarakanov for only a short time at a time under life-threatening conditions, now had the task of decontaminating the remaining area. The liquidators were partly recruited from among the evacuees from the 30-km exclusion zone, but soldiers and reservists, among others, were also deployed.

The next major countermeasure was to clean the roof of the fourth reactor unit of highly contaminated material. This was the first step to ensure long-term protection against radiation. A steel and concrete sarcophagus was built over the damaged reactor. This was done with the help of helicopters and cranes, which were protected from radiation with steel and lead plates. The work on the roof of the reactor, where radiation levels were highest, was initially to be done by remote-controlled vehicles. However, after these failed under the extreme conditions, humans were used here as well. According to official figures, 97% of the fuel rods are buried under the sarcophagus; however, this figure is in doubt, as more than 3% of their mass was ejected from the fourth reactor block during the explosion.

In order to bind the radioactive dust on the ground, a sticky polymer-based substance was spread around the reactor by helicopters, which was given the name Burda (Russian for "thin broth"). In the settlements, the roofs of all buildings were cleaned. At the reactor site, 300,000 m³ of contaminated soil was removed, pushed into trenches and sealed with concrete.

A new sarcophagus (New Safe Confinement; also known as "Arka") was slid over the old cracked casing, which no longer offers sufficient protection, in November 2017 directly next to the damaged power plant unit after many years of construction. Nowadays, about 300 liquidators live in the exclusion zone. These are involved in the new construction and maintenance of the old sarcophagus.

Present situation

The neighbouring town of Prypiat is now a ghost town and forms the centre of the restricted zone. Many buildings in the town have been renovated to serve as accommodation for the workers and engineers of the former Prypiat power plant park, for soldiers, police officers and firefighters. Around 700 of the 14,000 people who once lived in the Chernobyl area either refused to leave or returned to their villages after the 1986 disaster. In its list published in 2006, 2007 and 2013, the environmental organization Blacksmith Institute ranked Chernobyl among the ten most polluted places in the world.

Final shutdown of the power plant

All three units that were still operational were restarted after the cleanup was completed. After decontamination work in 1986 and 1987, the government considered that radiation had no further effect on personnel. The second reactor unit was shut down in October 1991 after a fire in the turbine hall.

At a meeting in Corfu in June 1994, the European Union decided to propose to Ukraine a programme of cooperation leading to the closure of the Chernobyl nuclear power station. The G7 countries, meeting in Naples in July 1994, supported this EU move. This eventually led to the signing of a Memorandum of Understanding by Ukrainian President Leonid Kuchma in Halifax, Canada, on 20 December 1995, which sought to close the reactors by the year 2000. Funding was provided through the EU's TACIS programme. Unit 1 was taken off line in November 1996, and in December 1997 the Ukrainian government decided to close reactor 3. Finally, in June 2000, the decision was taken to permanently close Unit 3 on 15 December 2000.

First protective cover

The damaged reactor block was covered by a temporary, permeable so-called "sarcophagus". Inside, the situation after the explosion is largely preserved in hot form. Of about 190 tons of reactor core mass, it is estimated that about 150 to 180 tons are still inside the sarcophagus: partly in the form of corium, partly in the form of dust and ash, washed-out liquids in the reactor sump and foundation, or in other forms.

In 1992, Ukraine, together with a French company, held a concept competition to find ideas for a long-term solution for Block 4. After a short time, the decision was made in favour of an effective protective shroud and a winner was selected. For this purpose, a complete shelter for Block 3 and Block 4 was to be built. However, since this concept would have required the shutdown of Block 3, which was still active at the time, this project was rejected. The costs for this were estimated at three to four billion US dollars.

In February 2013, due to large masses of snow, the roof of the machine hall collapsed, which is about 70 meters away from the sarcophagus. According to the Ukrainian Ministry of Civil Protection, no radioactive particles escaped.

Second protective cover ("New Safe Confinement")

The international Shelter Implementation Plan aimed to build a new, more durable sarcophagus: Initial measures included reinforcing the roof of the original sarcophagus and improving its ventilation system. The new sarcophagus was built from 2010 to 2016 about 200 meters next to the damaged reactor and then moved on plastic slide rails over the old sarcophagus. This should make it possible to remove the old sarcophagus without releasing more radioactive material. This is planned to be done with two cranes, which were specially manufactured on site for this purpose under high radiation exposure. Among other things, they can also crush radioactively contaminated materials. On 17 September 2007, the contract was awarded to the Novarka consortium.

As of March 2016, the German government has contributed approximately €97 million to the Chernobyl Shelter Fund (CSF), with outstanding contribution commitments amounting to approximately €19 million.

On 14 November 2016, work began on moving the new protective shell towards the old sarcophagus. It took up its final position on 29 November. On 25 April 2019, the European Bank for Reconstruction and Development (EBRD) announced the completion of a 72-hour test operation of the protective shell. The official commissioning in the presence of Ukrainian President Volodymyr Selenskyj took place on 10 July 2019.

"New sarcophagus" under construction, June 2013


New protective shell in final position over the damaged reactor unit, October 2017.


Sarcophagus of the 4th block in March 2006