Chernobyl disaster

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RBMK reactor at the Leningrad Nuclear Power Plant, nearly identical to the one at Chernobyl

A map of caesium-137 contamination in 1999, a decade after the Chernobyl crisis. Restriction orders are still in place for the production, transportation and consumption of food contaminated by Chernobyl fallout

Chernobyl Nuclear Power Plant reactor number 4, the enclosing sarcophagus and the memorial monument, 2009

The Chernobyl disaster^[1] was a nuclear disaster. It happened on April 26, 1986 at the Chernobyl nuclear power plant near the town of Pripyat, Ukraine. At that time, Ukraine was part of the Soviet Union. The station is 110 kilometres north of the nation's capital, Kyiv.

The event was one of the worst accidents in the history of nuclear power. It was rated level 7, the most severe level, on the International Nuclear Event Scale. The only other accident with a level 7 rating is Fukushima. The RBMK reactors that were used at the plant had no containment building to keep the radiation in. Radioactive fallout drifted over parts of the western Soviet Union, Eastern Europe, Scandinavia, the United Kingdom, and the eastern United States. Large areas of Ukraine, Belarus, and Russia were badly contaminated. About 60% of the radioactive fallout landed in Belarus.^[2]^[3] About 360,000 people needed to be moved to other places where they could live after the accident. Many people suffered from acute radiation poisoning and long-term illnesses such as thyroid cancer.^[4]^[5]

Cause of the accident

On the day of the incident, there was a planned power reduction. By the beginning of the day shift, the power level had reached 50%. Then, one of the regional power stations went offline. On the afternoon of April 25 there was a request that the further power reduction would be postponed. Further power down was allowed after 22:00.

A turbine generator run down test was to take place before the routine shutdown. At 00:05 on April 26, the power was at around 23%. 30 minutes later, the power fell to near zero, probably due to change of regulator. At 01:00 the power stabilized at around 6%. The test would be run at that level. A turbine was switched off and its oscillations were measured. Shift foreman A.F. Akimov reported to deputy-chief engineer A.S. Dyatlov (who supervised the power down) that the oscillations were measured. A final briefing was given. Everyone went to their instruments and at 01:23:04 the turbine generator run down test began. Everything went completely normally.

The reactor showed a tendency to accelerate as the cooling pumps connected to the turbine generator were slowing, and due to the property called positive void coefficient of reactivity; as number of so-called ‘voids’ (e.g. steam bubbles) increases due to increased boiling or loss of coolant, so does the reactivity. But the operators successfully kept the reactor in control and it did not accelerate. At 01:23:40 senior reactor chief control engineer L.F. Toptunov pressed the emergency protection button as planned for the shutdown, at the end of the test. The power was at 7% when the button was pressed. Instead of shutting down the reactor suffered a power surge. This was due to the reactor being unstable at low power. The shutdown system had severe design flaws. At 01:23:43 the power jumped to 17%. Due to the power increase, the automatic control rods suffered damage and they jammed. Fuel channels ruptured and at 01:23:47 the reactor exploded.

The explosion was so powerful that it blew the 1000 ton steel lid off the reactor. The explosion released large amounts of radioactive materials and fuel. This made the neutron moderator, made of graphite, to start to burn. The fire released more radioactive fallout, which was carried away by the smoke of the fire.

After the accident, Reactor 4 was covered by a "sarcophagus" made from steel and concrete to stop the escape of more radiation from the lost corium as well as radioactive dust. The sarcophagus was covered in 2016 with the New Safe Confinement structure.^[6]

Disputes

The accident raised concerns about the safety of the Soviet nuclear power industry. The Soviet Union slowed the process of making its nuclear industry bigger for some time. The government also had to become less secretive as a result of the accident. Since 1991, Russia, Ukraine and Belarus have become separate countries. Those countries have continued to pay the high costs for decontamination (removing the radioactive material) and health care because of the accident. Exposure to radiation leads to a higher risk of getting cancer.

Deaths and longterm effects

It is difficult to count the number of deaths caused by the events at Chernobyl. A 2005 IAEA report tells of 56 direct deaths. 47 were accident workers, and 9 were children who died of thyroid cancer. The report thinks that up to 4,000 people may die from long term diseases related to the accident. However, other estimates range from 4,000 to 27,000 by the Union of Concerned Scientists. Greenpeace estimate that between 93,000 - 200,000 people died as a result of the disaster. Following the disaster, babies and animals born around Chernobyl had defects, such as cerebrovascular diseases, physical deformities, and mental disorders. The animals exposed to high levels of radiation were born with physical deformities and mutations. Birds were born with smaller brains and less viable sperm.

Aftermath

The other three reactors at Chernobyl continued to operate after the disaster because there were not enough other power plants in Ukraine to meet energy demands. Reactor 2 was permanently turned off and stopped being used in 1991 after a fire in its turbine hall. Reactor 1 was decommissioned in 1996, and reactor 3 was decommissioned in 2000. In 2018, a 3800 panel, 1 megawatt solar plant was opened next to the former nuclear plant.

Gallery

Chernobyl Disaster Media

Reactor decay heat shown as % of thermal power from time of sustained fission shutdown using two different correlations. Due to decay heat, solid fuel power reactors need high flows of coolant after a fission shutdown for a considerable time to prevent fuel cladding damage, or in the worst case, a full core meltdown.
Process flow diagram of the reactor
Comparative Generation II reactor vessels size comparison, a design classification of commercial reactors built until the end of the 1990s.
Pripyat with the Chernobyl Nuclear Power Plant in the distance
Ruins of abandoned house in Chernobyl, 2019
Abandoned objects in the evacuation zone