Chernobyl disaster

The Chernobyl disaster 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 kilometers north of the nation's capital, Kyiv. The world learned of the event two days later, when radioactivity triggered alarms in a nuclear power plant in Sweden.

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. Approximately 60% of the radioactive fallout landed in Belarus. About 360,000 people needed to be relocated after the accident. Many people suffered from acute radiation poisoning and long-term illnesses such as thyroid cancer.

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 about 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, 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. 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 shutdown button (also called 'SCRAM', or 'AZ-5' in RBMK reactors), causing control rods to rapidly insert into the reactor core to absorb neutrons (intended to immediately stop the nuclear chain reaction and thus shut down the reactor). 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 graphite-tipped control rods first displaced neutron-absorbing coolant with neutron-moderating graphite, which briefly increased reactivity instead of reducing it. This, now known to be a design flaw, caused a sudden power spike, and 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 01:23:47:53 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. The neutron moderator, made of graphite, started to burn. The fire released more radioactive fallout, which was carried away by the smoke.

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.

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 cancer.

Deaths and longterm effects
It's 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 estimates 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 to 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.

 2025 drone crash and effects 
On Friday, February 14, 2025, a drone armed with a warhead struck the outer shell of the Chernobyl sarcophagus, damaging it. So far, the radiation levels are normal, with no dramatic increases.  While no extremely major damage was done, it will take time to repair the sarcophagus.

Gallery

Related pages
 Fukushima nuclear disaster
 Nuclear disasters
 Ukraine
 Nuclear power
 Soviet Union
 Chernobyl

References

 Other websites 

 The National Museum Chernobyl Accident
 IAEA Website about the Chernobyl Accident 

Category:1986 in Europe
Category:1980s explosions in Europe
Category:1980s in the Soviet Union
Category:20th century in Ukraine
Category:Disasters in the Soviet Union
Category:Explosions in 1986
Category:Nuclear accidents
Category:Chernobyl
Category:April 1986 events