The Chernobyl disaster was a nuclear reactor accident in the Chernobyl Nuclear Power Plant in the Soviet Union. It was the worst nuclear power plant accident in history and the only instance so far of level 7 on the International Nuclear Event Scale, resulting in a severe release of radioactivity into the environment following a massive power excursion which destroyed the reactor. Thirty people died in the explosion, but most deaths from the accident were attributed to fallout. (Fallout is the name for radioactive dust spread by the explosion on the surroundings, a kind of radioactive contamination.)

On 26 April 1986 at 01:23:44 a.m. (UTC+3) reactor number four at the Chernobyl plant, near Pripyat in the Ukrainian SSR, exploded. Nearly thirty to forty times more fallout was released than had been by the atomic bombings of Hiroshima and Nagasaki. That resulted in the evacuation and resettlement of over 336,000 people... It is difficult to accurately tell the number of deaths caused by the events at Chernobyl, as the Soviet-era cover-up made it difficult to track down victims. Lists were incomplete, and Soviet authorities later forbade doctors to cite "radiation" on death certificates.

The Chernobyl station is located near the town of Pripyat, Ukraine, 18 km northwest of the city of Chernobyl, 16 km (10 mi) from the border of Ukraine and Belarus, and about 110 km (68 mi) north of Kiev. The station consisted of four reactors of type RBMK-1000, each capable of producing 1 gigawatt (GW) of electric power, and the four together produced about 10% of Ukraine's electricity at the time of the accident. Construction of the plant began in the 1970s, with reactor no. 1 commissioned in 1977, followed by no. 2 (1978), no. 3 (1981), and no. 4 (1983). Two more reactors, no. 5 and 6, capable of producing 1 GW each, were under construction at the time of the accident.


During the daytime of April 25, 1986, reactor 4 was scheduled to be shut down for maintenance as it was near the end of its first fuel cycle. An experiment was proposed to test a safety emergency core cooling feature during the shut down procedure. Very large amounts of cooling water are needed to cool the nuclear core to maintain safe temperatures. The reactor consisted of about 1,600 individual fuel channels and each operational channel requires a flow of 28 tonnes of water per hour. Then a regional power station unexpectedly went offline, so the test was postponed as electricity was needed to satisfy the evening peak demand.

On April 26, 1986

At 1:23:04 a.m. the experiment began. The extremely unstable condition of the reactor was not known to the reactor crew. With reactor output rapidly increasing, the operators pressed the AZ-5 ("Rapid Emergency Shutdown 5") panic button at 1:23:40 (36 seconds into the experiment), that ordered a "SCRAM" - an emergency shutdown of a nuclear reactor. (photo - possibly the Chernobyl reactor 4 control room)


AT 1:23:45 a.m., reactor 4 suffered a massive, catastrophic power excursion, resulting in a steam explosion, which tore the top from the reactor, exposed the core and dispersed large amounts of radioactive particulate and gaseous debris, allowing air (oxygen) to contact the super-hot core containing 1,700 tonnes of combustible graphite moderator. The burning graphite moderator increased the emission of radioactive particles. The radioactivity was not contained by any kind of containment vessel and radioactive particles were carried by wind across international borders. Although much of the nuclear fuel in the reactor core did ultimately melt, it should be noted that the disaster was not a "nuclear meltdown" in the usual sense; the fuel melting was not a significant contribution to the radiological consequences of the accident, and the accident was not caused by a loss of coolant.



The radiation levels in the worst-hit areas of the reactor building have been estimated to be 5.6 röntgen per second (R/s), which is equivalent to 20,000 röntgen per hour (R/h). A lethal dose is around 500 röntgen over 5 hours, so in some areas, unprotected workers received fatal doses within several minutes. However, a dosimeter capable of measuring up to 1,000 R/s was inaccessible due to the explosion, and another one failed when turned on. All remaining dosimeters had limits of 0.001 R/s and therefore read "off scale". Thus, the reactor crew could ascertain only that the radiation levels were somewhere above 0.001 R/s (3.6 R/h), while the true levels were 5,600 times higher in some areas. Because of the inaccurate low readings, the reactor crew chief Alexander Akimov assumed that the reactor was intact. The evidence of pieces of graphite and reactor fuel lying around the building was ignored, and the readings of another dosimeter brought in by 4:30 a.m. were dismissed under the assumption that the new dosimeter must have been defective. Akimov stayed with his crew in the reactor building until morning, trying to pump water into the reactor. None of them wore any protective gear. Most of them, including Akimov, died from radiation exposure within three weeks.

Shortly after the accident, firefighters arrived to try to extinguish the fires. The first one to the scene was a Chernobyl Power Station firefighter brigade under the command of Lieutenant Vladimir Pravik, who died on May 9, 1986 of acute radiation sickness. They were not told how dangerously radioactive the smoke and the debris were, and may not even have known that the accident was anything more than a regular electrical fire: "We didn't know it was the reactor. No one had told us."
The immediate priority was to extinguish fires on the roof of the station and the area around the building containing Reactor No. 4 in order to protect No. 3 and keep its core cooling systems intact. The fires were extinguished by 5 a.m., but many firefighters received high doses of radiation. 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.[31] The fire was extinguished by a combined effort of helicopters dropping over 5,000 tonnes of materials like sand, lead, clay and boron onto the burning reactor and injection of liquid nitrogen. Ukranian filmmaker Vladimir Shevchenko captured film footage of a MI-8 helicopter as it lost its bearings while dropping its load and got its rotors tangled in the gibbets of a nearby construction crane, causing the wrecked copter to fall into the damaged reactor building and killing its two-man crew.

The explosion and fire threw particles of the nuclear fuel and also far more dangerous radioactive elements like caesium-137, iodine-131, strontium-90 and other radionuclides into the air: the residents of the surrounding area observed the radioactive cloud on the night of the explosion.

In order to evacuate the city of Pripyat, the following warning message was reported on local radio, "An accident has occurred at the Chernobyl Nuclear Power Plant. One of the atomic reactors has been damaged. Aid will be given to those affected and a committee of government inquiry has been set up." This message gave the impression that any damage and radiation was localized, although it was not. The government committee formed to investigate the accident, led by Valeri Legasov, arrived at Chernobyl in the evening of April 26. By that time two people were dead and 52 were hospitalized. During the night of April 26 / April 27 - more than 24 hours after the explosion - the committee, faced with ample evidence of extremely high levels of radiation and a number of cases of radiation exposure, had to acknowledge the destruction of the reactor and order the evacuation of the nearby city of Pripyat.

The evacuation began at 14:00, April 27. In order to reduce baggage the residents were told that the evacuation would be temporary, lasting approximately three days. As a result, Pripyat still contains personal belongings.

The worst of the radioactive debris was collected inside what was left of the reactor, much of it shoveled in by liquidators wearing heavy protective gear (dubbed "bio-robots" by the military); these workers could only spend a maximum of 40 seconds at a time working on the rooftops of the surrounding buildings due to the extremely high doses of radiation given off by the blocks of graphite and other debris. The reactor itself was covered with bags containing sand, lead and boric acid thrown off helicopters (some 5,000 metric tonnes during the week following the accident). By December 1986 a large concrete sarcophagus had been erected, to seal off the reactor and its contents.

Many of the vehicles used by the "liquidators" remain parked in a field in the Chernobyl area to this day, most giving off doses of 10-30 röntgen/hr. over 20 years after the disaster.



The nuclear meltdown provoked a radioactive cloud that floated not over just the modern states of Russia, Belarus, Ukraine and Moldova, but also Turkish Thrace, Macedonia, Serbia, Croatia, Bulgaria, Greece, Romania, Lithuania, Estonia, Latvia, Finland, Denmark, Norway, Sweden, Austria, Hungary, the Czech Republic and the Slovak Republic, The Netherlands, Belgium, Slovenia, Poland, Switzerland, Germany, Italy, Ireland, France, the United Kingdom and the Isle of Man.

In Western Europe, measures were taken including seemingly arbitrary regulations pertaining to the legality of importation of certain foods but not others. In France some officials stated that the Chernobyl accident had no adverse effects.

In the aftermath of the accident, 237 people suffered from acute radiation sickness, of whom 31 died within the first three months. Most of these were fire and rescue workers trying to bring the accident under control, who were not fully aware of how dangerous the radiation exposure (from the smoke) was. 135,000 people were evacuated from the area, including 50,000 from Pripyat.



The Chernobyl nuclear power plant lies next to the Pripyat River which feeds into the Dnieper River reservoir system, one of the largest surface water systems in Europe. The radioactive contamination of aquatic systems therefore became a major issue in the immediate aftermath of the accident. In the most affected areas of Ukraine, levels of radioactivity (particularly radioiodine: I-131, radiocaesium: Cs-137 and radiostrontium: Sr-90) in drinking water caused concern during the weeks and months after the accident. After this initial period however, radioactivity in rivers and reservoirs was generally below guideline limits for safe drinking water. Groundwater was not badly affected by the Chernobyl accident.

After the disaster, four square kilometres of pine forest in the immediate vicinity of the reactor turned ginger brown and died, earning the name of the "Red Forest". Some animals in the worst-hit areas also died or stopped reproducing. Most domestic animals were evacuated from the exclusion zone, but horses left on an island in the Pripyat River 6 km from the power plant died when their thyroid glands were destroyed by radiation doses of 150-200 Sv. Some cattle on the same island died and those that survived were stunted because of thyroid damage. The next generation appeared to be normal.

In the years since the disaster, the exclusion zone abandoned by humans has become a haven for wildlife, with nature reserves declared (Belarus) or proposed (Ukraine) for the area. Many species of wild animals and birds, which were not seen in the area prior to the disaster, are now plentiful due to the absence of humans in the area.

A robot sent into the reactor itself has returned with samples of black, melanin-rich fungi that are growing on the reactor's walls.



The Ukrainian government continued to let the three remaining reactors operate because of an energy shortage in the country. A fire broke out in the turbine building of reactor 2 in 1991; the authorities subsequently declared the reactor damaged beyond repair and had it taken offline. Reactor 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. On December 15, 2000, then-President Leonid Kuchma personally turned off Reactor 3 in an official ceremony, effectively shutting down the entire plant transforming the Chernobyl plant from energy producer to energy consumer.



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Source: Wikipedia article "chernobyl disaster"

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