Chernobyl reactor accident, 1986 compiled by Wm. Robert Johnston last modified 11 June 2006

Date: 26 April-6 May 1986

Location: Chernobyl nuclear power station, Prypyat, Ukraine, USSR

Type of event: steam/chemical explosion and subsequent fire in graphite-moderated power reactor 

Description:

The Chernobyl nuclear power station included four RBMK-1000 reactors at a site 3 km from Prypyat (pop. 45,000) and 15 km from Chernobyl (pop. 12,500). The RBMK-1000 reactor is a graphite-moderated, water-cooled reactor producing 1 MW of power. The design lacks a Western-style containment vessel, instead being based on older Soviet designs optimized for production of plutonium. Unit 4 (housed in the same building as Unit 3) was completed in December 1983 and fully brought on line in March 1984. The early commissioning had left several safety tests undone. In April 1986 Unit 4 was nearly through its first fuel cycle, with the result that a variety of fission products were built up in the core.

On 25 and 26 April 1986 operators were attempting to perform safety tests. On the morning of 26 April they circumvented safety systems to conduct a test at 1:23 AM. The test resulted in a power excursion; without a rapid ability to reinsert control rods the reactor reached 100 times its design thermal output about 1:23:40 AM. A steam explosion at 1:24 AM lifted the 1000-ton upper reactor cover, allowing all the water in the core to escape. This was followed by a second explosion, possibly a hydrogen explosion fueled by reaction of steam from ruptured pipes with zirconium and/or graphite in the reactor core. The second explosion ruptured the roof of the reactor building and expelled about 25% of the reactor core within and beyond the building. One person above the reactor was killed instantly by the explosions. A second person in the reactor building was severely burned by steam; he was found partly buried by debris and died within a few hours. Burning graphite and hot core material ejected by the explosions started about 30 fires, including some on the combustible tar roof of Unit 3. Many plant workers displayed symptoms of radiation sickness within hours of the explosion. In addition to plant workers, firefighters were also exposed significantly while successfully extinguishing fires started on the roof of Unit 3. Total number of people hospitalized was 108 by 6:00 AM and 132 by the end of the first day. Air flow in the damaged reactor building fed a fire in the graphite in the core. About 12 megacuries of radioactivity was released on the first day. Over the following days, attempts to extinguish the fire by dropping material onto the reactor core through the ruptured roof tended to insulate the core, allowing temperatures and emissions to increase after 31 April. The fire was extinguished on 6 May after about 40 megacuries had been released by the fire (in addition to releases on 26 April).

Three individuals died of non-radiation effects: the two killed on 26 April and a third who died of a heart attack. Another 28 individuals (6 firefighters and 22 power plant workers) died of acute radiation sickness, and 238 others survived with acute radiation sickness. Of 22 exposed to 600 to 1,600 rad, 20 died of radiation (one additional death may be due to heart attack associated with the accident); two of these deaths were individuals with total body thermal burns. Survival times for these 20 radiation deaths were 10, 14, 14, 14, 15, 17, 17, 18, 18, 18, 20, 21, 23, 24, 24, 25, 30, 48, 86, and 91 days. Exposures ranging from 400 to 600 rad occurred for 23 individuals, of whom 7 died, all of whom had severe beta burns (covering at least 30% of the body). Survival times for these seven were 16, 18, 21, 23, 32, 34, and 48 days. Exposures of 200 to 400 rad were incurred by 53 individuals, one of whom died 96 days after the accident. Some deaths involved complications of emergency transplants; 13 bone marrow transplants and 6 human embryo liver transplants were performed in the weeks following the accident. Survivors with radiation sickness included about 31 with beta radiation burns. In these cases burns developed one to three weeks after exposure over 1 to 30% or more of the body. A total of about 710 plant workers and emergency workers received doses over 50 rad. Workers who received doses between 25 and 50 rad during response or recovery operations include about 60 workers during the accident, about 2,900 workers during recovery through 1987, and about 16,000 military servicemen. Evacuation of residents under the plume was delayed by the government's unwillingness to publically acknowledge the accident. Throughout Europe many abortions of normal pregnacies were obtained out of fears of radiation from Chernobyl; studies suggest about 100 excess abortions in Italy and 400 excess abortions in Denmark in the months following the accident. Over the following years the principal observed chronic affect has been a significant increase in childhood thyroid cancer, affecting 700-1400 children with 10 deaths reported; these figures are far above background rates, whereas other cancers are not significantly different than background levels. 

Consequences: 31 acute fatalities--2 from explosion, 1 from heart attack, 28 from radiation (maximum exposure 1,600 rem); 238 acute radiation injuries; additional chronic radiation injuries (primarily thyroid injuries) with 10 probably chronic deaths (from thyroid cancer). 


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