2 The accident

On April 26 1986 at 1:23:58 am local time, the Chernobyl-4 nuclear reactor (i.e., the fourth reactor of the Chernobyl power plant) suffered a catastrophic nuclear meltdown that resulted in a series of explosions and fire.

The catastrophe is attributed to a flawed reactor design and mistakes made by the tired plant operators, who violated procedures intended to ensure safe operation of the plant. As at Three Mile Island, a secondary factor contributing to the accident was the fact that plant operators were insufficiently trained and unfamiliar with many characteristics of the reactor.

Several procedural irregularities contributed to the cause of the accident. One was insufficient communication between the safety officers and the operators in charge of an experiment being run that night. Moreover, because of insufficient training, the operators had only a very imperfect understanding of how the reactor worked under the low amount of reactivity caused by the experiment. Many of the reactor's engineering features, such as the dangerously large positive void coefficient (see below), were virtually treated as military secrets, and the operators were unaware of them. In addition, several safety systems were bypassed and ignored in order to conduct the experiment.

The reactor was undergoing an experiment to test the electrical backup supply which allows the reactor to run safely during a power loss. The power output of the reactor was reduced from its normal capacity of 3.2 to 1 GW in order to conduct the test at a safer, low power. The actual power output fell to 30 MW, however, allowing the concentration of the neutron absorbing fission product xenon-135 to rise; this product is typically consumed in a reactor under higher power conditions. As the operators attempted to restore the power to the desired 1 GW, the concentration of Xenon-135 limited the power output to around 200 MW. In order to overcome the neutron absorption of the Xenon-135, the control rods were pulled out of the reactor farther than normally allowed under safety regulations.

The coolant flow increased, and the coolant was heated rapidly so that much of it began to boil. As the coolant heated, pockets of steam formed in the coolant lines. The particular design of the RBMK graphite moderated reactor at Chernobyl has a large positive void coefficient, which means that the power of the reactor increases rapidly in the absence of the coolant. As the power output rapidly increased the operators attempted to shut down the reactor by ordering an emergency manual "scram"--or rapid full insertion of the control rods. However due to the slow speed of the control rod insertion mechanism, the graphite tips of the rods and the temporary displacement of coolant the scram actually caused the reaction rate to increase. Within seconds the reactor jumped to around 30 GW, ten times the normal operational output. The fuel rods began to melt and the steam pressure rapidly increased causing a large explosion, displacing and destroying the reactor lid, rupturing the coolant tubes and then blowing a hole in the roof. When outside air contacted the graphite moderator of the core, the graphite began to burn. The fire dispersed most of the radioactive contamination.

To reduce costs, and because of its large size, the reactor only had partial containment. This allowed the radioactive contaminants to escape into the atmosphere after the steam explosion burst the primary pressure vessel. After part of the roof blew off, the inrush of oxygen combined with the extremely high temperature of the reactor fuel and graphite moderator sparked a graphite fire. This fire greatly contributed to the spread of radioactive material.

In order to try to limit the scale of the disaster, the Soviet government immediately sent in workers to try to clean up. Firefighters, for example, were sent in to try to extinguish the fires; they were not told how dangerously radioactive the smoke was. In the next months, many "liquidators", members of the army and other workers were sent in as cleanup staff; again, most were not told anything about the danger, and effective protective gear was unavailable. The worst of the radioactive debris was collected inside what was left of the reactor, and a large steel sarcophagus was hastily erected to seal off the reactor and its contents.

203 people were hospitalized immediately, of whom 31 died (28 of them died from acute radiation exposure). Most of these were fire and rescue workers trying to bring the accident under control, and 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 the nearby town of Pripyat, Ukraine. Health officials have predicted that over the next 70 years there will be a 2% increase in cancer rates in much of the population which was exposed to the 5-12 (depending on source) E Bq of radioactive contamination released from the reactor. An additional 10 individuals have already died of cancer as a result of the accident.

In January 1993, the IAEA issued a revised analysis of the Chernobyl accident, attributing the main root cause to the reactor's design and not to operator error. The IAEA's 1986 analysis had cited the operators' actions as the principal cause of the accident.

Soviet scientists have reported that the Chernobyl Unit 4 reactor contained about 190 metric tons of uranium dioxide fuel and fission products. Estimates of the amount of this material that escaped range from 13 percent to 30 percent.

Contamination from the Chernobyl accident was not evenly spread across the surrounding countryside, but scattered irregularly depending on weather conditions. Reports from Soviet and Western scientists indicate that Belarus received about 60 percent of the contamination that fell on the former Soviet Union. But a large area in the Russian Federation south of Bryansk was also contaminated, as were parts of northwestern Ukraine.

Chernobyl was a secret disaster at first. The initial evidence that a major nuclear accident had occurred came not from Soviet sources, but from Sweden, where on April 27 workers at a nuclear power plant were found to have radioactive particles on their clothes. It was Sweden's search for the source of radioactivity, after they had determined there was no leak at the Swedish plant, that led to the first hint of a nuclear problem in the Soviet Union.

3 Short-term impact

Workers involved in the recovery and cleanup after the accident received high doses of radiation. In most cases, these workers were not equipped with individual dosimeters to measure the amount of radiation received, so experts can only estimate their doses. Even where dosimeters were used, dosimetric procedures varied. Some workers are thought to have been given more accurate estimated doses than others. According to Soviet estimates, between 300,000 and 600,000 people were involved in the cleanup of the 30 km evacuation zone around the reactor, but many of them entered the zone two years after the accident. (Estimates of the number of "liquidators" - workers brought into the area for accident management and recovery work - vary; the World Health Organization, for example, puts the figure at about 800,000, also Russia lists as liquidators some people who did not work in contaminated areas.) In the first year after the accident, the number of cleanup workers in the zone was estimated to be 211,000, and these workers received an estimated average dose of 165 millisievert (16.5 rem).

Some children in the contaminated areas were exposed to high thyroid doses up to 50 gray (Gy) because of an intake of radioactive iodine, a relatively short-lived isotope, from contaminated local milk. Several studies have found that the incidence of thyroid cancer among children in Belarus, Ukraine and Russia has risen sharply. The IAEA notes "1800 documented cases of thyroid cancer in children who were between 0 and 14 years of age when the accident occurred, which is far higher than normal" but fails to note the expected rate. The childhood thyroid cancers that have appeared are of a large and aggressive type, and if detected early, can be treated. Treatment entails surgery followed by iodine-131 therapy for any metastases. To date, such treatment appears to have been successful in all diagnosed cases.

4 Longer-term impact

Right after the accident, the main health concern involved radioactive iodine, with a half-life of eight days. Today, there is concern about contamination of the soil with strontium-90 and caesium-137, which have half-lives of about 30 years. The highest levels of caesium-137 are found in the surface layers of the soil, where they are absorbed by plants and mushrooms and enter the local food supply. Recent tests have shown that caesium-137 levels in trees of the area are continuing to rise. The main source of elimination is predicted to be natural decay of caesium-137 to stable barium-137, since runoff by rain and groundwater has been demonstrated to be negligible.

The IAEA notes that, while the Chernobyl accident released as much as 400 times the radioactive contamination of the Hiroshima bomb, it was 100 to 1000 times less than the contamination caused by atmospheric nuclear weapons testing in the mid-20th century.

According to reports from Soviet scientists at the First International Conference on the Biological and Radiological Aspects of the Chernobyl Accident ( September 1990), fallout levels in the 10 km zone around the plant were as high as 4.81 G Bq/m². The so-called "red forest" of pine trees killed by heavy radioactive fallout lies immediately behind the reactor complex within the 10 km zone. The "red forest" covered some 4 km² and only pine trees died while birch and aspen survived. The "red forest" is so named because it was reported by evacuees that in the days following the accident the trees glowed red, apparently due to heavy radioactive fallout.

Soviet authorities started evacuating people from the area around Chernobyl within 36 hours of the accident. By May 1986, about a month later, all those living within a 30 km (18 mile) radius of the plant-- about 116,000 people-- had been relocated.

According to reports from Soviet scientists, 28,000 km² (10,800 mile²) were contaminated by caesium-137 to levels greater than 185 kBq/m². Roughly 830,000 people lived in this area. About 10,500 km ² (4,000 mile²) were contaminated by caesium-137 to levels greater than 555 kBq/m². Of this total, roughly 7,000 km² (2,700 square miles) lie in Belarus, 2,000 km² (800 square miles) in the Russian Federation and 1,500 km² (580 square miles) in Ukraine. About 250,000 people lived in this area. These reported data were corroborated by the International Chernobyl Project .

The Chernobyl accident was a unique event, on a scale by itself. It was the first time in the history of commercial nuclear electricity generation that radiation-related fatalities occurred. (note: an accident at the Japanese Tokaimura nuclear fuel reprocessing plant on September 30, 1999 resulted in the radiation related death of one worker on December 22 of that same year)

Epidemiological studies have been hampered in the former Soviet Union by a lack of funds, an infrastructure with little or no experience in chronic disease epidemiology, poor communication facilities and an immediate public health problem with many dimensions. Emphasis has been placed on screening rather than on well-designed epidemiological studies. International efforts to organize epidemiological studies have been slowed by some of the same factors, especially the lack of a suitable scientific infrastructure.

An increased incidence of thyroid cancer among children in areas of Belarus, Ukraine and Russia affected by the Chernobyl accident has been firmly established as a result of screening programs and, in the case of Belarus, an established cancer registry. The findings of most epidemiological studies must be considered interim, say experts, as analysis of the health effects of the accident is an ongoing process.

The activities undertaken by Belarus and Ukraine in response to the accident--remediation of the environment, evacuation and resettlement, development of noncontaminated food sources and food distribution channels, and public health measures-- have overburdened the governments of those countries. International agencies and foreign governments have provided extensive logistic and humanitarian assistance. In addition, the work of the European Commission and World Health Organization in strengthening the epidemiological research infrastructure in Russia, Ukraine and Belarus is laying the basis for major advances in these countries' ability to carry out epidemiological studies of all kinds.

In marked contrast to the human cost, the evacuation of the area surrounding the plant has created a lush and unique wildlife refuge. It is unknown whether fallout contamination will have any long-term adverse affect on the flora and fauna of the region, as plants and animals have significantly different and varying radiologic tolerance compared with humans. However, it seems that the biodiversity around the massive radiation spill has increased due to the removal of human influence (see the first hand account of the wildlife preserve below). There are reports of mutations in some plants in the area, leading to unsubstantiated tales of a "forest of wonders" containing many strangely mutated plants. Also, the area is reported to be silent, suggesting that birds have not yet recolonised it.

The CIH computer virus was given the name "Chernobyl Virus" by many in the media, since the v1.2 variant activates on April 26 of each year.

5 Chernobyl after the accident

The trouble at the Chernobyl plant itself did not end with the disaster in Reactor No. 4. The Ukrainian government continued to let the three remaining reactors operate because of an energy shortage in the country. A fire broke out in Reactor No. 2 in 1991; the authorities subsequently declared the reactor damaged beyond repair and had it taken offline. Reactor No. 1 was decomissioned 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. In November 2000, Ukrainian President Leonid Kuchma personally turned off the switch to Reactor No. 3 in an official ceremony, effectively shutting down the entire plant.

The sarcophagus is not an effective permanent enclosure for the destroyed reactor. Its hasty construction, in many cases conducted remotely with industrial robots, means it is aging badly, and if it collapses, another cloud of radioactive dust could be released. A number of plans have been discussed for building a more permanent enclosure, but all of the plans so far have been too costly and dangerous to be attempted.

6 Chernobyl and the Bible

Because of a controversial translation of "chernobyl" as wormwood, an urban myth started among English-speaking people (probably mostly Christians) that the Chernobyl accident was mentioned in the Bible:

And the third angel sounded, and there fell a great star from heaven, burning as it were a lamp, and it fell upon the third part of the rivers, and upon the fountains of waters; and the name of the star is called Wormwood: and the third part of the waters became wormwood; and many men died of the waters, because they were made bitter. -- Revelation 8:10-11

The story appears to have originated - or at least spread to the West - with a New York Times article by Serge Schmemann (Chernobyl Fallout: Apocalyptic Tale, July 25, 1986) in which an unnamed "prominent Russian writer" was quoted as claiming the Ukrainian word for wormwood was chernobyl.

7 Political Outcome

The Chernobyl accident was clearly a major disaster, and it received worldwide media attention. Public awareness of the risks of nuclear power increased signficantly. Organizations, both pro- and anti-nuclear, have made great efforts to sway public opinion. Casualty figures, reactor safety estimates, and estimates of the risks associated to other reactors differ greatly depending on which position is favored by the author of any given document. For example, the UN scientific committee on the effects of radiation has pulblicly criticized the UN office on humanitarian affairs with respect to some of its publications. The true facts of the affair are therefore rather difficult to uncover.

8 Related articles

• List of nuclear accidents
• Nuclear reactor
• Pripyat, Ukraine which turned into a ghost town after the Chernobyl accident.
• Radiation
• Yuri Bandazhevsky
• Elena Filatova

9 External links

• The International Chernobyl Research and Information Network
• Frequently Asked Chernobyl Questions
• Detailed analysis of the events
• UN Chernobyl site
• World Nuclear Association account of the accident
• First hand account of the wildlife preserve
• Zone of Exclusion: Pripyat and Chernobyl - Review of this book, with sample photo gallery shot in the dead zone in 2001
• More Dead Zone pictures - Commentary in Russian
• Photos from the deserted town of Pryp'yat
• Video from inside the sarcophagus (AVI 4.1 MB)
• Chernobyl Heart (2003) - Academy Award-winning documentary
• Ukrainian regions - Post Chernobyl accident area
• A female tourist's account, in photos, see Elena Filatova
• Cheating Chernobyl [1] - Interview with Alexander Yuvchenko, an engineer working in Chernobyl on 26th April 1986
• The Chernobyl Disaster - including some discussion of the disagreements within the UN organization about its publications
• Chernobyl - A Canadian Perspective ( PDF 405 KB) - A brochure describing nuclear reactors in general and the RBMK design in particular, focusing on the safety differences between them and CANDU reactors. Published by the CANDU organization.

Environment Engineering failures Nuclear accidents

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