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Chornobyl (Ukrainian transliteration of Чорнобил) or Chernobyl (Russian transliteration) is a town in northern Ukraine not far from Kyiv. A major communications node and an important centre of trade and commerce, especially in 19th century. The name Chornobyl means Wormwood. The town is known primarily for the nuclear accident which occurred there in 1986.

Chornobyl accident

On April 26 1986, the Chornobyl-4 nuclear reactor suffered a catastrophic nuclear meltdown and fire, resulting from a flawed reactor design, and mistakes made by the tired plant operators, who violated procedures intended to ensure safe operation of the plant.

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 3200 MW to 1000 MW in order to conduct the test at a safer, low power. The actual power output fell to only 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 1000 MW, 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.

Procedural irregularities helped cause the accident. One was insufficient communication between the operators in charge of the experiment and the safety officers. Several safety systems were bypassed and ignored in order to conduct the experiment. 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 modified reactor at Chornobyl has a positive void coefficient, which means that the power of the reactor increases in the absence of the coolant. The power increase due to the steam voids combined with the retracted control rods caused the reactor power to quickly spike to around 30000 MW, ten times the normal operational output. The fuel rods began to melt and the steam pressure rapidly increased, 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 radioactivity.

To reduce costs, the reactor was intentionally not encased in an expensive secondary containment structure as are most western reactors. 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.

203 people were hospitalized immediately, of which 31 died. 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 45,000 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 80 megacuriess of radioactivity 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 Chornobyl 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 Chornobyl 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 Chornobyl 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.

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-kilometer 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.) 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 5,000 rad) because of an intake of radioiodine, 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 (see World Health Organization, page 154; Ivanov, Tsyb Studies, page 159; European Commission Program, page 159; Ukrainian Studies, page 160; and Swiss-Belarussian Paper, page 162). 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.

Longer-term impact

Right after the accident, the main health concern involved radioiodine, with a half-life of eight days. Today, in addition to radioiodine, there is concern about contamination of the soil with cesium-137, which has a half-life of about 30 years.

According to reports from Soviet scientists at the First International Conference on the Biological and Radiological Aspects of the Chornobyl Accident (September 1990), fallout levels in the 10-kilometer zone around the plant were as high as 130,000 curies per square kilometer. The so-called "red forest" of pine trees killed by heavy radioactive fallout lies within the 10-kilometer zone.

Chornobyl 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.

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

According to reports from Soviet scientists, 28,000 square kilometers (10,800 sq.mi.) were contaminated by cesium-137 to levels greater than five curies per square kilometer. Roughly 830,000 people lived in this area. About 10,500 square kilometers (4,000 sq.mi.) were contaminated by cesium-137 to levels greater than 15 curies per square kilometer. Of this total, roughly 7,000 kmē (2,700 sq.mi.) lie in Belarus, 2,000 kmē (800 sq.mi.) in the Russian Federation and 1,500 kmē (580 sq.mi.) in Ukraine. About 250,000 people lived in this area. These reported data were corroborated by the International Chornobyl Project.

The Chornobyl accident was a unique event, on a scale by itself. It was the only time in the history of commercial nuclear electricity generation that radiation-related fatalities occurred.

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 Chornobyl 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.

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

See also: nuclear reactor, radiation, list of nuclear accidents

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