chernobyl accident

Chernobyl Accident

• The Chernobyl accident in 1986 was the result of a flawed reactor design that was operated with inadequately trained personnel and without proper regard for safety.
• The resulting steam explosion and fire released about five percent of the radioactive reactor core into the atmosphere and downwind.
• 30 people were killed, and there have since been up to ten deaths from thyroid cancer due to the accident.
• An authoritative UN report in 2000 confirmed that there is no scientific evidence of any significant raadiation-related health effects to most people exposed.

The April 1986 disaster at the Chernobyl nuclear power plant in the Ukraine was the product of a flawed coupled with serious mistakes made by the plant operators in the context of a system where training was minimal. It was a direct consequence of Cold War isolation and the resulting lack of any safety culture.

The accident destroyed the Chernobyl-4 reactor and killed 30 people, including 28 from radiation exposure. A further 209 on site were treated foor acute radiation poisoning and among these, 134 cases were confirmed (all of whom recovered). Nobody off-site suffered from acute radiation effects. However, large areas of Belarus, Ukraine, Russia and beyond were contaminated in varying degrees.

Reactor diagram

The Chernobyl disaster was a

unique event and the only accident in the history of commercial nuclear power where radiation-related fatalities occurred.*
* There have been fatalities in military and research reactor contexts, eg Tokai-mura.
The accident
On 25 April, prior to a routine shut-down, the reactor crew at Chernobyl-4 began preparing for a test to determine how long turbines would spin and supply power following a loss of main electrical power supply. Similar tests had already been carried out at Chernobyl and other plants, despite the fact that these reactors were known to be very unstable at low power settings.
A series of operator actions, including the disabling of automatic shutdown mechanisms, preceded the attempted test early on 26 April. As flow of coolant water diminished, poower output increased. When the operator moved to shut down the reactor from its unstable condition arising from previous errors, a peculiarity of the design caused a dramatic power surge.
The fuel elements ruptured and the resultant explosive force of steam lifted off the cover plate of the reactor, releasing fission products to the atmosphere. A second explosion threw out fragments of burning fuel and graphite from the core and allowed air to rush in, causing the graphite moderator to bu
urst into flames.
There is some dispute among experts about the character of this second explosion. The graphite burned for nine days, causing the main release of radioactivity into the environment. A total of about 12 x 1018 Bq of radioactivity was released.
Some 5000 tonnes of boron, dolomite, sand, clay and lead were dropped on to the burning core by helicopter in an effort to extinguish the blaze and limit the release of radioactive particles.

Immediate impact
It is estimated that all of the xenon gas, about half of the iodine and caesium, and at least 5% of the remaining radioactive material in the Chernobyl-4 reactor core was released in the accident. Most of the released material was deposited close by as dust and debris , but the lighter material was carried by wind over the Ukraine, Belarus, Russia and to some extent over Scandinavia and Europe.
The main casualties were among the firefighters, including those who attended the initial small fires on the roof of the turbine building. All these were put out in a few hours.
The next task was cleaning up the radioactivity at the site so that the remaining three reactors could be restarted, and the damaged reactor shielded more pe

ermanently. About 200,000 people (“liquidators”) from all over the USSR were involved in the recovery and clean up during 1986 and 1987. They received high doses of radiation, around 100 millisieverts. Some 20,000 of them received about 250 mSv and a few received 500 mSv. Later, the number of liquidators swelled to over 600,000 but most of these received only low radiation doses.
Many children in the surrounding areas were exposed to radiation doses sufficient to lead to thyroid cancers (usually not fatal if diagnosed and treated early). Initial radiation exposure in contaminated areas was due to short-lived iodine-131, later caesium-137 was the main hazard (both are fission products dispersed from the reactor core). On 2-3 May, some 45,000 residents were evacuated from within a 10 km radius of the plant, notably from the plant operators’ town of Pripyat. On 4 May, all those living within a 30 kilometre radius – a further 116 000 people – were evacuated and later relocated. About 1,000 of these have since returned unofficially to live within the contaminated zone. Most of those evacuated received radiation doses of less than 50 mSv, although a few received 100 mSv or more.
In the years following the accident a further 210 000 people were resettled into less contaminated areas, and the initial 30 km radius exclusion zone (2800 km2) was mo
odified and extended to cover 4300 square kilometres.
Environmental and health effects
Several organisations have reported on the impacts of the Chernobyl accident, but all have had problems assessing the significance of their observations because of the lack of reliable public health information before 1986. In 1989 the World Health Organisation (WHO) first raised concerns that local medical scientists had incorrectly attributed various biological and health effects to radiation exposure
An International Atomic Energy Agency (IAEA) study involving more than 200 experts from 22 countries published in 1991 was more substantial. In the absence of pre-1986 data it compared a control population with those exposed to radiation. Significant health disorders were evident in both control and exposed groups, but, at that stage, none was radiation related.
Subsequent studies in the Ukraine, Russia and Belarus were based on national registers of over 1 million people possibly affected by radiation. These confirmed a rising incidence of thyroid cancer among exposed children. Late in 1995, the World Health Organisation linked nearly 700 cases of thyroid cancer among children and adolescents to the Chernobyl accident, and among these some 10 deaths are attributed to radiation.
So far no increase in leukaemia is discernible, but this is expected to be evident in the next few years along with a greater, though not statistically discernible, incidence of other cancers. There has been no substantiated increase attributable to Chernobyl in congenital abnormalities, adverse pregnancy outcomes or any other radiation-induced disease in the general population either in the contaminated areas or further afield.
Psycho-social effects among those affected by the accident are emerging as a major problem, and are similar to those arising from other major disasters such as earthquakes, floods and fires.
The most recent and authoritative UN report has confirmed that there is no scientific evidence of any significant radiation-related health effects to most people exposed to the Chernobyl disaster. The UNSCEAR* 2000 Report is consistent with earlier WHO findings. The report points to some 1,800 cases of thyroid cancer, but “apart from this increase, there is no evidence of a major public health impact attributable to radiation exposure 14 years after the accident. There is no scientific evidence of increases in overall cancer incidence or mortality or in non-malignant disorders that could be related to radiation exposure.” As yet there is little evidence of any increase in leukaemia appears to be increased, even among clean-up workers where it might be most expected. However, these workers remain at increased risk of cancer in the long term.
* the United Nations Scientific Commission on the Effects of Atomic Radiation, which is the UN body with a mandate from the General Assembly to assess and report levels and health effects of exposure to ionizing radiation.
Chernobyl today
The Chernobyl unit 4 is now enclosed in a large concrete shelter which was erected quickly to allow continuing operation of the other reactors at the plant. However, the structure is neither strong nor durable and there are plans for its reconstruction. The international Shelter Implementation Plan involved raising US$715 million for remedial work including removal of the fuel-containing materials. Some work on the roof has already been carried out.
In March 2001 a US$36 million contract was signed for construction of a radioactive waste management facility to treat spent fuel and other operational wastes, as well as material from decommissioning units 1-3.
In the early 1990s some US$400 million was spent on improvements to the remaining reactors at Chernobyl, considerably enhancing their safety. Energy shortages necessitated the continued operation of one of them (unit 3) until December 2000. (Unit 2 was shut down after a turbine hall fire in 1991, and unit 1 at the end of 1997.) Almost 6,000 people worked at the plant every day, and their radiation dose has been within internationally accepted limits. A small team of scientists works within the wrecked reactor building itself, inside the shelter.
Workers and their families now live in a new town, Slavutich, 30 km from the plant. This was built following the evacuation of Pripyat, which was just 3 km away.
Ukraine depends upon, and is deeply in debt to, Russia for energy supplies, particularly oil and gas, but also nuclear fuel. Although this dependence is gradually being reduced continued operation of nuclear power stations, which supply 45% of total electricity, is now even more important than in 1986. Ukraine is also planning to develop its own nuclear fuel cycle facilities to further increase its independence.
When it was announced in 1995 that the two operating reactors at Chernobyl would be closed by 2000, a memorandum of understanding was signed by Ukraine and G7 nations to progress this, but its implementation remained in doubt until 2000. Alternative generating capacity was needed, either gas-fired, which has ongoing fuel cost and supply implications, or nuclear, by completing Khmelnitski unit 2 and Rovno unit 4 in Ukraine. Construction of these was halted in 1989 but has since resumed, with financing which had been contingent upon Chernobyl’s closure.
What has been gained from Chernobyl’s disaster?
Leaving aside the verdict of history on its role in melting the Soviet iron curtain, some very tangible practical benefits have resulted from the Chernobyl accident . The main ones concern reactor safety.
While no-one in the West was under any illusion about the safety of early Soviet reactor designs, some lessons learned have also been applicable to western plants. Certainly the safety of all Soviet-designed reactors has improved vastly. This is due largely to the development of a culture of safety encouraged by increased collaboration between East and West, and substantial investment in improving the reactors.
Since 1989 over 1,000 nuclear engineers from the former Soviet Union have visited Western nuclear power plants and there have been many reciprocal visits. Over 50 twinning arrangements between East and West nuclear plants have been put in place. Most of this has been under the auspices of the World Association of Nuclear Operators, a body formed in 1989 which links 130 operators of nuclear power plants in more than 30 countries.
Many other international programmes were initiated following Chernobyl. The International Atomic Energy Agency (IAEA) safety review projects for each particular type of Soviet reactor are noteworthy, bringing together operators and Western engineers to focus on safety improvements. These initiatives are backed by funding arrangements. The Nuclear Safety Assistance Coordination Centre database lists Western aid totalling almost US$1 billion for more than 700 safety-related projects in former Eastern Bloc countries. The Nuclear Safety Convention is a more recent outcome.
In 1998 an agreement with the US provided for the establishment of an international radioecology laboratory inside the exclusion zone.

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