chernobyl accident

Chernobyl Accident

Chernobyl

• 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 eevidence of any significant radiation-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 eexposure. A further 209 on site were treated for 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 ccontaminated 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 aattempted test early on 26 April. As flow of coolant water diminished, power 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 ccore and allowed air to rush in, causing the graphite moderator to burst 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 tthe site so that the remaining three reactors could be restarted, and the damaged reactor shielded more permanently. 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 oof 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 modified 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 ...