"Good evening comrades. All of you know that there has been an incredible misfortune - The accident at the Chernobyl nuclear plant. It has painfully affected the Soviet people, and shocked the international community. For the fist time, we confront the real force of nuclear energy, out of control."
Mikhail Gorbachev
The accident
On 25 April 1986, 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, 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 core 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 radiation was released.
Some 5000 tons 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 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 km 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 modified and extended to cover 4300 square kilometres.
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.
- The releases contaminated an estimated 17 million people to some degree.
- 143,000 people have been evacuated from contaminated areas of Ukraine
- 600,000 people took part in liquidating effects of the disaster, 100,000 of which already died or are now handicapped
- Cases of leucosis and thyroid cancer exceed average by 2 and 5 times correspondingly among the Chernobyl victims.
- There are 1.8 million people residing on the territories of Ukraine, Russia, and Belarus, which are still defined as contaminated
- For the 14 years since the disaster 300,000 died in Ukraine alone from the radiation sickness
