A flawed reactor design operated by poorly trained staff resulted in the biggest ever nuclear accident when one of the four reactors exploded at Chernobyl’s power plant in April 1986.
The effects of the disaster are still felt today with experts and the public questioning the safety of nuclear power. A 30km exclusion zone remains in place around the plant in the Ukraine and a multi-billion pound decommissioning operation is still underway.
The accident occurred in the early hours of 26 April when its number four reactor exploded following an experiment to determine how long turbines would spin and supply power to the main circulating pumps in the event of a loss of main electrical power supply.
Control rods, which regulate the fission process in a nuclear reactor by absorbing neutrons and slowing the chain reaction, were lowered to reduce output to about 20% of normal output.
However, too many rods were lowered and output dropped too quickly, resulting in an almost complete shutdown.
Concerned by possible instability, engineers began to raise the rods to increase output. At this point the reactor began to overheat and its water coolant turned to steam.
Engineers attempted an emergency shutdown but the reactor became extremely unstable and there were two explosions.
The uranium fuel in the reactor overheated and melted through protective barriers. As the reactor did not have a containment structure - common in all western nuclear power stations - radioactive material from the explosion, including plutonium, iodine, strontium and caesium were scattered as far as Scandinavia and the UK.
The accident raised concerns about Soviet-designed nuclear power stations and nuclear power in general.
The initial explosions killed two workers and another 28 firemen and emergency workers died from acute radiation sickness within three months. The accident also caused serious health problems for the local population and the displacement of some 200,000 people.
The Chernobyl site stopped generating power in 2000 but there still remains a multibillion pound clean-up bill.
Following the accident a massive concrete and steel “sarcophagus” was quickly built around the damaged reactor but this has deteriorated over the years.
Plans are now in place for a £1.35bn New Safe Confinement and Spent Fuel Storage Facility.
This is a new 257m wide and 105m high steel arch that will be slid over the damaged reactor and a new facility to safely and securely store spent nuclear fuel from reactors one to three.
These facilities should be constructed by 2015.