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25-26 April 1986

Engineers on the evening shift at Chernobyl's number four reactor began an experiment to see whether the cooling pump system could still function using power generated from the reactor under low power should the auxiliary electricity supply fail.

At 2300 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 required for the test.

However, too many rods were lowered and output dropped too quickly, resulting in an almost complete shutdown.

Safety systems disabled

Concerned by possible instability, engineers began to raise the rods to increase output. At 0030 the decision was taken to carry on.

By 0100 power was still only at about 7%, so more rods were raised. The automatic shutdown system was disabled to allow the reactor to continue working under low power conditions.

The engineers continued to raise rods. By 0123, power had reached 12% and the test began. But seconds later, power levels suddenly surged to dangerous levels.


The reactor began to overheat and its water coolant started to turn to steam.

At this point it is thought that all but six control rods had been removed from the reactor core - the minimum safe operating number was considered to be 30.

The emergency shutdown button was pressed. Control rods started to enter the core, but their reinsertion from the top displaced coolant and concentrated reactivity in the lower core.


With power at roughly 100 times normal, fuel pellets in the core began to explode, rupturing the fuel channels.

At about 0124, two explosions occurred, causing the reactor's dome-shaped roof to be blown off and the contents to erupt outwards.

As air was sucked in to the shattered reactor, it ignited flammable carbon monoxide gas causing a reactor fire which burned for nine days.

Because the reactor was not housed in a reinforced concrete shell, as is standard practice in most countries, the building sustained severe damage and large amounts of radioactive debris escaped into the atmosphere.

Firefighters crawled onto the roof of the reactor building to fight the blaze while helicopters dropped sand and lead in an effort to quell the radiation.


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