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.
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 that 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.
During the maximum credible accident at the Chernobyl nuclear power station on 26 April 1986, an estimated 50 to 250 million Ci of radiation was released. Many publications compare the accident with the atom bombs dropped on Hiroshima and Nagasaki. In Chernobyl the destructive potential of at least one hundred atom bombs was unleashed. So far, the majority of experts have assumed that the explosion released only part of the reactor fuel. Most estimates give the amount as between 3.8 and 20 per cent. At the time of the accident there were 200 tonnes of uranium in the reactor.
These figures may have to be revised upwards. In early 2002 the debate took a new turn following research by the Soviet nuclear physicist Konstantin Checherov of the Kurchatov Institute in Moscow and his German colleague Sebastian Pflugbeil, Director of the Society for Radiation Protection in Berlin. The two nuclear physicists claimed, in a documentary broadcast on German national television (ZDF), that most of the fuel had been released into the environment, with only an insignificant amount remaining in the reactor itself.
The Ukrainian government agency Chernobyl Interinform, however, contends that studies of the reactor over 15 years indicate that 95 per cent of the fuel still remains within the reactor. The issue of how much fuel escaped is especially relevant in the light of plans to construct a second protective sarcophagus for the stricken reactor.