The radiolysis of water leads to the formation of a detonating mixture of hydrogen and oxygen. Small explosions occur, ejecting the control rods from the reactor.
In 3 to 5 seconds, the power of the reactor increases a hundredfold.
The 1,200 tonnes of the concrete slab covering the reactor are thrown into the air and fall at an angle onto the reactor core, which is fractured by the impact. A very large fire breaks out, while a light with blue reflections emerges from the hole formed (the Vavilov-Cherenkov effect).
The technicians present on site, notably the deputy chief engineer, do not immediately grasp the extent of the catastrophe and think that the reactor is still intact, as does Brioukhanov, the director of the power station, who was awakened at 1:30 am. Briukhanov, the director of the power station, was awakened at 1:30 a.m. He also called the Ministry of Energy at 4:00 a.m. and declared that the reactor core is probably not damaged
He receives the order to maintain the reactor's water cooling; this order, which Briukhanov will have enforced all day, will only have the effect of releasing more radio-elements into the atmosphere and drowning the underground installations common to Reactors 3 and 4, seriously threatening the operation and integrity of Reactor 3. Consequently, the chief engineer in charge of Reactor 3, Yuri Eduardovich Bagdassarov, who has correctly assessed the situation, will, during the course of the day and against Briukhanov's directives, take the decision to put this reactor on cold shutdown, thus saving it from certain destruction, given the progressive deterioration of the installations. notably due to the flooding of the lower parts of the power station by the intervention of the fire brigades' water hoses.