The problem of identifying explosive processes parameters for the Unit 4 accident taken place at the Chornobyl NPP on April 26, 1986 was considered in many publications, some of them are included into the book “Shelter Object. Its history, state and perspectives” by the authors V.N. Gerasko, A.A. Klyuchnikov, A.A. Korneyev, A.V. Nosovskii /Kiev, Intergraphik, 1997/.
Accident scenarios considered by many authors at the initial stage of development certify that the explosive process can be relatively divided into two stages. At the first stage, approximately 6-7 sec by duration, after pushing the A3 -5 button, the reactor power increased to the quantity from 50 to 100 times more than rating value, that is up to 160,000-320,000 megawatt, and it caused the increasing of vapor pressure (up to 10-30 mega Pascal) and fuel temperature in the most energy-intensive areas of core up to melting point (2650 ј С). By the time 7.5 sec the fuel in the lower part of the core melted and partially vaporized. These processes caused the damage of fuel elements and fuel channel tubes, the beginning of the reactor upper metal structures break (Assemble Е), and also they intensified steam-zirconium reactions, reactions between steam and iron, interaction between melted fuel and water and steam, accompanied by hydrogen evolving. Probably, ruptures in contour of multiple forced circulation became the reason for the first explosion confirmed by the witnesses. At this point, the first stage could be considered relatively completed.
The second stage, approximately 3 sec by duration, is characterized by hydrogen formation (according to some estimations up to 200 kg), its outlet into the central reactor hall and formation of the detonation oxygen and hydrogen mixture (detonating mixture) and its detonation. According to the estimation the trotyl equivalent of explosion is equal to 300-600 kg of trinitrotoluene. The character of damages confirms its large scale.