During some hours after the 4th Power Unit destruction the ChNPP firemen and personnel managed to liquidate numerous inflammations at the Unit that prevented the threat of fire spreading to other Power Units. Just after the accident at first the 3rd Power Unit, which along with the 4th Power Unit makes the second ChNPP generation, and then the 1st and 2nd Power Units were shutdown.
The Government Commission on Investigation of Causes of the Accident at the Chornobyl NPP was established under the order of the USSR Council of Ministers. The main tasks of the Government Commission were determination of the accident scales, development and implementation of measures on its localization and overcoming its consequences, health protection and population aiding, as well as the detailed study of the accident causes and the development based on the carried out analysis of immediate and long-term measures on preventing such accidents in future. The activity of the Government Commission was carried out in extreme conditions caused both by complexity of the problem occurred and lack of experience of actions in such situations.
At the initial stage of beyond designed accident management (from 26.04.86 till 06.05.86) the most important tasks were: the termination of self-sustaining chain reaction, cooling of the irradiated fuel, reduction of radioactive products release in environment, prevention of the subsequent development of the accident.
Since the very beginning attempts were made to reduce a temperature in the reactor shaft using the remained technological systems of the Nuclear Power Plant by water delivery in reactor core. It has been made the decision to isolate the destroyed power unit from environment with the purpose to create barriers on a releases way. The materials dropped from helicopters have covered the Central Hall with a layer in the thickness from 1 m at northern wall to 15 m at a southern wall. Helicopter brigades of army aircraft dropped about 5 000 tons of different materials, including 40 tons of boron compounds (an effective neutrons absorber), 600 tons of dolomite and 1800 tons of clay and sand in the destroyed unit since April, 27th till May, 10th, 1986. Almost 2400 tons of lead should take up generated heat, preventing melting of constructional materials and moving of the melted fuel under the reactor foundation.
At the same time it has been carried out measures to remove water from the suppression pool and to cool reactor core by liquid nitrogen delivery in it.
Water has been removed from suppression pool on May, 6th, 1986. By this time experts issued the conclusion about impossibility of self-sustaining chain reaction in the destroyed reactor, and release of radioactive substances in atmosphere was reduced in few thousand times.
One of the first issues faced by the Government Commission was determination of destiny of the town of Pripyat located 4 km from the ChNPP. Since morning of April 26 a permanent monitoring of radiation situation was established in the town. By evening of April 26 the radiation levels increased and in some areas reached hundreds of milliRoentgens per hour. In this connection the Government Commission took the decision on preparation for evacuation of Pripyat residents. On the night of 26th/27th April 1390 buses and 3 railway trains arrived from Kyiv and other neighbor’s towns. Regions and settlements were specified to accommodate the evacuated people. The order of their registration and settling was determined. The special groups to solve the urgent problems were organized.
The evacuation began at 14.00 on April 27, 1986, and was conducted for about 3 hours. That day around 45 thousand people were transported out of the town. Within first hours after the accident the population of the ChNPP adjacent (10-kilometer) zone was evacuated. On May 2 the decision on the evacuation of population from the Chernobyl NPP 30-km zone and some settlements beyond the bounds of it was approved. Later on by the end of 1986 about 116 thousand of people were resettled from 188 settlements (including the town of Pripyat).
On May 7, 1986, the Decree of the CC CPSU and Council of Ministers of the USSR for the issues of job placement and domestic arrangements of the population evacuated from the dangerous areas was approved. It specified specific measures on the placement of families of the Chornobyl NPP workers in Kyiv and other settlements, construction of domestic buildings and facilities used for conditions would be improved. Thus, the real hazard of melting was only for floor structure between premise 305/2 and steam distribution corridor.