The main dates of the first stage

April 26 – Accident at the Unit 4. Governmental Commission for investigation of the accident causes and elimination of Chornobyl accident consequences had been created by the USSR Council of Ministers Order No 830. B.E.Shcherbina - Deputy Chairman of USSR Council of Ministers has been appointed as the commission head.

April 27 - Evacuation of the Pripyat and 10-km zone inhabitants.

The reasons of the accident were analyzed by organizations and individual experts both in the former USSR and abroad. One can formulate three main reasons set conditions for pre-accident reactor status and its catastrophic power runaway:
- Before the accident the reactor facility was in such physical and thermo-hydraulic state of stability which could be disturbed even by minor excitations. Such state of the reactor was caused by personnel actions and occurred prior to the tests of generator running-out operation. All reactor parameters before start of the tests, except for operating reactivity margin, were within the limits authorized by the process regulations;
- A direct impulse for the accident occurrence was emergency reactor shutdown system activation that due to defective design of control and protection rods led to positive reactivity insertion and start of reactor power excursion;
- This runaway reached disaster proportions because of a large steam coefficient of reactivity peculiar to high power channel-type reactors (RBMK-1000), and impact of which is notably strong at a low level of power (small void fraction).

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Accident at Chornobyl NPP Unit 4 has occurred on April 26th, 1986 at 01. 23.40 a.m. (Moscow time) during design tests of one of the safety systems. The given safety system provided use of mechanical rotation energy of shutdowning turbogenerators (so-called running-out) for electrical energy generation in the conditions of two emergencies combination.

One of them - full loss of NPP power supply, including the main circulating pumps (MCP) and pumps of emergency core cooling system (ECCS); another - the maximum design accident (MDA) for which it has been considered blowout  of the big diameter pipeline in reactor loop circulation. The design stipulated that in case of  external power supply switching-off the electrical energy generated by turbogenerators due to running-out is supplied for start-up of the ECCS pumps, that would provide the guaranteed core cooling. The proposals to use TG running-out come in 1976 from the RBMK reactor Chief designer. This concept has been recognized and included in civil-engineering designs for the NPPs with such type reactors.

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The first stage of ACE (accident consequences elimination) was completed since Chornobyl NPP transition to a non-rotation based work.

The main objectives of work performance of ACE 2nd stage at Chornobyl NPP were:
- Prevention of radionuclide spreading from radioactive contamination area;
- Re-disposal of solid RAW;
- Creation of conditions within contamination area for radiation-safe vital activity of population;
- Preparation of the Exclusion Zone and resettled territories of strict control zone to the return to national economy;
- Preparation to Chornobyl NPP decommissioning.

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