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.
However ChNPP Unit 4, as well as other RBMK power units, has been accepted in operation without approbation of this mode though such tests should be a component of preoperational tests of the main design modes of the Unit. Besides Chornobyl NPP, any others NPP with RBMK-1000 type reactors after its commissioning doesn’t performed design tests using the TG running-out. Such tests have been carried out at Chornobyl NPP Unit 3 in 1982. They have shown that requirements to characteristics of electric current generated as a result of TG running-out during established time weren't meeting and it was necessary to improve the system of TG excitation regulation.
Programs of tests 1982-1984 stipulated to connect one MCP from each of two reactor circulation loops to running out TG, and programs of 1985 and April 1986 – connection of two MCP. Thus emergency situation modelling has been stipulated with switched-off by manual gate valves ECCS. Test at Unit 4 has been planned on April 25, 198 in the afternoon under thermal capacity of the reactor 700 MW then it was planned to shutdown the reactor for maintenance. It should be noted, that the tests program met the effective at that time requirements. Thus, tests should be carried out in a lowered capacity mode for which characteristic raised in comparison to nominal heat-carrier rate through the reactor, insignificant underheating of heat-carriers to boiling temperature at an input in the core and minimum steam quality. These factors have made direct impact on the accident severity.
Test at Unit 4 has been planned on April 25, 198 in the afternoon under thermal capacity of the reactor 700 MW then it was planned to shutdown the reactor for maintenance. However, because of prohibition of Kievenergo supervisor to reduce a capacity, the reactor was in operation till 23:00 under thermal capacity 1600 MW. At 23:10 it has been started capacity decreasing. During transition from the system of local automatic capacity control to automatic capacity control of the basic range, due to gang error in measuring part, unexpectedly occurred thermal capacity decreasing up to 30 MW (neutron capacity to zero). Capacity increasing was started and at 01:03 a.m. thermal capacity has been stabilized at the level 200 MW, that is less than 0,1 from nominal. There is an intensive xenon poisoning of reactor core, and for its compensation almost all control roads have been removed from reactor core. As executed later calculations have shown, the operative reactivity margin before the tests beginning made about 7-8 control roads. All MCP are put into operation because upon tests completion at four MCP operation in running-out mode the others should remain connected for reliable core cooling.
Tests have been started at 1.23.04 a.m. and four MCP have started to work from "running-out" turbogenerator, in this connection the water supply has started to decrease, and steam quality - to increase. At 1.23.39 a.m. the emergency protection (АЗ-5) button has been pressed, and all control and protection system roads have gone to a core. At 1.23.43 a.m. prompt capacity increasing with the period of doubling less than 20 seconds then two or three explosions which have completely destroyed reactor core and reactor building, the fire occurred at Unit 4.On April 26, 1986 till 3 p.m. it has been firmly established that the reactor is destroyed, and large quantities of radioactive substances are released in environment from its debris.
According to the international rating scale of NPPs events accident at Chornobyl NPP Unit 4 is qualified as the seventh - the highest level accident.
As a result of explosion Unit 4 as constructive system has been significantly destroyed. The reactor construction was completely destroyed, the roof of the Central Hall (CH) and drum separator premises and northern group of the main circulating pumps was collapsed and scattered. The top floors of deaerator stack were completely destroyed.
Huge territories were contaminated with radionuclides. The explosion of the reactor which has followed by fire, and the processes occurred in irradiated fuel, have caused enormous releases of a radio-activity. The territory surrounding the destroyed Unit, adjacent buildings and constructions have been contaminated by fragments of fuel elements, graphite cladding, fragments of reactor installation. Around Unit 4 gamma dose rates reached 2000 R/hour, and inside – up to 50 000 R/hour. Chornobyl origin aerosols have been found out in Japan and Canada. The territory surrounding ChNPP became "contaminated", so-called "exclusion zone", the major quantity (approximately 95 %) of nuclear fuel in the form of reactor core fragments, lava-like fuel containing materials, fuel dust remained within the Shelter object, constructed in 1986.