Avaria on Chornobyl АЭС is the largest in history radiationaccident, taking place on April, 26, 1986.
During the tests of the system explosions in power unit,resulting in contamination parts of Ukraine, Belorussia,Russia and other countries of Europe happened from theincorrect actions of employees and unsuccessful structuralfeatures of the station. Radioactive clouds got through thewhole world, and in 10 days after an accident the radiation ofenhanceable degree was looked after already in the UnitedStates of America.
Due to timely liquidation of consequences of accident it wassucceeded to avoid the second explosion, in 10 times morepowerful than first, that would result in the radiation infectionof greater part of Ukraine and Belorussia, and alsoconsiderable part of Russia. For prevention of distribution ofradiation a sarcophagus was built from durable.
Construction features of АЭС 
are Structure of reactor of РБМК- 1000
Fourth power unit was the reactor of boiling type. In thereactors of such type a radioactive fuel, id est enriched 2wounds, is used for transformation of evaporation of water,that sets turbines in motion, generating electric power. Onfourth power unit two turbogenerators were set - ТГ- 7 andТГ- 8 by power for 500 electric МВт, or 1600 thermal МВт,each.
The reactors of the same type were set on Leningrad, Kursk, Smolensk АЭС. Active zone
The main constituent of reactor was reactor space, namedthe active zone of reactor. His height makes 7 meters, anddiameter are 11,8 meters. Into an active zoneтепловыделяющие elements , that carried out thenuclear chain reaction, accompanied by heating of water,were situated. Except in a reactor bars of controlsystem are defence, or SOUSAS, that could be fully or partlysubmerged in the active zone of reactor, or fully withdrawnfrom her. Bars braked a reactor at immersion in an activezone and dispersed it at extraction. In all there was 211 in areactor. Part from them was automatic (ARE);
Also there were bars of the hand adjusting (РР);
In addition, стрежни of УСП, that moved from a top tobottom, was present, and also the so-called "blackabsorbers" are ДП.
However the personnel of АЭС could reprogram the bar ofautomatic control in a bar hand and vice versa. There is aprocess of vaporization exactly in an active zone.
The effective number of tomentous bars of SOUSAS isnamed the operative supply of reactivity. According toregulation, in a reactor it had there is 26-30 стрежней hand. on April, 25, 1986 must be stopped working Chornobyl АЭС 4th power unit in connection with next плановым repair.During such repairs, as a rule, substituting of equipment bythe station and verification of her work is produced foravoidance of the emergency states. On April, 25 verificationof the called mode of "stopway of rotor of turbogenerator"was pre-arranged, that was offered by an institute. On their calculations, it had to improve thesystem of emergency energy supply. The mode of "stopway"would allow to use kinetic energy of rotor of turbogeneratorfor providing the power supply of nourishing and maincirculation pumps, that allowed to hold a situation undercontrol in case of deenergizing of the station.
In spite of all efforts of specialists, the system did notdeclare oneself properly. First made an effort set the systemas early as 1982, but then appeared, that tension at astopway falls quicker, than planned. Subsequent tests wereconducted in 1983, 1984 and 1985, but also. April, 26, 1986 During approximately two hours thermal power of reactorwas mionectic to 700 МВт, level envisaged by the program.Then, on the unstated reason, she went down to 500 МВт. In0: 28, on April, 26, in transition from the system of localautomatic control to the automatic regulator of generalpower, an operator did not pull off adjusting of power andshe went down sharply - thermal to 30 МВт and neutron tothe zero)[4][6]. A personnel being on БЩУ- 4 made decisionabout renewal of power reactor and, extracting theabsorptive bars of reactor,[4][7] in a few minutes obtainedher height in future stabilized thermal power on 160-200 МВт. Thus the operative supply of reactivity continuously wentdown from the proceeding poisoning. Accordingly, the barsof the hand adjusting continued to be extracted[6].
After the achievement of thermal power 200 МВт wereincluded additional main circulation pumps, an amount ofworking pumps was accordingly. Infection of territories a radiation
As a result of accident out of agricultural turn about 5 millionha of earth was shown, round АЭС the 30-kilometre zone ofalienation is created, hundreds of shallow settlements aredestroyed and buried. Before an accident in the reactor of fourth block there was180-190 т of nuclear fuel, dioxide of uranium. Onestimations that are presently considered most reliable, inan environment it was thrown out from 5 to 30 т of thisamount. Some researchers contest these data, alluding tothe present photos and supervisions of eyewitnesses, thatshow that a reactor is practically empty. Follows, however, totake into account that the volume of 180 т dioxide of uraniummakes only insignificant part from the volume of reactor. Areactor was mainly filled by a graphite; it is considered thathe burned out in the first days after an accident. In addition,part of the contained reactor was melted and moved throughbreak a secret the corps of reactor for his limits.
Except a fuel, in an active zone in the moment of accidentthere were foods of division and transuraniums are differentradio-nuclidess accumulating during work of reactor. Exactlythey present a most radiation danger. 
Closing of power units. After an accident on 4th power unit work of power-stationwas stopped from a dangerous radiation situation. Howeveralready in October, 1986, after vast works ondecontamination of territory and building of "sarcophagus", 1th and 2th power units were again brought into an action;work of 3th is renewed in December, 1987.
on Memorandum was December, 25, 1995 signed about themutual understanding between Government of Ukraine andgovernments of countries "Large seven" and Commission ofthe European union according to that development of theprogram of the complete closing of the station began to2000.
Decision about the final stop of power unit № 1 it is acceptedon November, 30, 1996, power unit № of March, 2 - 15, 1999.
on the decision of Cabinet of Ministers of Ukraine № isMarch, 29, 2000 accepted 598 "About the pre-schedulestopping of exploitation of power unit № 3 and final closingChornobyl
