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At 01 hr 23 min 49 sec Moscow time on April 26, 1986, the world’s worst-ever nuclear disaster occurred at Power Generation Unit No.4 of the Chornobyl NPP.

At 01 hr 23 min 49 sec Moscow time on April 26, 1986, the world’s worst-ever nuclear disaster occurred at Power Generation Unit No.4 of the Chornobyl NPP. As a result, the reactor core was destroyed completely, and almost all its supporting structures were severely damaged.

The reactor and its building were destroyed by two powerful explosions. The upper biological shielding of the reactor (scheme “E”) was torn off and turned at 15° to the vertical. All high-pressure pipes were destroyed. Hot graphite blocks, fragments of pipelines and equipment were thrown outside. Most of the debris piled up at the north wall. The explosion destroyed the separator room.

All the four separators, each weighing 130 tons, were moved from their supports and torn off from the pipelines. The roof collapsed and hung down on the separators. The main circulation pump room on the north side was destroyed completely, and all its equipment was blown outside. Pieces of hot fissile fuel and graphite fell on the roof of the turbine room, causing a fire which destroyed most of the roof. Two upper floors of the deaerator stack were destroyed, the framework columns were shifted toward the turbine room. The explosion broke the radiation safety barriers and protective shields, which protected the environment from irradiated fuel radionuclides. Radiation emission rate reached millions of curies per day in the period from April 26 to May 6, 1986. Afterwards, the radiation level dropped thousands of times, and continued decreasing gradually.

The adjacent territory was contaminated by scattered debris: fragments of fuel elements, pieces of graphite stack, and radioactive flinders of structures. They fell on the roof of the turbine room, the deaerator stack, and the building of Reactor 3, on the metal supports of the ventilation stack and other structures. During the active phase of the accident, radioactive dust – finely dispersed fuel elements – fell out on the premises of the station, the walls and roofs of its buildings. Early radiation measurements near the destroyed reactor and on the adjacent territory were striking. Near the reactor building the gamma field showed 2,000 R/hr. Gamma radiation was basically emitted from the reactor, inside which the dose rate was 50,000 R/hr. During the summer of 1986, 4 sq.km of forest around the station died. By the character of destructive processes and the magnitude of consequences, the accident was classed at Level 7 (grave accidents) on the INES scale. The immediate task of emergency works at Reactor 4 were to bring the situation under control, to stop radioactive emissions, and to reduce the radiation level at the Chornobyl NPP site.



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