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Chernobyl Nuclear Power Plant sarcophagus
The Chernobyl Nuclear Power Plant sarcophagus or Shelter Object ((ウクライナ語:Об'єкт "Укриття")) is a massive steel and concrete structure covering the nuclear reactor #4 building of the Chernobyl Nuclear Power Plant. It was designed to limit radioactive contamination of the environment following the 1986 Chernobyl disaster, by encasing the most dangerous area and protecting it from climate exposure.〔(【引用サイトリンク】title=Chernobyl Sarcophagus )〕〔 It is located within a large restricted area known as the Chernobyl Exclusion Zone. The original Russian name is "Obyekt Ukrytiye" (Объект "Укрытие"),〔code name given under the Soviet secrecy procedures〕 which means "sheltering" or "covering", as opposed to sarcophagus.〔 The sarcophagus locked in 200 tons of radioactive corium, 30 tons of highly contaminated dust and 16 tons of uranium and plutonium.〔 In 1996 it was deemed impossible to repair the inside of the sarcophagus as radiation levels were estimated to be as high as 10,000 röntgens per hour (normal background radiation in cities is usually around 20-50 microröntgens per hour, and a lethal dose is 500 röntgens over 5 hours). A decision to replace the sarcophagus with the New Safe Confinement was taken, and a project to reconstruct the enclosure is underway.
==Construction==
The designing of the sarcophagus started on May 20, 1986. Subsequent construction lasted for 206 days, from June to late November of the same year.〔 The first task before construction started was to build a cooling slab under the reactor to prevent the hot nuclear fuel from burning a hole in the base. Coal miners were called up to dig the necessary tunnel below the reactor and by June 24, 1986 four hundred coal miners had dug the 168 metre (551 ft) long tunnel.〔 When the building became overly radioactive it became impossible to directly screw down the nuts and bolts or apply any direct welding to the sarcophagus, so this work was done by robots.〔 The seams of the sarcophagus, however, were not properly sealed. The entire construction process consisted of eight stages: clearing and concreting of territory around reactor unit 4, erection of initial reinforced concrete protective walls around the perimeter, construction of separation walls between units 3 and 4, cascade wall construction, covering of the turbine hall, mounting of a high-rise buttress wall, erection of supports and installation of a reactor compartment covering and finally the installation of a ventilation system.
More than 400,000 m3 of concrete and 7,300 tonnes of metal framework were used during the erection of the sarcophagus. The building ultimately enclosed 740,000 m3 of heavily contaminated debris inside,〔 together with contaminated soil.〔 On October 11, 1986 the Soviet Governmental Commission accepted "Conclusion on Reliability and Durability of a Covering Constructions and Radiation Safety of Chernobyl NPP Unit 4 Reactor Compartment". The sarcophagus has over 60 bore holes to allow observation of the interior of the core. In many places the structure was designed to have ventilation shafts to allow some convection inside.〔 Filtration systems have been put in place so that no radioactive material will escape through these holes.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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