Chernobyl Nuclear Power Plant sarcophagus

Chernobyl Nuclear Power Plant sarcophagus
Ukrainian: Укриття Чорнобильської АЕС

The sarcophagus in 2006. The tall chimney is an original part of the reactor building.
Location in Ukraine
Alternative names Chernobyl Nuclear Power Plant Object Shelter
General information
Type Confinement shelter
Location Covering Reactor 4 of the Chernobyl Nuclear Power Plant, within the Chernobyl Exclusion Zone, near Pripyat
Country Ukraine
Coordinates 51°23′23″N 30°05′57″E / 51.389636°N 30.099033°E / 51.389636; 30.099033Coordinates: 51°23′23″N 30°05′57″E / 51.389636°N 30.099033°E / 51.389636; 30.099033
Construction started June 1986
Completed November 1986
Demolished To partially start in 2017 (estimated)

The Chernobyl Nuclear Power Plant sarcophagus or Shelter Object (Ukrainian: Об'єкт "Укриття") is a massive steel and concrete structure covering the nuclear reactor No. 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.[1][2] It is located within a large restricted area known as the Chernobyl Exclusion Zone. The original Russian name is "Obyekt Ukrytiye" (Объект "Укрытие"),[3] which means "sheltering" or "covering", as opposed to sarcophagus.[2] The sarcophagus locked in 200 tons of radioactive corium, 30 tons of highly contaminated dust and 16 tons of uranium and plutonium.[1] 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).[4] A decision to replace the sarcophagus with the New Safe Confinement was taken, and a project to reconstruct the enclosure has since been completed.

Construction

The design of the sarcophagus started on May 20, 1986, 24 days after the disaster. Subsequent construction lasted for 206 days, from June to late November of the same year.[2] 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.[5] 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.[1] 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 400000 m3 of concrete and 7,300 tonnes of metal framework were used during the erection of the sarcophagus.[2] The building ultimately enclosed 740000 m3 of heavily contaminated debris inside,[1] together with contaminated soil.[5] On October 11, 1986 the Soviet Governmental Commission accepted a report entitled "Conclusion on Reliability and Durability of a Covering Constructions and Radiation Safety of Chernobyl NPP Unit 4 Reactor Compartment".[6] The sarcophagus has over 60 bore holes to allow observation of the interior of the core.[7] In many places the structure was designed to have ventilation shafts to allow some convection inside.[7] Filtration systems have been put in place so that no radioactive material will escape through these holes.[7]

Ongoing issues

The present shelter is constructed on top of the ruins of the reactor building. The two "Mammoth Beams" that support the roof of the shelter rest partly on the structurally unsound west wall of the reactor building that was damaged by the accident.[8] The western end of the shelter roof is supported by a wall at a point designated axis 50. This wall is reinforced concrete, and was cracked by the accident.

Designed Stabilisation Steel Structure

The DSSS is a yellow steel object that has been placed next to the wrecked reactor; it is 63 meters (207 ft) tall and has a series of cantilevers that extend through the western buttress wall, and is intended to stabilize the sarcophagus.[9] This was done because if the wall of the reactor building or the roof of the shelter were to collapse, then large amounts of radioactive dust and particles would be released directly into the atmosphere, resulting in a large new release of radioactivity into the environment. In December 2006 the "Designed Stabilisation Steel Structure" (DSSS) was extended until 50% of the roof load (about 400 tons) was transferred from the axis 50 wall to the DSSS.

Upper Biological Shield

A further threat to the shelter is the concrete slab that formed the "Upper Biological Shield" (UBS), situated above the reactor prior to the accident. This concrete slab was thrown upwards by the explosion in the reactor core and now rests at approximately 15° from vertical. The position of the upper bioshield is considered inherently unsafe, as only debris supports it in its nearly upright position. A collapse of the bioshield would further exacerbate the dust conditions in the shelter, possibly spreading some quantity of radioactive materials out of the shelter, and could damage the shelter itself. The UBS is a circle 15 meters in diameter, weighing 1000 tons and consisting of 2000 cubes, each located above a fuel channel. The shield, called Pyatachok (from word "pyatka" meaning "heel") before the disaster, was afterwards named Component "E" and nicknamed "Elena"; the twisted fuel bundles still attached to it are called "Elena's hair."[10][11][12]

Replacement

Main article: New Safe Confinement

On December 22, 1988, Soviet scientists announced that the sarcophagus would only last 20–30 years before requiring restorative maintenance work. In 1998, with the help of the European Bank for Reconstruction and Development, a conservation programme was completed which included securing the roof beams from collapsing. Nonetheless the rain-induced corrosion of supporting beams still threatens the sarcophagus's integrity.[5] It was revealed that the water is leaking through the sarcophagus via holes in its roof, becoming radioactively contaminated, and then seeping through the reactor's floor into the soil.[5] The new containment device will allow the sarcophagus to be dismantled and radioactive material to be removed.[13] The New Safe Confinement was expected to replace the existing sarcophagus in 2015. However, delays and a €100 million funding gap have seen the completion date of the New Safe Confinement set back to August 2017. [14]

References

Notes

  1. 1 2 3 4 "Chernobyl Sarcophagus". Chernobyl International. Retrieved 2010-11-30.
  2. 1 2 3 4 Ebel 1994, p. 1
  3. ↑ code name given under the Soviet secrecy procedures
  4. ↑ Marples 1996, p. 30
  5. 1 2 3 4 "What happened in Chernobyl?". Greenpeace. Retrieved 2010-11-30.
  6. ↑ Объект "Укрытие": Сооружение (in Russian). Chernobyl.by. Retrieved 2010-12-02.
  7. 1 2 3 Marples & Risovanny 1996, p. 17
  8. ↑ See BBC documentary Archived February 29, 2012, at the Wayback Machine.
  9. ↑ Nuclear Engineering International, July 2007, page 12.
  10. ↑ "Chernobyl Glossary". Angelfire.com. Retrieved 2010-03-22.
  11. ↑ Decade of disaster - Google Books. Books.google.com. 2000. ISBN 978-0-252-06820-1. Retrieved 2010-03-22.
  12. ↑ "Chernobyl Tour | Radiation Protection | US EPA". Epa.gov. 2006-06-28. Retrieved 2010-03-22.
  13. ↑ Wood 2007, p. 63
  14. ↑ ""NOVARKA and Chernobyl Project Management Unit confirm cost and time schedule for Chernobyl New Safe Confinement"". Archived from the original on 2011-09-18.

Further reading

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