Sarcophagus

The sarcophagus that currently encases Unit 4 of the Chernobyl Nuclear Power Plant is a giant metal concrete and structure quickly constructed as an emergency measure in 1986 to halt the release of radiation into the atmosphere following the explosion. The official Russian name is “Obyekt Ukrytiye” which means shelter or covering.

It is estimated that within the shelter there is 200 tons of radioactive corium, 30 tons of contaminated dust and 16 tons of uranium and plutonium (source Wikipedia). In 1996 it was considered impossible to repair the sarcophagus as radiation levels within it were as high as 10,000 röntgens per hour (background radiation in cities is around 20-50 microröntgens per hour, a lethal dose being 500 röntgens over 5 hours). A decision to replace the sarcophagus with a “New Safe Containment” was taken and construction of the new structure is now well underway. Originally planned to be in place by 2005, the New Shelter is expected to be completed by the French consortium Novarka in 2015.

 

Cross section of the sarcophagus over reactor 4

Cross section of the sarcophagus over reactor 4

 

Reactor 4 following the explosion

The enormity of the challenge ahead is clear.

Construction of the sarcophagus

The sarcophagus was constructed under extremely dangerous conditions, with very high levels of radiation, and severe time constraints. Design of the sarcophagus started on May 20 1986, a little over three weeks on from the disaster. The construction lasted for 206 days, from June to late November of the same year. It was first necessary to build a cooling slab under the reactor to prevent the hot nuclear fuel from burning through the foundations. Four hundred coal miners were called upon to dig the required tunnel below the reactor and by June 24 the necessary 168 metre long tunnel was in place.

More than 400,000 m3 of concrete and 7,300 tons of metal framework were used during construction with the building ultimately enclosing 740,000 m3 of heavily contaminated debris and soil inside. The high levels of radiation made it immensely dangerous for humans to carry out work on the sarcophagus and robots were used for joining and welding where possible. The extreme conditions made it impossible to completely seal the seams of the sarcophagus.

The sarcophagus has over 60 bore holes to allow observation of the interior of the core. In places the structure incorporated ventilation shafts to allow for some convection inside. Filtration systems were put in place to prevent radioactive material escaping through these holes.

The construction process consisted of eight stages:

  1. Clearing and concreting the area surrounding reactor unit 4
  2. erection of initial reinforced concrete protective walls around the perimeter
  3. construction of separation walls between units 3 and 4
  4. cascade wall construction
  5. covering of the turbine hall
  6. construction of a high-rise buttress wall
  7. erection of supports and installation of a reactor compartment covering
  8. installation of the ventilation system.

 

 

 
The existing Object Shelter is primarily supported by the damaged remains of the Unit 4 Reactor Building, which are largely considered to be structurally unsound as a result of explosive forces caused by the accident. Three major structural members support the roof of the Object Shelter. Two beams, usually referred to as B-1 and B-2, run in an east-west direction and support the roof beams and panels. A third, more massive member, the “Mammoth Beam”, spans the largest distance across the roof from east to west and assists in supporting the roof beams and panels. The roof of the shelter itself consists of 1 metre diameter steel pipes laid horizontally north to south and steel panels that rest at an angle, also in the north-south direction.

The south wall of the Object Shelter is formed by the steel panels of the roof as they make an angle of approximately 15 degrees from vertical. The east wall of the shelter is formed by the reactor building itself, and the north wall by a combination of the reactor building and concrete segments. The west wall is constructed of large concrete sections reinforced by buttresses. The complexity of the segments of the west wall necessitated their construction off-site; they were then lifted into place by a remotely operated tower crane. It is these buttressed sections of the Object Shelter that are most often recognized in photographs of the sarcophagus.

Inside the sarcophagus

On December 22 1988, Soviet scientists announced that the sarcophagus would only last 20–30 years before requiring restorative maintenance work. The Object Shelter was never intended to be a permanent containment structure. Its continued deterioration has increased the risk of its radioactive contents leaking out. In 2010 it was revealed that water leaking through the sarcophagus roof was becoming radioactively contaminated before seeping through the reactor’s floor into the soil.

 

 

 

Designed Stabilisation Steel Structure

The Designed Stabilisation Steel Structure (DSSS) is the yellow metal work that can be seen against the sarcophagus. It is 63 metres tall and has a series of cantilevers that extend through the western buttress wall, and is intended to stabilise the sarcophagus. This DSSS was put in place 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 thrown into the atmosphere. 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 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. If the bioshield were to move it would disturb the radioactive dust, resulting in a release of material, and could potentially 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 (“five kopek coin”) before the disaster, was afterwards named Component “E” and nicknamed “Elena”; the twisted fuel bundles still attached to it are called “Elena’s hair.

The recent collapse

On Tuesday 12 February 2013 a 600 m2 section of the roof of the turbine-building, adjacent to the sarcophagus, collapsed. Initially it was assumed that the roof collapsed because of the weight of the snow on it. However the amount of snow was not exceptional, and the report of a Ukrainian fact-finding panel concluded that the part collapse of the turbine-building was the result of sloppy repair work and aging of the structure. Experts such as Valentin Kupny, former deputy director of the nuclear plant, did warn that the complex was on the verge of a collapse, leaving the building in an extremely dangerous condition. After the 12 February incident, radiation levels were up to 19 becquerels per cubic meter of air: 12 times normal. The report assumed radioactive materials from inside the structure spread to the surrounding area after the roof collapsed. All 225 workers employed by the Chernobyl complex and the French company, Novarka, that is building the new shelter were evacuated shortly after the collapse. The managers of the complex stated that radiation levels around the plant were at normal levels (between 5 and 6 mS/h) and should not affect workers’ health. According to Valentin Kupny the situation was underestimated by the Chernobyl nuclear complex managers and information was kept secret.

 

2013 roof collapse. © chnpp.gov.ua

New Safe Confinement

The New Safe Confinement (NSC or New Shelter) is the structure, paid for by the Chernobyl Shelter Fund, intended to fully contain the damaged nuclear reactor and prevent the reactor complex from leaking further radioactive material into the environment for the next 100 years. The confinement is expected to be completed by the French consortium Novarka in 2015.

Photographs of construction progress can be seen at www.chnpp.gov.

The word “confinement” is used rather than the traditional “containment” to emphasize the difference between the “containment” of radioactive gases that is the primary focus of most reactor containment buildings, and the “confinement” of solid radioactive waste that is the primary purpose of the New Safe Confinement.
Objectives of the NSC:

  • Make the destroyed ChNPP Unit 4 environmentally safe (i.e. contain the radioactive materials at the site to prevent further environmental contamination)
  • Reduce corrosion and weathering of the existing shelter and the Unit 4 reactor building
  • Mitigate the consequences of a potential collapse of either the existing shelter or the Unit 4 reactor building, particularly in terms of containing the radioactive dust that would be produced by such a collapse.
  • Enable safe deconstruction of unstable structures (such as the roof of the existing shelter) by providing remotely operated equipment for their deconstruction.

International competition

In 1992, the Ukraine Government held an international competition for proposals to replace the existing sarcophagus. Of the 394 entries 19 entries were examined in detail, with only the British submission proposing a sliding arch approach. There was no overall winner with the French submission came 2nd with the UK and German proposals coming joint 3rd.

Subsequently, a pan-European study (the TACIS programme) re-examined the proposals of the top three finalists of the competition and the study selected the sliding arch proposal as the best solution for their further investigations and recommendations. By using the sliding method there is much less chance of the construction workers receiving a harmful dose of radiation.
The advantages of a sliding arch include:

  1. Off site construction limits the radiation doses of the construction workers to a minimum.
  2. An arch fits snugly over the damaged reactor (minus its chimney).
  3. An arch is easier to slide than a square box.

The arch-shaped steel structure will have an internal height of 92.5 metres (303.5 ft) and the internal span of the arch is to be 245 metres (803.8 ft). The dimensions of the arch were determined based upon the need to operate equipment inside the new shelter and decommission the existing shelter. The overall length of the structure is 150 metres (492.1 ft). The ends of the structure will be sealed by vertical walls assembled around, but not supported by, the existing structures of the reactor building.

The NSC is to being constructed 180 metres (590 ft) west of unit four and slid into place.

 

The final phase of construction of the NSC involves the deconstruction of the unstable structures associated with the original Object Shelter. The goal of deconstruction has imposed significant requirements upon the load carrying capacity of the arches and foundation of the NSC, as these structures must carry the weight of not only the suspended cranes to be used in deconstruction, but also the loads of those cranes. The NSC design includes two bridge cranes suspended from the arches. These cranes travel east to west on common runways and each has a span of 84 metres (276 ft).

The following elements of the Object Shelter are planned for deconstruction:

 

Element

Quantity

Mass of each
(metric tons)

Length of each
(meters)

Length of each
(feet)

Southern roof flat panels

6

31

28.7

94.2

Southern roof flat panels

6

16

28.7

94.2

Southern hockey stick panels

12

38

25.5

83.7

Mammoth beam

1

127

70

229.7

Northern beam B1

1

65

55

180.4

Southern beam B1

1

65

55

180.4

Northern hockey stick panels

18

9

18

59.1

Eastern hockey stick panels

1

7.25

7

23.0

Light roof

6

21

36

118.1

Piping roof

27

20

36

118.1

Northern beam B2

1

57

40

131.2

Southern beam B2

1

57

40

131.2

TOTALS:

85 elements

2024 tons

N/A

N/A

Workers carry two dosimeters, one showing real-time exposure and the second recording information for the worker’s dose log. Workers have a daily and annual radiation exposure limit. Their dosimeter beeps if the limit is reached and the worker’s site access is cancelled.

Project status

The New Safe Confinement (NSC) was originally intended to be completed in 2005, but the project has gone through several delays. In June 2003 the projected completion date was slated for February 2008. In 2009, planned completion was projected for 2012; the same year, progress was made with stabilisation of the existing sarcophagus, which was then considered stable enough for another 15 years. On February 2010 the reported completion date of the NSC was pushed back to 2013, then summer 2015. The estimated completion date is now estimated to be 2017.

30 September 2013 – The international team of engineers completes the first stage of the protective sarcophagus.

 

24 July 2015 – The ‘western’ and ‘eastern’ halves of the arch of the New Safe Confinement (NSC) are joined together.

Joining of the western and eastern parts of the arch

Joining of the western and eastern parts of the arch (Image: ChNPP)

 

December 2015 – Drone footage of the New Safe Confinement released