The ‘sarcophagus’ that encased Unit 4 of the Chernobyl Nuclear Power Plant is a giant metal concrete and structure that was hastily 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.
The sarcophagus has now been encased within a new, larger and safer, structure called the “New Safe Containment“. The old structure, along with the damaged reactor building it covers, will be gradually disassembled as part of the ongoing decontamination of the Zone.
Reactor 4 following the explosion
The enormity of the challenge is clear.
Construction of the sarcophagus
The sarcophagus was built under extremely dangerous conditions. Radiation levels were extremely high and there was great pressure to complete it quickly.
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. 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 sarcophagus.
The sarcophagus contained over 60 bore holes to allow observation of the interior of the core. In places, the structure incorporated ventilation shafts to allow for some movement of material inside. Filtration systems were put in place to prevent radioactive material escaping through the shafts.
The construction process consisted of eight stages:
- Clearing and concreting the area surrounding 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
- construction of a high-rise buttress wall
- erection of supports and installation of a reactor compartment covering
- 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 original rusting sarcophagus.
At 63 metres tall, it was intended to stabilise the sarcophagus via a series of cantilevers that extend through the western buttress wall. The DSSS was put in place in an effort to prevent the wall of the reactor building and the roof of the shelter from collapsing. If this were to happen 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
One of the many threat’s within the sarcophagus 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 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.
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 began. Originally planned to be in place by 2005, the giant arch, built by the French consortium Novarka, was slid into place in 2017.
2013 roof 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 partial collapse was the result of sloppy repair work and ageing 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 was 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 the actual levels kept secret.