Entombing the tragedy

A new 'sarcophagus' to protect the Chernobyl reactor's remains is on its way.

When a reactor at the Chernobyl atomic energy plant in Ukraine caught fire and exploded on 26 April 1986, sending a radioactive cloud across large parts of Europe, it produced the world's worst civil nuclear disaster. Huge areas, mostly in Ukraine, Belarus and Russia, but also other parts of Europe as far off as the UK, were contaminated.

A hastily constructed  concrete cover, officially known as the 'object shelter' and dubbed 'the sarcophagus', built to contain the remnants of the devastated reactor, left the prospect of yet another catastrophe dangling over Europe.

The sarcophagus was intended only as a temporary measure, but as wrangling over a more permanent solution continued, cracks emerged, and the deteriorating structure became a giant timebomb containing around 200t of radioactive fuel and irradiated materials. The sarcophagus itself was contaminated.

No contingency plans existed to deal with the 1986 eruption, and the Communist authorities used desperate ad hoc measures.  Helicopters dumped hundreds of tonnes of concrete, mixed with boron and other protective materials, to quell the blaze and dampen the effects of radiation.

The present sarcophagus was constructed within months of the disaster, with helicopters lifting slabs of concrete into place to shield the devastated reactor building. International donors have spent tens of millions of dollars to stabilise the sarcophagus, but a lasting solution has been elusive.

The donors contributed to the construction of a new shelter more than a decade ago, and the new sarcophagus should have been completed in 2003. But bitter arguments over the design and who was to take the lead in building the structure stalled the project. Ukrainian politicians and officials, who have an entirely warranted reputation for corruption and greed for hefty kickbacks from the massive construction budget, also slowed progress. A former Ukrainian minister was accused of confusing his personal finances with those of the funds for the shelter.

Acrimonious debate

In Ukraine there was debate - often acrimonious - about who should have the right to draw up terms for the tender and who should award the project. Ukrainian businesses demanded a bigger share of the work. Officials were lobbied furiously by Ukrainian and foreign construction companies for a chance to get a slice.

Ukranian responsibility for the project shifted confusingly from one ministry to another and even to non-government parties. Ukrainian authorities challenged the tender procedure in 2005 where French companies Vinci and Bouygues, of the 'Novarka', consortium won a tender over American company CH2M Hill. In 2006, the director general of the Chernobyl nuclear power plant claiming rights as the 'customer' tried to annul the results of the tender. The Ukrainian government later rescinded the decision.

Finally, there seems to be an agreement to commence work on a new shelter which will see one of the world's most ambitious engineering projects unfold. 

Last September, Ukraine signed a $505m contract with a French-led consortium Novarka for construction of a New Safe Confinement (NSC) made of steel to encase the existing 'object shelter' over the devastated fourth reactor. The giant vault is the largest scheme in a $1.4bn Shelter Implementation Plan incorporating various elements to make Chernobyl safe and including expenditure in past years to temporarily stabilise the sarcophagus.

The work will be financed by an international fund compris‑ ing 28 countries, including the G8 nations, managed by the London-based European Bank for Reconstruction and Development. The Novarka consortium, which designed and will build the NSC, includes the French companies Bouygues SA and Vinci SA, Britain's Nukem Ltd. as well as German and Ukrainian firms.

"By the end of the sarcophagus' lifespan, Chernobyl will not exist," said Vinci chairman Yves-Thibault de Silguy. "The end goal of the shield is to allow [Chernobyl's] total dismantling."

The EBRD and Ukrainian officials also signed a $200m contract with the American Holtec International Inc to decommission the power plant's three other reactors, which kept working until 2000, and to construct a storage facility for the spent nuclear fuel. A previous scheme was halted amidst rancour between the contractors and the Ukrainian government when it emerged the specifications were wrong for the type of radioactive waste. The contractors and the Ukrainian government blame each other for the blunder.

The planned NSC is a giant structure whose shimmering hemispherical steel silhouette, at 150m long, 257m wide and 105m tall, will dwarf everything else on the skyline. Not only is the project an extraordinary engineering challenge, but those building it will have to employ ingenuity and cutting edge science to overcome the dangers on one of the planet's most hazardous worksites.

Exclusion zone

When Chernobyl exploded, after a fatally misconceived experiment during which safety measures were disabled, the ruined plant released 100 times more radiation into the air than the combined fallout from the nuclear bombs at Hiroshima and Nagasaki. Environmental scientists say the 30km exclusion zone around the plant, emptied of inhabitants after the accident, will remain contaminated for tens of thousands of years.

Radiation levels in the exclusion zone vary enormously according to where radioactive debris fell, with some parts showing normal background radiation while others register 1,000 times higher levels.

However, some people have returned to their deserted villages with authorities turning a blind eye. Wildlife, mostly undisturbed by humans, has flourished.

The arched structure of the NSC, estimated at 20,000t, will be built some distance from the reactor, then fitted out and slid over the sarcophagus. The double-skin vault will be large enough to allow for the existing crumbling concrete sarcophagus and the wrecked reactor to be dismantled and for them and other debris to be safely entombed for at least a century. The project is scheduled for completion within five years. According to the designers, the word 'confinement' features in the structure's name to emphasize that the primary role of the NSC is the confinement of solid radioactive waste rather than the 'containment' of radioactive gases.

The NSC is also intended to permit work by remote control or specially trained teams inside it to eventually dismantle and safely store all the dangerous material within the original sarcophagus.

Large prefabricated tubular steel portions of the ark will be brought to Chernobyl and assembled in two halves. The final operation to lock the two parts together will be performed within 24 hours by sliding them into place on a specially constructed railway.

The NSC was designed to contain the radioactive materials at the site. It is meant to prevent further environmental contamination by reducing corrosion and weathering of the existing shelter and the remains of the reactor building which could lead to their collapse, and to enable the safe deconstruction of unstable structures such as the sarcophagus roof. If such a collapse happened, it would produce a large amount of radioactive dust.

The existing sarcophagus is precariously unstable. It is primarily supported by the damaged remains of the Unit 4 Reactor building. These are considered to be structurally unsound because of weaknesses caused by the explosion. Three beams, including a massive 'mammoth beam' running east to west, support the roof and panels. The roof is made of one-metre diameter steel pipes, laid horizontally north to south, and steel panels at an angle also running north to south.

The shelter's south wall is formed by these steel roof panels which transform from an angle of around 115 degrees to vertical. The east wall is formed by the reactor building and the north wall - by the reactor building and concrete slabs. The west wall is made of concrete slabs reinforced by buttresses.

To carry out its double role of containing and also dismantling the ruined reactor, the structure has been designed to hold the weight of four 155t bridge cranes, suspended from the arches with spans of 42m, with telescoping arms extending the range to 75m. The cranes will each be able to lift 100t in two carriages. They will also have radiation-shielded carriages to transport personnel deep into the bowels of the new sarcophagus.

"No one should underestimate the challenge of constructing and positioning the NSC," commented Len Green, CEng MEI, a UK-based independent consultant who has worked on nuclear-related projects in Ukraine and Eastern Europe over periods spanning ten years. "What is being planned is to move in a single day what has been described as the 'largest moveable structure in the history of mankind'. Its successful completion will, by any standards, be a remarkable achievement and testament to those who put engineering excellence above the political  wangling."

The NSC will be assembled in the following steps:

  1. Stabilisation of the 'object shelter' in order to prevent collapse during construction;
  2. Excavation and construction of foundation;
  3. Assembly of first and second arches to form Bay 1, installation of east wall on Arch 1;
  4. Bay 1 will be slid east to accommodate the construction of Arch 3 and Bay 2;
  5. Subsequent sliding of the complete structure and adding of arches and bays to complete the structure;
  6. Installation of cranes and large maintenance equipment;
  7. Installation of the west wall;
  8. Final slide into place over Unit 4;
  9. Construction of the fragmentation, decontamination, and auxiliary buildings.

As each bay is completed, infrastructure equipment including that for ventilation systems, radiation monitoring, plumbing, and electrical will
be installed.

Positioning of the NSC

The New Safe Confinement (NSC) is to be constructed 180m west of Unit 4 and slid into place. The actual sliding of the structure along foundation rails is a difficult process. The system to be used in construction of the NSC is derived from civilian bridge launching and bridge cantilever methods.

Two options were initially considered for moving the structure: hydraulic jacks to push the structure forward, or pulling the structure with large, multi-stranded steel cables. However, the first option would require the relocation of the hydraulic jacks after each push. This relocation process would necessitate more worker interaction with the system and a greater worker exposure to radiation. The second option was chosen because it would expose workers to a lower radiation dose, and would move the structure into its final position in just less than 24 hours.

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