Roof of the world: sheltering Chernobyl

In November 2012, a 15-year plan to cover the damaged Chernobyl unit four nuclear reactor moved a step closer to reality, when the first 5,300t section of an airplane-hangar-style curved roof was lifted 22m off the ground.

After two more major lifts later this year, the structure will be slid to the side and a second, identical section built in its place. The two sections will be joined together and slid over the unit four sarcophagus to form the environmentally sound, gastight 'New Safe Confinement', which will house further deconstruction work. The roof covering the existing temporary shelter continues to let rainwater in and radioactive dust out.

The lift was significant, not only because it was the biggest test so far of the concept of building at a virtually unprecedented scale, but also because it marked the end of the beginning: the end of the deliberations, fundraising and preparations that began as far back as 1992, when the dust from the collapse of the Soviet Union began to settle and a longer-term solution for the Ukrainian reactor was developed, funded by 46 countries and organisations, and overseen by the European Bank of Reconstruction and Development (EBRD).

The sarcophagus

The explosion in April 1986 blew the roof completely off the reactor, opening it up to wind and rain, which stirred up radiation and radioactive materials, and also let radiation and radioactive materials escape into the surrounding environment. A cover, or object shelter, was begun, under horrendous conditions, and completed on 30 November.

Owing to the difficulties of working in such a contaminated environment, structural elements of the roof could not be fastened to supports, but were simply laid on top of them. Significant uncertainty about the state of internal supporting structures and debris remains to this day; much of the interior remains off-limits because of radiation. For more than a decade, workers could not reach holes in certain sections of the roof structure, and run-off from water and snow still collects in lower reactor compartments, and is monitored and pumped out.

All four Chernobyl RBMK reactors were originally built in a row; unit four is located at its western end, while to the south, they share an 800m-long turbine hall. Although unit four was a disaster zone, electricity generation had to return to normal on the rest of the site. Chernobyl reactor units one and two restarted six months after the accident and continued to operate for years; even the neighbouring unit three started up in December 1987 and was not shut down until 15 December 2000.

The main source of radiation from unit four is the exploded core, which contains 200t of nuclear materials with a total radioactivity of about 20MCi, most of which remains deep within the reactor building. Therefore, radiation around the unit generally increases with proximity and with height above the ground; but what is surprising is that exposure rates are actually highly variable: doses to the west are fairly low, but are higher to the south. Many areas of the reactor building remain too contaminated to enter; in other places, where entry is permitted, workers can accumulate a year's worth of dosage in a matter of minutes. Ukraine's maximum annual radiological dose rates for nuclear workers are quite stringent: 20mSv a year, the same as in France, compared with 50mSv a year in the US.

In 1997, an international panel of experts outlined the Shelter Implementation Plan to stabilise the containment and make the site environmentally safe by building a monolithic shelter. Practical site activities began in 1998, and included repair of concrete beams supporting the roof and stabilisation of the vent chimney stack shared between the shelter and unit three.

In 2001, the roof was repaired to reduce water ingress into the sarcophagus and help protect nuclear materials. From 2004-06, a Russian-Ukrainian consortium including Atomstroyexport and Rovno NPP Construction Administration installed a tower of structural scaffolding to the west of the unit four object shelter to take the weight of the roof. This $50 million project was intended to protect the sarcophagus from collapse, since the stability of the walls on which it rested was uncertain.

By 2008, 80% of the roof load had been shifted from the reactor walls to the external support structure. Other object shelter work carried out included patching the roof, installing structural supports inside the de-aerator, and installing an integrated monitoring system of fuel-containing materials, a radiological monitoring system including neutron flux, a structural monitoring system, dust suppression technology, a fire protection system and seismic monitoring equipment.

Meanwhile, the land immediately west of unit four was cleared and prepared to become the New Safe Confinement building site. The consortium NOVARKA won the tender for the construction project in 2007. It is a 50:50 joint venture involving a lead partner and French firms Vinci Construction Grands Projets and Bouygues Travaux Publics.

In 2010, the area was cleared of buildings and debris, and digging began for excavations for 22,000m3 foundations. The work was punctuated by unhappy surprises: contaminated materials and equipment buried in the 1980s had to be carefully unearthed and removed. The topsoil itself near the trenches remained contaminated and radioactive, so 55,000m3 was dug up and removed, and clean fill was trucked in and spread about 0.5m deep. To provide a stable and safe surface for work, 40,000m3 of concrete to a depth of about 0.5m was poured, excluding the foundations.

The end result is that these preparations have successfully protected the work area. In other words, just 300m from the site of the world's worst nuclear disaster, construction staff without special equipment can work without radiological restrictions. Although workers must carry dust masks with them, they only need to wear them in case of a radiation alert. The average worker dose rate, from January to autumn 2012, was 0.6mSv a month.

Site workers wear two dosimeters: one records the monthly radiation dose received; the other measures the actual dose in real time. The actual-dose measurements are compared with a dose calculated according to their role in the project. During the planning stages, as part of an 'as low as reasonably achievable' (ALARA) exercise, project engineers compared multiple construction/engineering solutions, and the chosen solution was assigned a so-called committed dose budget. Construction workers go through a comprehensive medical examination that NOVARKA says fails one out of three applicants. All personnel at the site are monitored annually; those working in higher-risk areas are checked every three months.

The roof construction project

When complete, the roof will span 257m, and measure 164m across and 110m high. Starting in April 2012, steel components fabricated by Italian firm Cimolai were bolted together before being transported to the assembly area. The upper segments of the roof were positioned in place first, four arches across, then braced together and clad. Two lifting towers were assembled between each arch. The 45m-tall towers were set up near the ends of the arches, 107m apart in the long dimension and 25m in the short. Site construction cranes lifted and placed shipping containers with 900t-capacity strand jacks on top of 45m-high lifting towers.

From 22-24 November 2012, the strand jacks completed hundreds of tensioning cycles, ratcheting the strands upwards over their 400mm stroke, and the roof section gradually rose, with portions eventually reaching 22m high.

The second lift of the eastern part of the arch began on 12 June 2013; after a final lift at the end of the year, the arch will reach a final installation height of 110m.

The arches rest on horizontal concrete beams with piled foundations, 1m-diameter cylindrical steel piles driven 25m into the ground. The roof does not rest on the tops of the concrete beams, but on a plane inclined inwards by 33°, to best counteract the forces caused by the arch's 29,000t weight. When complete, the first roof section will be skidded to the east, toward the western face of unit four, and a second identical roof section will be constructed in its place. Then, the first section will be skidded back west for permanent coupling with the second arch.

After arch assembly ends next year, system fit-out begins. Once the New Safe Confinement is complete, both arches will be skidded east in a three-day operation, up to and then over the unit four reactor and its turbine building.

The gap between the walls installed on the open ends of the roof arch and the buildings it covers will be sealed to form a contamination-confining environment. An internal ventilation system will move enough air to prevent condensation on the inside of the roof. Two 50t-capacity overhead travelling cranes from PaR Systems - tested before the roof is slid into position - will run on 100m-long rails on the flat interior ceiling of the roof, 85m above the ground. The highest point of the sarcophagus building is 74.5m.

The cranes will be able to dismantle unstable parts of the sarcophagus, piece by piece. They will be controlled remotely from an new shielded auxiliary building outside the New Safe Confinement next to the sarcophagus, which will also house other control systems.

The new roof structure will not shield the radiation emitted from the sarcophagus. It will confine the radioactive dust emitted from the fuel-containing material, but is not expected to make the area immediately around the reactor any less radioactive until the cause of that radiation (the melted fuel at the reactor's core) is removed, work that remains outside the scope of the project.

Although the €1 billion New Safe Confinement itself is the purpose of the €1.54 billion Shelter Implementation Project (SIP), it is just one of the structures built during the implementation of the entire project. Modern infrastructure has been required to support the roof construction; new power lines, water, sewerage, a new road, offices, a new site entrance, a changing facility for 1,430 workers, a training centre, three concrete batching plants, a repair shop, warehousing and a canteen have all been built. At the peak of construction, 2,000 workers will be on site in two alternating teams, managed by 200 expatriate employees. The town of Chernobyl, which was evacuated after the accident, now houses site workers in renovated apartments.

The major players

The Ukrainian Government's Exclusion Zone Administration controls the site-management body State Specialised Enterprise Chernobyl NPP (SSE ChNPP). This is ultimately in charge of the SIP, which consists of 300 subprojects including the New Safe Confinement. A division of SSE ChNPP is the 200-strong SIP project-management unit, based at the Chernobyl worksite and also the nearby town of Slavutich. The SIP is funded by the Chernobyl Shelter Fund set up in 1997 by Ukraine, and the EU and G7 groups of nations, and is administered by the EBRD.

The EBRD does not dictate how the SIP runs, but the terms of its funding do dictate certain important aspects of the project. First, the EBRD requires foreign leadership of the project-management unit. That takes the form of a small team of about 25 contractor employees from US construction firm Bechtel and Battelle Memorial Institute. The EBRD also has strict criteria about methods of procurement, and closely monitors all the tenders and contracts proposed by the PMU; it also manages compliance of environmental protection and public information work. The project is essentially financed as a cash business, in which the EBRD pays for completed contracts with donated cash.

A fundraising milestone was achieved in June 2011, when an EBRD-organised donor conference secured an extra €550 million for the SIP and another Chernobyl project, the Nuclear Safety Account, raising the total budget of the two projects to about €1.8 billion. Toward the end of 2012, more than €1 billion had been promised and mostly delivered by more than 40 countries; in addition, the EBRD itself has contributed €325 million from the profits of its other work for the SIP and the Nuclear Safety Account, which is financing an interim spent-fuel store.

The State Nuclear Regulatory Inspectorate of Ukraine (SNRIU) has reviewed plans for each stage of the SIP project before work has begun. The SIP project has been broken down into six licensing packages corresponding to the sequence of the project, starting from the groundworks and ending with the commissioning of the structure. In 2009, the 'concept design safety' document was approved. Licensing package five, which provided the design of the main structure and crane system beams, as well as foundation excavations in the assembly zone, was approved in November 2011.

Each package consists of the main high-level licensing documents, but may also require further development of the technical specifications and work execution plans, some of which will require regulatory approval.

Like the SIP project-management unit, the regulator is supported by technical support organisations, domestic and foreign; these include the Ukrainian State Scientific and Technical Center for Nuclear and Radiation Safety, and international consultancies Scientech, IPSN and GRS.

Looking forward

Vince Novak of the EBRD said that finishing the shelter over unit four by 2015 was viable, but remained 'a very difficult challenge' to achieve on budget and on schedule. Although the design is largely complete, elements of it remain under regulatory review, after which components still need to be procured, delivered and installed. Many elements of the project - its vast scale, its purpose - have never been attempted before.

A difficult job is extending the skidding rails and their foundations right up to the western wall of the unit four object shelter, an area of high radiological dose where violent earthworks could potentially destabilise the wall or the new supports. To reduce the impact of foundation drilling, NOVARKA is planning to use continuous-flight augers, in which wet concrete is pumped into the hole as the auger is withdrawn. The reinforcement cage is placed as soon as the auger is out.

Another tricky job is dismantling the old chimney stack. A Russian-Ukrainian consortium has built a replacement to the east. The iconic structure has had its external steel support framework reinforced. As the stack protrudes above the reactor, it is in a hazardous zone and is itself radioactive. Local Ukrainian company Uktransbud has been contracted to remove the 76m-tall, 9m-diameter stack. The plan now is to cut the 330t stainless steel chimney horizontally into seven sections. A large crane will lift cut sections to the ground. During those lifting operations, non-essential site personnel will be cleared as a precaution in case of an unexpected release of contamination.

This article first appeared in Nuclear Engineering International. To find out more, visit www.neimagazine.com.