Raising the roof over Chernobyl

1 April 2013



In November a 15-year plan to cover the damaged Chernobyl unit 4 reactor came one significant step closer to reality, when the first 5300t section of a huge airplane-hanger-style curved roof was lifted 22m off the ground. By 2015, a EUR1 billion 100-year confinement roof should be ready to slide over the damaged reactor, through whose existing temporary shelter roof rainwater continues to leak in, and radioactive dust continues to leak out. By Will Dalrymple


After two more big lifts later this year, the structure will be slid to the side so a second identical section can be built in its place. The two sections will be joined together and both slid over the unit 4 sarcophagus to form an environmentally sound gas-tight 'new safe confinement' structure that will house further deconstruction work.

The simplicity of the gross movements of the roof sections conceals hundreds of smaller tasks, not only in terms of present construction, but also past preparation. The lift was significant not only because it was the biggest test so far of a concept of building at a largely unprecedented scale, but also because it marked the end of the beginning; the end of the deliberations, the end of the fundraising and the preparations that began as far back as 1992, as the dust from the collapse of the Soviet Union began to settle, when major international activities began to develop a longer-term solution for the reactor.

I visited the site a few days after the lift, a guest of the European Bank of Reconstruction and Development, which administers the funds donated by 46 countries and organisations for the project.

The sarcophagus

The April 1986 explosion blew the roof completely off the reactor, and opened it up not only to rain and wind, which stirred up radiation and radioactive materials, but also opened up a way for radiation and radioactive materials to escape into the surrounding environment. A cover, or object shelter, was begun, under horrendous conditions, and completed 30 November 1986. Due to the difficulties of working in such a contaminated environment, structural elements for the roof could not be fastened to supports, but were simply laid on top of them. The design evolved as work progressed. Designers suggested that it would last 30 years. 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, there were holes in the roof structure in sections that workers could not reach (and still runoff from water and snow collect in lower reactor compartments, and is monitored and pumped out).

All four Chernobyl RBMK reactors were originally built in a row; unit 4 is located at its western end. To the south, they share an 800m-long turbine hall. Although unit 4 was a disaster zone, electricity generation had to return to normal on the rest of the site; Chernobyl 1-2, also RBMKs, restarted six months after the accident, and continued to operate for years; even next-door neighbour unit 3 started up in December 1987 and was not shut down until 15 December 2000.

The main source of radiation from unit 4 is the exploded core; some 200 tons of nuclear materials with a total radioactivity of about 20 MCi, most of which remains deep within the reactor building. Therefore it makes sense that radiation around unit 4 generally increases with proximity and with height above the ground. What is surprising is that exposure rates are actually highly variable; doses to the west are fairly low, but higher to the south. I understand that many areas of the reactor building remain to this day too contaminated to enter; in some other places where workers are permitted to go, they can accumulate a year's worth of dose in a matter of minutes. (Ukraine's maximum annual radiological dose rates for nuclear workers are quite stringent: 20 mSv/year, the same as France, compared with 50 mSv/year in the USA).

In 1997, an international panel of experts, including Ukrainians, developed the outlines of the Shelter Implementation Plan to stabilise the containment and make the site environmentally safe by building a monolithic shelter. Practical site activities started in 1998, and included repair of concrete beams supporting the roof and stabilisation of the vent chimney stack shared between the shelter and unit 3. In 2001, the roof was repaired to reduce water ingress into the sarcophagus and help protect nuclear materials.

From 2004-2006, a Russian-Ukrainian consortium including Atomstroyexport and Rovno NPP Construction Administration installed a tower of structural scaffolding to the west of the unit 4 object shelter to take the weight of the roof. This $50 million project was not intended so much to prepare for the new containment structure as to protect the sarcophagus from the possibility of collapse, since the stability of the walls it was resting on 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, installing an integrated monitoring system of fuel-containing materials, a radiological monitoring system including neutron flux (although the likelihood of a recriticality incident is now thought to be virtually non-existent), a structural monitoring system, dust suppression system, fire protection system, and a seismic monitoring system (although fortunately the site is not in an active seismic zone).

Meanwhile, the land immediately west of unit 4 has received concentrated attention for several years, as it 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 of French firms Vinci Construction Grands Projets (the lead partner) and Bouygues Travaux Publics. In 2010, the area was cleared of buildings and debris, and digging began for excavations for NSC foundations (final volume: 22,000 m3). There were unhappy surprises during this work: contaminated materials and equipment buried in the 1980s had to be unearthed and removed, carefully. The topsoil itself near the trenches remained contaminated and somewhat radioactive, so 55,000 m3 was dug up and removed; clean fill was trucked in and spread about half a metre deep. To provide a stable and safe surface for work, 40,000 m3 of concrete to a depth of about half a metre was poured (excluding the foundations). The end result is that these preparations have protected the work area. In other words, 300 metres from the site of the world's worst nuclear disaster, construction staff without special gear 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. (Closer to the object shelter, workers may need to work behind concrete or lead screens). The average worker dose rate, from January to autumn 2012, was 0.6 mSv/month.

Site workers wear two dosimeters; one records monthly dose received, the other measures actual radiation dose in real time (and is recorded twice daily). The actual dose measurements are compared with a calculated dose predicted based on their role in the project. During the planning stages, as part of an ALARA exercise, project engineers compared multiple construction/engineering solutions to overcome the problems posed by the goals of the project, and the chosen solution was assigned a so-called committed dose budget. Construction workers hired for the project go through a comprehensive medical examination ('BIOMED') that NOVARKA says fails one out of three applicants. All personnel working at the site are monitored annually; those working in higher-risk areas are monitored every three months. Medical monitoring is paid for by EBRD.

The roof construction project

The roof itself is more on the scale of a bridge than a building; when complete it will span 257m, and measure 164m across by 110m high. Starting in April 2012, steel components fabricated by Italian firm Cimolai were transported to an assembly site near the power plant, where they were bolted together before transport in large sections 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 (a total of five per end). 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 up the strands upwards over their 400mm stroke, and the roof section gradually rose upwards, portions of which eventually reached 22m high.

The section I saw suspended in the air on 27 November 2012 was in the process of being tied down on to interim supports until May 2013, by which time the worst of the winter weather will have passed. After a third and final lift toward the end of 2013 the arch will reach its final installation height of 110m.

The arches rest on horizontal concrete beams. These have 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 29,000-ton weight. When complete, the first roof section will be skidded to the east, toward the western face of unit 4, 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 4 reactor and its turbine building (although the west end of the turbine hall will protrude slightly, and at the eastern end the new safe confinement will only reach the intersection between units 3&4).

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 has been designed to move enough air to prevent the formation of condensation on the inside of the roof, but not enough to stir up the radioactive dust emitted from the object shelter. 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, dimensions 106m wide by 79m long, 85m above the ground. The highest point of the sarcophagus building is 74.5m; the crane system hook will hang just 2.5m above this.

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 located outside the new safe confinement next to the sarcophagus that will also house other control systems.

Of all of the many expectations for the new roof structure, one thing it will not do is shield the radiation emitted from the sarcophagus; it will confine the radioactive dust emitted from the fuel-containing material, but it is not expected to make the area immediately around the reactor any less radioactive. Or at least, not until the cause of all of that radiation, the melted fuel at the reactor's core, is removed, work that remains outside the scope of the shelter implementation project. In fact, during construction, the intention is to minimise any disturbance to those materials that would force site evacuation, or otherwise interfere with operations.

Although the EUR 1 billion new safe confinement itself is the purpose of the EUR 1.54 billion Shelter Implementation Project, it is just one of the structures built during the implementation of the entire project. As the four-reactor site has been surrounded by land gradually returning to wilderness over the past 25 years, modern infrastructure has been required to support the roof construction; new power lines, water, sewerage, a new road, offices, a new site entrance, a 1430-worker changing facility, a training centre, three concrete batching plants, a repair shop, warehousing, and a canteen have all been built. At the peak of construction, 1200 Ukrainian workers will be on site (2000 in total 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.

Who is in charge

The Ukrainian government's Exclusion Zone Administration controls the site management body State Specialised Enterprise Chernobyl NPP. The site is separate from the Ukrainian state electrical utility Energoatom, which manages the country's four other nuclear power plants. Chernobyl NPP is ultimately in charge of the SIP, which consists of 300 sub-projects including the new safe confinement. A division of Chernobyl NPP is the SIP project management unit, based at the Chernobyl worksite and also the nearby town of Slavutich, of about 200 staff (mostly Ukrainian). The SIP is funded by the Chernobyl Shelter Fund, which was set up in 1997 by Ukraine and the EU and G7 groups of nations, and is administered by the European Bank for Reconstruction and Development. Although the EBRD does not dictate how the SIP runs (that is the role of the Assembly of Donors, chaired by Hans Blix), 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. French nuclear utility EDF was part of that original team, but later pulled out. The EBRD also has strict criteria about methods of procurement, and closely monitors all the tenders and contracts proposed by the PMU for compliance with its terms; 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.

That means that the EBRD, which is based in London, with a local presence in Kiev, is the central external organising force in the SIP project. Its main job is to funnel money from donor countries to the project. A milestone in fundraising was achieved in June 2011, only a few months after the Fukushima disaster, and 25 years after the explosion, when an EBRD-organised donor conference managed to secure an extra EUR 550 million for the SIP and another Chernobyl project, the Nuclear Safety Account, raising the total budget of the two projects to about EUR 1.8 billion. I understand that the extra money was not needed because of cost overruns, but because detailed costings for the project could not be completed until detailed design is completed and major parts of the programme received regulatory approval. At a Chernobyl site press conference, I heard Vince Novak, director of the EBRD nuclear safety department, say that no more money was needed from donors. Towards the end of 2012, more than EUR 1 billion had been promised and mostly delivered by more than 40 countries; in addition, the EBRD itself has contributed EUR 325 million from the profits of its other work worldwide for the SIP and Nuclear Safety Account, which is financing an interim spent fuel store. EBRD has also been involved in other projects in the region, such as a 2011 EBRD/Euratom proposal for safety improvements at other Ukrainian nuclear power plants, which is due to be put for consideration at an EBRD board meeting in late February.

The Ukrainian regulator, 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 licencing packages corresponding to the sequence of the project, starting from groundworks and ending with commissioning the structure. In 2009 the concept design safety document was approved. Licencing package 5, 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 licensing high-level documents, but may also require further development of the technical specifications and work execution plans, some of which will require regulatory approval; for example, there were 19 sub-packages associated with LP5, and each must be concurred with before work related to it can start.

Like the SIP project management unit, the regulator is supported by technical support organisations, both domestic and foreign; they include the Ukrainian State Scientific and Technical Center for Nuclear and Radiation Safety, and also international consultancies Scientech, IPSN and GRS. Technical support bills are also paid by the EBRD. In January 2013, the regulator was scheduled to approve LP6, systems design for the safe confinement (including ventilation, the auxiliary building, power supply, monitoring and control), according to NOVARKA.

Looking forward

In November, the EBRD's Vince Novak said that the project to finish the SIP over unit 4 by 2015 was viable, but remained 'a very difficult challenge' to achieve on budget and on schedule, with no room for error. 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 scheduled to start in spring 2013 will be extending the skidding rails and their foundations right up to the western wall of the unit 4 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 that is scheduled for late 2013 is dismantling the old chimney stack, which will not fit under the new safe containment arch. A Russian-Ukrainian consortium has built a replacement slightly to the east. The iconic structure, whose red and white stripes have faded with time (and exposure), has had its external steel support framework reinforced. As the stack protrudes above the reactor, it is in a radiologically-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 strategically-calculated number that involves a tradeoff between speed and worker dose on one hand and feasibility of lift on the other). A large crane will lift cut sections to the ground, although workers will still have to be on the roof. During those lifting operations nonessential site personnel will be cleared as a precaution in case of an unexpected release of contamination.


Will Dalrymple, editor of Nuclear Engineering International visited the Chernobyl site in November 2012.

 


 

Western side of Chernobyl object shelter Western side of Chernobyl object shelter
Chernobyl unit 4 reactor model Chernobyl unit 4 reactor model
Radiation dose map Radiation dose map
The assembled roof section The assembled roof section
Central roof section Central roof section
Raising the roof section Raising the roof section
Central section lift Central section lift


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