New safe century6 April 2006
Chernobyl 4’s object shelter is now at risk of collapse. In March 2004, the European Bank for Reconstruction and Development invited tenders for detailed design and construction for a new enclosure. By Eric Schmieman, Matthew Wrona, Philippe Convert and Daniel Couch
Within seven months of the April 1986 accident at Chernobyl, the damaged unit 4 was enclosed within a massive steel enclosure. The structure was named the ‘sarcophagus’ by the international press, but it is officially known in Ukraine as the object shelter and commonly referred to as simply the shelter.
Computer generated image of the new safe confinement structure, which will enclose the damaged Chernobyl 4 and the deteriorating object shelter ('sarcophagus')
The worst accident in the history of civil nuclear energy left a ruined reactor rapidly spreading radioactive contamination across the former Soviet Union. Workers rushed in to extinguish the fires and to stop the spread of contamination by enclosing the damaged unit.
In an heroic effort, workers quickly constructed a steel shelter over the ruins of the unit 4 reactor and auxiliary buildings. This shelter served to limit the continued escape of radioactive material into the environment and protect workers in adjacent areas from radiation exposure. Built directly over, and sometimes on, the highly contaminated accident debris, construction of the shelter was an extremely difficult task. High radiation levels required remote construction techniques involving long-reach cranes and shielded vehicles.
A vast construction force was employed because each person could only work for a very limited time in the construction area without accumulating high radiation doses. Building to conventional architectural standards was impossible.
Although the radiological safety goals of the shelter were met in the short term, the passage of time revealed severe problems. The walls of the object shelter were weakening; load-bearing structures built on top of debris, imprecise construction methods, and the ageing of joints threatened the long-term structural integrity of the shelter. Critical components could not be monitored because of the unstable structure and high radiation areas.
Water, condensation, and corrosion continue to take their toll on the shelter. Rainwater leaked through cracks and wall openings, speeding up deterioration of structural members. A mobile radioactive ‘soup’ formed in the basement from a combination of water, rust, and the remnants of the reactor core.
Radioactive dust escaping through holes in the shelter posed a hazard to workers and the surrounding area. Over 1000m2 of holes in the shelter allow birds and other animals to come and go freely, potentially spreading contamination. Recent maintenance activities have reduced these openings by an order of magnitude, but condensation and high humidity inside the shelter continue to corrode load-bearing steel components.
In 1997, the US Department of Energy and the European Commission Technical Assistance to the Commonwealth of Independent States (TACIS) programme sponsored a group of international experts to prepare a study known as the Shelter Implementation Plan (SIP). The plan’s intent was to address the problems associated with making the shelter environmentally safe.
The plan described measures to protect workers and the environment, stabilise the shelter, and construct a new confinement building within which future environmental remediation activities could be safely performed. International support, including funding, was necessary to achieve the plan’s goal.
The course of action was straightforward: reduce the potential for accidental collapse of the shelter; reduce consequences of an accidental collapse of the shelter; improve nuclear safety; improve worker and environmental safety; and develop a long term strategy for making the site environmentally safe.
The international shelter project, including the group of seven most industrialised nations (the G7), the European Union, Russia and Ukraine, immediately set up the Chernobyl shelter fund. The group arranged for the European Bank for Reconstruction and Development (EBRD) to manage the work, establish a management programme, and provide funding.
Estimating costs at a little less than $800 million, the bank started to solicit donations. Pledging began with a 1997 conference held in the USA. The Ukrainian government pledged $50 million in completed work. Between them, the G7, representatives of 22 other countries and the Ukrainian government pledged sufficient funds to begin work.
With funding on track, the bank invited competitive tenders for a contractor to operate the SIP project management unit (SIP-PMU). From the qualified proposals, the EBRD selected a consortium of Battelle Memorial Institute, Bechtel and Electricité de France (EdF) to operate the SIP-PMU. Working closely with Chernobyl plant managers and staff, the consortium members provided an integrated PMU team for technical oversight, operations and business development, and licensing and regulatory strategies.
The initial work managed by the PMU, known as the early biddable projects, was dominated by emergency repair work and investigations to develop the technical information needed to support later engineering design activities. The SIP-PMU provided contract management, technical evaluation of contractor products, and preparation of the overall SIP cost and schedule estimates. Bechtel provided overall business and operations management. EdF provided technical management, and Battelle provided safety and regulatory management.
The team’s first technical task was to place contracts for 17 urgently needed early biddable projects to determine technical solutions for additional stabilisation, confinement, and various engineered and management support systems.
The immediate concern was addressing structural stability issues such as strengthening pillars and stabilising the 45m ventilation stack. Other projects included worker and environmental protection, safety operations, and monitoring of the fuel mass.
After these preliminary studies, the SIP was divided into three main areas: preparatory work, stabilisation, and the design and building of a ‘new safe confinement’ (NSC) to enclose the damaged unit 4 and the deteriorating object shelter.
Several measures are underway to stabilise the shelter. The largest task consists of strengthening the load-bearing supports by adding a space frame superstructure at the shelter’s western wall. A Russian and Ukrainian consortium is responsible for the design and construction of the majority of current stabilisation projects.
100 YEARS OF SAFE CONFINEMENT
A freestanding arch was selected as the most suitable concept for the NSC design. The structure was designed to provide protection from weather and condensation and to minimise further corrosion. Equipment such as cranes could be remotely operated within the arch for orderly deconstruction of unstable structures. Radioactive dust resulting from deconstruction activities or from a possible collapse would be confined within the arch structure which is designed for a 100-year lifespan. When completed, the NSC will be one of the largest ‘free volume’ spaces in the world.
In enormous buildings of this type, interior weather patterns such as fog and rain can develop. To address this concern, the conceptual design process included modelling the interior environment to design a ventilation system that could move sufficient air to prevent condensation, while allowing radioactive dust to settle, and minimising the operating power of an active ventilation system that would have to operate for 100 years.
The arch structure was designed with an internal height of 98.3m, a 12m distance between the top and bottom chord centrelines, an internal span of 245m, and an external span of 270m. The dimensions of the arch were optimised to accomplish deconstruction activities under the arch.
The structure will be 150m long with plane vertical walls at either end. The 150m length comprises 12 bays and 13 arch frames that are placed every 12.5m from the centre. The end walls will be built around, but not supported by, the existing structures of the reactor building. The end walls, where possible, will be laterally supported at the base to reduce the need for cantilevered sections.
The arch will be constructed of tubular steel section members and have an external cladding of three-layer sandwiched panels. Similar panels are to be used for external cladding of the end walls and a cladding over the internal chord of the arch will prevent accumulation of contaminated dust on the framing members. The dead air space between the internal and external surfaces will protect the structural elements and provide some insulation for controlling the environment inside the arch.
Important decision criteria during the selection of arch shape included:
- Minimising the mass of steel.
- Minimising the horizontal reaction forces.
- Optimising constructability.
- Arch shape (circular or parabolic).
- Thickness (constant or tapered).
- Attachment to the foundation (hinged connection on the outer arch chord attachment or hinged connection and simple support on the outer and inner arch chord attachment).
To evaluate competing variants on the basis of constructability, arch member sizes were constrained to provide similar diameters along the chords. Also, the bracing and the post members were grouped to minimise sizes. Parabolic and tapered arches were eliminated because of the large amount of variation in member lengths of braces and struts. In addition, the parabolic arch would be 6m higher than the circular arch, which increases wind loading and siding loads significantly. A circular arch allows all roof panels to be identical, improving constructability.
A single variant was selected for further design development because it minimised materials consumption, minimised horizontal reaction forces on the foundation and simplified fabrication and construction. The selection was confirmed by independent calculations.
Seismic, wind, and snow loads for the structure were analysed during the conceptual design phase. Seismic loads used were equivalent static loads based on a simplified approach from the 1997 Uniform Building Code (UBC) which indicates that Ukraine is in Seismic Zone 0. The Seismic Zone 1 and soil type parameters were conservatively selected to establish the horizontal acceleration design load. Using an importance factor of 1.5, the maximum horizontal acceleration was determined to be 0.08g. Detailed design will include dynamic seismic analysis.
Wind loads govern arch structural design and were determined using Ukrainian national standards. Snow loads and thermal loads were based on site meteorological data and Ukrainian design normatives.
After the conceptual design had started, the standard for tornado loads was revised by the State Committee of Construction and Architecture of Ukraine (Derzhbud), increasing the load from a Class F-1.5 to a Class F-3.0 tornado. The conceptual design was completed based on original design loads, and the effect of a Class F-3.0 tornado was analysed as a beyond design basis accident by the Kiev National University of Construction and Architecture.
Building the arch
The arch-shaped structure will be constructed at a distance to minimise radiation exposure to construction workers. Large parts of the arch sections will be shop fabricated and transported to the assembly area.
Two methods of arch section assembly were investigated during conceptual design: using a mobile ‘goliath’ crane and using mobile strand jacks. Strand jacks were found to be preferable to a crane because of reduced worker dose and effects from decontamination efforts. Worker dose is reduced because the dose rate in the construction area increases with altitude. The strand jack construction methodology allows more arch structural elements to be assembled at ground level and fewer joints to be made at altitude.
Arch sections would be individually erected and connected to form an arch bay. The eastern wall, which is a skyline cutout of the shelter shape, is attached to the first bay. Once three adjacent arch bays have been constructed and connected, the partially constructed arch would be slid to the east sufficiently to allow construction of adjacent bays.
Plumbing, lighting, electrical, and ventilation systems would be installed in each bay during erection. The final bay erection would also involve the installation of the western wall – another cutout of the existing shelter shape. Cranes would then be installed in the construction before the entire building is slid into its final position.
The initial member type for the arch was a hollow tubular section. The arch was also analysed assuming rolled wide flange sections of high-strength steel with yield strength of approximately 3500kg/cm2 and pipe sections of both high-strength steel and low-strength steel with yield strength of 2500kg/cm2. Even with the high-strength steel, the weight of the arch is considerably greater when constructed of wide flange sections than when constructed of low-strength tube. Low-strength steel pipe was used to provide a conservative capital cost estimate.
Construction of the immense building, which should be visible from the International Space Station, is expected to take about four years.
A unique aspect of foundation construction at the Chernobyl site is the radioactive contamination of excavated material. Because the upper layers of the soil are contaminated, special measures are necessary to protect workers and to protect against further contamination of groundwater. Both concerns favour construction methodologies that minimise excavation.
Detailed calculations were performed for several types of foundation to support the arch at its final location. A foundation system consisting of three lines of two 4.5x1.0m foundation panels 21m long, and a 4m-high pile cap with its top at elevation 120m was found to minimise cost, volume of cuts in active layers, dose uptake, and risk to the environment. This system provides foundation panels every 12.5m (the distance between two consecutive arch supports) along its length.
The foundation designed for the arch assembly and sliding areas is slightly different from that for the final resting place of the arch. This foundation consists of three lines of two 4.5x1.0m foundation panels, 18.5m long, and, a 3m-thick pile cap with its top elevation at 120m along the assembly area. The foundation panels are embedded into the layers of floodplain, gully, beach, riverbed, and washout facilities. The foundation panel toe rests at elevation 95m. The pile toe is into the middle quaternary alluvial deposits.
For piles construction, conceptual design recommends excavation of the first 0.3m by rope-operated grabs followed by hydraulically operated clamshells under bentonite slurry protection. The thixotropic properties of the support fluid can be designed to form a filter cake, preventing radioactive substance transfer by clogging the surrounding soil column. Early removal of the most contaminated upper soil layers to reduce risk to construction workers will also be evaluated.
Sliding into place
The arch sliding system is derived from the incremental bridge launching and bridge cantilever methods. Several technologies developed for these civil applications were found to be suitable for sliding the arch into place. Two movement options are available:
- Pulling with post-tensioned bundled multi-strand tendons attached at one end by dead-end anchorages to the arch base, and at the other end by active anchorages to a pulling jack supporting structure.
- Pushing the structure by direct action on the lifting jacks on a spreader beam.
Pulling jacks would be installed at the east end of the sliding way on a pulling jack supporting structure. A 2.5m-wide by 2m-high lateral guiding corbel was found sufficient to resist horizontal forces during sliding. Pulling the constructed arch from its assembly area to its final resting place is expected to take slightly less than 24 hours.
Inside the confinement area, unstable shelter components to be deconstructed include fragments of steel structures and equipment, reinforced concrete structures, roof panels, pipes, trusses, beams, and materials added after the explosion to mitigate accident consequences.
The structures scheduled first for deconstruction after completion of the NSC will be decontaminated to the maximum extent practical. These are primarily components installed after the accident, so while they may be heavily contaminated, they have not been exposed to any neutron fluence and are not activated. Therefore, it is reasonable to expect most of these components to be classified as exempted wastes after decontamination.
Deconstruction scenarios were developed to help plan the effort and establish the design requirements for the confinement, such as the laydown area, swing radius, and transportation corridors. These set functional parameters that were captured in design criteria for the facility such as the basic arch dimensions. Because cranes are suspended from the arch structure, their use imposes significant loads on the arch structure and on foundation design.
Four bridge cranes are required for deconstruction, each with a span of 42m and a lifting capacity of 50t, suspended from the arch with an east-west travel. The total weight of each bridge is 155t.
Cranes are specified with carriages that can move from one crane to another. Three types of carriages have been designed:
- Two ‘classic’ lifting carriages, each with a 50t capacity.
- One secure lifting carriage for shielded transportation of personnel, with a 40t capacity.
- One carriage with a telescoping arm (75m extension and fitted with a variety of end effectors useful for deconstruction).
The decontamination, fragmentation, and packaging facilities extending through the west wall of the arch have a footprint of 4350m2. The covered buffer storage provided under the arch is 1815m2. Consideration was given to using the cascade wall as storage area (3658m2).
Currently available technologies were evaluated for use in deconstruction activities. Selection criteria included minimisation of individual and collective doses and minimisation of cost of operation. The most important factors were the amount of secondary waste generated, the feasibility of remote operation, the cutting rate, fire safety, and capital and operating costs.
Selected fragmentation technologies included plasma arc cutting torches for metal beams, diamond circular cutting wheels for medium-sized concrete elements, and diamond wire cutting for large elements. The greatest amount of radioactive contamination on deconstructed material will be loose surface contamination (dust). Selected decontamination technologies include vacuums with HEPA exhaust filters for general use, grit blasting for metallic elements, and scarifying for concrete elements.
The Chernobyl SIP has made excellent progress in spite of the project’s complexity and significant technical challenges. Preparatory work is nearly complete, and stabilisation is well underway. The Cabinet of Ministers of Ukraine approved the conceptual design for the NSC, and the international tender process for detailed design, procurement, and construction of the NSC is nearly complete. Mobilisation of the contractors’ forces will begin immediately after contract awards, and major construction activities for the NSC are expected to begin in the spring of 2007.
The NSC project demonstrates successful international cooperation in resolving a complex technical problem and although significant work remains, once completed, the arch will provide secure, stable confinement of the radioactive debris left as a legacy of the Chernobyl accident.
Based on presentations given by the authors at the American Nuclear Society topical meeting on Decontamination, Decommissioning and Reutilization, Denver, Colorado, 7-11 August 2005 and on Conceptual Design of the Chernobyl New Safe Confinement – an Overview in Canadian Nuclear Society Bulletin, Bulletin De La Société Nucléaire Canadienne, 2:2:9-16 (2004). Eric Schmieman, Daniel Couch, Battelle Memorial Institute, PO Box 999, M/S K1-64, Richland, WA 99352, USA. Matthew Wrona, Bechtel International Systems, 50 Beal Street, San Francisco, CA 94105, USA. Philippe Convert, Electricité de France, Chernobyl SIP PMU, 7/1 Gvardeyskaya Divizya St., Slavutich, Kiev Region, Ukraine
|Worker and environmental safety|
Major efforts have been undertaken to clear and refurbish the original power station constructors’ area near the shelter where new construction activities will be based. Electrical power, water, fire mains, domestic and active waste water systems have been upgraded.