Chernobyl New Safe Confinement

Existing shelter

The turtle shelter, formally referred to as the Object Shelter and often called the sarcophagus, was constructed between May and November 1986 as an emergency measure to contain the radioactive materials within reactor unit 4 at the Chernobyl nuclear power plant (ChNPP). The shelter was constructed under extreme conditions, with very high levels of radiation, and under extreme time constraints. The Object Shelter was moderately successful in containing radioactive contamination and providing for post-accident monitoring of the destroyed nuclear reactor unit.

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 (3 ft 3 in) 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.

The Object Shelter was never intended to be a permanent containment structure, despite rumors to the contrary. Its continued deterioration has increased the risk of its radioactive inventory leaking out into the environment. Upgrades to the site made sometime prior to 2007 include pathways for roof access, roof repairs, the installation of a dust control system, and the installation of a long-term monitoring system. However, substantial upgrade or replacement of the shelter will be necessary in the near future in order to continue containing the radioactive remains of ChNPP reactor 4. It has been estimated that up to 95% of the original radioactive inventory of reactor unit 4 still remains inside the ruins of the reactor building.

International competition

In 1992, Ukraine's government held an international competition for proposals to replace the hastily constructed sarcophagus.

In the autumn of 1992, Design Group Partnership (DGP) of Manchester was invited to assist the Atomic Energy Authority (AEA) for the UK's submission for the international competition organized by the Ukrainian government.

DGP's senior management were assembled to generate a solution. David Haslewood suggested an arch, built off site, and then slid over the existing Russian built sarcophagus because:

1. Off site construction would limit the radiation doses of the construction workers to a minimum.
2. An arch fitted snugly over the damaged reactor (minus its chimney).
3. An arch would be easier to slide rather than a square box.

Of the 394 entries, only the British submission proposed a sliding arch approach. The outcome of the competition was no overall winner, but the French submission came 2nd with the UK and German proposals coming joint 3rd.

Since the competition the world has adopted the sliding arch concept but it now has members supporting a dismantling crane which was not a requirement for the 1992 competition.

Subsequently, a pan-European study (the TACIS programme) re-examined the proposals of the top three finalists of the competition. The study selected the sliding arch proposal as the best solution for their further investigations and recommendations, primarily to reduce the chance of the construction workers receiving a harmful dose of radiation.

On 17 September 2007 Vinci Construction Grands Projets and Bouygues Travaux Publics announced that they won the contract to build the New Safe Confinement as 50/50 partners of a French consortium named Novarka. The original 432 million euros contract comprises the design and construction of the NSC and planned to employ 900 people at its peak.

Design goals

The New Safe Confinement (NSC) was designed with several design goals in mind:

Structural design

The NSC design is an arch-shaped steel structure with an internal height of 92.5 metres (303.5 ft), and a 12-metre (39.4 ft) distance between the centers of the upper and lower arch chords. The internal span of the arch is to be 245 metres (803.8 ft), and the external span is to be 270 metres (885.83 ft). The dimensions of the arch were determined based upon the need to operate equipment inside the new shelter and decommission the existing shelter. The overall length of the structure is 150 metres (492.1 ft), consisting of 13 arches assembled 12.5 metres (41 ft) apart to form 12 bays. The ends of the structure will be sealed by vertical walls assembled around, but not supported by, the existing structures of the reactor building.

The arches are constructed of tubular steel members, and are externally clad with three layer sandwich panels. These external panels will also be used on the end walls of the structure. Internally, each arch will be covered in polycarbonate (Lexan) to prevent the accumulation of radioactive particles on the frame members themselves.

Large parts of the arches will be shop fabricated and transported to the assembly site, 180 metres (590 ft) west of reactor unit 4. Each of the steel tubes will be high-strength steel in order to reduce cost and assembly weight. The steel used in construction of the tubular members will have a yield strength of no less than 2,500 kg/cm² (250 MPa; 36,000 psi).

Warm, dry air will be circulated in the gap between inner and outer roof sections in order to prevent corrosion and to stop drops of condensation falling into the interior.

Foundation design

The foundations of the NSC must meet the primary design requirements:

The site of the NSC itself is slightly sloped, ranging in elevation from +117.5 metres (385 ft) on the eastern side to +144 metres (472 ft) on the western side. The foundation must account for this difference without extensive site leveling.

The ground upon which the foundation must be built is unique in that it contains a "technogenic layer" just below the surface that is approximately 2.5 to 3 metres (8 to 10 ft) in overall depth. The technogenic layer was created by radioactive contamination from the accident and consists of various materials including nuclear material, stone, sand, loamy sands, concrete (probably unreinforced), and construction wastes. It is considered unfeasible to determine the geotechnical characteristics of this soil layer. As a result of this, the load-bearing properties of the technogenic layer are unassumed by the design of the foundation.

The water table at ChNPP fluctuates from +109.9 metres (360.6 ft) on average in December to +110.7 metres (363.2 ft) on average in May.

Several options were considered for the foundation design for the NSC, and the final design was specified as consisting of three lines of two 4.50-by-1.00-metre (14.76 by 3.28 ft) foundation panels 21 metres (68.9 ft) in length and a 4-metre (13.1 ft) high pile cap that reaches to a height of +118 metres (387 ft) of elevation. This option was selected in order to minimize the cost of the foundation, the number of cuts into radioactive soil layers, dose uptake of workers, and risk to the environment from further contamination. The foundation differs slightly between the area in which the NSC will be constructed and the final resting area around unit 4.

Special consideration is necessary for the excavation required for foundation construction due to the high level of radioactivity found in the upper layers of soil. The use of rope operated grabs for the first 0.3 metres (11.8 in) of pile excavation has been recommended for the Chernobyl site by the conceptual designers of the NSC. This will reduce the direct exposure of workers to the most contaminated sections of the soil. Deeper excavation for the foundation piles will be accomplished using hydraulic clam shells operated under bentonite slurry protection.

The foundation is designed to withstand horizontal acceleration structural loads of up to 0.08 g, as well as to withstand a tornado of up to Class F-1.5. However, the design requirement for the structure was later raised to withstand a Class F-3.0 tornado, resulting in a beyond-design-basis analysis that was carried out independently to evaluate the effects of a Class F-3.0 tornado upon the structure.

Assembly process

The NSC was assembled in the following steps:

1. Stabilization 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 (29 November 2016).
9. Deconstruction of the fragmentation, decontamination, and auxiliary buildings

This process of assembly was deemed advantageous because it took advantage of the designed mobility of the structure to maximize the distance between workers and the reactor building, thereby minimizing their uptake dosage of radiation.

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

Positioning

The NSC was constructed 180 metres (590 ft) west of unit four and slid into place. The actual sliding of the structure along foundation rails was a difficult process. The system used in construction of the NSC 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. 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 initially chosen because it would expose workers to a lower radiation dose, and would have moved the structure into its final position in less than 24 hours. However, the structure was moved using hydraulic jacks making the 327-meter move starting on 14 November 2016 and ending on 29 November.

Demolition of existing structures

The final phase of construction of the NSC involves the demolition of the unstable structures associated with the original Object Shelter ("Sarcophagus"). The goal of demolition has imposed significant requirements upon the load carrying capacity of the arches and foundation of the NSC, as these structures must carry the weight of not only the suspended cranes to be used in demolition, but also the loads of those cranes.

Demolition equipment

The NSC design includes two bridge cranes suspended from the arches. These cranes travel east to west on common runways and each has a span of 84 metres (276 ft).

Each crane can carry a variety of interchangeable carriages. Three types of carriages have been designed for the NSC:

The cranes' carriage interchangeability allows the rotation of the largest members to be demolished, reducing the overall size of the NSC by approximately one arch bay.

After the members to be demolished are removed by crane they must be fragmented into pieces small enough to decontaminate. It is expected that the primary contamination of most demolished elements will be loose surface contamination (mostly dust) and can largely be removed. Decontamination will take place using vacuum cleaners with HEPA filters, grit blasting (for steel elements), and scarifying (for concrete elements). Once decontaminated to the maximum extent practical, pieces will be further fragmented for eventual disposal. Fragmentation tools include plasma arc cutting torches, diamond circular cutting wheels, and diamond wire cutting. The tools selected for the demolition process were selected upon the basis of a number of factors, including minimization of individual and collective radiation exposure, the amount of secondary waste generated, the feasibility of remote operation, the cutting efficiency, fire safety, capital cost and operating costs.

The exact methods for disposing of wastes generated by the demolition process have not yet been determined, and may include on-site burial outside the NSC for low-level waste, and long term storage inside the NSC for medium and high level wastes. At this time no policy has been made as to the disposal and processing of fuel containing materials.

Elements to be demolished

The following elements of the Object Shelter are planned for demolition:

Types of materials to be demolished

The elements that are to be demolished fall into several broad material types:

Waste storage

Near to the Chernobyl site, the Vektor Radioactive Waste Storage Facility is being built, consisting of the Industrial Complex for Solid Radwaste Management (ICSRM), a nuclear waste storage site. It is being constructed by Nukem Technologies, a German nuclear decommissioning company which is a subsidiary of the Russian Atomstroyexport. This storage is reported to be able to contain 75,000 cubic meters. The storage is both for (temporary) high level waste as well as low and intermediate level waste storage.

Planned schedule

The New Safe Confinement (NSC) was originally intended to be completed in 2005, but the project has suffered lengthy delays. In June 2003 the projected completion date was slated for February 2008, according to the following schedule:

(The actual sliding took place in the second half of November, 2016.)

In 2009, planned completion was projected for 2012; the same year, progress was made with stabilization of the existing sarcophagus, which was then considered stable enough for another 15 years. On February 2010 the reported completion date of the NSC was pushed back to 2013. As of April 2011, the estimated completion date has been updated to Summer 2015. In November 2016, the planned completion date was stated as November 2017.

The project has involved workers and specialists from at least 24 countries in addition to Ukraine.

Progress

Responsible organizations

The European Bank for Reconstruction and Development (EBRD) is responsible for managing the Shelter Implementation Plan, including overseeing the construction of the New Safe Confinement.

Worker safety and radioactive exposure

Radioactive dust in the shelter is monitored by hundreds of sensors. Workers in the 'local zone' carry two dosimeters, one showing real-time exposure and the second recording information for the worker's dose log.

Workers have a daily and annual radiation exposure limit. Their dosimeter beeps if the limit is reached and the worker's site access is cancelled. The annual limit (20 millisieverts) may be reached by spending 12 minutes above the roof of the 1986 sarcophagus, or a few hours around its chimney.