New Safe Confinement
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The New Safe Confinement (NSC or New Shelter) is the structure intended to contain the nuclear reactor at Chernobyl, Ukraine, which was destroyed by a catastrophic nuclear accident in 1986. (See Chernobyl disaster.) The idea is to prevent the reactor wreck from leaking radioactive material into the environment.
A part of the Shelter Implementation Plan funded by the Chernobyl Shelter Fund, the NSC is designed to contain the radioactive remains of Chernobyl Unit 4 for the next 100 years. It is intended to replace the present containment structure, also known as the sarcophagus or Object Shelter, that was hastily constructed after a "beyond design-basis accident" destroyed reactor 4 on April 26, 1986.
The picture shows a concept rendering of what the New Safe Confinement will look like when it is completed. Note that the flat end wall is shown transparent so that buildings inside may be seen. The turbine hall protrudes from the NSC at the right of the image. The buildings protruding from the NSC at the left of the image are maintenance buildings that are being constructed as part of the NSC.
It may seem odd that the word "confinement" is used to refer to the new shelter rather than the traditional "containment" associated with nuclear power. According to the designers, this is intentional and was done with the intent to emphasize the difference between the "containment" of radioactive gases that is the primary focus of most Reactor Containment Buildings, and the "confinement" of solid radioactive waste that is the primary purpose of the New Safe Confinement.
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[edit] The existing shelter at Chernobyl
The existing shelter, formally referred to as the Object Shelter and often referred to in the vernacular as the sarcophagus, was constructed between May and November of 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 meter 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 115 degrees to become 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 and 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. Recent upgrades to the site 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 remain inside the ruins of the reactor building.
[edit] The Design and Construction of the NSC
[edit] Design goals of the NSC
The New Safe Confinement (NSC) was designed with several design goals in mind:
- Convert the destroyed ChNPP Unit 4 into an environmentally safe system (i.e. contain the radioactive materials at the site to prevent further environmental contamination)
- Reduce corrosion and weathering of the existing shelter and the Unit 4 reactor building
- Mitigate the consequences of a potential collapse of either the existing shelter or the Unit 4 reactor building, particularly in terms of containing the radioactive dust that would be produced by such a collapse.
- Enable safe deconstruction of unstable structures (e.g. The roof of the existing shelter) by providing remotely operated equipment for their deconstruction.
[edit] Structural design of the NSC
The NSC design is an arch shaped steel structure with an internal height of 92.5 meters, a 12 meter distance between the centers of the upper and lower arch chords. The Internal span of the arch is to be 245 meters, and the external span is to be 270 meters. 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 meters consisting of 13 arches assembled 12.5 meters 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 a 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 meters 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 2500 kg/cm² (245 MPa).
More extensive detail of the structural composition and design of the arches can be found in Section II.B. Structural Design Process of Conceptual Design of the Chornobyl New Safe Confinement—An Overview.
[edit] Foundation design of the NSC
The foundations of the NSC must meet the primary design requirements:
- They must support the weight of the arches of the NSC
- They must support rail tracks across which the NSC can roll 180 meters from the construction site into place over Unit 4.
- They must minimize the amount of digging and cutting into the upper layers of the ground, as the upper soil is heavily contaminated with nuclear material from the disaster.
The site of the NSC itself is slightly sloped, ranging in elevation from +117.5 meters on the eastern side to +144 meters 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 meters 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 meters on average in December to +110.7 meters 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 4.50m by 1.00m foundation panels 21 meters in length and a 4m high pile cap that reaches to a height of +118 meters 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 meters 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 0.08g, 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.
[edit] Assembly process of the NSC
The NSC will be assembled in the following steps:
- Stabilization of the Object Shelter in order to prevent collapse during construction.
- Excavation and construction of Foundation.
- Assembly of first and second Arches to form Bay 1, installation of East Wall on Arch 1.
- Bay 1 will be slid East to accommodate the construction of Arch 3 and Bay 2.
- Subsequent sliding of the complete structure and adding of Arches and Bays to complete the structure.
- Installation of cranes and large maintenance equipment.
- Installation of the West wall.
- Final slide into place over Unit 4.
- Construction of the fragmentation, decontamination, and auxiliary buildings
This process of assembly is advantageous because it takes 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 is completed infrastructure equipment including that for ventilation systems, radiation monitoring, plumbing, and electrical will be installed.
[edit] Positioning of the NSC
The New Safe Confinement (NSC) is to be constructed 180 meters west of unit four 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.
[edit] Deconstruction of existing structures
The final phase of construction of the NSC involves the deconstruction of the unstable structures associated with the original Object Shelter. The goal of deconstruction 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 deconstruction, but also the loads of those cranes.
[edit] The deconstruction equipment
The NSC design includes 4 bridge cranes suspended from the arches. These cranes all travel East to West and each has a span of 42 meters. The total weight of each bridge crane (including beams, carriage, and full load) is 155 tons.
Each crane can carry a variety of interchangeable carriages. Three types of carriages have been designed for the NSC:
- Two typical lifting carriages, each with a 50 ton carrying capacity.
- One secure lifting carriage for shielded transportation of personnel, with a 40 ton carrying capacity.
- One carriage with a telescoping arm, extending up to 75 meters, that can be fitted with a variety of end actuators useful for deconstruction.
The cranes' carriage interchangeability allows the rotation of the largest members to be deconstructed, reducing the overall size of the NSC by approximately one arch bay.
After the members to be deconstructed are removed by crane they must be fragmented into pieces small enough to decontaminate. It is expected that the primary contamination of most deconstructed 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 deconstruction 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 deconstruction 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 (FCM).
[edit] Elements to be deconstructed
The following elements of the Object Shelter are planned for deconstruction:
| Element | Quantity | Mass of each (metric tons) |
Length of each (meters) |
| Southern roof flat panels | 6 | 31 | 28.7 |
| Southern roof flat panels | 6 | 16 | 28.7 |
| Southern hockey stick panels | 12 | 38 | 25.5 |
| Mammoth beam | 1 | 127 | 70 |
| Northern beam B1 | 1 | 65 | 55 |
| Southern beam B1 | 1 | 65 | 55 |
| Northern hockey stick panels | 18 | 9 | 18 |
| Eastern hockey stick panels | 1 | 7.25 | 7 |
| Light roof | 6 | 21 | 36 |
| Piping roof | 27 | 20 | 36 |
| Northern beam B2 | 1 | 57 | 40 |
| Southern beam B2 | 1 | 57 | 40 |
| TOTALS: | 85 elements | 2024 tons | N/A |
[edit] Types of materials to be deconstructed
The elements that are to be deconstructed fall into several broad material types:
- Steel
- Flat (roof panels)
- Three dimensional (pipes, trusses, beams)
- Reinforced concrete
- Pre-cast
- Cast in place
- Debris
- Fragments of steel structures and equipment
- Fragments of reinforced concrete structures
- Materials added after the Chernobyl accident to mitigate its consequences.
[edit] Progress of the Project
The structure was originally intended to be completed in 2005, but has since been postponed and, as of June 2003, is expected to be completed in February 2008.
The following Schedule was released in June 2003:
- 12 February 2004 - complete the NSC conceptual design.
- 13 March 2004 - Ukraine government to approve the conceptual design.
- 13 June 2004 through 13 September 2004 - conduct a tender and sign a contract with the winner to proceed with relevant engineering and construction work.
- 16 April 2006 through 20 May 2007 - lay foundations for the NSC.
- 16 April 2006 through 22 October 2007 - fabricate steel arch segments, assemble, move in contact and secure arch sections.
- 23 October 2007 through 19 February 2008 - install cranes, piping and lighting fixtures under the arch.
- 20 February through 29 February 2008 - slide the arch structure in place over the existing Shelter.
On 11 March 2004 the international tender for NSC design and construction was announced.
On 16 November 2004 technical proposals from potential contractors were opened. After evaluation two candidates were identified from the proposals. These candidates were invited to submit commercial proposals for the NSC.
On 28 September 2005 the evaluation of the commercial proposals began, and was expected to be carried out until the end of 2005.
In September 2006, the plant's general director Ihor Hramotkyn announced his intent to annul all bids on the project. No general contractor has yet been selected.[1]
[edit] 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. The EBRD assigned the Shelter Implementation Plan to project number 4807 in the country of Ukraine. The EBRD project site can be found here. However, research has been unable to turn up any information from the EBRD as to the status of the project. Contact with the EBRD by Wikipedia Editors in search of recent information is ongoing as of January 2006
[edit] See also
- Chernobyl disaster
- Shelter Implementation Plan
- Chernobyl Shelter Fund
- List of Nuclear and Radiation Accidents
[edit] References
- Conceptual Design of the Chornobyl New Safe Confinement— An Overview (2004) by Charles Hogg—Bechtel Corp., Matthew Wrona—Bechtel National, Inc., Philippe Convert, Yuriy Nemchinov, Pascal Belicard, Valery Kulishenko, Eric Schmieman, Michael Durst published by Pacific Basin Nuclear Conference
- Chornobyl: Five-Year Schedule set for New Safe Confinement Over Wrecked Unit (June 9, 2003)
- Project Implementation Phase 2 from Chernobyl Nuclear Power Plant
- SIP Project Summary Document from The European Bank for Reconstruction and Development
- ^ "Ukraine may hold new tenders on Chernobyl safety facility", BBC Monitoring International Reports, 27 September 2006. Retrieved via Lexis-Nexis on 21 October 2006.
