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NUCLEAR CONTAINMENT STRUCTURES

VSL offers a complete range of post-tensioning materials and services complying with the international nuclear standards to guarantee that the nuclear containment safety requirements are achieved.

  • Nuclear Plant
    Strengthening water tanks with carbon fibre tendons for seismic upgrade.
    Switzerland - 2005 read more

    Nuclear Plant

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  • Nuclear Containment Vessels
    VSL actively participates in the post-tensioning work for the containment vessels presently being built in France.
    France - 1985 read more

    Nuclear Containment Vessels

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Reliable solutions for nuclear containments

VSL offers a complete range of services for the design and construction of post-tensioning system for the concrete nuclear containment vessels that use post-tensioning. The post-tensioning is the critical and essential component that insures the air tightness of the structure under a LOCA (Loss of cooling accident) and as such that stops the contaminated particles to dissipate into the atmosphere.

Strong of its experience gained in the construction of 59 containment structures worldwide since 1974, VSL provides:

  • efficient and reliable systems designed specifically for nuclear applications
  • technical support to designers and contractors to develop optimised solutions
  • state of the art equipment and construction methods providing a full and transparent traceability to the entire production and construction process.

VSL has developed a specialized range of data acquisition systems attached to the installation equipment that keeps track of in-situ construction data. This allows traceability of every step of the works, and as such allows isolating problems and issues associated with the installation of large tendons and insures that the works have been carried out to the highest standards which ultimately translate in durability for the nuclear containment structure.

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  • a strong commitment to R&D, to continue developing systems, equipment and methods to keep VSL its leading position in the post-tensioning industry

Full-scale mock up

VSL has completed a full-scale mock-up of a slice of an EPR®® - like containment to demonstrate new developments for the latest generation of nuclear containments. It is the largest R&D investment ever undertaken by VSL. The testing programme and its results have demonstrated the efficiency of VSL’s range of post-tensioning solutions for the latest nuclear containment designs, which suit power stations ranging from 1,000MW to 1,600MW. The technology can be applied to European Pressurised Reactors (EPR®), the Advanced Pressurized Water Reactor (APWR), Russia’s VVER design and the CPR 1000 in China. The mock-up was designed as a 2.75m-high slice of a nuclear containment vessel and includes six different types of post-tensioning tendons using various systems. The tendon radius was chosen to match the 24.46m outer tendon radius of an EPR®. In accordance with current nuclear designs, the high density of reinforcement within the buttresses led to the anchorages being cast into prefabricated elements to guarantee the accuracy of installation and the quality of concreting.

New installation and monitoring methods for nuclear applications were checked and confirmed to meet the most stringent criteria for post-tensioning applications. Construction of the mock-up demonstrated

  • the duct waterproofing as well as adjustment and placement of three different types of ducts: PT-Plus®, low friction steel sheet ducting and steel tubes.
  • Four tendon types were installed, using different anchorages, different type and sizes of ducts and different strands.
  • For these tendons, three new designs of post-tensioning bearing plates were developed, all of them meeting nuclear requirements. The tendon sizes were 6-37 and 6-55, using a maximum of 37 or 55 strands of 15.2mm and 15.7mm diameter.
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  • a dedicated supply chain for nuclear works to ensure system treatability and safety

VSL offers components, equipment and installation methods that are customized and approved for all current nuclear containment designs.

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  • turnkey services customised for each project

Complete turnkey services can be customized to each project, covering design, approval assistance, supply and installation for new containments as well as monitoring and maintenance of existing structures.

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  • specialised and skilled staff trained at the VSL Academy

VSL staff dedicated to nuclear works are trained to carry out the specific installation methods and to operate the sophisticated equipment required for nuclear applications. Trained personnel are the key to guaranteeing the performance of post-tensioning systems and thus ensuring the performance and reliability of the nuclear containment structure.

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VSL’s products and services

VSL offers a full range of services to guarantee that the post-tensioning system fulfills all the containment safety requirements.

These services include :

  • engineering and design assistance to design, detail and integrate of the post-tensioning system into the structure.
  • validation of the supply chain, by qualifying internal and external suppliers and carrying regular audits
  • supply of all post-tensioning materials including strands and grout
  • installation, including all the testing and checks required to ensure complete traceability of materials and site operations
  • specifications and construction of monitoring systems to keep track of the containment’s structural behaviour and performance. That allows controlling safety, and optimizing maintenance works throughout the structure service life.
  • Supply, installation and commissioning of the specialised working platforms to access the post-tensioning anchorages along the buttresses

Local regulations do not always require the hiring of a specialized contractor for the installation of the post-tensioning materials. However, VSL strongly recommends that the full scope of post-tensioning supply and installation scope is carried out by a specialist, to insure the highest quality standards are achieved to safeguard the long-term critical performance of a nuclear containment. Post-tensioning is the essential part of that scheme. VSL is indeed fully geared to provide that entire service.

Associated Services

Upgrading the structural capacity to meet new seismic requirements

VSL has developed effective strengthening and repair strategies specifically for the complex environment of nuclear facilities. These operations restore and/or enhance structural functions. After years of service, the containment walls may need to be strengthened if they no longer meet the latest design standard requirements. VSL’s strengthening systems use the principle of post-tensioning. Strengthening can be carried out by the use of either post-tensioning steel strand or by bonding fibre reinforced polymer materials such as carbon fibre plates onto weak spots in a structure. The advantages of that latter strengthening technique are manifold: low weight, low structural height, high geometric flexibility, no corrosion, very fast application and efficient handling. Strengthening or alterations may be needed if there has been a change in use and/or loading or if the structural system has been modified or needs repairs. Upgrading of structures in seismically endangered zones may also be required to meet the latest standards.

Custom-engineered solutions for heavy lifting works

VSL Group’s Heavy lifting division covers a wide range of services from initial concept through engineering, planning, equipment supply and execution of heavy lift projects. It has a proven track record within the nuclear power industry. VSL Heavy lifting offers tailored solutions to accomplish the most difficult works in the most stringent situations during the construction, maintenance, refurbishment or decommissioning of nuclear power plants. VSL Heavy lifting is specialized in:

  • Nuclear commissioning or decommissioning projects
  • Installation or replacement of steam generator and reactor vessel heads, or handling of any other heavy components within or outside of the nuclear structure
  • Specialised transportation of heavy loads
  • Design and supply of any special equipment and temporary structures related to heavy lifting
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Lifting a steam generator at the nuclear power plant Beznau I, Switerzrland, 1993

All work executed by VSL Heavy lifting follows a very stringent quality assurance programme to ensure that the temporary works are built to the required standards and that the processes follow established and checked plans. Before the lift proceeds, there are systematic load tests of the entire system of heavy lifting equipment and its associated temporary structures.

VSL’s Heavy lifting equipment
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VSL’s equipment range includes a large variety of hydraulic jacks with piston strokes between 160mm and 550mm and capacities from 100kN to 5,800kN. These units can be used in tandem in cases where high speeds are required. Equipment such as hydraulic coilers, emergency devices and sensor-controlled measuring units are also available to complete the system. VSL can design and supply custom-built hydraulic systems as required for special applications.

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Infrastructure Protection

VSL Infrastructure Protection is a division of VSL specialising in the analysis, design, development and implementation of solutions to safeguard critical infrastructure against blast, weapons and other extreme events. VSL has the experience and the capability to provide boundary deterrence to meet antiram, intrusion and blast protection requirements. It can also provide blast resilience measures for control buildings and associated structures at nuclear power plants, as well as protecting against aircraft impact. VSL’s solution portfolio includes a range of proven technologies and tested products for blast mitigation measures.

Enhanced structural durability with Ductal®
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Cattenom France

VSL can provide solutions that use Ductal® material to protect or replace concrete structures subjected to extreme aggressive conditions. Ductal® has very high mechanical properties combined with extremely low porosity and a very high resistance to chemicals and abrasion. The material has been used successfully in the Cattenom nuclear power plant, where it forms a beam grid at the bottom of the cooling tower. It was chosen instead of conventional concrete or steel beams because of the very aggressive environment featuring saturated humidity.

Maximum versatility through a combination of high-grade bar systems with VSL’s expertise

VSL offers a high-grade bar system (grade 900 to 1050) in both coarse-threaded and cold-threaded options. The bars can be used in short straight lengths in place of post-tensioned strand tendons. This provides better efficiency due to the short draw-in and the ability to re-stress the bars during stressing.

Grade 650 reinforcement bars

Nuclear containment structures are often very congested with rebar, with ratios locally exceeding 500kg/m3. VSL provides design assistance and materials to reduce the congestion by replacing the standard grade 500 reinforcement with higher-grade material.

Coarse threaded SAS and CT Bars – the key to efficiency around the world

Bars with coarse threads offer important advantages in all applications, particularly geotechnical ones:

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  • good bond behaviour
  • threaded along the full length, allowing cutting or extension at any position
  • robust
  • economical and readily available
Metric thread bars – great value for specific needs
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  • metric threading at both ends of the bar, with architectural benefits
  • no need to use lock nuts
  • non post-tensioned bars can be welded
  • wide variety of accessories and fittings available

Advantages of VSL’s offer

Several reactor technologies are available worldwide and the most common technology used is the Pressurised Water Reactor (PWR). The containments are designed to withstand the high internal pressures that would result from a loss of coolant accident (LOCA).

Applying such pressure to the inside surface of the containment causes tensile forces inside its walls, which are offset by the use of post-tensioning. As such, the post tensioning is the critical component of the containment that provides structural integrity to the containment vessel and ensures that it remains leak-tight.

Meeting the most stringent standards

VSL’s post-tensioning systems meet the advanced technical and practical requirements of today’s engineers and construction professionals. The systems comply with national and international standards such as those of European Technical Approval Guide ETAG 013, ASME (American Society of Mechanical Engineers) and the French technical code for civil works, ETC-C, produced by AFCEN. These standards define stringent test criteria, approval procedures, manufacturing certification and periodic audit requirements to insure the post-tensioning quality and performance, and thus its durability.

Post-tensioning anchorages undergo stringent tests including:

  • Static load tests
  • Load transfer tests
  • Fatigue load tests

VSL’s Post-tensioning materials are manufactured under strictly defined and controlled conditions, with:

  • Approved factories
  • Approved material
  • Standardized QA / QC
  • Continuous internal and external auditing

Supply chain continuity guarantees safety

For nuclear projects, the list of materials to be supplied is very large, ranging from the steel strip used in manufacturing the corrugated ducts, through the components of an anchorage system to the cement or grease that will ensure the system’s final corrosion protection. The VSL supply chain is organised for smooth integration of all the different steps that guarantee the timely supply of components to the right quality. Component production originates from three sources:

  • In-house factories in Europe and Asia
  • Approved suppliers
  • On-site facilities

Each relies on different production processes, all must comply with the project specifications and regulations. VSL has developed the necessary QA/QC procedures to meet these requirements.

Installation: Specialist required

The assembly and installation of the post-tensioning components is carried out on site, in accordance with stringent procedures, tolerances and quality control. This covers aspects including duct placement, tendon installation, stressing and filling with the final protection material. The installation works must be carried out by a specialist contractor in order to guarantee the system’s efficiency and safety as per the design. Specialist contractors must have qualified and experienced supervisors and well-trained labour knowledgeable about all installation aspects of the post-tensioning system. As a specialist contractor, VSL undertakes this entire scope of post-tensioning works.

Contributing to sustainable solutions

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The use of VSL Post-tensioning delivers the maximum cost-benefit for a project by enhancing the durability of a structure and has as well a beneficial impact on its sustainability and CO2 emissions during construction. Compared with conventional reinforced concrete structures, the use of post-tensioning results in more durable structures with reduced concrete volumes, thus lowering the CO2 emissions in some cases by up to 37%

Durability

Every step of implementation must be carefully planned and controlled in order to ensure durability of the nuclear post-tensioning and its long term performance:

  • At the design stage - by defining quality standards and procedures to ensure control and traceability at every stage of the supply and installation processes, as well as in the provision of long-term monitoring systems
  • During manufacturing of the components and their installation - by strict implementation of processes and quality procedures
  • By the development and use of specialised equipment to enhance accuracy and quality
  • Through the training and qualifications of skilled personnel
  • By the careful and thorough implementation of corrosion protection systems such as cementitious grout, wax or grease
  • In due course, by regular monitoring, inspection and maintenance procedures VSL has the experience to implement all of these steps, guaranteeing that the expected post-tensioning durability is achieved

How it works

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The Paluel Nuclear Power Plant ©EDF

Nuclear power plants use high-pressure heavy water or steam to transfer the energy provided by the nuclear reaction to a turbine activating a generator to generate electricity. In the event of a major leakage, the energy released by the coolant must be kept safely within a leak tight space. That space is the nuclear containment structure. The design depends on the coolant type and the working pressure that is expected in the event of an accident. Most nuclear plants with pressurised steam generators are designed to withstand a pressure of between 3 and 5 bars inside the containment vessel.

Post-tensioned tendons are the critical members of the nuclear containment that keep the concrete leak-tight, with a resultant stress in the concrete of about 1MPa at the design pressure. These stresses are balanced by the post-tensioning adequately laid out and installed into the containment walls that maintain the concrete in compression.

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Illustration of a section of an EPR® used for a VSL R&D test site installation

VSL has been active in the nuclear prestressed concrete containment vessel sector since its first project in 1974. The company has supplied and installed approximately 10 different designs of post-tensioned concrete containments using many types and sizes of tendons. The various designs have included horizontal and vertical tendons for both shallow-domed and semi-spherical dome-shaped roofs on cylindrical walls. The tendon technology, based on either bonded or unbonded systems, shall comply with various international standards issued by clients, states and professional organisations, including ASME, Atom Energo Projekt, EDF, KHNPC, NPCIL and RCCG.

VSL is constantly developing and enhancing its proprietary systems and new installation tools and procedures to meet the safety requirements of nuclear projects. As such, it focuses on two key points:

Quality control engineer carrying out measurements on the 55 individual strands of a mock-up nuclear tendon
  • Simplifying the installation process to avoid mistakes and provide higher quality and
  • improving the traceability of the works, checking and record-keeping of the operation to give
  • the owner and safety authorities the maximum assurance of the work’s quality.

Main components

The Anchorages

The anchorages are the key elements of the post-tensioning system. They must guarantee:

  • Safe load transfer to the concrete structure
  • Long-term performance under maximum load and fatigue
  • A reliable installation

For nuclear applications, VSL offers two types of anchorages which have been tested according to international standards. VSL also provides assistance to owners to define and choose the most appropriate anchorage for the project.

The NC range
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This Nuclear Casting system (NC) is an evolution of the well-proven EC Anchorage, which has been widely and successfully used since the 80s. The anchorage has high performances with regards to load transfer of the post-tensioning force to the concrete and is very robust.

The E range
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These anchorages are designed with a large steel bearing plate. They have been used by VSL since the mid-60s and have therefore a long track record of more than 50 years throughout the world. The design can accommodate a long trumpet with a constant cross section. This allows for detensioning of the tendons by sliding the anchor head into the trumpet without removing the anchor and the wedges. This is a particular benefit for unbonded system to carry out surveillance operations, or for tendon replacement, which is required by the ASME design.

The tendons
Bonded technology

This solution is primarily used in Europe and West Asia. It provides high durability and high performance at ultimate state for a given tendon size. The design is based on nuclear-specified strands, which are installed in ducts and stressed with specific tools to mitigate the force dispersion at the anchorage. The tendons are permanently bonded to the structure with high performance cementicious material, which provides long-term corrosion protection.

A 6-55 Nc casting with individually sheathed and greased strands just after initial grouting

VSL recommends that the PT-Plus® duct system is used with this bonded technology to reduce friction during the stressing operations. These ducts also provide leak tight environment in the duct allow monitoring of the corrosion protection.

Unbonded technology

The unbonded post-tensioning system - as used in US, Eastern Europe and east Asia particularly - offers very high performance with low friction as well as long-term service. It enables monitoring of the forces in all tendons and allows the replacing any of them, at any time, if this is eventually required.

High performance injection (HPI®) strategy
Grout mixing

The quality of the grouting is a key element to ensure the durability of bonded post-tensioning tendons. In this case, cementitious grout provides bonding and corrosion protection for the prestressing steel. VSL has developed a comprehensive process, VSL HPI®, which is dedicated to improving and ensuring the quality of grouting materials and grouting activities on site.


Slice of a 6-55 mock-up nuclear
tendon duct filled with HPI® Grout

Optimised grout mixes are designed for the specific use and environment, using selected local cements and admixtures that are analysed and optimized for exclusive use on the project. The VSL HPI® grout also delivers specific rheologic properties over a period of several hours, which is ideal for nuclear tendon layout and the typical access conditions.

Full-scale site testing is implemented for validation of grout properties and installation methods whenever there are new conditions such as different materials or for specific layouts.

DUCTING

The sheath layout can be made up from any of the following materials, which might also be used in combination within a single structure:

  • Corrugated steel sheath ducting is generally used for horizontal tendons with low deviation they are formed by spiral rolling of steel strips. For nuclear applications, their mechanical properties are higher than those required for standard post-tensioning works. They are either hot-dip galvanised or electrolytically zinc coated to provide the corrosion protection required during the time between installing the duct in the concrete and grouting the tendon.
  • Steel pipes are generally used for vertical tendons as well as for any areas with tight radiuses
  • VSL PT-PLUS® plastic pipes may also be used and have the advantages of reducing friction coefficients and providing additional corrosion protection.

The ducts can be lubricated, either at the time of manufacture or after installation, to reduce tendon friction values.

MONITORING DURING CONSTRUCTION AND SERVICE LIFE

Specific tools have been developed to follow the tendon installation with exhaustive monitoring and to report on the processes to the client. For certain clients, VSL follows the ASME licence requirements; other clients’ requirements may be different and are always respected.

Tendon force monitoring
Schematic view of a CV8 type load cell

When combined to monitoring of structural deformation, this allows observing the structure’s behaviour. This method is particularly suitable for checking the structure as a whole. For nuclear containments, it is important to check whether the post-tensioning in the structure changes over time. The force monitoring system can only be installed with unbonded tendons.

The VSL Grout Sensor

The void sensor provides key information at important stages in the structure’s life:

  • during grouting, it is used to check the filling of the duct
  • during the structure’s life, it can check the passivation of the strands.
The Electrically Isolated Tendons (EIT) system
Sample of a CS2000 6-31 anchorage with EIT detailing

During the life of the structure, this system allows monitoring the ingress of external agents inside the tendon that could originate steel corrosion. The system works by measurement of the electrical resistance between the strands and the structure. The use plastic duct is necessary for the tendon and the anchorage connecting to the duct is detailed to encapsulate entirely the tendon in order to guarantee its complete isolation.

Cutting edge equipment to guarantee safety and durability of nuclear containments
Special equipment and stressing processes

Installation of post-tensioning in nuclear containments requires special equipment and methods due to the exceptional size of the tendons and specific nuclear requirements. The equipment and methods are designed to address each step of the installation process:

  • Strand installation
  • Take-up of slack between the strands and equalise the initial force

VSL has also developed a special hydraulic jack capable of pulling each strand simultaneously and individually with the same force. This equipment is used prior to the post-tensioning operation to remove the slack of each strand and equalise their initial tension.

  • Stressing and logging of tendon forces and elongation
  • Injection of filling materials

Full traceability of the works must be insured during these operations and VSL has developed automatic data acquisition systems to achieve this. For each operation, VSL provides all the access equipment and tools needed to carry out the work with a minimum of disruption to other adjacent activities.

Special working platforms

VSL designs and builds special working platforms for the post-tensioning installation and inspection at the buttresses. The platforms are capable of running along the curvature of the dome while keeping the working area horizontal. The equipment is designed to meet all international safety standards. They can work between the inner and the outer containments for tendon installation and maintenance.

Traceability and site efficiency

VSL DAS - the Data Acquisition System for post-tensioning - automatically collects data including tendon forces and elongation during stressing. It features an industrial touch-panel and processes the information for further use in quality control, providing transparency to the client. The system is designed to record the appropriate stressing parameters to meet the project’s requirements. This includes:

  • Global forces, using an electrical load cell or the pressure in the jacks (global load cell)
  • Elongation of the strand (displacement sensors)
  • Individual strand forces (magnetic load cells on each individual strand)
Individual force monitoring

The DAS allows measurement of the force distribution and standard deviation between the strands. The system also provides the assurance that none of the strands has reached its actual yielding limit under the influence of differential friction. The information is provided as part of VSL’s quality programme.

Post-tensioning is designed and detailed to allow compression of the concrete in all directions to keep the containment air tight.

Depending on the design, different types of tendons are installed :

  • The horizontal tendons, anchoring into the vertical buttresses and running between 270 and 360 degrees.
  • Pure vertical tendons, running from the tendon gallery below to the beam at the dome level.
  • Dome tendons, sometimes combined with vertical tendon on one leg
  • Inverted U tendons.

Each Nuclear system has developed its dome and post-tensioning configuration and therefore,

  • the tendon size, between 19 and 55 strands
  • tendons layout, with a combination of vertical, horizontal, dome and U tendons
  • bonded or unbonded strand system

Tendons layout is also detailed to void the numerous openings in the containment, including the main hatch.

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Different tendon layouts allow for tri-axial compression of the concrete within the containment structure.

LAES Nuclear power plant, St Petersburg, Russia, 2 containments, 1170 MW – AES-2006 VVER technology

Construction of 2 nuclear containments of 1170 MW each using the nuclear technology VVER from AtomEnergoprojet.

The nuclear system uses 55 greased and sheathed unbonded strand tendons of 15.7 mm diameter, 279 kN capacity each. The tendon layout includes:

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  • Horizontal tendons running 360 degrees, and overlapping from one layer to the next by 180 degrees.
  • Inverted U tendons running from the tendons gallery into the dome down in the tendon gallery.

SHIN Ulchin 1-2, Korea and Barraka 1-2, United Arab Emirates with 4 containments OPR 1400 from - KEPCO technology

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Construction of 4 nuclear containments of 1400 MW each using the nuclear technology OPR 1400 from Kepco (Korea).

The nuclear system uses 42 bare unbonded strands tendons of 15.2 mm diameter, 265 kN capacity each. The tendon layout includes:

  • Horizontal tendons running 240 degrees, and overlapping from one layer to the next by 120 degrees.
  • Inverted U tendons running from the tendons gallery into the dome down in the tendon gallery.

FLAMANVILLE 3, France, 1 containment of 1450 MW – EPR® technology

Construction of one nuclear containment of 1450 MW of capacity, the second EPR® in the world.

The post-tensioning system uses 55 bonded strand tendons of 15.7 mm diameter, 279 kN capacity each. The tendon layout includes:

  • horizontal tendons running 360 degrees, and overlapping from one layer to the next by 180 degrees.
  • Pure vertical tendons
  • Dome tendons that are combined on one leg with vertical tendons, making them running from the tendons gallery into the dome beam.

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