Construction Practices in Nuclear Power Plants

Construction Practices in Nuclear Power Plants

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Construction Practices in Nuclear Power 1It was recently announced that India is discussing the setup of Russian assisted 22 nuclear power projects in the country. This may not come as a surprise as fossil fuel and carbon prices are on the rise and several countries are looking at nuclear to provide a low cost alternative to fossil fuels. The cost of nuclear power generation is already competitive compared to other forms of low-carbon energy technologies such as wind power and coal fired generation with carbon capture and storage, but nuclear power has additional advantages – unlike carbon capture and storage, nuclear power generation is a fully proven technology and nuclear power provides base load generation capacity, which has yet to become a reality for either wind or solar power generation.

Relative to coal fired and natural gas fired power plants, nuclear power plants are more expensive to build. However, there have been numerous improvements in construction methods in the past few years, and recent experience in nuclear power plant construction has shown that those advanced methods are fully applicable and can help shorten construction schedules. Advances in materials technology, manufacturing, heavy equipment handling, transportation, and 3D computer simulation have also proved that nuclear power plants can be built and operated in a cost-competitive manner when compared to other electricity production sources.

Recent nuclear construction projects have been completed in as little as four years. The decision to apply some of these methods must be made in the conceptual design stage and then followed through consistently. Some advanced construction methods require earlier investments for factories and workshops and earlier outlays of funds to purchase materials, although they later save time and labour. Masterbuilder takes a look at some of the typical construction practices that are being embraced by nuclear power plants.

Open Top Construction

Open top construction facilitates installation of large components and large modules. In the past, the walls of the reactor and containment building were constructed with temporary openings to allow the entry of large equipment. In open top construction, the reactor building is partially completed and left open at the top and large components can be lowered into place from above with very heavy lift (VHL) cranes and then installed. Open top construction permits more activities to be progressed in parallel because the placement and installation of modules can occur through the open top of the structure with the use of the heavy lift cranes.

Once the equipment is placed inside, piping and electrical systems can be in-stalled at the same time that construction of the reactor and containment building is being finished, including the replacement of the temporary roof by a permanent containment dome.

During the construction of Tarapur-3 and -4 in India, open top installation was used to position about 50 pieces of equipment, including the steam generators, moderator heat exchangers, several other heat exchangers, pressurizer, calandria, primary circuit headers and fuelling ma-chine. The lowering and positioning of each steam generator was completed in less than a day, much less than the installation time of more than one month required by other methods.

Open top installation has been used successfully with modularization to shorten construction schedules. VHL cranes add additional costs, but these are more than compensated for by the shortened construction time. VHL cranes also add to planning requirements as it is vital to en-sure that they are strategically placed to conduct multiple lifting activities including the installation of heavy equipment in other buildings of the plant or to provide lifting capabilities for two units being built concurrently next to each other.

Modularization

Modular Construction allows parallel construction activities to proceed with significant reductions in construction scheduleModular construction allows parallel construction activities to proceed with significant reductions in construction schedule. It also reduces site congestion, improves accessibility for personnel and materials, and can shorten the construction schedule. It can also significantly reduce on-site workforce requirements.

In past construction practices, the fabrication of the mechanical and electrical systems and components was done on-site and typically awaited until the civil work on the reactor building was complete.

Modularization allows maximum utilization of parallel construction activities in civil, mechanical and electrical areas to proceed. Many of the mechanical/and electrical modules for equipment, piping, I&C, and electrical systems can be built off-site. The interfacing systems are typically included in the modules and can facilitate installation.

At KashiwazakiKariwa-7 in Japan the seven floors of the reactor building were divided into three modules and fabricated in a pre-assembly yard before the pieces were successively lifted into place by a VHL crane. The heaviest, most complicated module was the ‘upper drywell super large scale module’ which consisted of a H-shield wall, pipes, valves, cable-trays, air-ducts and their support structures and weighed 650 tonnes.

Open top installation has been used successfully with modularization to shorten construction schedulesAt Lingao-4 in China the containment dome was assembled on the ground at the site and installed as a single module weighing 143 tons with a diameter of 37 metres and height of 11 metres. Previously, the dome would have been assembled by moving sections into position a process that normally took about two months.

The Shin-Kori-1 and -2 projects in the Republic of Korea modularized the fabrication and installation of the containment liner plate. This forms the inner structure of the containment building for the Korean Optimized Power Reactor. Normally, the installation process would have fifteen stages, each involving the installation of one containment liner plate ring. At Shin-Kori-1 and -2, except for the first ring, all the other rings were modularized into two-ring sections and installed with one lift for each section. The number of lifts is reduced, and the overall construction period is shortened. This method also simplifies connections with auxiliary buildings since connecting provisions, such as penetration sleeves for piping and electrical wire, are attached to the ring modules before installation.

As a final example of modularization, at Tarapur-3 in India, the prefabrication of piping was increased to 60-70%, compared with approximately 40% for previous plants in India. This reduced field welding by 30-40%.

Automatic Welding

Nuclear power plant construction involves numerous welds to connect both components of structures and components of pressurized systems. It also involves weld cladding, which refers to one meta lbeing deposited onto the surface of another to improve its performance characteristics. Quality welding is both crucial and time consuming, and techniques to increase the rate at which weld metal can be deposited while maintaining high quality can reduce construction times. Recent advanced welding technologies that meet this objective include gas metal arc welding, gas tungsten arc welding and submerged arc welding.

In addition, automatic welding equipment that makes it easier to weld in narrow spaces can further decrease construction times. Automatic welding equip-ment has been used to weld titanium tubes to condenser tube sheets at Tarapur-3 in India and to weld piping at Kashiwazaki Kariwa-7 in Japan.

Steel Plate Reinforced Concrete

Steel plants reinforced concrete is an alternative to conventionally reinforced concrete and can be used for most floors and wallsReinforced concrete is used in the foundations of nuclear power plants and in structures such as reactor containments, auxiliary buildings, turbine buildings and spent fuel storage areas. Conventionally reinforced concrete is fabricated in place using reinforcing bars (‘rebar’) with external forms to frame the structure prior to pouring the concrete. The time required to place the reinforcing bars and to construct and remove the forms into which the concrete is poured is considerable. It is a major part of the construction schedule.

Steel plate reinforced concrete is an alternative to conventionally reinforced concrete and can be used for most floors and walls. The concrete is placed between permanent steel plate forms with welds to tie the steel plates, rebar and tie-bars together. The forms can include any necessary penetrations and piping runs. Because of structural credit for the steel plateconcrete combination, the amount of rebar may be reduced, and because the steel plate structure can be self-suppor-ting, reinforced concrete sections can be modularized and prefabricated off-site, followed by placement and welding on site.

Steel plate reinforced concrete has been used to significantly shorten construction schedules at plants recently constructed in Japan.

Slip-Forming

Construction schedules can also be shortened by slip-forming with modular floor design technology. Slip-forming is the continuous pouring of concrete at a very specific, calculated and monitored rate that is achieved by continuous hydraulic lifting and moving of a short section (preferably less than two metres) of formwork while inserting steelwork and pouring concrete through the top. Using slip-forming, vertical walls can be constructed at a rate of about two metres per day compared to a typical value of 1.21.5 metres per day without slip-forming. Slip-forming re-quires a heavy lift crane to lift the heavy steelwork that is inserted while the concrete is being poured.

Modular floor design and installation are used in conjunction with slip-forming for the walls. After the outer vertical walls of a building are installed by slip-forming, the modular floors can be installed through the open top of the building by means of a heavy lift crane. Modular floors consisting of steel modules, which include rebar but no concrete, are placed on supports embedded in the concrete walls during the slip-forming process. The modular floors, which are designed to be transported from the site assembly shop and installed by cranes, are welded to the supports embedded in the walls and then filled with concrete.

Rebar Installation

Rebar installation by individual placement of bars is quite time consuming. Large amounts of rebar are needed in the base mat, containment walls, containment dome, and structural walls of the reactor and turbine buildings. The use of prefabricated modular rebar assemblies for these areas can shorten construction schedules.

Automation is another way to speed the installation of rebar. There are several techniques such as an automatic scaffold that moves vertically while horizontally feeding rebar into place. It both speeds the process and reduces labour requirements. Another technique is the use of a machine that automatically assembles rebar according to instructions from a 3-dimensional computer design model.

High Performance Concrete

Recent advances in the composition of concrete aggregates have allowed improved strength, corrosion resistance, curing at low temperatures, and better workability. They have also facilitated the application of slip forming and modular construction of structural elements. The low-heat concretes also makes the large volume pours practical and pours up to several meters deep can be utilized for construction sections such as the base slab.

Project Management and Information Technology

Recent advances in the composition of concrete aggregates have allowed improved strength, corrosion resistance, curing at low temperatures, and better workabilityAs large capital projects have become more complex and pressures to reduce construction schedules have intensified, the ability to effectively plan across an entire construction life cycle has become all the more critical. Advances in material flow modeling, construction sequence modeling, scheduling tools, construction equipment and personnel equipment have had a significant positive impact on construction efficiency and safety since commissioning of the first generation of nuclear power plants.

Modern nuclear plant construction techniques – in particular open top construction, slip forming and modularisation – require considerable planning. Decisions to apply modularization, open top construction and other advanced technologies must be made early in the project, ideally at the conceptual design stage. The equipment modules should be designed to fit into their spaces in the appropriate structures or structural modules. The structural modules must be designed taking into account the lift capacity of the cranes to be used at the site and other logistics such as transportation to the site.

All Weather Construction

To assure that work can be conducted continually, an ‘all-weather’ construction method may be applied to the major buildings of the nuclear power plant to protect the worksite from weather conditions. The all-weather construction method provides an environment that is enclosed and isolated from the ambient weather, and is equipped with cranes and/or hoists to install rebar, forms and mechanical bulk commodities. The all-weather construction method shall not prevent modular construction and open top construction, even though the building has temporary enclosures.

In this method, the side of the building is protected by a tent attached to the steel frame. Either temporary or structural steel can be used, which is taken into account in the building structure design.  A temporary roof on a steel framework covers the entire building. The roof over the containment in the reactor building should be fully movable to provide space for the steel liner module, piping modules and the heavy components inside the containment to be lifted by open top. The roof can be moved using a VHL crane located outside the building, or be transported on rails using motor/manual winch operation. The roof over the perimeter of the containment in the reactor building consists of temporary steel, such as deck steel plates that also can be moved when equipment is lifted with the VHL crane, using the open top method.

Monorails and overhead cranes in-stalled inside this temporary structure are used to bring in smaller items of equipment and materials. This horizontal and vertical lifting equipment is installed crosswise to handle rebar, forms, embedment as well as small equipment and materials. As the overhead crane in the turbine building has a wide range to lift, it is a good idea to pre-install the overhead crane in the all-weather steel frame. The heavy components in the turbine building such as feed water heaters and pumps can be installed using the overhead crane, without moving the temporary roof.

Permanent staircases are pre-installed inside the all-weather structure, allowing the construction crew easy floor-to-floor access during construction.

The all-weather construction method was used in Japan at Kashiwazaki-Kariwa Unit No. 6 for the reactor building and at Higashidori Tohoku Electric Power Co. Unit No. 1 for the reactor building, the turbine building and the auxiliary building.

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