Using modularization in the construction of nuclear containment vessels can be one way to control both cost and schedule when building the next generation of U.S. nuclear power plants. Although the advantages of modularization can be significant, each new reactor design and plant site poses unique construction challenges and must be individually analyzed to determine the benefits of this approach.
For currently operating U.S. nuclear plants, the average construction period was 9.3 years; the longest was 23.5 years. In Japan, close attention to modularization and construction sequencing has reduced construction times for the ABWR reactor design. In fact, units in Japan have been constructed in less than four years! One of the secrets to this speed was using modular construction techniques.
Standardization is the key to controlling both cost and schedule in building the next generation of nuclear power plants in the U.S.: standardized designs, standardized construction techniques, standardized equipment, and standardized plant layouts.
Once designs, details, plans, and procedures for the first unit are completed, they can be reused, enabling subsequent units to be built at a lower cost. Additionally, the learning curve for fabrication, construction, and project management decreases with each unit built, which improves the schedule for subsequent units.
How Modular Construction Works
One construction methodology that works particularly well with this standardized approach is modularization. By definition, modularization is the process of engineering and fabricating construction projects into shippable packages or segments that can be installed economically at the job site. The task of fabricating and assembling these segments can be carried out in a shop environment or on a nearby construction pad, if on-site space is available (Figure 1).
1. Prefabricating rings. Construction pads located adjacent to a project site are used for prefabricating containment vessel sections. Courtesy: CB&I
There is precedent for modularizing components and systems of nuclear plants. Containment vessels — the leak-tight steel structures used to enclose the reactor and contain any radioactive release in the event of an accident — are particularly well suited for modular construction. Because of the shape of the vessels, they can be fabricated in large rings or panels and then lifted one on top of the other to assemble the structure (Figure 2).
2. Lifting rings. A containment module is lifted from the construction pad.
Courtesy: CB&I
CB&I used this technique in the construction of nuclear containment vessels built from the 1950s to the 1980s, although it was not called modularization at that time. Construction pads were built adjacent to the nuclear island. Large pieces were assembled on these pads and then lifted into final position. A number of countries have used these techniques since that time, most notably Japan, where modular construction techniques have been enhanced to include structural segments that include mechanical components, piping, and electrical and control wiring.
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