Evaluating Modular Construction as an Option
Using modularization for constructing containment vessels has many advantages, but it is not the solution for every situation. For all the advantages of modularization, this technique does not guarantee lower cost or a shorter schedule. Each new project must be evaluated individually to determine whether the advantages of modularization can be used to reduce cost and shorten the schedule.
At times, modular construction may not be an option. But whenever modularization is considered for a construction project, a complete analysis must be performed and a number of factors must be evaluated, such as those that follow.
Site evaluation. How much space is available for on-site construction? The more congested the site, the more desirable modularization becomes.
Site access. What transportation options are available for accessing the plant via truck, rail, or barge? Certainly, barge access provides the optimal situation for modularization. If both the shop and the plant are connected via a waterway, enormous modules can be fabricated in the shop, shipped to the plant, and lifted into place. If barge access isn’t an option, the size of the modules will need to be orders of magnitude smaller to ship by truck or rail. Nevertheless, these modules are still quite large and require careful planning to transport, as permits for using roadways and bridges must be obtained.
Module lifting. What is the lifting capacity at the site? Large, heavy modules that are constructed in a shop or on a nearby construction pad must be lifted into place. Ideally, if a construction pad is used, it is best to have a crane that can access both the construction pad and the area where the finished containment vessel will be placed on-site. As each piece of the vessel is assembled, the crane lifts it into place. If modules are shipped via truck, rail, or barge, appropriate lifting capacity must be available to lift the pieces into place (Figure 3).
3. Stacking rings. A modularized ring of a containment vessel is set on the nuclear island containment vessel. Courtesy: CB&I
On-site transportation. What space is available to transport the modules from one part of the site to another? If the modules are built on a construction pad and need to be transported to the nuclear island, a path as wide as the unit must be available.
Cost savings. Can you save money by modularizing the containment vessel? The answer to this question varies, depending on the considerations just discussed. Modularization increases the amount of engineering that must be performed, as each module must be designed to be safely transported. A framework for transporting and lifting the modules must be engineered along with the unit itself, increasing both engineering and material cost. On the other hand, modularization can reduce the schedule, and schedule reductions, in turn, can reduce the cost of construction.
If You Choose Modularization
Although all five of the next-generation nuclear power plant concepts in the U.S. include standardized designs and support modularization of the containment vessels, careful analysis will be necessary for each individual project to determine if modularization is the best approach. If modularization is used, that decision must be made early to realize optimal benefits.
Engineering of the modules must be done early in the engineering phase so that all the details of transporting and lifting the modules can be considered and planned. Tolerances for containment construction are critical and must be established during the design phase. If due consideration is not given during the design phase, the erected modules could conceivably not match up to the piping and other connections.
Additionally, all of the parties must be involved in developing and coordinating the modularization effort, including owners, nuclear steam supply system manufacturers, fabricators, erectors, the logistics team, and even the Nuclear Regulatory Commission, whose staff will need to perform inspections in an off-site shop — a challenge not encountered during construction of the currently operating facilities.
—Lee Presley (lpresley@cbi.com) is vice president, nuclear operations, and Barbara Weber (bweber@cbi.com) is senior director, corporate communications for CB&I.
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