Advantages and disadvantages

Prefabrication and preassembly of modules are construction techniques used in many industries, including NPPs.

FIG. 111. Configuration of containment dome liner plates, Shin-Wolsong 1 and 2, Republic of Korea.

FIG. 112. Modularization of containment dome liner plate assembly, Shin-Wolsong 1 and 2, Republic of Korea.

8.4.1. Advantages

Modules may be fabricated in a controlled environment in a factory or in a workshop on the plant site.

Multiple modules can be fabricated in factories or workshops, while the civil work is progressing on the site in preparation to receive the modules. On the site, only sequential assembly of the modularized assemblies is required.

FIG. 113. Large module structure — main steam and feedwater piping module (credit: Mitsubishi Heavy Industries, Ltd).

FIG. 114. Modularized gas insulated switchgear in India on steel-plate reinforced concrete structure construction (NPCIL).

This reduces on-site congestion, improves accessibility for personnel and materials, and can improve the construction schedule. It can also significantly reduce the manpower needs for the site work at an NPP.

Other important attributes and advantages of modularization include:

— Capability for mass production of modules for several units, with the associated benefit of reducing production time and labour requirements;

— Provision of a controlled environment for module production;

— Manufacturing of modules before the site becomes available;

— Potential to utilize accelerated curing techniques for concrete modules;

— Reduced schedule (if the module is applied to the critical path);

— Reduced field work and levelized on-site manpower;

— Increased productivity and quality under factory environment;

— Increased safety and efficiency at ground level work.

The chief benefit of modularization is that it shortens schedules by:

— Creating parallel construction activities;

— Increasing the productivity of workers by allowing assembly in controlled shop environments as opposed to construction sites;

— Reducing work-site congestion so that on-site craft is more productive;

— Allowing construction of modules at grade and in easy-to-reach positions (e.g. vertical wall reinforcement constructed in horizontal position on ground);

— Removing or reducing the effects of weather at the construction site (if module assembly occurs in indoor facilities);

— Reducing construction time of some systems and equipment, since testing may be conducted within the shop.

With the application of modularization techniques, a reduction of four to five months could be achieved in the construction schedule.

The decision as to whether to apply the modular approach should be made in the conceptual design stage, and it must be then followed throughout the project: from detailed design, engineering, procurement, fabrication and installation, to completion of commissioning. In this way, equipment can be designed to be located so that it conveniently fits into a module. In addition to the capacity of the VHL crane, considerations in sizing modules include whether the site is accessible from the sea, or if it is necessary to fabricate sub-modules to be shipped by rail to the site. Examples of modules include structural modules; containment liner modules; electrical equipment modules with their own structural frames; cable tray modules; piping and valve modules; and stairwell modules.

Modularization can also influence testing procedures, as preliminary tests can be done on many components at the fabrication facility to help eliminate potential sources of error before conducting formal tests after installation at the construction site.

8.4.2. Disadvantages

The use of modularization places several requirements on the project schedule. Engineering design must be complete prior to module procurement. The quality of the modules off-site and on-site must be strictly ensured for successful modularization to take place. The schedule for component procurements will also be affected: materials required for modules will have to be ordered earlier than was necessary for conventional stick-build construction.

The use of multiple module vendors will also require strict coordination to ensure proper delivery times. Finally, modularization will require a detailed plan for how to sequence and schedule connections between adjoining modules.

Modularization does introduce challenges to project schedules. These challenges include, but are not restricted to:

— Design schedules may increase because of additional upfront work;

— Extensive design effort may be required;

— Each module might have a unique rigging strategy;

— In some cases, the size and weight of large modules require that modules be delivered by barge to the site;

— Construction of temporary transportation infrastructure and lay-down areas will be required during the site preparation phase to stage and move large modules once delivered on-site;

— Late delivery of modules can cause schedule delays and setbacks;

— Installation of modules must be highly structured and prioritized so connections can be made expeditiously;

— Damage to modules during shipment to the site can cause delays.

In addition to the impact on construction schedule and reduction of site labour requirements, some important factors which need to be examined in making the decision to modularize include:

— The necessity to complete the total design of the plant before module fabrication;

— Construction of factories or workshops for fabrication of modules;

— The qualification of separate vendors has an influence on nuclear quality programmes and thus influences and complicates module fabrication;

— The necessity for earlier outlay of funds for engineering, materials and components before module fabrication;

— Increased engineering for modules regarding transportability and handling at site;

— Increased engineered temporary support steels;

— The need for expensive heavy lift cranes and the complexity due to placement, movement and establishing these cranes at site;

— Transportation costs due to size of modules.