Modular construction in the nuclear industry is intended to reduce Total Capital Investment Cost of a Nuclear Power Plant, and consists of shifting labor from the reactor site to an off-site factory. Through modularization, modules are manufactured in off-site factories and transported to the site. On-site, they are assembled into super modules in the on-site area, a factory-like section of the site that allows a high level of parallelism. The super modules are then lifted and moved to the on-site construction hole where they are assembled to form the nuclear island. This process allows a cost reduction according to the 1-3-8 rule of thumb: a task that requires 1 labor hour to be completed in the factory, will require 3 and 8 hours if performed in the on-site assembly area and in the on-site hole, respectively.

In Refs [1-3], we evaluated the cost benefits of fabricating modules off-site as opposed to on-site for the Westinghouse Small Modular Reactor (WEC-SMR). In that analysis, we created a detailed construction model for the WEC-SMR in Microsoft Project 2007, consisting of about 2,000 activities. In this paper, the analysis is further developed, utilizing the model to evaluate the cost benefits of fabricating the super modules in off-site factories as opposed to the on-site. This strategy may provide cost reductions for those sites that allow transportation of super modules, i.e. with barge access and special transport vehicles from the jetty to the construction hole.

Modules are defined as transportable pieces of equipment (structural, mechanical, or instrumentation and control, composite) whose size is limited by the largest transportation method available. In case the super modules are assembled where the modules are fabricated, a further cost reduction is obtained as some labor and equipment is not needed. The assembly of modules into super modules mainly consists of performing connections between the modules through the use of composite modules. If the assembly is performed in the factory, the modules design can be changed to accommodate a higher amount of equipment, and their number can be reduced. Under this assumption, the labor associated with the fabrication of composite modules is reduced, as they are integrated in the new modules design.

This construction methodology was applied to the WEC-SMR. Activities in the on-site assembly stage were moved to the off-site fabrication stage reducing the amount of labor-hour according to the 1-3-8 rule, i.e., the labor cost and the assembly activity durations were divided by a factor of 3. Through this calculation, we estimated a 5.8% saving in Total Capital Investment Cost (TCIC) and a shortening in project duration equal to 81 days. A sensitivity analysis was conducted on the 3-to-1 factor of the 1-3-8 rule, evaluating labor-hours reduction factor in the range 2-4.

Assuming that the amount of labor in the fabrication of composite modules is also reduced, TCIC is further reduced. A sensitivity analysis on the amount of factory labor that is eliminated was conducted. As a fraction of composite modules is no longer fabricated, a saving up to 18% can be reached.