Transitioning From Top–Down to Bottom–Up Cost Assessments in Fusion

In the early stages of any large engineering/construction project, a standard way of estimating the cost is a top–down approach, comparing the design with similar existing projects with known costs. As many international fusion power plant projects are currently in their concept design phase or early engineering design phase any whole program cost estimates can be expected to transition from the top–down to more bottom–up cost approaches. While multiple publications of top–down cost estimates for fusion power plants exist in the literature see and some isolated bottom–up cost models for components have been presented, Bonavolonta et al., 2023 are not aware of any published work that demonstrates how to transition from the top–down to the bottom–up cost estimates and how these are used within the design of the fusion plant. The Spherical Tokamak for Energy Production (STEP) program has just completed its concept design phase where top–down parametric costing has been used in addition to comparable costing to generate broad estimates. It consciously uses these estimates throughout its design phase to ensure a cost-optimized design. To achieve a smooth transition, the total project cost is split up according to a unique cost breakdown structure (CBS) (such as the one defined in STARFIRE). As the STEP design matures and becomes more defined, it is beneficial to replace sections of the estimate with more detailed bottom–up estimates as the design detail is developed. The Excel add-in software CALC4XL supports this approach, by enabling the whole project cost to be split into sections, estimated in top–down or bottom–up methods (incorporating costs of materials and manufacturing). CALC4XL can switch between the two calculation methods for an easy comparison and each section is assigned an uncertainty class to calculate a high/low view of the costs. STEP uses a special bespoke version of the software, adapted for the particular needs of the unique STEP project, making it flexible to any level of design detail. The development and accuracy level of the project estimate can be tracked and plotted via this software, where it can easily be seen how the estimates are generated. A quality control procedure has been employed to make sure all cost changes can be attributed accurately. This system could be employed in many other complex mega-projects, especially fusion.