Jack Foster

The Spherical Tokamak for Energy Production (STEP) programme is a world-leading fusion power plant programme that has embedded a cost conscience in its design from the early phases. This firmly addresses the attitude of cost complacency of which many major infrastructure projects have historically been accused. While a detailed and highly accurate whole life cycle cost analysis is not possible, or even valuable, during the conceptual design stage, this early design phase is still the most critical programme phase where a focus on costs can drive longer term reductions and impact whole life cycle costs at the high level. Consequently, appropriate estimating methods for these early-stage designs and lessons learned from other industries are used to inform design decisions and ensure cost is part of the overall option analysis. Hence, while the overall programme cost estimate is too immature to be a reliable indicator for the final programme costs, significant effort has been undertaken to understand the major cost drivers and take action to make the STEP design as cost-effective as possible. This article is part of the theme issue ‘Delivering Fusion Energy – The Spherical Tokamak for Energy Production (STEP)’.

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.

For mega-projects like fusion power plants, modularity is a key enabler to cost and schedule efficiency. One way of achieving more modularity is aiming for higher numbers of smaller fusion reactors. Previous work has demonstrated that the levelized cost of electricity (LCOE) of commercial magnetic confinement fusion power plants falls at a decreasing rate with increasing net electric power. Furthermore, net electric power increases more rapidly than size/cost. This is because as fusion power increases the proportion of energy being exported as net electric power plateaus but the size of the plant required increases linearly. Increases in plant size increase upfront capital costs and project complexity. Therefore, there is an optimal design point beyond which any increases in net electric power continue to increase the project cost and complexity but deliver only marginal gains in LCOE. This helps identify a sweet spot between anticipated, better economy of size (cost per unit being smaller at larger unit sizes), and economy of scale (cost per unit being smaller at a higher scale of production).

We provide a cluster-algebraic approach to the computation of the recently introduced generalised biadjoint scalar amplitudes related to Grassmannians Gr(k, n). A finite cluster algebra provides a natural triangulation for the tropical Grassmannian whose volume computes the scattering amplitudes. Using this method one can construct the entire colour-ordered amplitude via mutations starting from a single term.