Estimating costs of prototype or demonstration fusion power plants is difficult enough due to the often still preliminary designs combined with a non-existing supply chain for many bespoke technologies or materials. Extrapolating to commercial fusion power plants without a clear design is even harder and uncertainties are large. As a result, forecasts of commercial viability of fusion are often built on many assumptions that cannot be validated or refuted until the next set of prototype plants have been built. However, it is crucial to understand which factors impact the costs of commercial power plants to address the right validations either on prototypes or separate rigs/facilities on the path to commercialisation. Relative costs can be used to determine expected cost drivers for commercial power plants and help determine decisions that affect the balance between operational and capital costs.

For Mega-projects like fusion power plants, modularity is a key enabler to cost and schedule efficiency (e.g. Flyvbjerg 2021). One way of achieving more modularity is aiming for smaller and higher numbers of fusion reactors. Previous work (Wade & Leuer 2021, Sheffield 1986) has demonstrated that commercial magnetic confinement fusion power plants have an optimal design point with respect to net electric output where the levelized cost of electricity (LCOE) is significantly lower in comparison to lower net electric plants and aiming for higher net electric outputs only marginally reduces LCOE. This determines a viable minimum size for commercial fusion power plants. This work will investigate the sensitivity of these results with respect to multiple assumptions and identify the key drivers that need to be addressed on the path to commercial viability of fusion.