Rhian Chapman

Systems models and associated cost analyses are widely used within the fusion community to analyse tokamak designs, from prototype and demonstrator machines to potential commercial fusion power plants. To ensure the design programmes of fusion prototype/demonstrator power plants deliver a cost optimised design (within existing uncertainty limitations) the use of integrated cost modelling during the design process is essential. This integration produces holistic solutions in which engineering design choices and changes are directly represented in the cost results, allowing alternative solutions to be tested technologically and financially in the same analysis and cost estimates to be directly aligned with each specific design solution. Using examples from the STEP (Spherical Tokamak for Energy Production) programme this paper shows how implementing such an approach allows an interrogation of the design through the lens of cost-effectiveness, enabling a systematic exploration of potential trade-offs between performance and cost, highlighting cost drivers and interrogating the design aspects underpinning them, and facilitating holistic comparisons between design options. Including cost analysis into early design decisions through integrated cost modelling will drive a cost-optimised design; this is vital in proving that fusion power plants can be an economically viable energy source. One top-level example of this approach is understanding the critical size drivers and therefore cost drivers of the design, such as the inboard radial build for the STEP design. This understanding enables optimisation of this parameter within the relevant margins required to ensure performance (within design uncertainties).

Deuterium (D) and beryllium (Be) fluxes are obtained in JET Low-confinement mode (L-mode) plasmas at the outer limiters of the first wall using calibrated visible cameras. They are inferred from the measured radiances using the spectroscopic S/XB method. From the fluxes, the effective gross erosion yield Y ^eff of the limiter surface is estimated. After discussing the uncertainties in the proposed methodology, we show the dependence of the deduced particle fluxes and Y ^eff of recent JET L-mode plasmas on: separatrix–limiter clearance, magnetic field and plasma current, neutral beam injection and ion cyclotron resonance heating power, average plasma density and majority ion mass, hydrogen (H), deuterium (D) and tritium (T). The results are in general accord with prior edge plasma L-mode understanding. Finally, the obtained Y ^eff yields are discussed in view of updated SDTrim surface–particle interaction code calculations. The possible contribution of parasitic light due to reflections from the divertor is examined.

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)’.

Being a novel technology, estimating costs for fusion power plants comes with large uncertainties. Cost uncertainties in prototypes arise from various sources and reduce with program maturity (including the selection of a site), technical and design maturity, as well as the maturity of the commercial strategy (e.g., make versus buy decisions and partnering decisions). In the early design phases, where not much detail is fixed, the cost is typically evaluated via analogy/comparator to existing technologies, which are typically associated with high uncertainties due to the innovative nature of fusion technology. Early cost estimates for fusion power plants are broken down using a top-level cost breakdown structure as this allows for cost uncertainty evaluation over the various fusion components. The STEP prototype fusion power plant is nearing the end of conceptual design with significant uncertainties but seeks to follow best practices to effectively manage costs. Best practice in cost estimating involves evaluating the base cost estimate, corresponding uncertainties, and project risks and correcting for optimism bias as defined by the Infrastructure and Project Authority (IPA). The overall objective is to illustrate the importance of uncertainty definition within fusion prototype/demonstration power plants and their impact on program costs.