STEP Fusion: Overview of the Concept Design for the Spherical Tokamak for Energy Production and Lessons Learnt

The STEP Programme began in 2019 with the mission to deliver a UK prototype fusion energy plant, targeting 2040, and a path to commercial viability of fusion. The STEP Prototype Plant (SPP) aims to demonstrate power plant characteristics by producing at least 100 MW_e net electrical output, being fuel self-sufficient, demonstrating high grade heat production, and a route to commercial availability. In this paper we give an overview of the concept design stage and discuss lessons learnt that are applicable to wider fusion power plant design. Concepts were initially generated using a combination of systems and equilibrium codes, that gave staring points for more detailed study and evaluation. From 69 different concepts explored, a down-selection was made to the SPP-1 baseline design. Further development identified challenges around the lifetime of the centre column magnet and the realisation of the blanket. This led to a pivot to the new SPP-2 design, increasing the major radius from 3.6 m to 4.275 m to increase the centre column lifetime from 0.22 FPY (Full Power Years) to 0.6 FPY , and changing from liquid lithium to a solid lithium oxide breeder blanket. The STEP programme will now look to increase the design definition and confidence as it moves to preliminary design.