The United Kingdom's Spherical Tokamak for Energy Production — a government-backed prototype fusion power plant targeting grid electricity in the 2040s.
STEP (Spherical Tokamak for Energy Production) is the United Kingdom's flagship fusion energy program, managed by the UK Atomic Energy Authority (UKAEA). The program aims to design, build, and operate a prototype fusion power plant that delivers net electricity to the grid, representing a critical bridge between today's experimental devices and tomorrow's commercial fusion fleet.[1]
STEP is based on the spherical tokamak (ST) configuration — a compact variant of the conventional tokamak with a very tight aspect ratio (the ratio of major radius to minor radius). Spherical tokamaks produce higher plasma pressure for a given magnetic field strength compared to conventional tokamaks, offering a potentially more economical path to fusion power. The design builds directly on decades of research at the MAST and MAST Upgrade facilities at Culham.[2]
The STEP reactor concept envisions a device capable of producing several hundred megawatts of net electrical power. The design incorporates high-temperature superconducting magnets, advanced divertor solutions for heat exhaust, and a tritium breeding blanket to achieve fuel self-sufficiency. The plant is intended to operate with deuterium-tritium fuel.[1]
The STEP program entered its conceptual design phase in 2019 with initial government funding of £220 million. The program follows a staged approach: conceptual design (through the mid-2020s), engineering design, construction, and commissioning targeting operations in the 2040s. The UK government has committed significant additional funding and has established a regulatory framework for fusion energy through the Energy Act 2023, which classifies fusion distinctly from fission.[3]
STEP is notable as one of the few nationally funded programs worldwide explicitly targeting a prototype fusion power plant rather than a scientific demonstration device. It positions the UK as a leader in the commercial fusion race and serves as an anchor for the country's broader fusion industry strategy, including supply chain development, skills training, and regulatory innovation.[1]