The UK's bold bet on a compact spherical tokamak to deliver fusion electricity to the grid by the 2040s — the first DEMO-class device to pick a site.
STEP — the Spherical Tokamak for Energy Production — is the United Kingdom’s national fusion power plant program, managed by the UK Atomic Energy Authority (UKAEA). Announced in 2019 and funded with an initial £222 million from the UK government, STEP aims to build a net-electricity-producing fusion power plant by the early 2040s. In October 2022, the UKAEA selected West Burton in Nottinghamshire — the site of a decommissioned coal-fired power station — as STEP’s home, making it the first DEMO-class fusion project in the world to choose a construction location.[1]
STEP departs from the conventional tokamak geometry used by ITER and EU-DEMO. A spherical tokamak (ST) has a much tighter aspect ratio — the ratio of major radius to minor radius is typically 1.5–2.0, compared with 3.0–3.5 for conventional designs. This gives the plasma a cored-apple shape rather than a doughnut. The physics advantage is that spherical tokamaks can achieve higher plasma pressure relative to magnetic pressure (higher beta) for a given field strength, potentially enabling compact, high-power-density reactors.[2]
The UK has decades of experience with the spherical tokamak concept through START and MAST at Culham, and MAST Upgrade (operational since 2020) is generating data directly relevant to STEP’s design. The Super-X divertor concept being tested on MAST Upgrade — which spreads exhaust heat over a larger area by extending the divertor leg — is a candidate technology for STEP’s power exhaust system.[3]
STEP’s design is still evolving through its conceptual phase, but publicly released parameters indicate a device with a major radius of approximately 3.6 m, an aspect ratio near 1.8, and a fusion power target in the range of 1–1.8 GW. The net electrical output goal is on the order of 100 MW or more — enough to prove commercial viability without requiring a full-scale power station. STEP plans to use high-temperature superconducting magnets, aligning with the broader industry shift away from conventional low-temperature superconductors.[4]
The spherical tokamak geometry that gives STEP its physics advantages also creates unique engineering difficulties. The narrow central column — the cylindrical core around which the plasma wraps — must carry the toroidal field current, withstand enormous electromagnetic forces, and survive intense neutron bombardment, all in a volume far smaller than a conventional tokamak provides. Shielding the central solenoid and breeding tritium in a compact inboard blanket are among the hardest design problems the STEP team faces.
As of mid-2026, STEP is in its conceptual design phase, with a planned transition to engineering design around 2027–2028. The UK government has committed to a regulatory framework for fusion energy and has designated the Environment Agency as the responsible regulator. Construction is targeted for the early-to-mid 2030s, with first operations expected around 2040. The siting decision, early regulatory engagement, and dedicated government funding make STEP one of the most concrete national DEMO programs in the world, though significant physics and engineering questions remain to be resolved.[5]