Tokamak Energy splits ST-E1 pilot and commercial power targets for the first time

The red and blue split in this ST-E1 cutaway is the design principle the whole collection is built around, permanent capital assets protected from the parts that get swapped out

(Image courtesy of Tokamak Energy)

Tokamak Energy has published the pre-concept design of its ST-E1 fusion power plant as an 11-paper Focus Collection in Nuclear Fusion, putting the design through peer review for the first time. The collection sets out assumptions, trade-offs and open uncertainties across plasma physics, magnet engineering, maintenance and nuclear systems. For readers who have tracked ST-E1’s numbers through earlier conference disclosures, the peer-reviewed figures add real specificity to what the company will commit to in print.

What the ST-E1 design now commits to

The reference design point is a steady state plasma tokamak with a 5 m major radius, an aspect ratio of 2.3, and an on-axis toroidal field of 5.25 T. Tokamak Energy frames ST-E1 around a “single device, two lives” concept, in which long-lived assets such as the magnet cage and vacuum vessel are deliberately separated from replaceable in-vessel systems.

The overview paper sets a commercial phase target of 800 to 1000 MW net electric power, at a normalised capital expenditure of $12,000 to $14,000 per kW of net electric power. A separate paper on thermal management and net-power evaluation gives the pilot phase, the nearer-term deliverable under the DOE milestone, its own figure: a net power target of 300 to 500 MWe. A radial maintenance scheme allows entire toroidal segments to be removed along a single axis, a design choice the papers link directly to plant availability and commercial viability.

The pilot and commercial targets are now split out separately

Tokamak Energy’s first public reveal, at APS DPP in October 2024, described a plant capable of 800 MW of fusion power and 85 MW of net electricity. A year later, at APS DPP 2025, the reference point known as revision D was quoted at a net power electric output of 450 to 750 MW, without a stated pilot and commercial split.

The July 2026 papers present a more structured pair of figures instead: 300 to 500 MWe net power for the pilot phase, and 800 to 1000 MW net electric power for the commercial phase. The two should be read as separate disclosures at different stages of the design, rather than a stated revision of the 2024 and 2025 figures. The split does clarify one thing the earlier disclosures didn’t: which target belongs to the near-term pilot plant, and which belongs to the longer-term commercial rollout.

Maintenance and nuclear engineering share billing with plasma physics

Reactor economics run through several papers beyond the headline capex figure. The tritium systems paper sets an initial tritium breeding ratio target of 1.20 for early scoping work, a figure the authors describe as a rough boundary rather than a settled design value. The neutronics paper identifies shielding effectiveness for the high-temperature superconductor magnet systems as a major driver of tokamak size, plant lifetime and balance of plant.

The maintenance and structural design paper estimates that structural optimisation, including compression rings around the toroidal field coil corners, could reduce required structural mass by up to 30% without compromising performance. That figure sits alongside an acknowledged open question: thermal contraction stresses during cool-down still need assessment against design limits.

A transatlantic delivery structure behind a UK company

The work was carried out under the U.S. Department of Energy’s Milestone-Based Fusion Development Program, funded in part under TIA DE-SC0024889, and Tokamak Energy is one of eight companies selected for the $46 million programme. The author lists span Tokamak Energy’s UK teams alongside researchers credited from Princeton Plasma Physics Laboratory and Oak Ridge National Laboratory, consistent with the collection’s own description of a coordinated effort across private industry, national laboratories and academia.

The papers are explicit that this is a pre-concept stage, not a construction commitment. Several, including the maintenance and structural design paper, flag specific follow-on work needed before the design can progress, from thermal stress qualification to further integrated assessment of tokamak and plant architecture. The next disclosure worth watching is whichever paper or milestone report narrows these ranges further, particularly on the pilot-phase target.

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