U.K. researchers this week offered the world a peek at their designs for a prototype reactor meant to show fusion power is practical, providing electricity to the grid by the early 2040s. To keep costs low, the Spherical Tokamak for Energy Production (STEP) may rely on electromagnets made from innovative superconducting tape, and its reactor vessel and magnets may be jointed to allow easy access to replace worn components, according to 15 papers published in a special edition of the Philosophical Transactions of the Royal Society A.
STEP is “a really important entry into the race for fusion,” says Dennis Whyte of the Massachusetts Institute of Technology. Steven Cowley, director of the Princeton Plasma Physics Laboratory (PPPL), agrees, saying the project “is probably one of the most advanced.”
All fusion reactors built to date have been research machines, designed to understand how to control and contain a roiling ionized gas, or plasma, at the hundred-million-degree temperatures needed for hydrogen nuclei to fuse. ITER, a massive international project under construction in France, aims to show that fusion reactions can produce a surplus of energy. Lately, developers have been planning the next step: building a demonstration power plant. Venture capitalists and wealthy individuals are bankrolling dozens of such efforts.
In contrast, STEP is funded by the U.K. government and led by scientists at a national fusion lab in Culham, part of the UK Atomic Energy Authority (UKAEA). With £300 million from the U.K. government, the project’s design phase began in 2019, and in 2022, the project selected a site for construction, a retired coal-fired power station at West Burton in Nottinghamshire. The project is now in talks with the government to fund its next 4 years to narrow down on a final design, according to Paul Methven, leader of the project and CEO of UK Industrial Fusion Solutions, a UKAEA subsidiary that will build STEP. Although STEP was launched by the previous Conservative administration, Methven says the project is a good fit with the newly elected Labour government’s emphasis on economic growth and achieving net zero carbon emissions. “So far, so good,” he says.
STEP’s design is a variant of the most successful reactor type, known as a tokamak. Tokamaks are doughnut-shaped reactor vessels enclosed by electromagnets that contain the plasma while other devices heat it. But the trend in tokamaks has been bigger and bigger. ITER’s tokamak is more than 19 meters across and the project will likely cost more than €25 billion. Europe’s planned next machine, known as DEMO, will be bigger still. Those sorts of expensive behemoths, Whyte believes, will be anathema to energy companies. “It’s going to be much more difficult [to interest them] at $10 billion than it is at $5 billion,” he says.
To keep costs lower, STEP’s tokamak will be spherical—more like a cored apple than a doughnut. This design, pioneered at UKAEA’s Culham lab, has been shown to stabilize plasmas and leads to an overall smaller, cheaper machine. But it presents a challenge in accommodating necessary magnet parts in a narrow, central core. In the papers published this week, the STEP team describes a machine that will be about 9 meters in diameter and will produce up to 200 megawatts of electricity.
Because it is not as well understood as conventional tokamaks, the spherical design poses some risks. The largest spherical tokamak currently in operation, the Mega Amp Spherical Tokamak (MAST) Upgrade, is only about 2.5 meters across, and machines can behave differently when scaled up. STEP also hopes to benefit from the experience of PPPL’s National Spherical Torus Experiment-Upgrade (NSTX-U) tokamak, which is similar in size to MAST-Upgrade but with higher heating power. NSTX-U has been offline for 8 years after suffering damage from a magnet fault in 2016. Cowley says the recovery is nearing completion and hopes it will soon achieve plasma conditions not too far from what STEP is aiming for. Once up and running, NSTX-U “will probably give them most of the information they need,” he says.
Like some private fusion efforts, STEP plans to construct its magnet coils with high-temperature superconductors grown as thin layers on metallic tape. This relatively new technology has yet to be tested in a fusion reactor, but should generate stronger magnetic fields and reduce power consumption. Also untested is the idea of having joints in the magnet coils so the reactor vessel can be opened for easier maintenance. In all previous tokamaks, the magnets have formed an almost impenetrable cage, leaving only small ports between magnets for access. That has meant that any maintenance, such as replacing the interior wall, can take years. STEP’s joints mean the upper part of the reactor vessel could be flipped open like a trash can. “STEP is one of the first places on the planet to actually take this seriously,” Cowley says.
Methven says developers are assessing about 60 variants of the design before deciding on a final configuration. “What we’re publishing at the moment is by no means the final destination,” he says.
