A U.K. spherical tokamak aims to generate 50 megawatts of fusion power when it starts up in 2040.
U.K. Atomic Energy Authority
The U.K. government today invited communities around the country to volunteer a site for a prototype fusion reactor, which would be the first—it is hoped—to put electricity into the grid. The project, called Spherical Tokamak for Energy Production (STEP), began last year with an initial £222 million over 5 years to develop a design. The U.K. Atomic Energy Authority (UKAEA), the government agency overseeing the effort, says construction could begin as soon as 2032, with operations by 2040.
“Any new device is welcome because it brings new insights,” says Tony Donné, director of EUROfusion, the European Union’s fusion program. But he suspects STEP won’t quite cut it as a power generator. “My impression is that it will be more of a component test facility.”
The race is on around the world to build the first fusion reactor that can generate excess energy. Fusion melds isotopes of hydrogen together in a superheated gas, or plasma—mirroring the process that powers the stars. The fuel sources are relatively plentiful and radiation concerns are slight compared with nuclear reactors powered by fission.
But as a practical power source, fusion has remained a distant dream. It requires temperatures of hundreds of millions of degrees. To prevent the hot plasma from touching and melting its containment vessel, engineers typically use powerful magnets that surround doughnut-shaped tokamaks. But no tokamak has generated more energy from fusion than is used to heat up the plasma. The ITER tokamak in France, due for completion in 2025, will be the first to demonstrate energy gain, although that won’t happen until after 2035 and even then, the fusion energy won’t be used to generate electricity.
The spherical STEP would look more like a cored apple than a doughnut. This confers more stability in the plasma so operators can achieve higher temperatures in a smaller device. Spherical tokamaks have been pioneered at UKAEA’s Culham Centre for Fusion Energy (CCFE), with a device called the Mega Amp Spherical Tokamak (MAST) Upgrade, and in the United States at the Princeton Plasma Physics Laboratory with its National Spherical Torus Experiment Upgrade device. The United Kingdom now hopes to capitalize on that experience with STEP, which would aim to generate 50 megawatts of electrical power. “STEP is a logical step after MAST Upgrade,” Donné says.
CCFE Director Ian Chapman says the small size of spherical tokamaks is a key advantage because the greatest cost in the $25 billion ITER is its gigantic magnets. With capital costs as low as a few billion dollars, Chapman says STEP would be far cheaper than ITER—necessary if fusion is ever to compete with fossil or renewable power stations that can be built for less and generate comparable amounts of energy.
But spherical tokamaks also come with drawbacks, Donné says. The hot dense plasma in a smaller device is more punishing on materials, so components may need to be replaced more often. And STEP is unlikely to be capable of breeding tritium, one of two hydrogen isotopes that fuels the reactor. Tritium is radioactive with a half-life of 12 years and global supplies are low. A working reactor will have to breed its own tritium by surrounding the vessel with patches of lithium that produce tritium when bombarded by neutrons from the fusion reaction. ITER will be the first attempt at demonstrating tritium breeding. STEP, Donné says, “couldn’t implement tritium breeding in such a short time.”
Donné also suspects there is a political element in the push for STEP. CCFE is also home to the Joint European Torus, now the world’s largest tokamak, which is nearing the end of its working life. Its demise could potentially leave a lot of fusion researchers with time on their hands. The United Kingdom’s future as a partner in the ITER project is also in question, if the country does not sign a trade agreement with the European Union. And CCFE has private sector rivals breathing down its neck. Tokamak Energy, a U.K. startup, is trying to build a compact spherical tokamak for energy production by 2030 and U.S. startup Commonwealth Fusion Systems has plans to start to build a similar working reactor by 2025.
That’ll be of little concern to the communities vying to host STEP, who will see it as a way to draw money and jobs to their region. They have until March 2021 to apply, and will need to offer 100 hectares of land, which will be vetted for geological suitability, access, and other criteria. UKAEA plans to choose a site by the end of 2022.