UK gives a taste of new apple-shaped fusion reactor

Nuclear fusion has enjoyed a renaissance in recent years, with a number of startups and governments taking the idea seriously. Now, British scientists have released a preview of a novel reactor design that could be supplying power to the grid by 2040.

Despite the technology’s reputation for always being 20 years away, recent years have seen a flood of investment as optimism grows that its time has finally come. Last year, $900 million in new funding brought the total to $7.1 billion, according to the Fusion Industry Association.

That optimism hasn’t been dampened by major delays in ITER, the international collaboration that has long been considered the flagship project for fusion. Building on knowledge gained from ITER and other publicly funded experiments, a number of startups are now betting they can deliver smaller fusion reactors in a fraction of the time and cost.

But it’s not just the private sector that’s pushing to commercialize the technology. In 2019, the UK government committed £300 million to a cutting-edge 200-megawatt reactor known as Spherical Tokamak for Energy Production (STEP). In a series of articles recently published in Philosophical Transactions of the Royal Society Aits designers have already revealed what they have managed to create.

The most common design of fusion reactor is the tokamak, which heats a cloud of ionized gas, known as a plasma, until the atoms fuse together, generating huge amounts of energy in the process. The plasma is held together by incredibly strong magnetic fields generated by coils of magnets wrapped around a doughnut-shaped reactor vessel.

STEP is based on similar principles but is tall and narrow, more like a hollowed-out apple, according to ScienceWhile this may seem like a small difference, it means that the distance between the center of the reactor vessel and the magnets surrounding it is smaller than in a classic tokamak.

This reduction in distance allows smaller, cheaper magnets to be used to hold the plasma, making the overall design more compact, according to Financial TimesThe spherical tokamak shape also produces an inherently more stable plasma, which should improve efficiency. However, the design has some drawbacks.

Fusion reactors typically use two isotopes of hydrogen as fuel, called deuterium and tritium. Tritium is extremely rare, however, so the reactors generate their own tritium through a reaction between the metal lithium and the neutrons released in the fusion reaction. This lithium is stored in tritium breeder blankets wrapped around the chamber, which also act as radiation shields, protecting the magnets.

The hole in the center of a tokamak typically houses large magnets and a breeding blanket. However, with the narrower design of a spherical tokamak, there is much less space, so the STEP reactor will have to get rid of the blanket and significantly reduce the size of the magnets, or even get rid of some of them.

Fortunately, a new high-temperature superconducting tape, which is also being used by a number of private startups, could help create more compact magnets. However, the reactor will have to generate enough tritium using only blankets on the outer wall of the chamber, meaning the team had to come up with an optimized design using liquid lithium and a vanadium alloy.

The reactor designers also opted for an ambitious architecture with joints in the magnets, which will allow the top of the vessel to be opened. This will significantly speed up maintenance work and thus reduce operating costs.

However, as project leader Paul Methven said, Science that the recently published designs are still a long way from being finalized. And while the project has already found a site – a closed coal-fired power station in Nottinghamshire – it is currently in talks with the UK government to secure four more years of funding to create a final plan.

So whether this reactor will ever see the light of day remains to be seen. But it is encouraging that the government is investing significant amounts of money to push the technology forward.

Image Source: STEP