Fusion may be an energy technology that seems to be perpetually coming over the horizon, but projects around the world such as SPARC are getting increasingly close to reactors that, although smaller in capacity, can produce net positive power in just a few years.

“The project [SPARC] is enabled by the arrival of a breakthrough technology, high-temperature superconductors, which opens the ‘smaller, faster, cheaper’ path we are pursuing,” says MIT Plasma Science and Fusion Center deputy director Martin Greenwald. “Our plan is to carry out R&D leading to a demonstration of a high-performance, high-temperature superconducting (HTS) magnet at the scale required for fusion, followed by construction and operation of SPARC, which would be the world’s first net-energy fusion experiment. SPARC is – roughly – the smallest and least expensive fusion experiment that could achieve this goal.”

In the UK there is MAST, or the Mega Amp Spherical Tokamak, an alternative fusion project that is making big steps forward, and is also focusing on smaller reactors.

“Essentially, the whole genesis of the spherical tokamak is to look at whether there's a way of doing fusion on a smaller and therefore we assume cheaper capital cost basis,” says UK Atomic Energy Authority CEO Ian Chapman. Elsewhere, Tokamak Energy is developing modular reactors, with the aim of having a grid-connected power plant by 2030.

“Tokamak Energy is working on the design of a small modular fusion reactor to produce 175MWe or 450MW of heat,” says Tokamak Energy executive vice chairman David Kingham. “We are confident that the technology will be commercially viable and that fusion will be an important part of the long-term solution for electricity generation. We also expect fusion to become an important source of industrial process heat, for example to produce hydrogen without any carbon emission.”

The largest project currently underway is the International Thermonuclear Experimental Reactor (ITER), which is being built in France. It is a huge collaboration, bringing together nuclear specialists from 35 different countries, and builds on the work of the Joint European Torus (JET) in Oxfordshire, UK, which first proved nuclear fusion was possible in 1997.

ITER is aiming to generate 500MW of fusion power from 50MW of input heating power by 2025. This puts it roughly in the same generation capacity bracket as fission SMRs, although it is much larger.

All of these reactors, should they prove successful, would mark hugely important steps towards achieving efficient nuclear fusion. But none could really compete with current fission reactors; as such, it is questionable whether they could provide a real alternative.

For commercial nuclear fusion to become a reality, it does not need to quite reach its holy-grail predictions but to overcome economics. While work is underway to reach a technology that can produce safe, predictable, positive net energy from fusion, its realisation would only get fusion so far.

In order for nuclear fusion technology to become commercially viable, it must be economical. ‘Perfect fusion’ would be intrinsically economical, because it is an endless source of power once running. But the fusion we will likely reach within the next few years may not be so, and will possibly include huge capital costs.

“There are two factors that I consider important in an economical source of electricity,” explains Chapman. “First is the capital cost and second is the levelised cost of electricity. Obviously the cost of electricity has to be competitive; at the moment we're working on the basis that fusion will happen in the middle of the century, when the economics of electricity provision will be significantly different to how they are today as the imperative to reduce our carbon emissions goes up. Therefore you might imagine that carbon pricing will change dramatically over those couple of decades.

“What the strike price needs to be to be competitive in that market is frankly just not clear at the moment. If we assume the strike price needs to be roughly similar to fission, sort of £90-£100 per megawatt hour, that's where we're aiming for fusion reactors to come in, and it's quite conceivable that they could come in at that level. The cost of electricity I think can be competitive, and then the big issue is the capital cost of the plant.”

There are benefits to keeping reactors small, similar to SPARC and tokamaks, as it reduces capex and potentially speeds up construction time.

“If you look at the cost breakdown, roughly speaking about a third of the money is in the magnets, and a third of the money is in the buildings used to house the tokamak,” says Chapman. “So if you can make more efficient use of your magnets and put them in a smaller building, you can strip out considerable cost – that was the idea behind the spherical tokamak way back in the 90s.”

As the technology enters the market in an imperfect and most likely smaller format, fusion will have to struggle to become economic.

So will large reactors eventually win out, and will their smaller counterparts ever be commercially viable?

The majority of commentators argue that we are unlikely to see commercial, grid-scale nuclear fusion power until the 2050s. While technological breakthroughs currently are making huge strides in proof of concept, the challenge of perfecting aspects of the reactors such as the magnetic field mean there is still a lot of work to be done before it is safe and effective enough for commercial use.

As such it makes it hard to predict whether these smaller, 100MW reactors will ever form an important part of international energy mixes. But many believe smaller reactors at varying levels will play an important role.

“I think there's a market for both [large and small], and that will be country-dependent and market conditions-dependent,” says Chapman. “All energy markets are local, depending on various conditions, and it all comes down to the free market, right? So it depends on what the grids are like in different countries, it depends on the population distribution and density; all of these determine which is the right solution for any given market. So it'll be market dynamics that determine which type happens where.”

The majority of fusion projects are currently aiming to increase in size once the technology is proven and tested. Many see nuclear fusion eventually taking over from fission and fossil fuels as an environmentally sustainable alternative. As such, while smaller reactors offer reduced capex, larger plants will undeniably be needed to meet demand.

“Once SPARC is successful, the next project would be to build and operate a fusion pilot plant, to put electricity from fusion power on to the grid,” says Greenwald. “The purpose of this plant would be to demonstrate all of the science and technology required for economically competitive, mass production of fusion energy.

The project would be carried out mainly as a commercial endeavour. While we have only an outline for such a device, we can get some idea about what it might look like from the ARC concept, a project undertaken by a group of MIT students in a fusion design course. The ARC design was intended to show the capabilities of the new magnet technology by developing a point design for a plant producing as much fusion power as ITER at the smallest possible size.”

Researchers and scientists are unlikely to stop searching for perfect fusion as the energy source to end all energy sources. But given the potential for clean and reliable power that smaller-scale reactors offer in the meantime, it seems likely that they can still be a hit, as long as developers are able to make the all-important business case.