In a serenely billowing cloud of gas and dust somewhere out in space, a swirl of turbulence brings enough particles together that the combined gravity begins to attract even more. As the lighter elements come together over the millennia, they eventually form a seething mass, crushing together around a dense core. More gas then pushes in from the outside, pressing down on the centre. At one brief moment in time, a threshold is crossed, and two particles join together to form one larger particle.…and a star is born.
Fusion for power
This process is called fusion. It is when lighter elements are pressed into forming progressively heavier elements, and each step releases a little bit of energy, which we experience on Earth as light and heat. Harnessing the power of fusion for commercial energy production has been a holy grail for science for over half a century. A star is the most abundant energy source we have encountered.
But the conditions in the stellar core are unimaginable. At the centre of our sun the temperature is around 15 million Kelvin, which is roughly the same in Celsius. And the density is around 160 grams per centimetre-cubed, which doesn’t sound like much, until you realise that iron is at less than 8 grams per centimetre-cubed, and even elements like gold and tungsten are less than 20. It is denser than the densest metals.
Britain is a small country, but even the largest could not feasibly contain a second sun, which is 109 times the diameter Earth. If we are to crack commercial fusion, we will need to do it on a far more compact scale meaning that numbers like 15 million Kelvin, are meagre. We need to go hotter… much, much hotter.
This is what the UK Atomic Energy Authority is seeking to accomplish. It aims to build a prototype fusion power plant by the 2040s. To do that they are currently working on a concept design, choosing from a menu of international technologies. Their work needs to be complete by spring 2024 to then work on and complete detailed design by 2030.The success of this project will be measured in one way, whether it can sustainably put electricity into the grid and this is a challenge that will push the limits of robotics, plasma science, materials science, nuclear science. But if we get it right, if all of the technologies fall into place, we have the potential to generate carbon neutral energy for millions of years.
Nuclear fission history
Historically to produce energy, we have used a process known as fission. “Fission is the process of taking a heavy atom that is unstable, bombarding it with a neutron, making a neutron hit that atom and splitting that atom into two,” says Nick Walkden, a nuclear physicist and Head of the Executive Office of the UK Atomic Energy Authority (UKAEA).
The two parts that it splits into, tend to have a slightly lighter mass than the original. “We’re lucky in this field, because we get to use the most famous equation in science, so E=MC2, where that change in mass creates energy. And it’s that that energy that we create out of the end of that, that reaction that we can end up harnessing as electricity,” says Nick.
Fission uses highly radioactive heavy elements such as uranium and this is what creates the heat, which converts water into steam which then turns a turbine in a conventional nuclear power plant.
“Fusion is almost exactly the opposite. So instead of taking one very heavy atom, we take two very light atoms and force them very, very close together. And they will bond and form a new atom, which is ever so slightly lighter than the two when they were separate,” says Nick.
Heavier than each individually, but lighter overall. That change in mass creates energy. And it’s that energy that’s released in the fusion process.
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And it is similar to the process undertaken in the sun, however Nick says that the process as it happens in the sun is actually quite inefficient so we have to use alternative materials. “Instead, we use deuterium and tritium, which are two isotopes of hydrogen on earth to do fusion because they react much more regularly. And they produce enough energy out that we can be reasonably assured that we’re going to get a decent return of energy,” says Nick.
But to make it workwe need to create temperatures 10 times hotter than the centre of the sun. 150 million degrees to be exact. “We can do it,” says Nick. “We’ve done it for the last 20 years or so, now what we need to do is focus on the engineering and the science that we need to take us from those sort of fusion lab experiments to deployable fusion power.”
From laboratory to deployment
Fusion research for energy itself has been going on since the 1950s. Among those concepts is a design called a Tokamak, started to emerge out of Russia, which is basically a toroidal chamber with magnetic coils. “What we’re really doing in a Tokamak is we’re replacing the effects that gravity has in the sun by using very, very strong magnetic fields to hold a superheated fuel in place,” says Nick.
Tokamaks have been developed for since the sort of late 1960s, early 1970s, through machines like JET, which offers high power fusion research. JET is the Joint European Torus project, an experimental fusion facility based in Culham, just to the south of Oxford. And in the future, scientists are looking towards machines like one being built in the south of France called ITER, which will be the first machine that proves that you can get more power out than you put into fusion.
New approaches emerge
In the 1980s a different approach started to emerge called the spherical tokamak, which still has all of the same features. “So it’s still got some vessel, it’s still got magnetic coils, it’s still got a hole in the centre. But what we’ve done is we’ve pushed everything much closer to the centre. So rather than a doughnut, it looks a bit more like a Terry’s chocolate orange that you’ve taken the centre out of,” explains Nick.
He explains that the smaller you make it, the cheaper it is to operate and build. So it’s a more cost-effective route to fusion power. But some of the challenges then become harder to deal with such as the excess heat issue is intensified.
The regular tokamak is the European approach. Whereas the British decision has been to go for the spherical tokamak design in a project called STEP – Spheroidal Tokamak for Energy production.
This is the only one of these experiments that intends to produce actual electricity for the grid, rather than just ‘net energy’. “We believe it’s the most cost-effective approach. So if you instead want to say, okay, we know a little bit less about the science and engineering, but we’ll put efforts in to try and close that gap. But what we do want to do is we want to focus on the design that gives us cost effective, cheaper, quicker, fusion power plants. This is the design that you go for.”
UK steps forward
Jenny Cane is product area lead for STEP. What that means is that I am responsible for the design of all the systems, all the components that sit right next to the middle of the reactor right next to the plasma, ones that take all the heat all the power and not only that, we have to extract the exhaust out of the reactor chamber,” she says. “We have to convert those high energy neutrons into the heat that is actually going to make the electricity. And we also produce the fuel for the reactor.”
Again the two different isotopes of hydrogen used are deuterium, which is one proton and one neutron, and tritium, which is far rarer and radioactive. Tritium is needed for a high-power reaction and needs to be generated on site in the plant itself. Jenny explains how this will work “So you’ve got your, your plasma changer, the centre where the actual plasma, the fusion reaction is happening, where the little star is. And then around that you at some point, you have to have a wall, we’ve got no choice. Now, within that wall, we not only have to capture those neutrons and turn them into heat. But we also need to breed the tritium.”
Incredibly this can be achieved by just having lithium in the walls. You can bombard it with neutrons, and you get tritium out. “It seems so simple, doesn’t it? It’s just you have to do it all in one place” says Jenny.
Deuterium coming from water and the lithium is also eventually expected to come from water, although lithium recovery from water is itself a problem that engineers and scientists are currently working on, primarily to supply the overwhelming growth in demand for batteries.
In designing the concept for this fusion plant, Jenny says the world’s many fusion research programmes offered up a menu of technologies to choose from with the US and South Korea for example. “And we have to decide what the best combination of those technologies are, to give us the best chance of a working power plant in the 2040s. So which ones are going to give us the best performance, but with the least amount of risk to be able to get us there?”
STEP involves around 200 people just at this concept phase, all with their own skills and focus areas… but Jenny says one of the most challenging problems, is the magnetism. “The magnets are a major challenge for the kind of fields that we need in Step or really any particularly spherical tokamak, then we need very high-powered magnets. And those magnets need to survive for a long time. That might mean that we’re either in low temperature superconductors or high temperature superconductors,” she says.
There is a lot of other work to do as well. The finished power plant will also need to be highly automated, the minimal human involvement requiring a lot of work in the field of robotics. Although the elements are much less radioactive than the fission alternatives uranium and plutonium. “So anywhere, you’ve got tritium, you can’t have humans there, unless they’ve got a lot of kit on,” says Jenny.
Another area of focus is materials, a hot topic within the fusion community. Not only are the components right in the middle of the reactor that sitting in 1000s of degrees of temperature they have got heat going in them, which is like having a blowtorch on them permanently, and are being bombarded continually by these high energy neutrons and particles that are trying to erode their surface and are basically changing those materials as the atoms get knocked about. “So that really limits the number of materials we’ve got to use, because most materials just cannot withstand those conditions,” says Jenny. “The good news is that already in the fusion community, there has been some work on tweaking materials. So trying to put different compositions of materials together. So you tweak from one material to something that works with those high energy neutrons coming in, and makes it so that it withstands the radiation loads that it’s getting. But there is much more work to do.”
Nuclear energy always makes people jumpy, and although there is radiation used for fusion, there is no runaway reaction to control. It takes activity to keep a fusion reaction going, and the moment you stop, the reaction stops. Which should make regulating it more straightforward. The existing fusion experiment Jet certainly operates in an easier regulatory environment that fission.
Commercial fusion is one of those technologies that has always been a few decades away, but with a projects now being designed to produce that first prototype, if all goes well, we could be entering the final stretch.
