Nuclear fusion 101

How do we, in theory, recreate the process which gives the sun its energy here on earth?
21 February 2023

Interview with 

Brian Appelbe, Imperial College London




To kick us off with some of the background to nuclear fusion, we’ve got Imperial College London physicist Brian Appelbe...

Brian - Yes, nuclear fusion at its most basic level is just a nuclear reaction in which you take two nuclei and these are light nuclei that come from near the start of the periodic table and you force them to join together. You fuse them. And when you do that, Einstein's famous equation, e = mc^2, possibly the most famous equation in physics, comes into play. A small amount of the mass of those nuclei that you're fusing together is converted into kinetic energy of the particles. So really you're not creating energy, you're just moving energy from being part of the rest mass of the particles to being a kinetic energy which you can then recover and use to create electricity. But yes, this is different to our current power plants which are all based on fission. Fission is whereby, again, it's a nuclear reaction, but it's when you take the heavy elements from the end of the periodic table, and you essentially cause them to break apart, you bombard them with neutrons, they break apart. And again, Einstein's equation means that that releases energy. So fusion is when we are joining the particles together. The real challenge though is, when we bring the particles close together, they start to repel each other electrically. And what we need to do is we need to make this thing that we call a hot plasma whereby the fusion reactions can actually happen.

Chris  -Yes, we know the physics works because we look up in the sky and there is a sun which has been there for billions of years and is responsible for all life on earth, or at least sustaining all life on earth. And therefore we know the physics works. But we basically have to try and recreate that here on earth.

Brian - I think that's very true. And sometimes us fusion physicists question, are we engineers or physicists? So this is the process by which the sun gets all its energy. So we know it works. What we are trying to do is do it in a controlled manner, at the moment in the laboratory. Can we essentially get these reactions to happen on a small enough scale such that we can actually recover the energy and convert it into electricity. So that's really challenging because when you make this plasma hot, it immediately wants to blow itself apart. The sun works because it is so massive that the gravitational force from all the massive material in the sun holds it together. But when we make our plasmas in the laboratory hot, they tend to just blow apart. So we must find ways of confining them and keeping them in place.

Chris - Why is this regarded as such an energy panacea? Why is this better than the fission process that we know we can already do, do well and produce lots of energy that way?

Brian - It's a nuclear form of energy. So that means it's not going to really emit many greenhouse gases as compared to fossil fuels. Then comparing it with fission, essentially you get more energy per unit mass from fusion than from fission energy. So it's a more efficient source of energy. And then also because you're taking nuclei that are near the start of the periodic table, they are less radioactive than the fission materials that you use. So the nuclear waste from fission, we have to store that for thousands of years. And I think the final point contrasting it with fission is we feel that it is a safer form of energy in the sense that there is zero chance by design and the way the fusion reactions work of any sort of nuclear meltdown in the types of power plants that we would have.

Chris - Interesting. When you mentioned that we're fusing smaller elements to make bigger ones, and scientists often talk about using particular forms of hydrogen, heavy hydrogen like deuterium and tritium to do this. Why are we fixated on that recipe? Can we not just take any old element from the periodic table and fuse it with another one and make a bigger one and get the same process?

Brian - Well, we could up to iron. So iron is element number 27 in the periodic table. All the elements up to iron, if we fuse them together, we will release energy. But there are two specific reasons why we take D and T, these isotopes of hydrogen. And that is because first of all, they've a very large cross section and what we mean by cross-section is the probability when you bring them close together that they will actually fuse.

Chris - This is deuterium and tritium?

Brian - The deuterium and tritium, yeah. They're isotopes of hydrogen. So that's one reason. And then the second reason is how much energy is released when you actually have the fusion reaction? And so the deuterium tritium D, T pair is the pair that releases the most energy per reaction happening.

Chris - So we get the best bang for our buck?

Brian - We get the best bang for our buck.

Chris - When we were thinking about harnessing energy that was in say, oil or coal hundreds of years ago, we built an engine that would liberate the fuel in the right way. What sort of engine should we be building for liberating the energy that's in fusion fuels like you've just been describing? What should that engine look like?

Brian - So I think the engine is probably really dictated by these constraints that we have that we need to make something very hot and then keep it in place for sufficiently long time for these reactions to happen. And so there's at the moment two different designs or concepts for these engines. There's one form called magnetic confinement - and I believe we'll have someone speaking a little bit more about that later - in which you have this hot deuterium, tritium plasma and then you wrap it in magnetic fields. And these magnetic fields keep it in place while the reactions are happening and it produces the energy. And the other form of engine is something that we call inertial confinement fusion, where we just compress the fuel as rapidly as possible. And doing that compression makes it very, very hot. And then we aim to get as much of the fuel to burn up in as short timescale as possible.

Chris - And what do you think are the big hurdles that need now to be surmounted to reach George Freeman's 2040 goal?

Brian - At the moment when these reactions happen, we produce a burst of neutrons, high energy neutrons, and going from creating a burst of neutrons to having electricity is a non-trivial challenge. It involves a lot of material science that we have yet to do.

Chris - Brian, thank you very much. That's Brian Applebee. He's staying with us across the programme. We'll hear more from him in a minute.


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