The Physics of Fusion
Chris - To give us the basics of nuclear fusion and what it is we have got with us Professor Steve Cowley. He's from the Culham Fusion science centre in Oxfordshire. Hello Steve.
Steve - Hello.
Chris - Welcome to the Naked Scientists. How does fusion differ from fission, the thing that powers the nuclear power stations we have here in Britain?
Steve - Behind all these ways of making energy is that fact that the most stable nucleus of an atom happens to be iron, right in the middle of the periodic table. It's a medium-sized nucleus. Any way you can go towards it you can gain energy. Fission is splitting very big atoms, nuclei of atoms to go towards iron. Fusion is joining very small ones together to make bigger nuclei to go towards iron. It turns out the easiest reactions to do with fusion are to join hydrogen together to make helium.
Chris - Is that why if you make a star like our sun as it ages it builds up a core of iron because it's fused all of its products to make iron and that's when it runs out of energy so it blows up?
Steve - Yes. Stars start with hydrogen and a little bit of helium. They join the hydrogens together to make helium. They join the heliums together and they make carbon and oxygen and all the things that life is made of and gradually work their way towards iron. When they burn out some of them burn out before they get to iron. The bigger ones go to iron. That's how you get iron that we have on Earth.
Chris - Let's have a look at the nuts and bolts of the fusion process then. You have a star which starts off as a big ball of gas which collapses in as everything rubs together and gets compressed. That's largely hydrogen. How does it join up and make these other things to end up as iron?
Steve - The problem with fusion is that in order to get them to stick together you have to get them really close. There's a force in the nucleus which is called the strong force. It only acts over a very short distance. There's another force which is the electric force which acts over a long distance. When you've got two nuclei far apart they repel each other because they're the same charge. Two nuclei are both positive charges. They repel. When you get them really close together they grab each other and stick. To get them that close you have to get over that repulsion all the way. I like to think it's a bit like having an enormous hill. At the middle of the top of the hill you've got an incredibly deep well. What you've got to do is get your two hydrogens up to the top of the hill and to drop into the well at the top of the hill. Then you can release lots of energy. The problem is you've got to fire them together hard enough that they get over this repulsion. It's sort of like playing golf in some crazy golf course. You've got to fire the golf balls up the hill and let them drop into this well. You've got to fire the hydrogen together really hard. Most of the time they just bounce off each other and then occasionally they get close enough that they stick. They grab each other, they make helium and you get fusion from that. It's a little more complicated than that but the key fusion reaction that you want to do, the on that's the easiest to do, is between two isotopes of hydrogen. Two kinds of hydrogen. One's called deuterium, the other one's tritium.
Chris - Why those and not standard, belt and braces hydrogen?
Steve - It's a complex process to take two hydrogens. Helium is actually four particles in the nucleus and ordinary hydrogen is just one proton. When normally you fire hydrogen into hydrogen you make a very slow reaction that really only goes on in the sun to make an intermediate stage which is deuterium. Deuterium is heavy hydrogen. It has one proton and one neutron. In order to fuse those together to make helium we do a reaction in our lab which is the fusion of deuterium with another kind of hydrogen called tritium which is one proton and two neutrons. You fire them together. For a moment they all stick together and then it disintegrates into helium and one neutron left over. Each time you do that you release enormous quantities of energy.
Chris - On the sun what sorts of conditions are there presiding over this reaction in order to make it happen? What have we got to try and aim at here on Earth to get the same thing going here?
Steve - On the sun you've got 10-15 million degree stuff there which is called a plasma. This means the nuclei are running around free. They're not in atoms any more. They don't have electrons going around them. Electrons are running around free, the nuclei are running around free. They're running very fast so they keep bumping into each other and every now and again a fusion reaction happens. The fusion reaction happens. It releases energy. It supplies energy to this very hot stuff in the middle of the sun and more reactions happen. Gradually, over time it releases that heat that works its way to the surface and comes out as light.
Chris - One thing that fusion has is a very clean image. It's viewed as a clean source of energy. The sun pumps out this cosmic wind which, if we get basted by it is fatal. Why has fusion got this clean image and why do we view it as this salubrious counterpart and reverse of fission?
Steve - So what you've gotta do to make fusion happen is you've got to make something that hot. In fact, in our experiments at Culham we get things up to 100-150 million degrees, actually much hotter than the centre of the sun, ten times hotter. At those temperatures obviously you've got to keep it away from the walls. If it touches the walls it'll get cold so we do that with magnetic fields. When we do that we can find this thing at 100 million degrees this plasma particles are bouncing into each other all the time and making fusion happen inside there. The radiation that comes off is confined both by the magnetic field and by the walls itself.
Chris - So nothing can escape, we hope?
Steve - Nothing can escape. With fusion we aim to produce a power that has no long-lived radioactive waste. The only problem with that is that you make a little bit of radioactivity in the walls. You design the walls so the radioactivity dies away very quickly.