Nuclear fusion is the power source inside the Sun - so can we copy the physics of the stars to generate energy here on Earth?& This week, we find out how nuclear fusion works, and how to mimic a star on Earth...
In this episode
How do blind people dream?
Chris - I know this because people have told me. My friend who I mentioned earlier was blind from birth and he tells me that his dreams were mainly words and sounds. In other words the kinds of things that you're exposed to that you have experience of experiencing. He would just experience those.
Why is there so much radiation in space?
Chris - The answer is that in space there's nothing to stop the radiation and so there's a million mile an hour maelstrom of cosmic radiation which is streaming off our own sun. If we didn't have the magnetic field of the Earth to deflect it away then it would be basting us in radiation. It would also be plucking away our water and our gases on our planet and we'd end up like Mars ultimately did. It isn't a safe and nice hospitable place in space. It's not just because of the use of fusion.
20:40 - The Physics of Fusion
The Physics of Fusion
with Steve Cowley, Culham Science Centre
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.
28:01 - Fusion Power at JET
Fusion Power at JET
with Andrew Kirk and Jef Ongena - JET
Meera - This week I'm at the Culham Science Centre in Oxfordshire which is home to the Joint European Torus Project, also known as JET. The world's largest nuclear fusion research facility. Nuclear fusion is the process that occurs in our sun to keep it burning. If at all possible on Earth it could provide us with vast amounts of energy. Current work on fusion involves heating the hydrogen isotopes deuterium and tritium to high enough temperatures that they fuse together to form helium, releasing more energy as a result of this fusion. It's proving to be a real challenge because whilst there are techniques to heat and energise the atoms such as current and beams of high energy atoms the real trick to actually maintain these temperatures long enough for fusion to occur continuously. With me now is Andrew Kirk, a senior scientists here at the Culham Science Centre. So Andrew, fusion happens so naturally in our sun. Why is it so hard to recreate here on Earth?
Andrew - To make fusion happen we have to get two atomic nuclei close enough together to make them fuse. That can only happen if the two nuclei collide at very high speeds, high temperatures. So tens to a hundred million degrees centigrade. Once you've got particles at these temperatures you've got to find a way of actually keeping them together and not making them melt any material surfaces. What we do is we use magnetic fields to actually constrain the charged particles in a plasma and keep them away from these surfaces while we try to heat them up to these extreme temperatures.
Meera - How do you actually go about doing this an creating fusion?
Andrew - We use a machine called the tokamac which is a Russian acronym which basically means a magnetic bottle. This allows us to actually constrain charged particles.
Meera - How does it go about doing that?
Andrew - A tokamac is a sealed vacuum vessel. The inside of a tokamac actually resembles a ring donut into which we inject a small amount of gas. Instead of using hydrogen we actually use the heavier forms of hydrogen called deuterium and tritium. We then take this gas and turn it into a plasma. A plasma is the fourth state of matter. You know you've got solids, liquids and gases. The next stage is a plasma in which you've stripped the electrons off from the atoms. You've got the positively-charged nuclei and the electrons together in effectively an electromagnetic gas.
Meera - What happens once you've created this plasma then?
Andrew - What we then do is we use the magnetic field to shape the plasma and to keep it away from touching the sides of the vessel. Then we actually start to heat it.
Meera - Why do you need to keep it away from the sides of the vessel?
Andrew - Because anywhere this plasma comes into contact with the vessel a) it would erode the material or damage the material of the vessel but more importantly it would actually cool down the plasma and it would stop the fusion happening. Or you'd have to put in a lot more energy to keep the plasma hot.
Meera - How does the tokamac actually do that?
Andrew - We generate the magnetic field in such a fashion that the charged particles would follow a magnetic field: spiral around and aournd the tokamac in a shap ethat resembles that of a slinky spring. They follow around in this helical pattern all around the tokamac. The slinky spring stops the charged particles escaping from the edge of the plasma and therefore keeps them away from the walls. We put a gap of about ten centimetres away from the plasma and the wall.
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Meera - So I've now come to the control room of the project. The tokamac isn't actually very far. It's about sixty metres away from us. With me now is Jeff Ongenar who's the task force leader on the JET project. How big is the actual tokamac here? We can only see a slight part of it but how big is the overall thing?
Jef - The overall thing is about 30 metres high and 30 metres in diameter. It has to be a certain size to produce a certain amount of energy. That is what physics teaches us. Small machines can only do a little bit, larger machines can do much better. Then a reactor will even be larger than JET.
Meera - So we're here now in the control room. What's monitored on the project here?
Jef - The control room is, in fact, the place where we control and plan the experiments. He set up all the physical parameters needed to run the machine for a particular experiment to start a particular idea. Every twenty minutes we can have a new experiment. A new experiment means that we change another parameter, see the effect. The final aim of all these experiments is to get to the best possible magnetic confining system that means we want to optimise the heat we need to get the reactions going. We want to optimise the time the heat stays in the machine because that will then allow to run as efficiently as possible.
Meera - So we heard a few minutes ago that a pulse had just taken place. We've got this screen here in front of us that has all the facts and the stats of the previous pulses that just happened in the past couple of hours. It's all to optimise the confinement of the plasma to keep it at as high a temperature as possible. What has the energy output in relation to input been so far with the project?
Jef - It's designed to show that fusion is scientifically possible. When JET was planned we had only small machines which fit more-or-less on the table. Europe decided to take a bold step and to build a much larger machine to show that the amount of heat produced by fusion reactors could be euqal to the amount of heat you need to get reactions going. We have proven that we get to about 70% of the heat back compared to the heat we put in. I think with current developments of the last years if we try again we will get much closer to one now. In fact the scientific possibility of fusion essentially is shown.
Meera - Are you now hoping to create more output - I.e. create more energy than you are putting in and therefore obviously having an energy source?
Jef - That will not be possible because JET is not built like this. To get more heat out you need a larger machine. This larger machine is designed and is now starting to be constructed in Cadarache in France. This machine is called ITER which stands for International Nuclear Experimental Reactor. ITER is there to show that science and technology now go together and can be used to realise a fusion reactor.
35:53 - ITER and the Future of Fusion
ITER and the Future of Fusion
with Steven Cowley, Culham Science Centre
Chris - They said size is important, Steve. This certainly is big budget. This is our best bet of trying to make fusion a reality, isn't it?
Steve - It's going to be a fabulous experiment to be involved in because it's going to produce 500MW of fusion power. When you think that a typical power station is about a gigawatt so about twice that. This is incredible. This is getting to industrial scale of fusion power. It's really showing that we can do it and that we're getting there. It won't produce any electricity but it'll get us right to the very edge of being able to produce electricity with fusion.
Chris - Could you just paint us a picture of what actually is ITER, how will it work and how will you get the energy that the fusion reaction makes out in the future to make things like electricity?
Steve - In the fusion reaction out comes a neutron. ITER will be a great big donut. It's a ring donut shape. It has six metres radius. It will produce fusions in the middle of that ring and they will come out as this neutron. The neutron doesn't get captured by the magnetic field because it doesn't have any charge and so it comes out and it comes into the wall. In an energy producing reactor we have something in the wall called a blanket. In the blanket the neutron hits actually lithium in the blanket. It makes tritium which is the fuel you put back in again and it makes energy which you extract as heat and you drive a normal turbine out of that heat. In the world there's so much lithium and deuterium that we'll be able to run fusion reactors for millions of years.
Chris - Assuming that we can get them working!
Steve - Yes!
Chris - Where actually in France is this happening and what stage of the build are you now at?
Steve - It's being built near Aix-en-Provence so a nice part of France, actually.
Chris - Good wines there.
Steve - Yes, a nice rose. Near Cadarache. It's going to take about ten years to build. During that time JET will continue to operate at Culham and we're hoping to break all our records in the near future - get more fusion power out of JET than we got in the 90s.
38:43 - Fusion Power with Lasers
Fusion Power with Lasers
with Kate Lancaster, STFC Rutherford Appleton Laboratory
Helen - Now we have Kate Lancaster in the studio with us. She's from the Rutherford Appleton Laboratory. She's looking at generating fusion using lasers. Hi, Kate. Thanks for coming on the Naked Scientists.
Kate - No problem.
Helen - Great to have you here. First of all, why lasers? Where do they fit in to this whole picture of fusion in the laboratory?
Kate - Ever since lasers were invented in the sixties there was an idea that you could use lasers to drive fusion. Essentially, there is more than one way to skin a cat with lasers but I'll describe the current most popular way of doing it. Essentially it's like a petrol engine where you have a compression phase where you use long-pulse lasers to compress a fuel capsule made of deuterium and tritium, the two isotopes of hydrogen that Steve was talking about earlier. Then the second phase is to heat this. The long pulse lasers that I'm describing are a billionth of a second so not particularly long n your field of view but actually acres of time in laser terms. The point of compressing the capsule is basically in order to move the atoms much closer together so you compress the material to hundreds of times solid density.
Helen - How are you doing that compression?
Kate - You've got symmetrically irradiated capsules so you irradiate all around a sphere. The laser hits the surface and heats up some of the surface which flies away. Basically, due to Newton's third law the rest flies forward. If you can do this symmetrically all of the material flies forward and compresses together to high density.
Helen - So it's like a sphere and it's all coming towards the core of the sphere.
Kate - Exactly.
Helen - You said the lasers are very quick and short in duration. Are there lots of them? Is it a continuous stream of them coming on and off?
Kate - No essentially this compression phase takes the duration of the laser pulse. A few nano-seconds which is this billionth of a second.
Helen - That's enough to heat up this sphere of matter?
Kate - The heat part comes next. Basically, as I said it's like a petrol engine so you've done the compression part but now you need the spark plug. What you do is you have an even more intense, more powerful laser beam which is injected in. What happens there is that actually when it interacts with the dense material it produces hot particles like electrons, for example. They stream in and deposit their energy to raise it to the 100,000,000 degrees centigrade that you need for fusion to occur. We know quite a lot about the compression side of things because, as I've said, this has been around since the 1960s. It's the spark plug bit which is the unknown thing. What I spend most of my time trying to investigate how these particles are generated - how they do the heating.
Helen - Have you got this to actually work yet or are you still fiddling around with that ignition part?
Kate - yeah so essentially laser facilities at the moment are only just being built which have any capability of really properly demonstrating such a technique. There were proof of geometry (I'm not even going to say proof of principle experiments) that you could compress and inject some short pulse heating beam in Japan back in 2000, 2001. They were very successful experiments and they sort of spawned this whole field of interest that really helped. You know, essentially we have a lot of work to do in order to demonstrate. But we're trying to get a laser facility built in Europe called HiPER laser. It's not the same scale as ITER but it's a huge facility which will try to test this technique and try to get gain out of it - actually get energy out. It won't generate electricity but again it's going to be one of those things where we can actually try to prove the principle. If you're interested in the details of the website it's
www.hiper-laser.org. There you'll find all the details of this project. We're currently in the preparatory phase at the moment. We've got money from Europe to try and design this laser. It's very exciting.
Helen - I'm quite keen to know what sort of scale this might be on. Also really when is it going to happen if you're going to take a guess? When are we going to see this?
Kate - The time scales are - Hyper is going to take 8 years to de-risk and design and should be operational by the early 2020s. After that it's going to be at least 20 years after that. It's a long term thing but the fact is that it's so attractive you have to continue to work on it.
As well as her work on making fusion power a reality with lasers, Kate is also part of the EPSRC's NOISEmakers campaign. To find out more, visit the
New Outlooks in Science and Engineering (NOISE) website.
44:22 - Letting Batteries Discharge?
Letting Batteries Discharge?
We put this to Patrick Palmer, University of Cambridge, Department of Engineering:
This is a very good question that exercises me most mornings after I've cleaned my teeth and I don't know whether to put my toothbrush back in its holder and charge it or whether to just leave it on the side. There is some truth in the fact that the nickel cadmium battery which is the light-weight one occasionally needs to be helped by being deep-discharged. Most of the time just discharging it 20% and recharging it is okay but it needs to be reset once every month or so, something like that. The lithium battery that's popular in telephones is also light-weight. These, however, do need considerable care. That's why you find lithium batteries in mobile phones and in laptop computers. Their charging and recharging has to be monitored very carefully. They have protection circuits in them usually. So occasionally it probably is a good idea to let your laptop run flat. Do that occasionally because that allows the computer to recalibrate itself and be up and running for the future. The other main type of battery is your lead acid battery in the car. In fact we know very well that lead acid batteries can work very well if they're just kept basically topped up the whole time. Care is required and occasional deep-discharge of nickel cadmium, and for that matter probably nickel metal hydride - probably less often - just by using it in the equipment 'til it's flat is probably not a bad idea.
How much juice is left in fuel rods?
Steve - Nuclear power plants aren't doing fusion, they're doing fission which is splitting big nuclei up: uranium. In a nuclear power plant you only use a very small fraction of the uranium so there's something like 50 times more energy left in there that we're unable to tap except through what's called fast breeder reactors which we don't currently do. Chris - How do they work?
Steve - This is a long question. They work by using the fast neutrons from nuclear reactions, from uranium, to attach to the inert part of uranium (uranium 238) to make plutonium. Plutonium can fission like uranium 235. That's a complicated answer.
Why does the sun take so long to burn out?
Steve - The sun's doing fusion very slowly. It doesn't have to do it any faster to keep itself held up. If the sun did fusion faster it would expand and then the fusion would go slower and it would contract back down again. If the sun was doing fusion too slowly it would contract until the fusion got fast enough to hold it up. The sun is just self-regulating and that's why it's going to last 5 billion years.
Can fuel from nuclear power plants be recycled?
Steve - With the current designs we have fusion power stations. These are conceptual. We haven't built one yet. With the current designs we burn deuterium which we extract from sea water and we burn lithium which you get from salts. There's millions of years-worth of this fuel. The by-product is helium which you can put in kid's balloons if you really like. They don't have any long-lived wastes from this. It's really an ideal way to make energy.
How will ITER work?
Kate - ITER as Steve pointed out is a completely different device. It's a big magnetic donut and it confines plasma for a length of time whereas the thing about lasers is the Reactor would be slightly different. You have to inject these fuel pellets and compress them. In order to support a 4GW reactor you'd have to do that four times a second. The designs of the two system'sare very different. That's not to say we don't have overlap and they're not complementary. They are, we have some of the same technology challenges.