IOP Schools Lecture – part 11

Our 2010 IOP schools lecturer Melanie Windridge on her tour and fusion physics:

Last week I started in my lectures in Tonbridge, Kent, then moved north in a loop encompassing Milton Keynes, Peterborough and Woodbridge, near Ipswich, before returning home. 

Moving on straight on to fusion.  Last time the subject was the robots used on JET, and this time we’ll be talking about why fusion releases energy.  This is all to do with something called binding energy. 
All nuclei have a binding energy, which says how tightly the nucleus is bound together.  Energy is required to split a nucleus apart, so when a nucleus is created it releases this energy.  Binding energy is really quite a confusing concept, but I like to think about it by likening the nucleus to an elastic band.

Take an elastic band and stretch it as far as you can – you have to put energy in to do that; you can feel it.   Now let one side go and the elastic band will ping back together, releasing energy.  You can hear it – snap!  This is what happens in the nucleus too.  Splitting a nucleus apart requires energy – the binding energy – but when a nucleus comes together it releases all that energy.

Different nuclei have different amounts of binding energy.  It’s clear, then, that if we start with something with a lower binding energy and we make something with a higher binding energy we are going to release all that extra energy.  The binding energy curve shows how binding energy varies with the size of the nucleus, and from this we can see why both nuclear fusion and nuclear fission release energy. 
Any time we move upwards on this curve – when we go from a nucleus with a lower binding energy to one with a higher binding energy – energy is being released.  In the case of nuclear fission (the splitting apart of heavy nuclei to make lighter ones) we start on the far right of the curve with a heavy nucleus such as uranium (marked U235).  This is split into two smaller fragments such as strontium (Sr90) and xenon (Xe143) about midway along the gentle downwards slope.  You can see that we have moved up the curve from a nucleus with a lower binding energy to make two with higher binding energies, so that extra energy is what is released from the fission reaction.

In Part 2 of this blog we talked about the fusion reaction in tokamaks, which is deuterium + tritium ? helium + neutron. Deuterium and tritium are isotopes of hydrogen – heavier hydrogen with extra neutrons in their nuclei.  On the binding energy curve, deuterium is marked as H2, tritium as H3 and helium as He4.  At this side the curve is very steep so there is a big difference in binding energy between the H2 and H3 and the He4.  Therefore, the fusion reaction, which makes He4, releases a lot of energy, more even than the fission reaction.   

Another way to think about why fusion releases energy is in terms of something called missing mass.  It turns out that if you weigh the deuterium and the tritium that you start with, and then you weigh the helium and the neutron and the end, then you’ll find that the deuterium and the tritium actually weigh MORE.  During the fusion reaction some mass has been lost.

It was Einstein who said that energy and mass are equivalent and they can be transformed into one another.  You’ve probably heard Einstein’s famous formula E=mc2.  This says that the amount of energy released will be the change in mass times the speed of light squared.  Light travels very fast – three hundred thousand km per second!  So c squared is a very big number, which means you only need a tiny change in mass to release a lot of energy.  This is why fusion releases so much.

You might remember that in Part 1 of this blog I said that just 1kg of fusion fuel releases as much energy as 10 million kg of fossil fuels – so ten million times as much energy.  That’s because when fossil fuels burn it is chemical bonds that are being broken and when a nuclear reaction occurs it is nuclear bonds that are being broken.  Nuclear bonds are much stronger than chemical bonds, so much more energy can be released, which is why nuclear reactions release so much energy.

Next time I’ll talk about how we hope to convert the energy released by the fusion reaction into electricity, and I’ll be travelling up to Newcastle.