2010 Schools Lecture – part two

Our 2010 School Lecture presenter Dr Melanie Windridge on her lecture tour and fusion reaction:

The second destination on my country-wide schools lecture tour was Liverpool.  I was speaking at the University of Liverpool to pupils of various different schools in the area.

Now, before we start to talk about the fusion reaction used in tokamaks, we’re going to recap some basic atomic physics.  In the talk I do this as a quiz to make it a bit more fun.  It’s not a very difficult quiz for GCSE students, because they should have already learnt all of this at school.
 
An atom consists of a nucleus at the centre and electrons that go round the outside.  Back in the late 1800s when J.J. Thomson first discovered electrons, it was thought that the atom looked a bit like a plum pudding – a positive pudding with little negative electron-raisins embedded in it. 

Then Rutherford and students discovered that the electron is actually mostly empty space and the model of the atom became more like that of the solar system, with a big, heavy nucleus at the centre and electrons orbiting around like planets around the Sun. 

The nucleus is only 100,000 of the size of the atom, which is like saying that if the nucleus were a pinhead at the centre of a football stadium then the electrons would be outside the back of the stands.  In fact, there is so much empty space in the atom that if you took out all the space from all the atoms in the human race we would all fit into a teaspoon! 
The solar system model gets us a bit closer to the picture of the atom, but this couldn’t be quite right either, since circling like this the electrons would gradually lose energy and spiral into the nucleus.  Atoms can only be explained by quantum theory, so we now think of an electron “cloud” surrounding the nucleus, where the electrons flit about, sometimes here, sometimes there, because in the quantum world particles can be in several places at once…. 

However, strange and interesting as it is, I’m not going to go into quantum theory here.  Instead, let’s talk about the nucleus, since it is the nuclei that fuse.  The nucleus is very small but contains 99.9% of the mass of the atom.  It is made up of protons (positively charged) and neutrons (neutral) and so has a net positive charge.  

The type of atom depends on the number of protons in the nucleus, for example hydrogen has one proton, helium has two, lithium has three.  But the same type of atom does not always have the same number of neutrons (pictured below)  These are all hydrogen, with one proton, but deuterium and tritium have extra neutrons and are known as isotopes of hydrogen.  These isotopes of hydrogen are what we fuse in the tokamak. 

In a neutral atom the positive charge of the protons is exactly balanced by the negative charge of an equal number of (equally, but negatively, charged) electrons.  If the atom is missing one or more electrons then it is charged and is called an ion.     

So now we come to the fusion reaction in tokamaks.  Fusion is the combining of two smaller nuclei into a larger one.  For fusion energy from tokamaks, deuterium and tritium will be combined to form helium and a neutron (pictured below).  This is not the same as the fusion reaction that occurs in the Sun. 

 
In the Sun, fusion of hydrogen to helium happens in three stages: two protons combine to form deuterium (one emits a positron and turns into a neutron); the deuterium combines with another proton to form helium-3 (an isotope of helium with only one neutron); two helium-3 nuclei combine to form helium-4. 

Fusion in the Sun proceeds very slowly, taking hundreds of million years for two protons to fuse.  This is pretty lucky for us, since otherwise all the fuel in the Sun would have burned out before life on Earth could evolve, but it’s not too useful a reaction for us do use on Earth to make energy. 

Fusing two protons into deuterium is the slowest step of the reaction, so starting with deuterium saves time, as now protons and neutrons only need to be rearranged rather than turned into other things.  There are a few different fusion reactions involving deuterium, but the deuterium-tritium (DT) reaction is considered the best for a power plant because it requires the lowest amount of energy to get it started and is the fastest.
   deuterium + tritium ? helium + neutron

So that’s the basic fusion reaction.  After our discussion of atoms, nuclei and charges you may have picked up on a small difficulty in getting fusion to occur.  We are fusing nuclei together, and the nuclei are both positively charged, so they will repel each other. Opposites attract, or like charges repel.  They don’t want to come together and fuse.  Next time, I’m going to tell you how we overcome this force of repulsion and get fusion to happen.