IOP Schools Lecture – part 11

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

It’s been a while since my last blog and I’ve been up and down the country, most recently to Swansea.  I visited Three Cliffs Bay on the Gower Peninsula, which was beautiful in the heavy frost.
In the last blog we talked about why fusion releases energy, and this time I’ll be telling you about how we get this energy out of the fusion reaction to make electricity.

You remember that the products of the tokamak fusion reaction are helium and a neutron.  The neutron has no charge and can therefore escape the magnetic fields that trap the charged plasma, while the helium remains in the machine. The neutron also carries away most of the energy that is released.  This is due to the conservation of momentum.

Momentum is the amount of motion that an object has and is defined as the mass of the object times its speed in a particular direction (momentum = mass x velocity).  The more momentum an object has – so the heavier or the faster it is – the harder it is to stop.

When two things collide in isolation, the total momentum of the system stays the same – this is the conservation of momentum.  The objects in the collision will transfer momentum to each other, and what one loses the other gains, so both the objects experience equal and opposite momentum changes.

To get an idea of what happens you can do a simple demonstration yourself.  All you will need are two balls of differing sizes.  I use a basketball and a soft toy football, but you can use any.  Just make sure that the smaller ball is not too hard (you don’t want it to hurt if it hits anyone) and you’ve got enough space  (it’s probably better to go outside where the bouncing balls won’t break anything).

First of all just drop – don’t throw – the smaller ball to see how bouncy it is.  Then stack the balls one on top of the other, see Picture 1, and drop them together.  If they bounce on top of each other you should see the small ball go flying off at speed – much more bouncy than when you dropped it on its own – and the big ball hardly bounces.   So you can see that the small ball takes away most of the energy in this collision. 

When the balls bounce, momentum is transferred from each ball to the other, but the big ball has more momentum and so transfers more to the small ball.  It gives it a kick.  The small, light ball then has a much bigger velocity in order to conserve momentum (momentum = mass x velocity).

So, just as the small ball takes away most of the energy of our collision, the small, light neutron takes away most of the energy of the fusion reaction.  The neutron takes away four fifths of the energy released (14.1MeV) and the helium nucleus takes just one fifth (3.5MeV), see Picture 2 (image courtesy of EFDA-JET). 

And because the charge-less neutron can escape the magnetic trap, it flies straight out of the machine and we can use it.  A fusion power station would use this neutron energy to make electricity.

In a future power station, after the neutron passes through the first wall of the machine it will pass into thick blanket, or layer, around the outside of the torus (doughnut shape), see Picture 3 (courtesy of S. Cowley). This simple diagram shows a cut through of the torus, with the pink plasma in the centre and the grey blanket layer around the outside.  The blanket will be made of lithium – the same material that is used in your mobile phone batteries – and it performs two functions. 

Firstly, the lithium reacts with the neutron to form tritium, which is one of the fuels of the fusion reaction.  Secondly, it heats up, and we can use that heat to make steam, drive turbines and make electricity in exactly the same way that conventional power stations make electricity now.  Let’s look at the two processes separately.
 
 The neutrons react with the lithium to make tritium.  There are two naturally occurring isotopes, or varieties, of lithium – 6Li and 7Li.  Both reactions produce tritium and helium; the reaction with 7Li also produces a neutron. 
6Li + n ? 4He + T + 4.8MeV
7Li  + n ? 4He + T + n – 2.5MeV
The reaction with 6Li occurs more easily, requires only a slow neutron to trigger it and releases energy, while for the reaction with 7Li to occur a fast neutron is needed and the reaction absorbs energy.  The issues associated with breeding the tritium fuel will be discussed in the next blog.
The neutrons carry away 80% of the power of the fusion reaction.  They pass easily through the first wall but are slowed down by collisions with atoms in the blanket, which heats up. 

At the same time, energy from the hot plasma heats up the first wall and a region at the bottom of the tokamak called the divertor, where the magnetic field channels lots of energetic particles.  So the blanket, the first wall and the divertor will all heat up, and they will be cooled by high-pressure water or helium gas.  This coolant will pass through a heat-exchanger and produce steam, which will drive the turbines to make electricity.
 
Lithium blanket designs will be tested in ITER during the later stages of the project, testing the heat-exchanging and tritium-breeding capabilities of different configurations.   The blanket will be modular – made of many different pieces – to make maintenance and replacement easier.  See Picture 4 (KIT/ IMF III  courtesy of EFDA) for a module design.  In ITER it will be 440 individual segments, each measuring 1×1.5m and weighing up to 4.6 tons.  But the ITER blanket will be made of high-strength copper and stainless steel – not lithium – and will act as neutron shield.  Lithium blanket modules will be tested individually.

The lithium blanket is the means by which the energy of the fusion reaction is converted to electricity for our use.  It is also a means of producing the tritium fuel for the fusion reaction.  So while the fuels of the fusion reaction are deuterium and tritium, the actual fuels required for operation of a fusion power plant will be deuterium and lithium.  The fuels will be discussed in more detail next time.