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NUCLEAR FUSION

Nuclear fusion is a way of producing energy by joining two light nuclei together to get one heavier nucleus.

It is nuclear fusion that powers the stars!

One simple reaction is to join two nuclei of heavy hydrogen (deuterium) together to make one nucleus of helium.
 
 

The reason that energy is produced is that the mass of the two deuterium nuclei is slightly greater than the mass of the one helium nucleus. This tiny difference becomes energy. You can work out the amount of energy released by using the famous equation proposed by Albert Einstein:


Energy (E) = mass (m) x (speed of light)2
E = mc2


You don't get much energy produced when two nuclei fuse as you do when one nucleus of uranium splits but beca use deuterium is so much lighter than uranium there are many more nuclei per kilogram of material and the energy released per kilogram is actually a little more for deuterium fusion than for uranium fission.

Deuterium is found in water (since a water molecule is made of two hydrogen atoms and one oxygen atom) and in fact one atom in every 5000 atoms of hydrogen in sea water is deuterium. This means that there is a vast supply of deuterium as heavy water that could be used for fusion.

schoolphysics nuclear fusion animation

To see an animation of nuclear fusion click on the animation link.

Chances of fusion po wer

It looks as if fusion would be a very good source of energy because:
(a) there is a plentiful supply of fuel
(b) in a simple fusion reaction there are no radioactive waste products like there are in the fission reaction
However there is a problem. The nuclei of all atoms are positively charged and to get two positive charges to join together is very difficult. They repel each other and this repulsion gets bigger and bigger the closer together they get.
The only way to get them to fuse is to
(a) make them move very fast or
(b) use enormous pressure or both

To obtain fast moving atoms and molecules means heating the gas to a high temperature. At the centre of the Sun fusion reactions are going on all the time, the gases there are at temperatures of about 4 million degrees centigrade and under enormous pressures.

In the laboratory on Earth scientists have decided to try to use lower pressures but because of this the temperatures needed have to be higher still. At JET, the fusion reactor at Culham in Oxfordshire, they have managed to achieve fusion for a few seconds using temperatures of nearly 200 million degrees! At these enormous temperatures the electrons are stripped from the nuclei and the resulting soup of nuclei and electrons is called a PLASMA.

One other problem is that you can't simply put a plasma at 200 000 000 oC into a can - the can would simply vaporise. However because it is charged it can be held in a specially shaped magnetic field in a doughnut shaped container called a TORUS.

A simplified diagram of a nuclear fusion research reactor is shown in Figure 2.



In the future it is most likely that the nuclei to be used in a fusion reactor will be two isotopes of hydrogen deuterium (1p,1n) and tritium (1p, 2n). Tritium has to be made artificially and so the plasma tube will be surrounded by a blanket of lithium to absorb neutrons from the reaction and breed tritium. The capture of these energetic neutron heats the blanket and this heat can be used to produce steam. The steam could then be used to drive turbines and generators and produce electricity. Unfortunately at this stage the material of the reactor will become radioactive and so the operators will have to be shielded but it does seem that even then there will be less radioactive waste to dispose of than with the fission reactor. The picture shows the JET research fusion reactor at Culham, UK.




 
A VERSION IN WORD IS AVAILABLE ON THE SCHOOLPHYSICS CD
 
 
 
© Keith Gibbs 2016