IOP’s Schools Lecture – final part

Pictures courtesy of http://tiny.cc/jogjx

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

Last time we spoke about how the energy of the fusion reaction is converted into electricity by way of a blanket of lithium surrounding the torus.  We also talked about how the lithium also reacts with the fusion neutrons to form tritium, one of the fuels of the fusion reaction.  This time we’re going to talk more about the fuels required for a fusion power plant.
The fusion reaction that will be used for a fusion power plant is the reaction between deuterium and tritium, because it is the easiest to do, ie. it requires the lowest temperature.  Remember that deuterium and tritium are heavy isotopes of hydrogen. 

Deuterium is found naturally in water in concentrations of about 1 part in 6700.  This may not sound like a lot, but when all the water in the oceans is considered there is about a million billions tons of deuterium, so a virtually inexhaustible supply.   Deuterium can be extracted from water easily using electrolysis.

Tritium is radioactive with a half-life of 12.3 years, so it does not occur naturally on Earth.  Small amounts of tritium are available as a by-product from some fission reactors, which can be used for experiments and for starting-up future fusion power stations, but for long-term fusion operation power stations will have to breed their own tritium fuel by reacting the fusion neutron with lithium.  So the second fuel required for a fusion power station is really lithium.

Lithium is found in the Earth’s crust in sufficient quantities to last for tens of thousand of years, but additional lithium could be extracted from seawater if necessary. Lithium exists in two natural forms, 6Li and 7Li, both with three protons and either three or four neutrons respectively.  The reactions of the neutrons with these isotopes of lithium are:
6Li + n ? 4He + T + 4.8MeV
7Li  + n ? 4He + T + n – 2.5MeV

The important issue with tritium breeding is that since each deuterium-tritium reaction produces only one neutron, that one neutron must breed at least one tritium nucleus in order for the power plant to be self-sufficient.  In tokamaks, not all of the plasma will be able to be completely surrounded by the breeder blanket, especially around the centre of the machine where space is tight.  Also, some of the neutrons will be absorbed in the structure of the machine rather than reacting with the lithium, so the others will have to make more than one tritium nucleus to make up for these losses. 
7Li is important here because if a neutron reacts with 7Li, then it produces not only tritium but also another neutron, which can then go on to react again.  (The reaction with 7Li requires energy of 2.5MeV to occur, but the neutrons start out with 14.1MeV so they have enough energy for this to happen). 

Another way to increase the number of available neutrons is to introduce another material, such as beryllium, into the blanket to produce neutrons, which can then go on to react with lithium and produce tritium.  Beryllium-9, see Picture 2, reacts with a neutron to form two helium nuclei and two neutrons:
   9Be  + n ? 4He + 4He + 2n

When ITER is running, experiments will be done to test different designs of lithium blanket, including what chemical form the lithium should take (such as lithium/lead or lithium/tin alloys, lithium oxide or other mixtures) and whether or not a neutron multiplier such as beryllium will be needed.

So we have seen that the fuels required for a future fusion power plant will be deuterium and lithium, both of which are abundant, spread evenly around the world and are relatively easy and cheap to extract.  Moreover, because of the huge amount of energy that fusion produces, very little fuel will be needed compared to fossil-fuel power plants.  The fuel costs for fusion will be very low indeed compared to the cost of constructing the fusion power plants. 

So now it’s the end of the IOP lecture tour 2010, and the last of these fusion blogs.  I’ve had a great year and a fantastic tour, and I hope you’ve enjoyed learning a bit about fusion along the way.