piner@CSvax:Pucc-H:pur-phy.UUCP (10/04/83)
I have a couple of quick notes on fusion. The proposed fuel for fusion reactors is deuterium and tritium. The fusion of these two produces helium and a neutron. The neutron is the problem. When it hits another nucleus two things can happen. The nucleus will split or it will absorb the neutron. Only heavy nuclei will fission. Since they won't be used to build reactors we don't need to worry about them. The lighter nuclei are the ones we will use to make reactors. Iron is a good example. When it absorbs a neutron it becomes radioactive. The half-life for radioactive iron is long. Aluminium on the other hand has a short life time. If you build your reactor out of iron, it will be centuries before it is "cool". If you build it out of aluminium it will be weeks. So the waste problem is really a materials problem. We know which elements have short life times after being activated be neutrons. The trick is to make materials out of them that can take the stress of being a fusion generator. Sadly, solid state physics is the least well funded branch of physics, so we may find our developement of fusion power delayed by a lack of knowledge in materials and not fusion physics. If it does turn out that some part of a fusion reactor has to made with iron or some other element with a long half life, then fusion power will be made much more expensive, because reactors will have to be repaired with precautions against radiation. If the average half life of reactor vessels is only a few days, then repairs could be done after only a few weeks wait with few precautions. Disposal of reactor parts is simple if they have short half lives. A safe site is needed if they stay hot for a long time. Now it should be noted that if pure deuterium is burned, the reaction only produces helium nuclei. There are no neutrons and hence no neutron induced radioactivity and no neutron damage to the materials. Also deuterium is stable while tritium is radioactive and must be made. Tritium does not exist in nature. So fuel is cheaper in a pure deuterium fueled machine. The trouble is that deuterium-deuterium reactions required higher temperatures and so it is more difficult to produce fusion. In the long run, it may pay to spend the extra time and effort to attempt a deuterium-deuterium machine. Right now more research is needed on all fronts. Rich Piner Purdue Physics Dept.
pct@dewey.UUCP (Pierre Trepagnier) (10/05/83)
With regard to fusion waste, you are encouraged to read the article "The Trouble with Fusion" by Lawrence Lidsky in the October Technology Review. The thrust of his article is that the current D-T program is shortsighted and will result in a reactor that no one will want. Neutron contamination is treated in detail.