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From: piner@CSvax:Pucc-H:pur-phy.UUCP
Newsgroups: net.physics
Subject: Re: fusion waste
Message-ID: <1020@pur-phy.UUCP>
Date: Tue, 4-Oct-83 02:02:51 EDT
Article-I.D.: pur-phy.1020
Posted: Tue Oct  4 02:02:51 1983
Date-Received: Thu, 29-Sep-83 07:21:54 EDT
Organization: Purdue Univ. Physics Dept., W. Lafayette, IN
Lines: 42


  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.