### Re: Wouldn't fusion energy deplete our water sources?

Date: Tue Apr 29 12:46:26 2003
Posted By: Steve Nelson, Grad student, nuclear astrophysics Ph.D. program, Nuclear Lab, Duke University
Area of science: Physics
ID: 1049969066.Ph
Message:
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There are a couple of things wrong with this idea.  The first is that you
only use the tiny fraction of water that contains deuterium.  By tiny, I
mean 0.015% (it's on the standard table of isotopes that you can find at
http://www2.bnl.gov/ton/ )
of hydrogen is deuterium.  Since you have 2
hydrogen atoms per water molecule, 0.03% of water will be heavy.  You'll
actually have to re-separate it later after you get rid of the 0.2% of
heavy water that results from having an oxygen-18 in it (8 protons, 10
neutrons) and 0.038% with O-17.  All told, it's still only about 0.3% of
the total water After the heavy water is separated from the normal water,
then the normal water can be returned to the source.  This will leave the
source relatively unchanged.  The total mass of deuterium from the 500 l
of water is only 165 g.

There's another reason, aside from that we're now only talking about 10
million cubic meters of water, or a layer of ocean which is 1 meter deep
and 10 square kilometers in area.  That may sound like a lot, really, but
there are 361,000,000 sq. kilometers of ocean in the world (result of a
google search).  That means, on average, you're lowering the ocean
surface by under 30 nanometers to power the world for everyone for a
level by several meters, or billions of times the change in the surface
level.  So no, even if we used all the water that was taken in for
deuterium extraction was somehow used and even if we ran for 100
lifetimes we would lower the ocean surface by 1 mm.  And the CO2 from
current power plants will cause enough global warming to counteract any
such effect many times over.

There are also advances in technology to be considered for the future.
Perhaps we will conceive of a better power source than fusion.  As soon
as people do achieve fusion, they'll want to step up to more advanced
versions (CNO cycle fusion and such).

As to the tritium, you said you need 30 grams of lithium, which is itself
a less common isotope (7.5% of natural lithium).  To get the right amount
of lithium, however, you actually probably need more like 480 grams to
get 1 6Li for every deuterium, meaning you need 6.4 kg of natural
lithium.  However, it's actually easier to extract tritium from the
cooling water of a reactor (deuterium is naturally converted to tritium
via neutron capture) than from lithium.  It's also environmentally safer,
since you don't have to mine lithium that way (mining is one of the most
destructive things we do to this planet).  After all, you need neutrons
to get tritium from lithium anyhow (although this process produces
significant heat and can be used to produce energy).

Power production is an enormous industry with wide-reaching implications
because of its massive scale.  You have to think about all of these parts
in detail before passing judgement on any one thing about it, as the
length of this question response will attest to.

```

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