[extropy-chat] Nuke 'em

Robert J. Bradbury bradbury at aeiveos.com
Tue Oct 25 17:55:06 UTC 2005

On Sun, 23 Oct 2005, Samantha Atkins wrote:

> I don't see how that is true in any meaningful way after reading
> this.  Please show me how a pebble bed reactor always makes plutonium
> or is even well suited for such use.

Samantha, Pu production is a natural consequence of the basic fission reaction.

Reactor grade fuel is a mixture of U-235 + U-238.  (Uranium is more than 99% U-238,
reactor grade fuel is somewhat enriched with U-235, weapons grade is highly
enriched with U-235.  This is why the current debate over what Iran is doing
is problematic.  If you can make reactor grade, all one has to do is run
the centrifuges somewhat longer to get weapons grade.)

The basic fission reaction in any nuclear reactor produces neutrons (this
is what makes the whole thing "go").  Neutrons that are captured U-238
which is present producing Np-239 which then decays into Pu-239.  If Pu-239
captures another neutron it produces Pu-240.  [See 1,2].  Argonne & the EPA
have good pages about various Uranium & Plutonium isotopes and half-lives [3,4].

The basic "nuclear" fission reaction is:
  U-235 + 1n --> Xe-134 + Sr-100 + 2n

Presumably the initial neutron is from either cosmic rays or the decay
of other naturally occuring radioactive elements.  Using 1 neutron and
getting 2 neutrons is what starts the chain reaction.  If any of those
neutrons are absorbed by any U-238 you end up with plutonium.  You can
structure the reactors to increase or decrease the production of Pu
by controlling the speed of the neutrons but I strongly doubt you
can eliminate Pu production entirely unless you completely replaced
the U-238 with some other isotope (presumably one that comfortably
absorbs neutrons without becoming radioactive).  That would probably
be an extremely expensive process due to the fact that natural uranium
is mostly U-238.  (You can't use pure U-235 as I'm fairly sure that
either gets very hot and melts or explodes.)


1. http://nuclearweaponarchive.org/Library/Plutonium/
2. http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/fission.html
3. http://www.ead.anl.gov/pub/doc/uranium.pdf
4. http://www.ead.anl.gov/pub/doc/plutonium.pdf

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