On The, 28 Aug 1997 "Eliezer S. Yudkowsky" <sentience@pobox.com> Wrote:
>So you don't need neutron reflectors; just ordinary water.
Lots of things reflect neutrons, the best is Beryllium because it has more
atoms per volume than any other element, but Beryllium is very light and a
good tamper needs to be heavy. Any heavy element, like lead, would fit the
bill but for reasons I will explain later, in a modern H bomb U238
(the common isotope) is used.
The chemical explosive used is HMX, also called OCTOGEN, it has a detonation
velocity of 9100 meters per second verses 7000 for TNT.
NO2 H2 NO2
\ | /
N - C - N
| |
H2-C C-H2
| |
N - C - N
/ | \
NO2 H2 NO2
HMX or OCTOGEN
The explosive does not push directly on the plutonium it pushed on the heavy
neutron reflector around the inner sphere of U235 (the rare hard to get
isotope) or Plutonium. This massive tamper has several jobs, it reflects
neutrons, it helps to make the spherical shock wave more even, and when the
plutonium or U235 starts to get hot its great mass slows the expansion and
so gives the plutonium more time to complete its nuclear reactions.
Water is in no way involved in the physics of a nuclear explosion but it is
involved in the manufacture of nuclear bombs, more specifically plutonium,
and plutonium works even better than U235 in a bomb. A neutron is more
likely to react to common U238 if it's moving slowly, and water slows down
neutrons. When a slow neutron hits U238 it does not undergo fission but it
turns it into U239, this has a half life of 24 minutes and it decays into
neptunium 239, this has a half life of 2.3 days and it decays into
plutonium 239, and plutonium 239 has a half life of 14,000 years and works
great in bombs.
Make a bunch of uranium rods in which the U235 has been enriched from its
natural .7% abundance to 2%, make sure there is water between the rods,
without it the reaction will not go and if the water is not flowing the
reactor will get very hot very fast and you're in trouble. Let the reactor
operate for several weeks then stop it and take the rods out. The slow
neutrons have turned much of the common U238 into exotic plutonium that can
be separated from the uranium by simple chemical means.
In a H bomb the fission trigger gets the deutroniun and tritium so hot they
fuse, when they do it produces a helium 4 nucleus with 3.5 mev of energy,
but more importantly a neutron with 17.6 mev of energy. When a neutron with
this enormous speed hits U238 things are very different than when a slow
neutron does in a reactor, the U238 fissions and releases a lot of energy in
the process. In fact, in a garden variety H bomb 70% of the energy released
comes from the fission of the heavy U238 tamper, although of course that
wouldn't be possible without the super high speed neutron produced from the
fusion reaction.
>it could be that no amount of water decreases the size of a critical
>mass.
Anything outside of the sphere that reflects neutrons will reduce the critical
mass, it also depends on purity, especially for uranium. For 100% U235 and
assuming no neutron reflection you would need a 15 kilogram sphere to go
critical, at 40% you would need 75. For plutonium at 100% just 4.4 kilograms
would do and even at 25% purity a 20 kilogram sphere would go boom. These
figures can be reduced with neutron reflection and if the chemical explosive
compresses the fissionable metal to greater than normal density. If the
tamper is massive it will slow the time it takes for the sphere to expand
enough to stop the reaction, so you get a bigger more efficient explosion.
John K Clark johnkc@well.com
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