ccoleman@gmuvax2.gmu.edu (Chuck Coleman) (11/07/90)
From: ccoleman@gmuvax2.gmu.edu (Chuck Coleman)
I'd like to clear up a little confusion that has crept into the net
about nuclear weapons. Briefly, there are two reactions which power
them: fission and fusion. Fission is the splitting of heavy nuclei to
create a self-sustaining "chain reaction". Fusion is the combining of
light nuclei to create heavier nuclei. Naturally, these reactions
release energy very rapidly, or else there wouldn't be any point to
this posting. This is all probably familiar.
Each nuclear weapons uses a fission trigger to start everything. A
chain reaction in a piece of fissile material by creating a critical
mass in which fission reactions become self-sustaining through the use
of neutrons released by previous reactions to fission more nuclei.
Any isotope which fissions when subject to low-energy ("thermal")
neutron flux may be used. These are generally transactinides with an
even number of protons and an odd number of neutrons. In practice,
only U-235 and Pu-239 are used. The critical mass may be achieved
either by combining smaller, subcritical masses together, as in the
Hiroshima bomb, or by imploding a subcritical mass, so that it reaches
criticality at a higher density. The latter method was used in the
Nagasaki bomb and is the only method currently used. Pure fission
devices only use this mechanism.
The energy output of a nuclear bomb may be enhanced by using the
energy of the fission trigger to compress and heat a mass of
fusionable material to start a fusion reaction. These are commonlyy
known as "thermonuclear" weapons. In general, any light isotope may
be used. In practice, only isotopes of hydrogen and lithium are used
because they require the least energy for fusion. I believe lithium
deuteride is the preferred fuel. The output of ordinary fission bombs
may be enhanced by "seeding" the fissile material with fusionable
isotopes.
Fusion is not the end of the story. Fusion reactions produce large
quantities of energetic neutrons, which may then be used to fission
nuclei which cannot be used in pure fission weapons. These weapons
are properly called "fission-fusion-fission" weapons. A shell of
fissionable material is placed around the weapons. Usually, U-238
("depleted uranium") is used. U-238 basically makes for a bigger
explosion, with lots of extra radioactivity. In "neutron bombs", the
shell is made of Be-9, which splits into 2 He-4 nuclei and a free
neutron when fissioned. The cascade of neutrons produced by this
mechanism gives the neutron bomb its name.
Chuck Coleman "Sorry, no concluding witticism"
School: Work:
Center for Study of Public Choice NPA Data Services, Inc.
George Mason University 1424 16th St. N.W. Suite 700
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BITNET: ccoleman@GMUVAX
Internet: ccoleman@gmuvax2.gmu.edu