144 lines
4.9 KiB
Plaintext
144 lines
4.9 KiB
Plaintext
"In thermonuclear weapons, radiation from a fission explosive can be contained
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and used to transfer energy to compress and ignite a physically separate
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component containing thermonuclear fuel."
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The 3 basic concepts of thermonuclear devices,
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U.S. DOE, Sept 1980, Duane Sewell,
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Assistant Secretary of Energy for Defense,
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Official Declassification Act.
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FUSION PRINCIPLES
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Under solar conditions (high temps of about 100 millions degrees C, 1 milion
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megabars pressure), H atoms fuse into He. Three isotopes of H exist:
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H1 (P) protium
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H2 (D) deuterium
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H3 (T) tritium.
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Protium reacts too slowly even in the sun so deuterium and tritium are used.
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Under solar conditions, the H atoms gain enough kinetic enery to overcome
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the electrostatic repulsion of their positive charges. The electrons which
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are normally found surrounding H nuclei have already been ionised. You have
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a plasma of positive nuclei. He is formed in a H-H reaction, releasing energy.
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Sources of D and T
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Heavy water (D2O) is present at 1 part in 6700 in normal tap water. You can
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separate the heavy water, and then obtain deuterium gas. D2 gas
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is obtained via electrolysis.
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Tritium is radioactive, and is obtained via bombardment of Li6 with thermal
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(slow) neutrons. It beta decays like: T -> He3 + e
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T fuses with D at a temperature an order of mag lower than for D-D fusion,
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hence its usefulness in a weapon.
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Lithium
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The lightest of metals, only 1/2 as dense as water. Found combined with
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other elements in igneous rocks and mineral spring water. Li7 is separated
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electrolytically from Li7Cl. Has several isotopes: Li5 to Li9. Li6 and Li7
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are used in weapons, and are naturally occuring. Li5, Li8, and Li9 are
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man-made radioisotopes.
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Li6 is present as 7.5% of all naturally occuring Li. Separation methods
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include electrolysis, distillation, chemical exchange, or EM methods. Li
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bonds with H to form the solid Li6D.
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Back to the Story
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Since the mass of the resultant He is less than the mass of the separate H,
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the excess energy is converted into radiation and kinetic energy of neutrons.
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The energy of these fast neutrons is high enough to split normal U-238. Slow
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neutrons only transmute U-238 into Np-239 (which then beta decays into
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Pu-239).
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One cubic metre of gaseous deuterium, when fused into helium, yields the
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equivalent of about 10 megatons of TNT.
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Deuterium and tritium are gases at room temperature, so their storage in a
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weapon would be cumbersome. Instead, a substance called lithium deuteride
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(Li6D or Li7D) is used. This material has the property of being a whitish,
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slightly-blue powdery light salt-solid (which is extremely hygroscopic) at
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room temperature. It is made by heating metal lithium in a vessel, into
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which deuterium gas is injected. It is then pressed and shaped into a
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ceramic.
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When a neutron is absorbed by a LiD molecule, the molecule breaks up into
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a He, H3, and a deuterium. The D can then reacts with the T in fusion. This
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releases enormous amounts of energy, much greater than you would get in
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a fission reaction. The end products include a free n, and a He. Schematically:
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U-238 fission releases fast neutrons and heat (thermal kinetic energy of
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neutrons).
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Li6 + n -> He4 + T + 4.7 MeV
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then
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D + T -> He4 + n + 17.6 MeV.
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n + U-238 -> neutrons + fission products + energy
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These reactions occur in under 1/10-6 secs. Additional reactions are:
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Li6 + D -> 2(He4) + 22.4 MeV
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Li6 -> 2(He4) + n
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Li6 + P -> He4 + He3 + 4.0 MeV
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Li7 + P -> 2(He4) + 17.3 MeV
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Li7 + D -> Li8 + P
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Li7 + n -> He4 + T + n
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D + Li7 -> Be8 + n + 15.1 MeV
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D + D -> T + H + 4.0 MeV
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D + D -> He3 + n + 3.25 MeV
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D + D -> He4 + 23 MeV
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T + T -> He4 + 2n + 12.2 MeV
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He3 + D -> He4 + H + 18.3 MeV
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D + n -> T
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Beryllium is useful in the core of a fission mass since you can use it to
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increase the neutron flux:
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Be9 + n -> Be8 + 2n
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Be9 + D -> Be8 + T + 4.53 MeV
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For a thermonuclear reaction, you have to compress the Li6D solid to 15-30
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times it's original uncompressed density at RTP (15lbs/foot^3). Compression
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is needed to:
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(1) increase fusion *probability*. You pack the molecules closer together.
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In the process, you pave the way to overcoming the electrostatic repulsion
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of the H atoms in the Li6D.
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(2) increase fusion *rate*, since you get quicker reactions when the reactants
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are packed closely together than far apart. The *time* for a reaction is
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inversely proportional to fuel density. Denser fuels mean shorter reaction
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times, and hence more chance of a larger number of reactions. The *rate* of
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reaction, on the other hand, is proportional to the square of the fuel density.
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Increase the density by a factor of 30, and your rate increases by a factor
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of 900.
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Compression is a form of inertial confinement fusion (ICF). You are in effect
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counteracting the explosive forces released in the fusion, by giving the
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reactants an inwardly directed momentum. So the whole mass of fuel stays
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together. It's collapsing in on itself; at the same time it wants to tear
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itself apart.
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PS, 1994
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