“In thermonuclear weapons, radiation from a fission explosive can be contained
\n and used to transfer energy to compress and ignite a physically separate
\n component containing thermonuclear fuel.”<\/p>\n
\t\t\t\tThe 3 basic concepts of thermonuclear devices,
\n\t\t\t\tU.S. DOE, Sept 1980, Duane Sewell,
\n\t\t\t\tAssistant Secretary of Energy for Defense,
\n\t\t\t\tOfficial Declassification Act.<\/p>\n
FUSION PRINCIPLES<\/p>\n
Under solar conditions (high temps of about 100 millions degrees C, 1 milion
\nmegabars pressure), H atoms fuse into He. Three isotopes of H exist:<\/p>\n
\t\tH1 (P)\tprotium
\n\t\tH2 (D)\tdeuterium
\n\t\tH3 (T)\ttritium.<\/p>\n
Protium reacts too slowly even in the sun so deuterium and tritium are used.
\nUnder solar conditions, the H atoms gain enough kinetic enery to overcome
\nthe electrostatic repulsion of their positive charges. The electrons which
\nare normally found surrounding H nuclei have already been ionised. You have
\na plasma of positive nuclei. He is formed in a H-H reaction, releasing energy.<\/p>\n
Sources of D and T<\/p>\n
Heavy water (D2O) is present at 1 part in 6700 in normal tap water. You can
\nseparate the heavy water, and then obtain deuterium gas. D2 gas
\nis obtained via electrolysis.<\/p>\n
Tritium is radioactive, and is obtained via bombardment of Li6 with thermal
\n(slow) neutrons. It beta decays like: T -> He3 + e<\/p>\n
T fuses with D at a temperature an order of mag lower than for D-D fusion,
\nhence its usefulness in a weapon.<\/p>\n
Lithium<\/p>\n
The lightest of metals, only 1\/2 as dense as water. Found combined with
\nother elements in igneous rocks and mineral spring water. Li7 is separated
\nelectrolytically from Li7Cl. Has several isotopes: Li5 to Li9. Li6 and Li7
\nare used in weapons, and are naturally occuring. Li5, Li8, and Li9 are
\nman-made radioisotopes.<\/p>\n
Li6 is present as 7.5% of all naturally occuring Li. Separation methods
\ninclude electrolysis, distillation, chemical exchange, or EM methods. Li
\nbonds with H to form the solid Li6D.<\/p>\n
Back to the Story<\/p>\n
Since the mass of the resultant He is less than the mass of the separate H,
\nthe excess energy is converted into radiation and kinetic energy of neutrons.
\nThe energy of these fast neutrons is high enough to split normal U-238. Slow
\nneutrons only transmute U-238 into Np-239 (which then beta decays into
\nPu-239).<\/p>\n
One cubic metre of gaseous deuterium, when fused into helium, yields the
\nequivalent of about 10 megatons of TNT.<\/p>\n
Deuterium and tritium are gases at room temperature, so their storage in a
\nweapon would be cumbersome. Instead, a substance called lithium deuteride
\n(Li6D or Li7D) is used. This material has the property of being a whitish,
\nslightly-blue powdery light salt-solid (which is extremely hygroscopic) at
\nroom temperature. It is made by heating metal lithium in a vessel, into
\nwhich deuterium gas is injected. It is then pressed and shaped into a
\nceramic.<\/p>\n
When a neutron is absorbed by a LiD molecule, the molecule breaks up into
\na He, H3, and a deuterium. The D can then reacts with the T in fusion. This
\nreleases enormous amounts of energy, much greater than you would get in
\na fission reaction. The end products include a free n, and a He. Schematically:<\/p>\n
U-238 fission releases fast neutrons and heat (thermal kinetic energy of
\nneutrons).<\/p>\n
\t\tLi6 + n -> He4 + T + 4.7 MeV<\/p>\n
then<\/p>\n
\t\t D + T -> He4 + n + 17.6 MeV.<\/p>\n
\t n + U-238 -> neutrons + fission products + energy<\/p>\n
These reactions occur in under 1\/10-6 secs. Additional reactions are:<\/p>\n
\t\tLi6 + D -> 2(He4) + 22.4 MeV<\/p>\n
\t\t Li6 -> 2(He4) + n<\/p>\n
\t\tLi6 + P -> He4 + He3 + 4.0 MeV<\/p>\n
\t\tLi7 + P -> 2(He4) + 17.3 MeV<\/p>\n
\t\tLi7 + D -> Li8 + P<\/p>\n
\t\tLi7 + n -> He4 + T + n<\/p>\n
\t\tD + Li7 -> Be8 + n + 15.1 MeV<\/p>\n
\t\tD + D\t-> T + H + 4.0 MeV<\/p>\n
\t\tD + D\t-> He3 + n + 3.25 MeV<\/p>\n
\t\tD + D\t-> He4 + 23 MeV<\/p>\n
\t\tT + T\t-> He4 + 2n + 12.2 MeV<\/p>\n
\t\tHe3 + D\t-> He4 + H + 18.3 MeV<\/p>\n
\t\tD + n\t-> T<\/p>\n
Beryllium is useful in the core of a fission mass since you can use it to
\nincrease the neutron flux:<\/p>\n
\t\tBe9 + n\t-> Be8 + 2n<\/p>\n
\t\tBe9 + D\t-> Be8 + T + 4.53 MeV<\/p>\n
For a thermonuclear reaction, you have to compress the Li6D solid to 15-30
\ntimes it’s original uncompressed density at RTP (15lbs\/foot^3). Compression
\nis needed to:<\/p>\n
(1) increase fusion *probability*. You pack the molecules closer together.
\nIn the process, you pave the way to overcoming the electrostatic repulsion
\nof the H atoms in the Li6D.<\/p>\n
(2) increase fusion *rate*, since you get quicker reactions when the reactants
\nare packed closely together than far apart. The *time* for a reaction is
\ninversely proportional to fuel density. Denser fuels mean shorter reaction
\ntimes, and hence more chance of a larger number of reactions. The *rate* of
\nreaction, on the other hand, is proportional to the square of the fuel density.
\nIncrease the density by a factor of 30, and your rate increases by a factor
\nof 900.<\/p>\n
Compression is a form of inertial confinement fusion (ICF). You are in effect
\ncounteracting the explosive forces released in the fusion, by giving the
\nreactants an inwardly directed momentum. So the whole mass of fuel stays
\ntogether. It’s collapsing in on itself; at the same time it wants to tear
\nitself apart.<\/p>\n
PS, 1994<\/p>\n
