Thermonuclear Weapons

Fusion and Thermonuclear Fuel

Thermonuclear weapons are frequently named hydrogen bombs or H-bombs. This is
because the thermonuclear reactions relevant for nuclear weapons involve isotopes of
hydrogen:

  1. D + T → He-4 + n (14.1 MeV) + 17.6 MeV
  2. D + D → He-3 + n (2.45 MeV) + 3.3 MeV
  3. D + D → T + p + 4.03 MeV
  4. He-3 + D → He-4 + p + 18.35 MeV

    There are other thermonuclear reactions which can occur. However the reaction rates are
    too low in order to be significant.

    The first thermonuclear fuel which was considered for a weapon was deuterium. This is:

    • (Relatively) easy to ignite and burn
    • Abundant in nature (in comparison with tritium)
    • (Relatively) inexpensive to produce

    One other thermonuclear fuel is easier to ignite- this is a mixture of deuterium and tritium.
    At thermonuclear temperatures, the D+T reaction rate (reaction 1) is two magnitude orders
    faster than D+D (reaction 2 or 3). However, tritium does not occur in nature and is very
    expensive to manufacture.

    The cheapest and most usual manufacturing on an industrial scale is to breed it in reactors.
    Generally it is produced in the same reactors which produce plutonium - thus the
    production of plutonium and tritium are usually in competition. Tritium has a half-life of
    approximately 12 years, thus it is expensive simply to maintain an inventory of tritium.

    The other method is to produce tritium in situ from other reactions in a functioning weapon.
    There are two main reactions which produce tritium; reaction 3 (D+D) produces a triton and
    lithium-6 and lithium-7 (7% and 93% of natural lithium respectively) can undergo reactions
    which produce tritium:

  5. Li-6 + n → T + He-4 + 4.8 MeV
  6. Li-7 + n → T + He-4 + n - 2.5 MeV

Because lithium-6 has a higher neutron cross-section than lithium-7, it is advantageous to
enrich the content of lithium-6 to use as thermonuclear fuel. Because there is always a large
excess of deuterium and as a result of the much higher D+T reaction rate in comparison with
the D+D rate, all the tritium produced becomes burned up.