Fusion Bombs

Fusion Bombs
Fission bombs worked, but they weren't very efficient. Fusion bombs, also called thermonuclear bombs, have higher kiloton yields and greater efficiencies than fission bombs. To design a fusion bomb, some problems have to be solved:

  • Deuterium and tritium, the fuel for fusion, are both gases, which are hard to store.
  • Tritium is in short supply and has a short half-life, so the fuel in the bomb would have to be continuously replenished.
  • Deuterium or tritium has to be highly compressed at high temperature to initiate the fusion reaction.

First, to store deuterium, the gas could be chemically combined with lithium to make a solid lithium-deuterate compound. To overcome the tritium problem, the bomb designers recognized that the neutrons from a fission reaction could produce tritium from lithium (lithium-6 plus a neutron yields tritium and helium-4; lithium-7 plus a neutron yields tritium, helium-4 and a neutron). That meant that tritium would not have to be stored in the bomb. Finally, Stanislaw Ulam recognized that the majority of radiation given off in a fission reaction was X-rays, and that these X-rays could provide the high temperatures and pressures necessary to initiate fusion. Therefore, by encasing a fission bomb within a fusion bomb, several problems could be solved.

Teller-Ulam Design of a Fusion Bomb
To understand this bomb design, imagine that within a bomb casing you have an implosion fission bomb and a cylinder casing of uranium-238 (tamper). Within the tamper is the lithium deuteride (fuel) and a hollow rod of plutonium-239 in the center of the cylinder. Separating the cylinder from the implosion bomb is a shield of uranium-238 and plastic foam that fills the remaining spaces in the bomb casing. Detonation of the bomb caused the following sequence of events:

  1. The fission bomb imploded, giving off X-rays.
  2. These X-rays heated the interior of the bomb and the tamper; the shield prevented premature detonation of the fuel.
  3. The heat caused the tamper to expand and burn away, exerting pressure inward against the lithium deuterate.
  4. The lithium deuterate was squeezed by about 30-fold.
  5. The compression shock waves initiated fission in the plutonium rod.
  6. The fissioning rod gave off radiation, heat and neutrons.
  7. The neutrons went into the lithium deuterate, combined with the lithium and made tritium.
  8. The combination of high temperature and pressure were sufficient for tritium-deuterium and deuterium-deuterium fusion reactions to occur, producing more heat, radiation and neutrons.
  9. The neutrons from the fusion reactions induced fission in the uranium-238 pieces from the tamper and shield.
  10. Fission of the tamper and shield pieces produced even more radiation and heat.
  11. The bomb exploded.

All of these events happened in about 600 billionths of a second (550 billionths of a second for the fission bomb implosion, 50 billionths of a second for the fusion events). The result was an immense explosion that was more than 700 times greater than the Little Boy explosion: It had a 10,000-kiloton yield.