Fission bombs worked, but they weren't very efficient. It didn't take scientists long to wonder if the opposite nuclear process -- fusion -- might work better. Fusion occurs when the nuclei of two atoms combine to form a single heavier atom. At extremely high temperatures, the nuclei of hydrogen isotopes deuterium and tritium can readily fuse, releasing enormous amounts of energy in the process. Weapons that take advantage of this process are known as fusion bombs, thermonuclear bombs or hydrogen bombs. Fusion bombs have higher kiloton yields and greater efficiencies than fission bombs, but they present some problems that must be solved:
- Deuterium and tritium, the fuels for fusion, are both gases, which are hard to store.
- Tritium is in short supply and has a short half-life.
- Fuel in the bomb has to be continuously replenished.
- Deuterium or tritium has to be highly compressed at high temperature to initiate the fusion reaction.
Scientists overcome the first problem by using lithium-deuterate, a solid compound that doesn't undergo radioactive decay at normal temperature, as the principal thermonuclear material. To overcome the tritium problem, bomb designers rely on a fission reaction to produce tritium from lithium. The fission reaction also solves the final problem. The majority of radiation given off in a fission reaction is X-rays, and these X-rays provide the high temperatures and pressures necessary to initiate fusion. So, a fusion bomb has a two-stage design -- a primary fission or boosted-fission component and a secondary fusion component.
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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 causes the following sequence of events:
- The fission bomb implodes, giving off X-rays.
- These X-rays heat the interior of the bomb and the tamper; the shield prevents premature detonation of the fuel.
- The heat causes the tamper to expand and burn away, exerting pressure inward against the lithium deuterate.
- The lithium deuterate is squeezed by about 30-fold.
- The compression shock waves initiate fission in the plutonium rod.
- The fissioning rod gives off radiation, heat and neutrons.
- The neutrons go into the lithium deuterate, combine with the lithium and make tritium.
- The combination of high temperature and pressure are sufficient for tritium-deuterium and deuterium-deuterium fusion reactions to occur, producing more heat, radiation and neutrons.
- The neutrons from the fusion reactions induce fission in the uranium-238 pieces from the tamper and shield.
- Fission of the tamper and shield pieces produce even more radiation and heat.
- The bomb explodes.
All of these events happen 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 is an immense explosion with a 10,000-kiloton yield -- 700 times more powerful than the Little Boy explosion.