By Craig Freudenrich, Ph.D. and John Fuller
Freudenrich, Ph.D., Craig, and John Fuller. "How Nuclear Bombs Work." 05 October 2000. HowStuffWorks.com. 26 February 2010.
Nuclear bombs involve the forces that hold the nucleus of an atom together, especially atoms with unstable nuclei. There are two basic ways that nuclear energy can be released from an atom. The first is nuclear fission, in which the nucleus of an atom is split into two smaller fragments and a neutron. The second method is nuclear fusion, in which two small atoms, usually hydrogen or its isotopes (deuterium, tritium), fuse together to form a larger one (helium or helium isotopes). Earth's sun produces energy through nuclear fusion. In either process, fission or fusion, staggering amounts of heat energy and radiation are given off.
To build an atomic bomb, you need a source of fissionable or fusionable fuel, a triggering device, and a way to allow the majority of fuel to fission or fuse before the explosion occurs so that the bomb doesn't fizzle out.
Fission bombs use uranium-235 as fuel. Uranium is the heaviest naturally occurring element on Earth, and it has two isotopes - uranium-238 and uranium-235, both of which are only barely stable because they have very high numbers of neutrons. U-235 is one of the few materials that can undergo induced fission. Instead of waiting more than 700 million years for uranium to decay naturally, the element can be broken down much faster if a neutron collides with a U-235 nucleus. The nucleus will absorb the neutron right away, causing it to become unstable and split. As soon as the nucleus captures a neutron, it splits into two lighter atoms, and emits two or three new neutrons. These two new atoms emit gamma radiation while they are settling into their new states. When a fission bomb is working properly, more than one neutron ejected from each fission causes another fission to occur in a chain reaction.
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. The fuels used for fusion, deuterium and tritium, are both gases, and are difficult to store. Tritium is scarce and has a brief half-life, so the fuel would have to be continuously replenished. The fuel, whether deuterium or tritium, must be highly compressed at a high temperature to initiate the fusion reaction.
Several of these problems can be solved by encasing a fission bomb within a fusion bomb. To visualize this design, imagine an implosion fission bomb in a bomb casing, along with a cylinder of uranium-238 (tamper). The tamper contains lithium deuteride (fuel), and a hollow rod of plutonium-239, in the center of the tamper cylinder. A shield of uranium-238 separates the tamper cylinder from the implosion bomb. Detonation of the bomb causes the following to occur:
- The fission bomb implodes, emitting 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 causess the tamper to expand and burn away, exerting pressure inward against the lithium deuterate.
- The lithium deuterate is compressed by about 30-fold.
- The compression shock waves initiate fission in the plutonium rod.
- The fissioning rod emits off radiation, heat, and neutrons.
- The neutrons enter the lithium deuterate, combine with the lithium and make tritium.
- The combination of high temperature and pressure cause tritium-deuterium and deuterium-deuterium fusion reactions to occur, producing more heat, radiation, and neutrons.
- The neutrons produced by the fusion reaction induce fission in the uranium-238 pieces from the tamper and shield.
- Fission of the tamper and shield pieces produced even more radiation and heat.
- The bomb explodes.
All of these events occur 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.
