A fission bomb uses an element like uranium-235 to create a nuclear explosion. If you have read How Nuclear Radiation Works, then you understand the basic process behind radioactive decay and fission. Uranium-235 has an extra property that makes it useful for both nuclear-power production and nuclear-bomb production -- U-235 is one of the few materials that can undergo induced fission. If a free neutron runs into a U-235 nucleus, the nucleus will absorb the neutron without hesitation, become unstable and split immediately.
This figure shows a uranium-235 nucleus with a neutron approaching from the top. As soon as the nucleus captures the neutron, it splits into two lighter atoms and throws off two or three new neutrons (the number of ejected neutrons depends on how the U-235 atom happens to split). The two new atoms then emit gamma radiation as they settle into their new states (see How Nuclear Radiation Works). There are three things about this induced fission process that make it interesting:
The probability of a U-235 atom capturing a neutron as it passes by is fairly high. In a bomb that is working properly, more than one neutron ejected from each fission causes another fission to occur. This condition is known as supercriticality.
The process of capturing the neutron and splitting happens very quickly, on the order of picoseconds (1*10E-12 seconds).
An incredible amount of energy is released, in the form of heat and gamma radiation, when an atom splits. The energy released by a single fission is due to the fact that the fission products and the neutrons, together, weigh less than the original U-235 atom.
The difference in weight is converted to energy at a rate governed by the equation e = m * c^2. A pound of highly enriched uranium as used in a nuclear bomb is equal to something on the order of a million gallons of gasoline. When you consider that a pound of uranium is smaller than a baseball and a million gallons of gasoline would fill a cube that is 50 feet per side (50 feet is as tall as a five-story building), you can get an idea of the amount of energy available in just a little bit of U-235.
In order for these properties of U-235 to work, a sample of uranium must be enriched . Weapons-grade uranium is composed of at least 90-percent U-235.
In a fission bomb, the fuel must be kept in separate subcritical masses, which will not support fission, to prevent premature detonation. Critical mass is the minimum mass of fissionable material required to sustain a nuclear fission reaction. This separation brings about several problems in the design of a fission bomb that must be solved:
The two or more subcritical masses must be brought together to form a supercritical mass, which will provide more than enough neutrons to sustain a fission reaction, at the time of detonation.
Free neutrons must be introduced into the supercritical mass to start the fission.
As much of the material as possible must be fissioned before the bomb explodes to prevent fizzle.
To bring the subcritical masses together into a supercritical mass, two techniques are used:
Neutrons are introduced by making a neutron generator. This generator is a small pellet of polonium and beryllium, separated by foil within the fissionable fuel core. In this generator:
The foil is broken when the subcritical masses come together and polonium spontaneously emits alpha particles.
These alpha particles then collide with beryllium-9 to produce beryllium-8 and free neutrons.
The neutrons then initiate fission.
Finally, the fission reaction is confined within a dense material called a tamper, which is usually made of uranium-238. The tamper gets heated and expanded by the fission core. This expansion of the tamper exerts pressure back on the fission core and slows the core's expansion. The tamper also reflects neutrons back into the fission core, increasing the efficiency of the fission reaction.