Nuclear Fission

You might wonder why 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 barely stable. Both isotopes also have an unusually large number of neutrons. Although ordinary uranium will always have 92 protons, U-238 has 146 neutrons, while U-235 has 143 neutrons.

Both isotopes of uranium are radioactive, and they eventually decay over time. U-235, however, 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. Instead of waiting more than 700 million years for uranium to naturally decay, the element can be broken down much faster if a neutron runs into a U-235 nucleus. The nucleus will absorb the neutron without hesitation, become unstable and split immediately.

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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. There are a couple of things about this induced fission process that makes 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. It helps to think of a big circle of marbles as the protons and neutrons of an atom. If you shoot one marble -- a single neutron -- in the middle of the big circle, it will hit one marble, which will hit a few more marbles, and so on until a chain reaction continues.
  • The process of capturing the neutron and splitting happens very quickly, on the order of picoseconds (0.000000000001 seconds).

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.

Critical Mass
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. Think about the marble analogy again. If the circle of marbles are spread too far apart -- subcritical mass -- a smaller chain reaction will occur when the "neutron marble" hits the center. If the marbles are placed closer together in the circle -- critical mass -- there is a higher chance a big chain reaction will take place. 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.

In the next section we'll look at how a fission bomb actually works.