The fission process

When bombarded by neutrons, certain isotopes of uranium and plutonium (and some other heavier elements) will split into atoms of lighter elements, a process known as nuclear fission. In addition to this formation of lighter atoms, on average between 2.5 and 3 free neutrons are emitted in the fission process, along with considerable energy. As a rule of thumb, the complete fission of 1 kg (2.2 pounds) of uranium or plutonium produces about 17.5 kilotons of TNT-equivalent explosive energy.

fission Sequence of events in the fission of a uranium nucleus by a neutron. Encyclopædia Britannica, Inc.

In an atomic bomb or nuclear reactor, first a small number of neutrons are given enough energy to collide with some fissionable nuclei, which in turn produce additional free neutrons. A portion of these neutrons are captured by nuclei that do not fission; others escape the material without being captured; and the remainder cause further fissions. Many heavy atomic nuclei are capable of fissioning, but only a fraction of these are fissile—that is, fissionable not only by fast (highly energetic) neutrons but also by slow neutrons. The continuing process whereby neutrons emitted by fissioning nuclei induce fissions in other fissile or fissionable nuclei is called a fission chain reaction. If the number of fissions in one generation is equal to the number of neutrons in the preceding generation, the system is said to be critical; if the number is greater than one, it is supercritical; and if it is less than one, it is subcritical. In the case of a nuclear reactor, the number of fissionable nuclei available in each generation is carefully controlled to prevent a “runaway” chain reaction. In the case of an atomic bomb, however, a very rapid growth in the number of fissions is sought.

Fission weapons are normally made with materials having high concentrations of the fissile isotopes uranium-235, plutonium-239, or some combination of these; however, some explosive devices using high concentrations of uranium-233 also have been constructed and tested.

The primary natural isotopes of uranium are uranium-235 (0.7 percent), which is fissile, and uranium-238 (99.3 percent), which is fissionable but not fissile. In nature, plutonium exists only in minute concentrations, so the fissile isotope plutonium-239 is made artificially in nuclear reactors from uranium-238. (See uranium processing.) In order to make an explosion, fission weapons do not require uranium or plutonium that is pure in the isotopes uranium-235 and plutonium-239. Most of the uranium used in current nuclear weapons is approximately 93.5 percent enriched uranium-235. Nuclear weapons typically contain 93 percent or more plutonium-239, less than 7 percent plutonium-240, and very small quantities of other plutonium isotopes. Plutonium-240, a by-product of plutonium production, has several undesirable characteristics, including a larger critical mass (that is, the mass required to generate a chain reaction), greater radiation exposure to workers (relative to plutonium-239), and, for some weapon designs, a high rate of spontaneous fission that can cause a chain reaction to initiate prematurely, resulting in a smaller yield. Consequently, in reactors used for the production of weapons-grade plutonium-239, the period of time that the uranium-238 is left in the reactor is restricted in order to limit the buildup of plutonium-240 to about 6 percent.