Nuclear fission

In physics, fission is a nuclear process, meaning it occurs in the nucleus of an atom. Fission is when the nucleus splits into two or more smaller nuclei plus some by-products. These by-products include free neutrons and photons (usually gamma rays). Fission releases substantial amounts of energy (the strong nuclear force binding energy).

Inducing fission

• Though fission is most often and most easily initiated by the absorption of a free neutron, it can also be induced by striking a fissionable nucleus with other particles. These other particles can include protons, other nuclei, or even high intensity high-energy photons (gamma rays).
• Very infrequently, a fissionable nucleus will undergo spontaneous nuclear fission without an incoming neutron.
• Inducing fission is easiest in heavy elements and generally, the more massive the nucleus the more likely it is able to be fissioned. Fission in any element heavier than iron produces energy, and fission in any element lighter than iron requires energy. The opposite is true of nuclear fusion reactions - fusion in elements lighter than iron produces energy, and fusion in elements heavier than iron requires energy. (see: the curve of binding energy)
• The most frequently used elements to produce nuclear fission are uranium and plutonium. Uranium is the heaviest naturally occurring element in any appreciable abundance; plutonium undergoes spontaneous fission reactions and has a relatively short half-life. So, although other elements can be used, these have the best combination of abundance and ease of fission. For further detail, see fissile.