fission, nuclear

fission, nuclear In physics, form of nuclear reaction in which a heavy atomic nucleus splits into two, with the release of two or three neutrons and large amounts of heat and nuclear radiation. Fission may occur spontaneously or be made to occur by bombarding certain nuclei with low-energy (slow) neutrons or protons. The only naturally occurring fissionable material is uranium-235. Natural uranium is not capable of sustaining a chain reaction because its major isotope, U²³⁸, tends to absorb neutrons before they can split another atom. Currently, there are two main methods of creating a sustained chain reaction in uranium fuel. In the first, the neutrons released by fission of U²³⁵ are slowed down by a moderator composed of light atoms, such as deuterium, which does not absorb neutrons. Most nuclear reactors use this method. Alternatively, the uranium may be enriched (increasing the amount of fissionable material). Fast-breeder reactors employ this process. Nuclear fission and nuclear fusion are used in nuclear weapons.