fission

In physics, the splitting of a heavy atomic nucleus into two or more major fragments. It is accompanied by the emission of two or three neutrons and the release of large amounts of nuclear energy.

Fission occurs spontaneously in nuclei of uranium-235, the main fuel used in nuclear reactors. However, the process can also be induced by bombarding the nuclei with neutrons because a nucleus that has absorbed a neutron becomes unstable and soon splits. For example:

²³⁵₉₂U + ¹₀n → ²³⁶₉₂U → 2 nuclei + 2–3 neutrons + energy

The neutrons released spontaneously by the fission of uranium nuclei may therefore be used in turn to induce further fissions, setting up a chain reaction that must be controlled if it is not to result in a nuclear explosion. In a nuclear power station, heat energy released from the chain reaction is used to boil water to produce steam. The steam is used to drive the turbine of a generator to produce electricity. An atomic bomb uses pure uranium-235 to start an uncontrolled nuclear reaction, producing large amounts of heat energy. The minimum amount of fissile material that can undergo a continuous chain reaction is referred to as the critical mass.