Nuclear fission

In nuclear fission, a nucleus separates into two or more smaller nuclei. In induced fission, an incoming particle—often a neutron—helps initiate the split; some unstable nuclei can also undergo spontaneous fission without an incoming particle. The smaller nuclei are called fission products, and a reaction may release neutrons, radiation and energy.

For a heavy nucleus, the combined mass of the products is lower than that of the starting nucleus and incoming particles. The difference corresponds to released energy. In a reactor, much of the energy from fission fragments becomes heat as they transfer kinetic energy to nearby material. Fission is not identical to radioactive decay generally: it is one kind of nuclear transformation, and its products and probability depend on the nuclide and conditions.

Some neutrons released in one fission can trigger further fissions. When successive generations of these reactions continue, the process is called a chain reaction. Uranium-235 is a common fissile fuel for such reactions; reactor designs and fuel compositions determine how neutrons are used. In many power reactors, a moderator slows neutrons, while control rods absorb neutrons and help operators regulate the reaction rate.

A nuclear reactor contains and controls the chain reaction. Fission heats the fuel and a coolant; in common water-cooled designs, this heat produces steam that turns a turbine connected to an electrical generator. The reactor’s purpose is to manage the rate of energy release, not to let the chain reaction grow without control.

Commercial fission power commonly uses uranium fuel containing uranium-235, although reactor designs and fuel cycles differ. Natural uranium contains a small proportion of uranium-235, so many reactor fuels undergo enrichment; some reactor designs can use other fuel compositions. Uranium-235 fission can yield different pairs of lighter nuclei rather than one fixed set of products, along with neutrons.

Some fission products are radioactive. Used reactor fuel also contains other radioactive materials formed during operation, and remains hot and highly radioactive after removal from a reactor. It therefore requires shielding and regulated storage or disposal. Some fuel can be reprocessed for reuse, but management of spent fuel and radioactive waste remains an important part of nuclear technology.

Fission splits nuclei; fusion combines lighter nuclei to form a heavier one. Both can release nuclear energy, but they are different physical processes. Electricity generated by commercial nuclear power plants today comes from controlled fission. As of the International Atomic Energy Agency’s 2025 overview, fusion electricity generation remained at the research-and-development stage. Fission is also studied in nuclear physics, where experiments examine how nuclei split and what their products reveal about nuclear matter.