Nuclear fission is a process in which an atomic nucleus divides into two or more smaller nuclei. Fission of heavy nuclei generally releases substantial energy, together with neutrons and gamma radiation. It may occur spontaneously or be induced by an incoming particle, most commonly a neutron. The possibility that emitted neutrons can cause further fissions makes the process the physical basis of nuclear reactors. Unlike nuclear fusion, which combines light nuclei, fission divides a heavier nucleus. (energy.gov)
Physical mechanism
A useful conceptual description comes from the liquid-drop model of the nucleus. Short-range nuclear attraction, associated with the strong interaction, binds nuclear constituents together, while electrical repulsion between protons favors separation. When a heavy nucleus becomes sufficiently excited, it can deform, elongate, develop a narrowing neck, and separate into fragments. Electrical repulsion then accelerates the fragments apart. This description explains the broad mechanism, although detailed predictions also require microscopic nuclear structure. (ncsp.llnl.gov)
In neutron-induced fission, neutron absorption commonly produces an excited compound nucleus that subsequently splits. For example, uranium-235 can absorb a neutron to form excited uranium-236. Absorption does not guarantee fission: competing processes include radiation emission without splitting. The fission cross-section describes the likelihood of the reaction and depends on both the target nucleus and incident-particle energy. Spontaneous fission needs no incoming projectile; quantum tunneling through the fission barrier helps explain how it occurs. (ncsp.llnl.gov)
Energy and products
Energy release arises because the resulting fragments are generally more tightly bound per nuclear constituent than the original heavy nucleus. Their greater nuclear binding energy corresponds to a reduction in total rest mass. Through mass–energy equivalence, the mass difference appears as released energy. A uranium-fueled reactor obtains approximately 200 million electronvolts of heat per fission when associated reaction and decay contributions are included. Most appears initially as the kinetic energy of the fragments. (ncsp.llnl.gov)
Fission does not produce one fixed pair of daughter nuclei. Different fragment combinations occur with different yields, and the split is often unequal. A commonly used illustrative reaction is
This is one possible channel, not a description of every uranium-235 fission. The equation conserves total nuclear charge and the number of nuclear constituents. (ncsp.llnl.gov)
Many fragment isotopes contain excess neutrons relative to stable nuclei of comparable size. They undergo beta decay and other transformations, producing additional radiation. Their radioactivity persists across widely differing half-lives. Heat from these decays continues after a reactor’s principal chain reaction has stopped, making decay heat distinct from ongoing fission power. (ncsp.llnl.gov)
Fissionable, fissile, and fertile materials
“Fissionable” denotes material capable of undergoing fission. “Fissile” has the narrower meaning of material that can fission through absorption of low-energy, thermal neutrons. Uranium-235, uranium-233, and plutonium-239 are important fissile isotopes. Uranium-238 can fission with sufficiently energetic neutrons but is not classified as fissile. (nrc.gov)
A fertile isotope can be transformed into a fissile isotope through neutron absorption followed by nuclear decay. Uranium-238 can thereby produce plutonium-239, while thorium-232 can produce uranium-233. These conversions connect fuel consumption with the creation of new fissile material. Natural uranium contains approximately 0.7 percent uranium-235 and 99.3 percent uranium-238, with a small uranium-234 component; enrichment increases the uranium-235 proportion. (nrc.gov)
Chain reactions and reactor control
A nuclear chain reaction occurs when neutrons released by fission induce subsequent fissions. Some neutrons instead escape or are absorbed without producing another fission. The effective multiplication factor, , expresses the balance between successive neutron generations. Below one, the chain reaction declines without an external source; at one, it is self-sustaining; above one, the neutron population tends to increase. “Critical” therefore describes a neutron balance, not necessarily a dangerous condition. (ncsp.llnl.gov)
Most fission neutrons are emitted promptly. A small fraction appear later through the decay of precursor nuclei. These delayed neutrons lengthen the response times relevant to reactor control, allowing regulated changes in power rather than changes governed solely by prompt-neutron timescales. (ncsp.llnl.gov)
A nuclear reactor combines fuel, cooling systems, monitoring equipment, and neutron-control mechanisms. Thermal reactors use moderators such as water or graphite to slow neutrons; fast reactors operate without this slowing requirement. Neutron-absorbing control rods regulate the reaction. In nuclear power generation, heat is transferred to a power-conversion system, commonly using a steam turbine and generator to produce electricity. Cooling remains necessary after shutdown because radioactive decay continues releasing heat. (nrc.gov)
Discovery and development
In December 1938, Otto Hahn and Fritz Strassmann identified barium among products of neutron-bombarded uranium. Lise Meitner and Otto Robert Frisch interpreted the result as the splitting of the uranium nucleus and explained its large energy release. Their interpretation appeared in 1939. Hahn received the 1944 Nobel Prize in Chemistry for the discovery of heavy-nucleus fission. (osti.gov)
On December 2, 1942, a team led by Enrico Fermi achieved the first controlled, self-sustaining nuclear chain reaction in Chicago Pile-1. Fission subsequently supported both reactor development and the weapons work of the Manhattan Project. Beyond electricity generation, fission processes and their products are used in research, isotope production, and nuclear-material analysis. (energy.gov)