Nuclear fission is a nuclear reaction in which the nucleus of a heavy atom splits into two or more smaller nuclei, usually after absorbing a neutron, releasing energy and commonly producing additional neutrons. The most important fissile materials used in energy systems include uranium 235 and plutonium 239, although other heavy nuclides can undergo fission under appropriate conditions. The original nucleus is called the parent nucleus, while the smaller products are called fission fragments.
Fission is fundamentally different from an ordinary chemical reaction because chemical reactions rearrange electrons and chemical bonds, whereas fission changes the atomic nucleus itself. The energy released comes from a reduction in the total nuclear mass of the initial system compared with the combined mass of the products. This mass difference is converted into energy according to the mass energy relationship described by Albert Einstein.
A single fission event releases an amount of energy vastly greater than the energy released by a typical chemical reaction involving one atom or molecule. Fission therefore provides a mechanism by which a relatively small quantity of nuclear fuel can release a very large quantity of usable energy.
A typical fission event begins when a suitable nucleus captures a neutron and becomes an excited compound nucleus. In the case of uranium 235, neutron absorption produces an excited uranium 236 nucleus, which can become sufficiently unstable that it deforms and separates into two major fragments. The fragments are usually radioactive isotopes located in the middle region of the periodic table, and the exact products vary from event to event rather than always being the same pair of nuclei.
Several fast neutrons are also normally emitted, together with substantial kinetic energy carried by the heavy fission fragments. Prompt gamma radiation is also associated with the process, and the radioactive fission products subsequently undergo further radioactive decays that continue producing heat. Most of the immediate fission energy appears as kinetic energy of the charged fission fragments. Because these fragments interact rapidly with surrounding atoms in the fuel, their kinetic energy is converted into thermal energy. This heat is ultimately the useful output exploited in conventional nuclear power reactors.
The reason fission releases energy can be understood through nuclear binding energy and the shape of the nuclear binding energy curve. Very heavy nuclei such as uranium have a lower average binding energy per nucleon than nuclei of intermediate mass. When a heavy nucleus divides into medium mass fragments, the resulting nucleons are generally more strongly bound on average. The difference in binding energy appears as released energy.
The products also experience electrical repulsion because both major fission fragments are positively charged, and this Coulomb repulsion contributes substantially to their kinetic energy as they separate. The strong nuclear force holds nucleons together over extremely short distances, while electrostatic repulsion acts between positively charged protons. In a sufficiently heavy nucleus, the balance between these effects allows large distortions to become energetically significant. Neutron absorption can provide the additional excitation needed for the nucleus to pass through the conditions required for fission. This explains why some isotopes are readily fissile with slow neutrons while others require much higher neutron energies or have different fission probabilities.
Fission must be distinguished from nuclear decay. Radioactive decay is a spontaneous transformation of an unstable nucleus, whereas induced fission commonly results from the interaction of a nucleus with an incoming neutron. Some very heavy nuclei can also undergo spontaneous fission without first capturing an external neutron, although spontaneous fission is not the dominant process used to operate most commercial nuclear reactors. Fission must also be distinguished from fusion, in which light nuclei combine to form a heavier nucleus.
Both processes can release nuclear energy because their products can have greater binding energy per nucleon than their starting nuclei, but they operate in different regions of nuclear mass and require different physical conditions. Fission should also not be confused with ordinary fragmentation caused by mechanical forces. A nuclear nucleus is not being physically broken apart like a macroscopic object. Instead, its quantum mechanical state changes and the nuclear interaction permits it to transform into different nuclei and emitted particles. The resulting products are governed by nuclear structure, conservation laws and probability.
Fission is central to the operation of nuclear reactors because the emitted neutrons can initiate additional fission events. If, on average, enough neutrons from one generation cause fission in subsequent generations, a self sustaining chain reaction can occur. Reactor systems are designed so that this process can be controlled rather than allowed to increase without regulation. Neutron absorbers, moderators, fuel geometry, coolant properties and reactor design all influence the neutron population.
In a thermal reactor, a moderator slows fast neutrons so that they are more likely to cause fission in fissile nuclei such as uranium 235. In a fast reactor, the neutron spectrum remains predominantly high energy and the reactor is designed around different neutron behaviour. The important quantity called effective multiplication factor describes the balance between neutron production and neutron loss. A reactor operating at critical conditions has a stable neutron population on the relevant timescale, while subcritical conditions cause the neutron population to decline and supercritical conditions cause it to increase.
The practical energy production sequence in a fission power plant involves several stages. Fission releases energy primarily inside fuel pellets, raising the temperature of the fuel. A coolant removes this heat and transports it through the reactor system. Depending on reactor type, the heated coolant may directly produce steam or transfer heat to a separate water circuit. Steam then drives turbines connected to electrical generators. The generator converts mechanical rotational energy into electrical energy through electromagnetic induction.
Thus nuclear fission does not directly produce electricity in the same way that a battery supplies electrical energy. It is principally a source of heat, and the power station uses thermal and mechanical engineering systems to convert that heat into electricity. Some reactor designs use the heat for district heating, industrial processes, desalination or other applications. The underlying energy conversion chain therefore resembles other thermal power stations, but the heat source is nuclear rather than combustion of fossil fuel or another conventional heat source.
Fission products create important engineering and safety considerations. Many fission fragments are neutron rich and therefore unstable, because stable nuclei of their mass generally require a different neutron to proton ratio. They consequently undergo beta decay and frequently emit gamma radiation as they move towards more stable nuclear configurations. Some fission products have relatively short half lives, while others remain significant for much longer periods.
Cesium 137 and strontium 90 are examples of fission products that are important in radiological assessments because of their radioactive properties and biological behaviour. The spent fuel therefore remains radioactive after the reactor has stopped operating. In addition to fission products, neutron activation can make structural materials and reactor components radioactive. Managing these materials requires shielding, cooling, monitoring, containment and regulated waste or fuel management systems. The existence of radioactive fission products is one of the major differences between nuclear fission energy systems and ordinary fossil fuel combustion systems, although fossil fuel combustion also produces pollutants and long lived environmental consequences through different mechanisms.
Nuclear fission has been studied scientifically since the discovery of radioactivity and the development of nuclear physics in the late nineteenth and early twentieth centuries. The discovery of the neutron in 1932 was particularly important because neutrons can enter atomic nuclei without experiencing electrostatic repulsion from the positively charged nucleus.
In 1938 Otto Hahn and Fritz Strassmann experimentally observed products associated with the splitting of uranium, while Lise Meitner and Otto Frisch provided the physical interpretation of the process and Frisch introduced the term nuclear fission by analogy with biological cell division. Enrico Fermi and other researchers had already investigated neutron bombardment and nuclear transformations, helping establish the experimental foundations for understanding neutron induced reactions.
Fission subsequently became important in both military and civilian contexts, including the first controlled chain reaction in the Chicago Pile 1 experiment in 1942 and the later development of nuclear power reactors. Modern fission science encompasses reactor physics, nuclear materials, fuel cycles, radiation protection, waste management, thermal hydraulics, probabilistic safety analysis and advanced reactor technologies.
