Editing Nuclear fission (section)

===Mechanism===
Younes and Loveland define fission as, "...a collective motion of the protons and neutrons that make up the nucleus, and as such it is distinguishable from other phenomena that break up the nucleus. Nuclear fission is an extreme example of large-[[amplitude]] collective motion that results in the division of a parent nucleus into two or more fragment nuclei. The fission process can occur spontaneously, or it can be induced by an incident particle." The energy from a fission reaction is produced by its [[fission products]], though a large majority of it, about 85 percent, is found in fragment [[kinetic energy]], while about 6 percent each comes from initial neutrons and gamma rays and those emitted after [[beta decay]], plus about 3 percent from [[neutrino]]s as the product of such decay.<ref name=ww/>{{rp|21–22,30}}

[[File:UFission.gif|250px|right|thumb|A visual representation of an induced nuclear fission event where a slow-moving neutron is absorbed by the nucleus of a uranium-235 atom, which fissions into two fast-moving lighter elements (fission products) and additional neutrons. Most of the energy released is in the form of the kinetic velocities of the fission products and the neutrons.]]
[[File:ThermalFissionYield.svg|thumb|300px|[[Fission product yield]]s by mass for [[thermal neutron]] fission of [[uranium-235]], [[plutonium-239]], a combination of the two typical of current nuclear power reactors, and [[uranium-233]], used in the [[thorium cycle]]]]

====Radioactive decay====
Nuclear fission can occur without neutron bombardment as a type of radioactive decay. This type of fission is called [[spontaneous fission]], and was first observed in 1940.<ref name=ww/>{{rp|22}}

====Nuclear reaction====
During induced fission, a compound system is formed after an incident particle fuses with a target. The resultant excitation energy may be sufficient to emit neutrons, or gamma-rays, and nuclear scission. Fission into two fragments is called binary fission, and is the most common [[nuclear reaction]]. Occurring least frequently is [[ternary fission]], in which a third particle is emitted. This third particle is commonly an [[Alpha particle|α particle]].<ref name=ww/>{{rp|21–24}} Since in nuclear fission, the nucleus emits more neutrons than the one it absorbs, a [[chain reaction]] is possible.<ref name=rr/>{{rp|291,296}}

Binary fission may produce any of the fission products, at 95±15 and 135±15 [[Dalton (unit)|daltons]]. One example of a binary fission event in the most commonly used [[fissile nuclide]], {{chem|235|U}}, is given as:

<math>\ {}^{235}\mathrm{U} + \mathrm{n} \longrightarrow {}^{236}\mathrm{U}^{*} \longrightarrow {}^{95}\mathrm{Sr} + {}^{139}\mathrm{Xe} + 2\ \mathrm{n} + 180\ \mathrm{MeV}</math>

However, the binary process happens merely because it is the most probable. In anywhere from two to four fissions per 1000 in a nuclear reactor, ternary fission can produce three positively charged fragments (plus neutrons) and the smallest of these may range from so small a charge and mass as a proton ([[Atomic number|''Z'']]&nbsp;=&nbsp;1), to as large a fragment as [[argon]] (''Z''&nbsp;=&nbsp;18). The most common small fragments, however, are composed of 90% helium-4 nuclei with more energy than alpha particles from alpha decay (so-called "long range alphas" at ~16 [[megaelectronvolt]]s (MeV)), plus helium-6 nuclei, and tritons (the nuclei of [[tritium]]). Though less common than binary fission, it still produces significant helium-4 and tritium gas buildup in the fuel rods of modern nuclear reactors.<ref>S. Vermote, et al. (2008) [https://books.google.com/books?id=6IkykKNob6gC&pg=PA259 "Comparative study of the ternary particle emission in 243-Cm (nth,f) and 244-Cm(SF)"] in ''Dynamical aspects of nuclear fission: proceedings of the 6th International Conference.'' J. Kliman, M. G. Itkis, S. Gmuca (eds.). World Scientific Publishing Co. Pte. Ltd. Singapore. {{ISBN|9812837523}}.</ref>

Bohr and Wheeler used their [[liquid drop model]], the packing fraction curve of [[Arthur Jeffrey Dempster]], and Eugene Feenberg's estimates of nucleus radius and surface tension, to estimate the mass differences of parent and daughters in fission. They then equated this mass difference to energy using Einstein's [[mass-energy equivalence]] formula. The stimulation of the nucleus after neutron bombardment was analogous to the vibrations of a liquid drop, with [[surface tension]] and the [[Coulomb force]] in opposition. Plotting the sum of these two energies as a function of elongated shape, they determined the resultant energy surface had a saddle shape. The saddle provided an energy barrier called the critical energy barrier. Energy of about 6 MeV provided by the incident neutron was necessary to overcome this barrier and cause the nucleus to fission.<ref name=ww/>{{rp|10–11}}<ref>{{cite journal |last1=Dempster |first1=A.J. |title=The Atomic Masses of the Heavy Elements |url=https://journals.aps.org/pr/abstract/10.1103/PhysRev.53.64 |journal=Physical Review |publisher=American Physical Society |access-date=9 October 2023 |date=1938|volume=53 |issue=1 |pages=64–75 |doi=10.1103/PhysRev.53.64 |bibcode=1938PhRv...53...64D }}</ref><ref>{{cite journal |last1=Feenberg |first1=eugene |title=On the Shape and Stability of Heavy Nuclei |url=https://journals.aps.org/pr/abstract/10.1103/PhysRev.55.504.2 |journal=Physical Review |publisher=American Physical Society |access-date=9 October 2023 |date=1939|volume=55 |issue=5 |pages=504–505 |doi=10.1103/PhysRev.55.504.2 |bibcode=1939PhRv...55..504F }}</ref> According to John Lilley, "The energy required to overcome the barrier to fission is called the ''activation energy'' or ''fission barrier'' and is about 6 MeV for [[Mass number|''A'']]&nbsp;≈&nbsp;240. It is found that the activation energy decreases as A increases. Eventually, a point is reached where activation energy disappears altogether...it would undergo very rapid spontaneous fission."<ref name="jl">{{cite book |last1=Lilley |first1=John |title=Nuclear Physics: Principles and Application |date=2001 |publisher=John Wiley & Sons, Ltd |isbn=9780471979364 |pages=7–9,13–14,38–43,265–267}}</ref>

[[Maria Goeppert Mayer]] later proposed the [[nuclear shell model]] for the nucleus. The nuclides that can sustain a fission chain reaction are suitable for use as [[nuclear fuel]]s. The most common nuclear fuels are <sup>235</sup>U (the isotope of uranium with [[mass number]] 235 and of use in nuclear reactors) and [[Plutonium-239|<sup>239</sup>Pu]] (the isotope of plutonium with mass number 239). These fuels break apart into a bimodal range of chemical elements with atomic masses centering near 95 and 135 daltons ([[fission products]]). Most nuclear fuels undergo spontaneous fission only very slowly, decaying instead mainly via an [[alpha particle|alpha]]-[[beta particle|beta]] [[decay chain]] over periods of [[millennium|millennia]] to [[eon (geology)|eons]]. In a nuclear reactor or nuclear weapon, the overwhelming majority of fission events are induced by bombardment with another particle, a neutron, which is itself produced by prior fission events.

[[Fissionable]] isotopes such as uranium-238 require additional energy provided by [[fast neutron]]s (such as those produced by nuclear fusion in [[thermonuclear weapons]]). While ''some'' of the neutrons released from the fission of {{chem|238|U}} are fast enough to induce another fission in {{chem|238|U}}, ''most'' are not, meaning it can never achieve criticality. While there is a very small (albeit nonzero) chance of a thermal neutron inducing fission in {{chem|238|U}}, [[neutron absorption]] is orders of magnitude more likely.