• The latter figure means that a nuclear fission explosion or criticality accident emits about 3.5% of its energy as gamma rays, less than 2.5% of its energy as fast neutrons
  (total of both types of radiation ~6%), and the rest as kinetic energy of fission fragments (this appears almost immediately when the fragments impact surrounding matter, as simple heat).

• On the other hand, so-called delayed neutrons emitted as radioactive decay products with half-lives up to several minutes, from fission-daughters, are very important to reactor
  control, because they give a characteristic “reaction” time for the total nuclear reaction to double in size, if the reaction is run in a “delayed-critical” zone which deliberately relies on these neutrons for a supercritical chain-reaction
  (one in which each fission cycle yields more neutrons than it absorbs).

• However, this process cannot happen to a great extent in a nuclear reactor, as too small a fraction of the fission neutrons produced by any type of fission have enough energy
  to efficiently fission 238 U (fission neutrons have a mode energy of 2 MeV, but a median of only 0.75 MeV, meaning half of them have less than this insufficient energy).

• Like nuclear fusion, for fission to produce energy, the total binding energy of the resulting elements must be greater than that of the starting element.

• The remaining energy to initiate fission can be supplied by two other mechanisms: one of these is more kinetic energy of the incoming neutron, which is increasingly able to
  fission a fissionable heavy nucleus as it exceeds a kinetic energy of 1 MeV or more (so-called fast neutrons).

• This was the first observation of a nuclear reaction, that is, a reaction in which particles from one decay are used to transform another atomic nucleus.

• Fission product yields 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.

• The fission process often produces gamma photons, and releases a very large amount of energy even by the energetic standards of radioactive decay.

• Energy input deforms the nucleus into a fat “cigar” shape, then a “peanut” shape, followed by binary fission as the two lobes exceed the short-range nuclear force attraction
  distance, and are then pushed apart and away by their electrical charge.

• [9] Carl Friedrich von Weizsäcker’s semi-empirical mass formula may be used to express the binding energy as the sum of five terms, which are the volume energy, a surface
  correction, Coulomb energy, a symmetry term, and a pairing term:[9] where the nuclear binding energy is proportional to the nuclear volume, while nucleons near the surface interact with fewer nucleons, reducing the effect of the volume term.

• Critical fission reactors are built for three primary purposes, which typically involve different engineering trade-offs to take advantage of either the heat or the neutrons
  produced by the fission chain reaction: • power reactors are intended to produce heat for nuclear power, either as part of a generating station or a local power system such as a nuclear submarine.

• About 6 MeV of the fission-input energy is supplied by the simple binding of an extra neutron to the heavy nucleus via the strong force; however, in many fissionable isotopes,
  this amount of energy is not enough for fission.

• In such isotopes, therefore, no neutron kinetic energy is needed, for all the necessary energy is supplied by absorption of any neutron, either of the slow or fast variety
  (the former are used in moderated nuclear reactors, and the latter are used in fast-neutron reactors, and in weapons).

• 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 neutrinos as the product of such decay.

• [17][18] Some processes involving neutrons are notable for absorbing or finally yielding energy — for example neutron kinetic energy does not yield heat immediately if the
  neutron is captured by a uranium-238 atom to breed plutonium-239, but this energy is emitted if the plutonium-239 is later fissioned.

• [23][24][25] The objective of an atomic bomb is to produce a device, according to Serber, “…in which energy is released by a fast neutron chain reaction in one or more of
  the materials known to show nuclear fission.”

• For uranium-235 (total mean fission energy 202.79 MeV[15]), typically ~169 MeV appears as the kinetic energy of the daughter nuclei, which fly apart at about 3% of the speed
  of light, due to Coulomb repulsion.

• In nuclear fission events the nuclei may break into any combination of lighter nuclei, but the most common event is not fission to equal mass nuclei of about mass 120; the
  most common event (depending on isotope and process) is a slightly unequal fission in which one daughter nucleus has a mass of about 90 to 100 daltons and the other the remaining 130 to 140 daltons.

• This extra binding energy is made available as a result of the mechanism of neutron pairing effects, which itself is caused by the Pauli exclusion principle, allowing an extra
  neutron to occupy the same nuclear orbital as the last neutron in the nucleus.

• [5] Eventually, in 1932, a fully artificial nuclear reaction and nuclear transmutation was achieved by Rutherford’s colleagues Ernest Walton and John Cockcroft, who used artificially
  accelerated protons against lithium-7, to split this nucleus into two alpha particles.

• Binding energy due to the nuclear force approaches a constant value for large A, while the Coulomb acts over a larger distance so that electrical potential energy per proton
  grows as Z increases.

• In a critical fission reactor, neutrons produced by fission of fuel atoms are used to induce yet more fissions, to sustain a controllable amount of energy release.

• [9] In fission there is a preference for fission fragments with even Z, which is called the odd–even effect on the fragments’ charge distribution.

• According to Younes and Loveland, “Actinides like 235 U that fission easily following the absorption of a thermal (0.25 meV) neutron are called fissile, whereas those like
  238 U that do not easily fission when they absorb a thermal neutron are called fissionable.

• Though less common than binary fission, it still produces significant helium-4 and tritium gas buildup in the fuel rods of modern nuclear reactors.

• Looking further left on the curve of binding energy, where the fission products cluster, it is easily observed that the binding energy of the fission products tends to center
  around 8.5 MeV per nucleon.

• 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 A ≈ 240.

• Both uses are possible because certain substances called nuclear fuels undergo fission when struck by fission neutrons, and in turn emit neutrons when they break apart.

• Nuclei that are neutron- or proton-rich have excessive binding energy for stability, and the excess energy may convert a neutron to a proton or a proton to a neutron via the
  weak nuclear force, a process known as beta decay.

• For heavy nuclides, it is an exothermic reaction which can release large amounts of energy both as electromagnetic radiation and as kinetic energy of the fragments (heating
  the bulk material where fission takes place).

• According to Lilley, “The radioactive decay energy from the fission chains is the second release of energy due to fission.

• Main article: Nuclear chain reaction John Lilley states, “…neutron-induced fission generates extra neutrons which can induce further fissions in the next generation and
  so on in a chain reaction.

• According to Rhodes, “Untamped, a bomb core even as large as twice the critical mass would completely fission less than 1 percent of its nuclear material before it expanded
  enough to stop the chain reaction from proceeding.

• [4]: 21–24  Since in nuclear fission, the nucleus emits more neutrons than the one it absorbs, a chain reaction is possible.

• 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 (Z = 1), to as large a fragment as argon (Z = 18).

• [4]: 21–22, 30  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.

• 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.

• In Chadwick’s words, “…In order to explain the great penetrating power of the radiation we must further assume that the particle has no net charge…” The existence of the
  neutron was first postulated by Rutherford in 1920, and in the words of Chadwick, “…how on earth were you going to build up a big nucleus with a large positive charge?

• Fission energy is released when a A is larger than 120 nucleus fragments.

• The thorium fuel cycle produces virtually no plutonium and much less minor actinides, but 232 U – or rather its decay products – are a major gamma ray emitter.

• This can be easily seen by examining the curve of binding energy (image below), and noting that the average binding energy of the actinide nuclides beginning with uranium
  is around 7.6 MeV per nucleon.

• Such high energy neutrons are able to fission 238 U directly (see thermonuclear weapon for application, where the fast neutrons are supplied by nuclear fusion).

• The two (or more) nuclei produced are most often of comparable but slightly different sizes, typically with a mass ratio of products of about 3 to 2, for common fissile isotopes.

• Energy of about 6 MeV provided by the incident neutron was necessary to overcome this barrier and cause the nucleus to fission.

• If, in a reactor, k is less than unity, the reactor is subcritical, the number of neutrons decreases and the chain reaction dies out.

• The amount of free energy released in the fission of an equivalent amount of 235 U is a million times more than that released in the combustion of methane or from hydrogen
  fuel cells.

• [4] The products of nuclear fission, however, are on average far more radioactive than the heavy elements which are normally fissioned as fuel, and remain so for significant
  amounts of time, giving rise to a nuclear waste problem.

• [9] According to Lilley, “The binding energy of a nucleus B is the energy required to separate it into its constituent neutrons and protons.

• Apart from fission induced by a neutron, harnessed and exploited by humans, a natural form of spontaneous radioactive decay (not requiring a neutron) is also referred to as
  fission, and occurs especially in very high-mass-number isotopes.

• Thus, the mass of an atom is less than the mass of its constituent protons and neutrons, assuming the average binding energy of its electrons is negligible.

• Fission into two fragments is called binary fission, and is the most common nuclear reaction.

• 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.

• The feat was popularly known as “splitting the atom”, and would win them the 1951 Nobel Prize in Physics for “Transmutation of atomic nuclei by artificially accelerated atomic
  particles”, although it was not the nuclear fission reaction later discovered in heavy elements.

• Fission is a form of nuclear transmutation because the resulting fragments (or daughter atoms) are not the same element as the original parent atom.

• Most of the energy released is in the form of the kinetic velocities of the fission products and the neutrons.

• The exact isotope which is fissioned, and whether or not it is fissionable or fissile, has only a small impact on the amount of energy released.

• In their second publication on nuclear fission in February 1939, Hahn and Strassmann predicted the existence and liberation of additional neutrons during the fission process,
  opening up the possibility of a nuclear chain reaction.

• Neutron absorption which does not lead to fission produces plutonium (from 238 U) and minor actinides (from both 235 U and 238 U) whose radiotoxicity is far higher than that
  of the long lived fission products.

• Nuclear fission is a reaction in which the nucleus of an atom splits into two or more smaller nuclei.

• When a uranium nucleus fissions into two daughter nuclei fragments, about 0.1 percent of the mass of the uranium nucleus[14] appears as the fission energy of ~200 MeV.

• This makes a self-sustaining nuclear chain reaction possible, releasing energy at a controlled rate in a nuclear reactor or at a very rapid, uncontrolled rate in a nuclear
  weapon.

• Additional terms can be included such as symmetry, pairing, the finite range of the nuclear force, and charge distribution within the nuclei to improve the estimate.

• [12] Among the heavy actinide elements, however, those isotopes that have an odd number of neutrons (such as 235U with 143 neutrons) bind an extra neutron with an additional
  1 to 2 MeV of energy over an isotope of the same element with an even number of neutrons (such as 238U with 146 neutrons).

• It is possible to achieve criticality in a reactor using natural uranium as fuel, provided that the neutrons have been efficiently moderated to thermal energies.”

• 238 U needs a fast neutron to supply the additional 1 MeV needed to cross the critical energy barrier for fission.

• In the words of Younes and Lovelace, “…the neutron absorption on a 235 U target forms a 236 U nucleus with excitation energy greater than the critical fission energy, whereas
  in the case of n + 238 U, the resulting 239 U nucleus has an excitation energy below the critical fission energy.”

• Without their existence, the nuclear chain-reaction would be prompt critical and increase in size faster than it could be controlled by human intervention.

• According to John C. Lee, “For all nuclear reactors in operation and those under development, the nuclear fuel cycle is based on one of three fissile materials, 235U, 233U,
  and 239Pu, and the associated isotopic chains.

• The binding energy formula includes volume, surface and Coulomb energy terms that include empirically derived coefficients for all three, plus energy ratios of a deformed
  nucleus relative to a spherical form for the surface and Coulomb terms.

• Spontaneous fission was discovered in 1940, in an experiment intended to confirm that, without bombardment by neutrons, the fission rate of uranium was negligible, as predicted
  by Niels Bohr; it was not negligible.

• The nuclides that can sustain a fission chain reaction are suitable for use as nuclear fuels.

• Absorption of any neutron makes available to the nucleus binding energy of about 5.3 MeV.

 

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Photo credit: https://www.flickr.com/photos/thebosque/6606820461/’]