Nuclear fission
Nuclear fission is a reaction in which the nucleus of an atom splits into two or more smaller nuclei. The fission process often produces gamma photons, and releases a very large amount of energy even by the energetic standards of radioactive decay.
Nuclear fission was discovered by chemists Otto Hahn and Fritz Strassmann and physicists Lise Meitner and Otto Robert Frisch. Hahn and Strassmann proved that a fission reaction had taken place on 19 December 1938, and Meitner and her nephew Frisch explained it theoretically in January 1939. Frisch named the process "fission" by analogy with biological fission of living cells. 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.
For heavy nuclides, it is an exothermic reaction which releases large amounts of energy both as electromagnetic radiation and as kinetic energy of the fragments. 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 fission barrier must also be overcome. Fissionable nuclides primarily split in interactions with fast neutrons, while fissile nuclides easily split in interactions with "slow" i.e. thermal neutrons, usually originating from moderation of fast neutrons.
Fission is a form of nuclear transmutation because the resulting fragments are not the same element as the original parent atom. The two 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. Most fissions are binary fissions, but occasionally, three positively charged fragments are produced, in a ternary fission. The smallest of these fragments in ternary processes ranges in size from a proton to an argon nucleus.
Apart from fission induced by an exogenous neutron, harnessed and exploited by humans, a natural form of spontaneous radioactive decay is also referred to as fission, and occurs especially in very high-mass-number isotopes. Spontaneous fission was discovered in 1940 by Flyorov, Petrzhak, and Kurchatov in Moscow. In contrast to nuclear fusion, which drives the formation of stars and their development, one can consider nuclear fission as negligible for the evolution of the universe. Nonetheless, natural nuclear fission reactors may form under very rare conditions. Accordingly, all elements which are important for the formation of solar systems, planets and also for all forms of life are not fission products, but rather the results of fusion processes.
The unpredictable composition of the products distinguishes fission from purely quantum tunneling processes such as proton emission, alpha decay, and cluster decay, which give the same products each time. Nuclear fission produces energy for nuclear power and drives the explosion of nuclear weapons. 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. 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.
The amount of free energy released in the fission of an equivalent amount of is a million times more than that released in the combustion of methane or from hydrogen fuel cells.
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. However, the seven long-lived fission products make up only a small fraction of fission products. Neutron absorption which does not lead to fission produces plutonium and minor actinides whose radiotoxicity is far higher than that of the long lived fission products. Concerns over nuclear waste accumulation and the destructive potential of nuclear weapons are a counterbalance to the peaceful desire to use fission as an energy source. The thorium fuel cycle produces virtually no plutonium and much less minor actinides, but - or rather its decay products - are a major gamma ray emitter. All actinides are fertile or fissile and fast breeder reactors can fission them all albeit only in certain configurations. Nuclear reprocessing aims to recover usable material from spent nuclear fuel to both enable uranium supplies to last longer and to reduce the amount of "waste". The industry term for a process that fissions all or nearly all actinides is a "closed fuel cycle".
Physical overview
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 neutrinos as the product of such decay.File:ThermalFissionYield.svg|thumb|300px|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
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.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 α particle. Since in nuclear fission, the nucleus emits more neutrons than the one it absorbs, a chain reaction is possible.Binary fission may produce any of the fission products, at 95±15 and 135±15 daltons. One example of a binary fission event in the most commonly used fissile nuclide,, is given as:
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 and the smallest of these may range from so small a charge and mass as a proton, to as large a fragment as argon. The most common small fragments, however, are composed of 90% helium-4 nuclei with more energy than alpha particles from alpha decay, plus helium-6 nuclei, and tritons. Though less common than binary fission, it still produces significant helium-4 and tritium gas buildup in the fuel rods of modern nuclear reactors.
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. 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. 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."
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 fuels. The most common nuclear fuels are 235U and 239Pu. These fuels break apart into a bimodal range of chemical elements with atomic masses centering near 95 and 135 daltons. Most nuclear fuels undergo spontaneous fission only very slowly, decaying instead mainly via an alpha-beta decay chain over periods of millennia to 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 neutrons. While some of the neutrons released from the fission of are fast enough to induce another fission in, most are not, meaning it can never achieve criticality. While there is a very small chance of a thermal neutron inducing fission in, neutron absorption is orders of magnitude more likely.
Energetics
Input
Fission cross sections are a measurable property related to the probability that fission will occur in a nuclear reaction. Cross sections are a function of incident neutron energy, and those for and are a million times higher than at lower neutron energy levels. Absorption of any neutron makes available to the nucleus binding energy of about 5.3 MeV. needs a fast neutron to supply the additional 1 MeV needed to cross the critical energy barrier for fission. In the case of however, that extra energy is provided when adjusts from an odd to an even mass. In the words of Younes and Lovelace, "...the neutron absorption on a target forms a nucleus with excitation energy greater than the critical fission energy, whereas in the case of n +, the resulting nucleus has an excitation energy below the critical fission energy."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. Uranium-238, for example, has a near-zero fission cross section for neutrons of less than 1 MeV energy. If no additional energy is supplied by any other mechanism, the nucleus will not fission, but will merely absorb the neutron, as happens when absorbs slow and even some fraction of fast neutrons, to become. 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. Such high energy neutrons are able to fission directly. 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.
Among the heavy actinide elements, however, those isotopes that have an odd number of 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. 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. 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.
According to Younes and Loveland, "Actinides like that fission easily following the absorption of a thermal neutron are called fissile, whereas those like that do not easily fission when they absorb a thermal neutron are called fissionable."