Fissile
Encyclopedia
In nuclear engineering
, a fissile material is one that is capable of sustaining a chain reaction
of nuclear fission
. By definition, fissile materials can sustain a chain reaction with neutrons of any energy. The predominant neutron energy may be typified by either slow neutrons (i.e. a thermal system) or fast neutrons. Fissile materials can be used to fuel thermal reactor
s, with a neutron moderator
; fast-neutron reactors, with no moderators; and nuclear explosive
s.
, heavy isotope
s with 90 ≤ Z ≤ 100 and 2 × Z - N = 43 ± 2, with few exceptions, are fissile (N – number of neutron
s and Z – number of proton
s).
"Fissile" is distinct from "fissionable." A nuclide
capable of undergoing fission after capturing a neutron is referred to as "fissionable." A fissionable nuclide that can be induced to fission with low energy thermal neutrons is referred to as fissile. Although formerly used as a synonym for fissile material, fissionable materials also include those (such as uranium-238
) that can be fissioned only with high-energy neutrons. As a result, fissile materials (such as uranium-235
) are a subset of fissionable materials.
Uranium-235 fissions with low-energy thermal neutrons because the binding energy resulting from the absorption of a neutron is greater than the critical energy required for fission; therefore uranium-235 is a fissile material. By contrast, the binding energy released by uranium-238 absorbing a thermal neutron is less than the critical energy, so the neutron must possess additional energy for fission to be possible. Consequently, uranium-238 is a fissionable material but not a fissile material.
An alternative definition defines fissile nuclides as those nuclides that can be made to undergo nuclear fission (i.e., are fissionable), and also produce neutrons from such fission that can sustain a nuclear chain reaction in the correct setting. Under this definition, nuclides that are only fissionable are those nuclides that can be made to undergo nuclear fission but produce insufficient neutrons, in either energy or number, to sustain a nuclear chain reaction
. As such, while all fissile isotopes are fissionable, not all fissionable isotopes are fissile. In the arms control
context, particularly in proposals for a Fissile Material Cutoff Treaty
, the term "fissile" is often used to describe materials that can be used in the fission primary of a nuclear weapon. These are materials that sustain an explosive fast fission chain reaction
.
Under all definitions above, uranium-238 is fissionable, but because U-238 cannot sustain a neutron chain reaction, U-238 is not fissile. Neutrons produced by fission of U-238 inevitably inelastically scatter to an energy below 1 MeV (i.e. a speed of about 14,000 km/s), the fission threshold to cause subsequent fission of U-238, so fission of U-238 does not sustain a nuclear chain reaction
.
Fast fission of U-238 in the secondary stage of a nuclear weapon contributes greatly to yield
and to fallout
. The fast fission of U-238 also makes a significant contribution to the power output of some fast-neutron reactors.
isotopes with an odd neutron number
are fissile. Most nuclear fuels have an odd atomic mass number (A = the total number of proton
s and neutron
s), and an even atomic number
(Z = the number of protons). This implies an odd number of neutrons. Isotopes with an odd number of neutrons gain an extra 1 to 2 MeV of energy from absorbing an extra neutron, from the pairing effect which favors even numbers of both neutrons and protons. This energy is enough to supply the needed extra energy for fission by slower neutrons, which is important for making fissionable isotopes also fissile.
More generally, elements with an even number of protons and an even number of neutrons, and located near a well-known curve in nuclear physics of atomic number vs. atomic mass number are more stable than others; hence, they are less likely to undergo fission. They are more likely to "ignore" the neutron and let it go on its way, or else to absorb the neutron but without gaining enough energy from the process to deform the nucleus enough for it to fission. These "even-even" isotopes are also less likely to undergo spontaneous fission
, and they also have relatively much longer half-lives
for alpha
or beta decay
. Examples of these elements are uranium-238 and thorium-232. On the other hand, isotopes with an odd number of neutrons and even number of protons (even Z, odd N) are short-lived because they readily decay by beta-particle emission
to an isotope with an even number of neutrons and an even number of protons - (even Z, even N) - becoming much more stable. The physical basis for this phenomenon also comes from the pairing effect in nuclear binding energy, but this time from both proton-proton and neutron-neutron pairing. The short half-life of such odd-odd heavy isotopes means that they are not available in quantity and are highly radioactive.
Fissile nuclide
s in nuclear fuel
s include:
Fissile nuclides do not have a 100% chance of undergoing fission on absorption of a neutron. The chance is dependent on the nuclide as well as neutron energy. For low and medium-energy neutrons, the neutron capture
cross section
s for fission (σF), the cross section for neutron capture
with emission of a gamma ray
(σγ), and the percentage of non-fissions are in the table at right.
Nuclear engineering
Nuclear engineering is the branch of engineering concerned with the application of the breakdown as well as the fusion of atomic nuclei and/or the application of other sub-atomic physics, based on the principles of nuclear physics...
, a fissile material is one that is capable of sustaining a chain reaction
Chain reaction
A chain reaction is a sequence of reactions where a reactive product or by-product causes additional reactions to take place. In a chain reaction, positive feedback leads to a self-amplifying chain of events....
of nuclear fission
Nuclear fission
In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts , often producing free neutrons and photons , and releasing a tremendous amount of energy...
. By definition, fissile materials can sustain a chain reaction with neutrons of any energy. The predominant neutron energy may be typified by either slow neutrons (i.e. a thermal system) or fast neutrons. Fissile materials can be used to fuel thermal reactor
Thermal reactor
A thermal reactor is a nuclear reactor that uses slow or thermal neutrons. Most power reactors are of this type. These type of reactors use a neutron moderator to slow neutrons until they approach the average kinetic energy of the surrounding particles, that is, to reduce the speed of the neutrons...
s, with a neutron moderator
Neutron moderator
In nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235....
; fast-neutron reactors, with no moderators; and nuclear explosive
Nuclear explosive
A nuclear explosive is an explosive device that derives its energy from nuclear reactions. Almost all nuclear explosive devices that have been designed and produced are nuclear weapons intended for warfare....
s.
Fissile vs fissionable
According to the Fissile ruleFissile rule
According to the Fissile rule, heavy isotopes with 90 ≤ Z ≤ 100 and 2Z-N=43 ± 2, with few exceptions, are fissile ....
, heavy isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
s with 90 ≤ Z ≤ 100 and 2 × Z - N = 43 ± 2, with few exceptions, are fissile (N – number of neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
s and Z – number of proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
s).
"Fissile" is distinct from "fissionable." A nuclide
Nuclide
A nuclide is an atomic species characterized by the specific constitution of its nucleus, i.e., by its number of protons Z, its number of neutrons N, and its nuclear energy state....
capable of undergoing fission after capturing a neutron is referred to as "fissionable." A fissionable nuclide that can be induced to fission with low energy thermal neutrons is referred to as fissile. Although formerly used as a synonym for fissile material, fissionable materials also include those (such as uranium-238
Uranium-238
Uranium-238 is the most common isotope of uranium found in nature. It is not fissile, but is a fertile material: it can capture a slow neutron and after two beta decays become fissile plutonium-239...
) that can be fissioned only with high-energy neutrons. As a result, fissile materials (such as uranium-235
Uranium-235
- References :* .* DOE Fundamentals handbook: Nuclear Physics and Reactor theory , .* A piece of U-235 the size of a grain of rice can produce energy equal to that contained in three tons of coal or fourteen barrels of oil. -External links:* * * one of the earliest articles on U-235 for the...
) are a subset of fissionable materials.
Uranium-235 fissions with low-energy thermal neutrons because the binding energy resulting from the absorption of a neutron is greater than the critical energy required for fission; therefore uranium-235 is a fissile material. By contrast, the binding energy released by uranium-238 absorbing a thermal neutron is less than the critical energy, so the neutron must possess additional energy for fission to be possible. Consequently, uranium-238 is a fissionable material but not a fissile material.
An alternative definition defines fissile nuclides as those nuclides that can be made to undergo nuclear fission (i.e., are fissionable), and also produce neutrons from such fission that can sustain a nuclear chain reaction in the correct setting. Under this definition, nuclides that are only fissionable are those nuclides that can be made to undergo nuclear fission but produce insufficient neutrons, in either energy or number, to sustain a nuclear chain reaction
Nuclear chain reaction
A nuclear chain reaction occurs when one nuclear reaction causes an average of one or more nuclear reactions, thus leading to a self-propagating number of these reactions. The specific nuclear reaction may be the fission of heavy isotopes or the fusion of light isotopes...
. As such, while all fissile isotopes are fissionable, not all fissionable isotopes are fissile. In the arms control
Arms control
Arms control is an umbrella term for restrictions upon the development, production, stockpiling, proliferation, and usage of weapons, especially weapons of mass destruction...
context, particularly in proposals for a Fissile Material Cutoff Treaty
Fissile Material Cutoff Treaty
The Fissile Material Cutoff Treaty is a proposed international treaty to prohibit the further production of fissile material for nuclear weapons or other explosive devices. The treaty has not been negotiated and its terms remain to be defined...
, the term "fissile" is often used to describe materials that can be used in the fission primary of a nuclear weapon. These are materials that sustain an explosive fast fission chain reaction
Chain reaction
A chain reaction is a sequence of reactions where a reactive product or by-product causes additional reactions to take place. In a chain reaction, positive feedback leads to a self-amplifying chain of events....
.
Under all definitions above, uranium-238 is fissionable, but because U-238 cannot sustain a neutron chain reaction, U-238 is not fissile. Neutrons produced by fission of U-238 inevitably inelastically scatter to an energy below 1 MeV (i.e. a speed of about 14,000 km/s), the fission threshold to cause subsequent fission of U-238, so fission of U-238 does not sustain a nuclear chain reaction
Chain reaction
A chain reaction is a sequence of reactions where a reactive product or by-product causes additional reactions to take place. In a chain reaction, positive feedback leads to a self-amplifying chain of events....
.
Fast fission of U-238 in the secondary stage of a nuclear weapon contributes greatly to yield
Nuclear weapon yield
The explosive yield of a nuclear weapon is the amount of energy discharged when a nuclear weapon is detonated, expressed usually in the equivalent mass of trinitrotoluene , either in kilotons or megatons , but sometimes also in terajoules...
and to fallout
Nuclear fallout
Fallout is the residual radioactive material propelled into the upper atmosphere following a nuclear blast, so called because it "falls out" of the sky after the explosion and shock wave have passed. It commonly refers to the radioactive dust and ash created when a nuclear weapon explodes...
. The fast fission of U-238 also makes a significant contribution to the power output of some fast-neutron reactors.
Fissile nuclides
In general, most actinideActinide
The actinide or actinoid series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium.The actinide series derives its name from the group 3 element actinium...
isotopes with an odd neutron number
Neutron number
The neutron number, symbol N, is the number of neutrons in a nuclide.Atomic number plus neutron number equals mass number: Z+N=A....
are fissile. Most nuclear fuels have an odd atomic mass number (A = the total number of proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
s and neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
s), and an even atomic number
Atomic number
In chemistry and physics, the atomic number is the number of protons found in the nucleus of an atom and therefore identical to the charge number of the nucleus. It is conventionally represented by the symbol Z. The atomic number uniquely identifies a chemical element...
(Z = the number of protons). This implies an odd number of neutrons. Isotopes with an odd number of neutrons gain an extra 1 to 2 MeV of energy from absorbing an extra neutron, from the pairing effect which favors even numbers of both neutrons and protons. This energy is enough to supply the needed extra energy for fission by slower neutrons, which is important for making fissionable isotopes also fissile.
More generally, elements with an even number of protons and an even number of neutrons, and located near a well-known curve in nuclear physics of atomic number vs. atomic mass number are more stable than others; hence, they are less likely to undergo fission. They are more likely to "ignore" the neutron and let it go on its way, or else to absorb the neutron but without gaining enough energy from the process to deform the nucleus enough for it to fission. These "even-even" isotopes are also less likely to undergo spontaneous fission
Spontaneous fission
Spontaneous fission is a form of radioactive decay characteristic of very heavy isotopes. Because the nuclear binding energy reaches a maximum at a nuclear mass greater than about 60 atomic mass units , spontaneous breakdown into smaller nuclei and single particles becomes possible at heavier masses...
, and they also have relatively much longer half-lives
Half-life
Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...
for alpha
Alpha decay
Alpha decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle and thereby transforms into an atom with a mass number 4 less and atomic number 2 less...
or beta decay
Beta decay
In nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted from an atom. There are two types of beta decay: beta minus and beta plus. In the case of beta decay that produces an electron emission, it is referred to as beta minus , while in the case of a...
. Examples of these elements are uranium-238 and thorium-232. On the other hand, isotopes with an odd number of neutrons and even number of protons (even Z, odd N) are short-lived because they readily decay by beta-particle emission
Beta decay
In nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted from an atom. There are two types of beta decay: beta minus and beta plus. In the case of beta decay that produces an electron emission, it is referred to as beta minus , while in the case of a...
to an isotope with an even number of neutrons and an even number of protons - (even Z, even N) - becoming much more stable. The physical basis for this phenomenon also comes from the pairing effect in nuclear binding energy, but this time from both proton-proton and neutron-neutron pairing. The short half-life of such odd-odd heavy isotopes means that they are not available in quantity and are highly radioactive.
Nuclear fuel
To be a useful fuel for nuclear fission chain reactions, the material must:- Be in the region of the binding energyBinding energyBinding energy is the mechanical energy required to disassemble a whole into separate parts. A bound system typically has a lower potential energy than its constituent parts; this is what keeps the system together—often this means that energy is released upon the creation of a bound state...
curve where a fission chain reaction is possible (i.e. above radiumRadiumRadium is a chemical element with atomic number 88, represented by the symbol Ra. Radium is an almost pure-white alkaline earth metal, but it readily oxidizes on exposure to air, becoming black in color. All isotopes of radium are highly radioactive, with the most stable isotope being radium-226,...
) - Have a high probability of fission on neutron captureNeutron captureNeutron capture is a kind of nuclear reaction in which an atomic nucleus collides with one or more neutrons and they merge to form a heavier nucleus. Since neutrons have no electric charge they can enter a nucleus more easily than positively charged protons, which are repelled...
- Release two or more neutrons on average per neutron capture (which means a higher average number of them on each fission, to compensate for nonfissions, and absorptions in the moderator)
- Have a reasonably long half life
- Be available in suitable quantities
| Thermal neutrons | |Epithermal neutrons | |||||
|---|---|---|---|---|---|---|
| σF | σγ | % | σF | σγ | % | |
| 531 | 46 | 8.0% | 233U | 760 | 140 | 16% |
| 585 | 99 | 14.5% | 235U | 275 | 140 | 34% |
| 750 | 271 | 26.5% | 239Pu | 300 | 200 | 40% |
| 1010 | 361 | 26.3% | 241Pu | 570 | 160 | 22% |
Fissile nuclide
Nuclide
A nuclide is an atomic species characterized by the specific constitution of its nucleus, i.e., by its number of protons Z, its number of neutrons N, and its nuclear energy state....
s in nuclear fuel
Nuclear fuel
Nuclear fuel is a material that can be 'consumed' by fission or fusion to derive nuclear energy. Nuclear fuels are the most dense sources of energy available...
s include:
- Uranium-235Uranium-235- References :* .* DOE Fundamentals handbook: Nuclear Physics and Reactor theory , .* A piece of U-235 the size of a grain of rice can produce energy equal to that contained in three tons of coal or fourteen barrels of oil. -External links:* * * one of the earliest articles on U-235 for the...
which occurs in natural uraniumNatural uraniumNatural uranium refers to refined uranium with the same isotopic ratio as found in nature. It contains 0.7 % uranium-235, 99.3 % uranium-238, and a trace of uranium-234 by weight. In terms of the amount of radioactivity, approximately 2.2 % comes from uranium-235, 48.6 % uranium-238, and 49.2 %...
and enriched uraniumEnriched uraniumEnriched uranium is a kind of uranium in which the percent composition of uranium-235 has been increased through the process of isotope separation. Natural uranium is 99.284% 238U isotope, with 235U only constituting about 0.711% of its weight... - Plutonium-239Plutonium-239Plutonium-239 is an isotope of plutonium. Plutonium-239 is the primary fissile isotope used for the production of nuclear weapons, although uranium-235 has also been used and is currently the secondary isotope. Plutonium-239 is also one of the three main isotopes demonstrated usable as fuel in...
bred from uranium-238Uranium-238Uranium-238 is the most common isotope of uranium found in nature. It is not fissile, but is a fertile material: it can capture a slow neutron and after two beta decays become fissile plutonium-239...
by neutron captureNeutron captureNeutron capture is a kind of nuclear reaction in which an atomic nucleus collides with one or more neutrons and they merge to form a heavier nucleus. Since neutrons have no electric charge they can enter a nucleus more easily than positively charged protons, which are repelled... - Plutonium-241Plutonium-241Plutonium-241 is an isotope of plutonium formed when plutonium-240 captures a neutron. Like Pu-239 but unlike 240Pu, 241Pu is fissile, with a neutron absorption cross section about 1/3 greater than 239Pu, and a similar probability of fissioning on neutron absorption, around 73%. In the non-fission...
bred from plutonium-240Plutonium-240Plutonium-240 is an isotope of the metal plutonium formed when plutonium-239 captures a neutron. About 62% to 73% of the time when Pu-239 captures a neutron it undergoes fission; the rest of the time it forms Pu-240. The longer a nuclear fuel element remains in a nuclear reactor the greater the...
by neutron capture. The Pu-240 comes from Pu-239 by the same process. - Uranium-233Uranium-233Uranium-233 is a fissile isotope of uranium, bred from Thorium as part of the thorium fuel cycle. It has been used in a few nuclear reactors and has been proposed for much wider use as a nuclear fuel. It has a half-life of 160,000 years....
bred from thorium-232 by neutron capture
Fissile nuclides do not have a 100% chance of undergoing fission on absorption of a neutron. The chance is dependent on the nuclide as well as neutron energy. For low and medium-energy neutrons, the neutron capture
Neutron capture
Neutron capture is a kind of nuclear reaction in which an atomic nucleus collides with one or more neutrons and they merge to form a heavier nucleus. Since neutrons have no electric charge they can enter a nucleus more easily than positively charged protons, which are repelled...
cross section
Neutron cross-section
In nuclear and particle physics, the concept of a neutron cross section is used to express the likelihood of interaction between an incident neutron and a target nucleus. In conjunction with the neutron flux, it enables the calculation of the reaction rate, for example to derive the thermal power...
s for fission (σF), the cross section for neutron capture
Neutron capture
Neutron capture is a kind of nuclear reaction in which an atomic nucleus collides with one or more neutrons and they merge to form a heavier nucleus. Since neutrons have no electric charge they can enter a nucleus more easily than positively charged protons, which are repelled...
with emission of a gamma ray
Gamma ray
Gamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency . Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei...
(σγ), and the percentage of non-fissions are in the table at right.
See also
- Fertile materialFertile materialFertile material is a term used to describe nuclides which generally themselves do not undergo induced fission but from which fissile material is generated by neutron absorption and subsequent nuclei conversions...
- Fission productFission productNuclear fission products are the atomic fragments left after a large atomic nucleus fissions. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons and a large release of energy in the form of heat , gamma rays and neutrinos. The...
- Special nuclear materialSpecial nuclear materialSpecial nuclear material is a term used by the Nuclear Regulatory Commission of the United States to classify fissile materials. The NRC divides special nuclear material into three main categories, according to the risk and potential for its direct use in a clandestine nuclear weapon or for its...
- Nuclear fusionNuclear fusionNuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...
- FissilityFissilityFissility may refer to:*Fissility , a property of shales*Fissility , referring to a nuclear process...
(disambiguation) - Fissile ruleFissile ruleAccording to the Fissile rule, heavy isotopes with 90 ≤ Z ≤ 100 and 2Z-N=43 ± 2, with few exceptions, are fissile ....