Wigner effect
Encyclopedia
The Wigner effect also known as the discomposition effect, is the displacement of atom
s in a solid caused by neutron radiation
.
Any solid can be affected by the Wigner effect, but the effect is of most concern in neutron moderator
s, such as graphite, that are used to slow down fast neutrons. The material surrounding the moderator receives a much smaller amount of neutron radiation, and from slower neutrons, and is not as worrisome.
An interstitial atom and its associated vacancy are known as a Frenkel defect
.
s that collide with the atoms in a crystal structure
must have enough energy
to displace them from the lattice. This amount (threshold displacement energy
) is approximately 25 eV
. A neutron's energy can vary widely but it is not uncommon to have energies up to and exceeding 10 MeV (10,000,000 eV) in the center of a nuclear reactor
. A neutron with a significant amount of energy will create a
displacement cascade in a matrix via elastic collision
s. For example a 1 MeV neutron striking graphite
will create 900 displacements, however not all displacements will create defects because some of the struck atoms will find and fill the vacancies that were either small pre-existing voids or vacancies newly formed by the other struck atoms.
The atoms that do not find a vacancy come to rest in non-ideal locations; that is, not along the symmetrical lines of the lattice. These atoms are referred to as interstitial atoms
, or simply interstitials. Because these atoms are not in the ideal location they have an energy associated with them, much like a ball at the top of a hill has gravitational potential energy. When large amounts of interstitials
have accumulated they pose a risk of releasing all of their energy suddenly, creating a temperature spike. Sudden unplanned increases in temperature can present a large risk for certain types of nuclear reactors with low operating temperatures and were the indirect cause of the Windscale fire
. Accumulation of energy in irradiated graphite has been recorded as high as 2.7 kJ/g, but is typically much lower than this.
Despite some reports, Wigner energy buildup had nothing to do with the Chernobyl disaster
: This reactor, like all contemporary power reactors, operated at a high enough temperature to allow the displaced graphite structure to realign itself before any potential energy could be stored.
. In graphite this occurs at 250°C
.
An accident during this controlled annealing was the cause of the 1957 Windscale fire
.
has been described in terms of a metastable interstitial-vacancy pair (Ewels et al., PRL2003). The interstitial atom becomes trapped on the lip of the vacancy, and there is a barrier for it to recombine to give perfect graphite.
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
s in a solid caused by neutron radiation
Neutron radiation
Neutron radiation is a kind of ionizing radiation which consists of free neutrons. A result of nuclear fission or nuclear fusion, it consists of the release of free neutrons from atoms, and these free neutrons react with nuclei of other atoms to form new isotopes, which, in turn, may produce...
.
Any solid can be affected by the Wigner effect, but the effect is of most concern in 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....
s, such as graphite, that are used to slow down fast neutrons. The material surrounding the moderator receives a much smaller amount of neutron radiation, and from slower neutrons, and is not as worrisome.
An interstitial atom and its associated vacancy are known as a Frenkel defect
Frenkel defect
The Frenkel Defect is shown by ionic solids. The smaller ion is displaced from its lattice position to an interstitial site. It creates a vacancy defect at its original site and an interstitial defect at its new location.-Definition:...
.
Explanation
To create the Wigner effect, neutronNeutron
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 that collide with the atoms in a crystal structure
Crystal structure
In mineralogy and crystallography, crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a pattern, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...
must have enough energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
to displace them from the lattice. This amount (threshold displacement energy
Threshold displacement energy
The threshold displacement energy T_d is the minimum kinetic energythat an atom in a solid needs to be permanentlydisplaced from its lattice site to adefect position.It is also known as "displacement threshold energy" or just "displacement energy"....
) is approximately 25 eV
Electronvolt
In physics, the electron volt is a unit of energy equal to approximately joule . By definition, it is equal to the amount of kinetic energy gained by a single unbound electron when it accelerates through an electric potential difference of one volt...
. A neutron's energy can vary widely but it is not uncommon to have energies up to and exceeding 10 MeV (10,000,000 eV) in the center of a nuclear reactor
Nuclear reactor
A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...
. A neutron with a significant amount of energy will create a
displacement cascade in a matrix via elastic collision
Elastic collision
An elastic collision is an encounter between two bodies in which the total kinetic energy of the two bodies after the encounter is equal to their total kinetic energy before the encounter...
s. For example a 1 MeV neutron striking graphite
Graphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...
will create 900 displacements, however not all displacements will create defects because some of the struck atoms will find and fill the vacancies that were either small pre-existing voids or vacancies newly formed by the other struck atoms.
The atoms that do not find a vacancy come to rest in non-ideal locations; that is, not along the symmetrical lines of the lattice. These atoms are referred to as interstitial atoms
Interstitial defect
Interstitials are a variety of crystallographic defects, i.e. atoms which occupy a site in the crystal structure at which there is usually not an atom, or two or more atoms sharing one or more lattice sites such that the number of atoms is larger than the number of lattice sites.They are generally...
, or simply interstitials. Because these atoms are not in the ideal location they have an energy associated with them, much like a ball at the top of a hill has gravitational potential energy. When large amounts of interstitials
Interstitial defect
Interstitials are a variety of crystallographic defects, i.e. atoms which occupy a site in the crystal structure at which there is usually not an atom, or two or more atoms sharing one or more lattice sites such that the number of atoms is larger than the number of lattice sites.They are generally...
have accumulated they pose a risk of releasing all of their energy suddenly, creating a temperature spike. Sudden unplanned increases in temperature can present a large risk for certain types of nuclear reactors with low operating temperatures and were the indirect cause of the Windscale fire
Windscale fire
The Windscale fire of 10 October 1957 was the worst nuclear accident in Great Britain's history, ranked in severity at level 5 on the 7-point International Nuclear Event Scale. The two piles had been hurriedly built as part of the British atomic bomb project. Windscale Pile No. 1 was operational in...
. Accumulation of energy in irradiated graphite has been recorded as high as 2.7 kJ/g, but is typically much lower than this.
Despite some reports, Wigner energy buildup had nothing to do with the Chernobyl disaster
Chernobyl disaster
The Chernobyl disaster was a nuclear accident that occurred on 26 April 1986 at the Chernobyl Nuclear Power Plant in Ukraine , which was under the direct jurisdiction of the central authorities in Moscow...
: This reactor, like all contemporary power reactors, operated at a high enough temperature to allow the displaced graphite structure to realign itself before any potential energy could be stored.
Dissipation of Wigner energy
This build up of energy referred to as Wigner energy can be relieved by heating the material. This process is known as annealingAnnealing (metallurgy)
Annealing, in metallurgy and materials science, is a heat treatment wherein a material is altered, causing changes in its properties such as strength and hardness. It is a process that produces conditions by heating to above the recrystallization temperature, maintaining a suitable temperature, and...
. In graphite this occurs at 250°C
Celsius
Celsius is a scale and unit of measurement for temperature. It is named after the Swedish astronomer Anders Celsius , who developed a similar temperature scale two years before his death...
.
An accident during this controlled annealing was the cause of the 1957 Windscale fire
Windscale fire
The Windscale fire of 10 October 1957 was the worst nuclear accident in Great Britain's history, ranked in severity at level 5 on the 7-point International Nuclear Event Scale. The two piles had been hurriedly built as part of the British atomic bomb project. Windscale Pile No. 1 was operational in...
.
Intimate Frenkel pairs
It has recently been postulated that Wigner energy can be stored by the formation of metastable defect structures in graphite. Notably the large energy release observed at 200-250°CCelsius
Celsius is a scale and unit of measurement for temperature. It is named after the Swedish astronomer Anders Celsius , who developed a similar temperature scale two years before his death...
has been described in terms of a metastable interstitial-vacancy pair (Ewels et al., PRL2003). The interstitial atom becomes trapped on the lip of the vacancy, and there is a barrier for it to recombine to give perfect graphite.