The Wigner effect (named for its discoverer, E. P. Wigner)[1], also known as the discomposition effect, is the displacement of atoms 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 moderators, 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.
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To create the Wigner effect, neutrons 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 collisions. 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.[2] Despite some reports[3], 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.
This build up of energy referred to as Wigner energy can be relieved by heating the material. This process is known as annealing. In graphite this occurs at 250°C. [4]
An accident during this controlled annealing was the cause of the 1957 Windscale fire.
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°C 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.