TheWigner effect (named for its discoverer,Eugene Wigner),^[1] also known as thediscomposition effect orWigner's disease,^[2] is the displacement ofatoms in a solid caused byneutron radiation.

Any solid can display the Wigner effect. The effect is of most concern inneutron moderators, such asgraphite, intended to reduce the speed offast neutrons, thereby turning them intothermal neutrons capable of sustaining a nuclear chain reaction involvinguranium-235.

To cause the Wigner effect,neutrons that collide with the atoms in acrystal structure must have enoughenergy to displace them from the lattice. This amount (threshold displacement energy) is approximately 25eV. 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 centre of anuclear reactor. A neutron with a significant amount of energy will create adisplacement cascade in a matrix viaelastic collisions.

For example, a 1 MeV neutron strikinggraphite will create 900 displacements. 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 avacancy come to rest in non-ideal locations; that is, not along the symmetrical lines of the lattice. Theseinterstitial atoms (or simply "interstitials") and their associated vacancies are aFrenkel defect. Because these atoms are not in the ideal location, they have aWigner energy associated with them, much as a ball at the top of a hill hasgravitational potential energy.

When a large number of interstitials have accumulated, they risk releasing all of their energy suddenly, creating a rapid, great increase in temperature. Sudden, unplanned increases in temperature can present a large risk for certain types of nuclear reactors with low operating temperatures. One such release was the indirect cause of theWindscale fire. Accumulation of energy in irradiated graphite has been recorded as high as 2.7kJ/g--enough to raise the temperature by thousands of degrees--but is typically much lower than this.^[3]

Despite some reports,^[4] Wigner energy buildup had nothing to do with the cause of theChernobyl 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.^[5] Wigner energy may have played some part following theprompt critical neutron spike, when the accident entered the graphite fire phase of events.

However, Wigner energy was the cause of theWindscale fire on 10 October 1957 at theSellafield nuclear site in UK.

A buildup of Wigner energy can be relieved by heating the material. This process is known asannealing. In graphite this occurs at 250 °C (482 °F).^[6]

In 2003, it was 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.^[7] The interstitial atom becomes trapped on the lip of the vacancy, and there is a barrier for it to recombine to give perfect graphite.

• Glasstone, Samuel, and Alexander Sesonske [1963] (1994).Nuclear Reactor Engineering. Boston: Springer.ISBN 0-412-98531-4.OCLC 852791143.