Wigner Effect in Metallic Lattices

[nickandre]

Hi,
I'm a bit confused with the Wigner effect concept. This effect is normally associated to damages in moderator material typically graphite.
But metallic cladding of the fuel element is also exposed to fast neutron, but Wigner effect is seldom being used as a term to explain the damages in the metallic lattice.
is Wigner effect only for giant crystal lattices, hence is not the suitable mechanism in metal lattice?
Hope someone can enlighten me on this

Thanks,

Confused

 

[Astronuc]

The atomic bonds in carbon in graphite are very different that metallic bonds in metal. Neutron radiation causes damage in both types of materials in the form of dislocations (with vacancies and interstial atoms).

When the carbon bonds in graphite reform in the proper lattice (hexagonal lattice), they release chemical energy. To reform the graphite, it must be heated (annealed) to about 250°C.

Energy stored in the irradiated graphite of a graphite reactor. The graphite atoms located at the interstitials cause this energy storage ( See 'Wigner effect'). At graphite temperatures of more than 250° C these voids recombine releasing energy, the Wigner energy.

http://www.euronuclear.org/info/encyclopedia/w/wigner-energy.htm

In metals, the radiation damage is also in the form of dislocations, but there are no chemical bonds in the sense of graphite. In order for the dislocations to 'disappear', i.e. for displaced atoms to move back into the correct lattice positions, the metal must be heated above normal operating temperature, which is ~300°C in BWRs and 340-350°C in PWRs. The annealing temperature is about 500°C for Zircaloys for long annealing times. Zr-2 is usually annealed above 560°C, and Zr-4 above 600°C.

An interstial atom may be displaced some number of atoms from it's original location, therefore an interstial atom next to or near a vacancy is not necessarily the atom which orginated at that vacancy location.

 

[Morbius]

Hi,
I'm a bit confused with the Wigner effect concept. This effect is normally associated to damages in moderator material typically graphite.
But metallic cladding of the fuel element is also exposed to fast neutron, but Wigner effect is seldom being used as a term to explain the damages in the metallic lattice.
is Wigner effect only for giant crystal lattices, hence is not the suitable mechanism in metal lattice?
Hope someone can enlighten me on this

nickandre,

Just to add to what Astronuc has already stated; metals also experience damage due
to neutron irradiation.

Fast neutrons can also cause dislocations in metals; which is why neutron-irradiated
metals become embrittled. Those dislocations decrease the ductility of the metal.
A metal can be annealed by heating it to high temperature and then cooling which gives
the atoms a chance to reconstitute the original crystal structure. However, in normal
operation of a reactor, temperatures that high won't be reached.

Graphite, however, anneals at a lower temperature - a temperature that can be found in
a reactor core. HTGRs - high temperature gas reactors operate at temperatures high
enough so that the graphite is continually annealing and one doesn't have problems with
a build-up of Wigner energy.

Dr. Gregory Greenman
Physicist

 

[nickandre]

Thanks Astronuc and Dr. Gregory greenman