Wigner effect
This effect is a very curious effect (and dangerous) that occurs in moderators who are trained by a crystal structure (beryllium and graphite, especially this last).
high-speed neutrons (those occurring after the bombing alpha and especially light elements produced in fission) have a lot of kinetic energy, sometimes up to 10 MeV.
In a moderator, these neutrons collide with light atoms, losing energy to be thermal neutrons.
But in each collision, neutrons can displace that atom.
graphite, for example, has this crystalline structure:
If a neutron collides with a carbon atom, this will be dislodged, will go out of its structure. And that's what happens with this atom?
Very simple: to "save" part of the kinetic energy that has given him the neutron. It's like potential energy: an object, for the simple fact of being 10 m above the ground, has more energy than one that is within 5 meters.
If we have a block of graphite acting as moderator of a neutron flux of high speed, the graphite will be saving energy can be very dangerous because you can shoot all at once, causing a huge explosion as the Windscale fire.
How about it? Very simple: having that block of graphite at high temperature. Contrary to what may seem, if high-temperature Wigner effect may not appear because the atoms have more freedom and can reodenar smoothly, without accumulating energy. In graphite is a minimum of 250 ยบ C.
Of course, at high temperatures with graphite care: in contact with oxygen can explode, as happened at Chernobyl.
If you have at home a block of graphite used as moderator, nothing happens, unless your neutron flux is high (I mean more than 10 ^ 5 neutrons / min for a long time).
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