To cause the Wigner effect, neutrons that collide with the atoms in a crystal structure must have enough energy to displace them from the lattice.
A neutron with a significant amount of energy will create a displacement cascade in a matrix via elastic collisions.
When a large number of interstitials have accumulated, they risk releasing all of their energy suddenly, creating a rapid, great increase in temperature.
Accumulation of energy in irradiated graphite has been recorded as high as 2.7 kJ/g--enough to raise the temperature by thousands of degrees--but is typically much lower than this.
[5] Wigner energy may have played some part following the prompt critical neutron spike, when the accident entered the graphite fire phase of events.