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 . An interstitial atom and its associated vacancy are known as a Frenkel defect . Because these atoms are not in the ideal location , they have an energy associated with them , much as a ball at the top of a hill has gravitational potential energy . This energy is referred to as '' Wigner energy '' . When a large number of interstitials have accumulated , they pose a risk of releasing all of their energy suddenly , creating a rapid , very 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 was 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 . Graphite , having a heat capacity of 0.720 J/g °C , could see a sudden increase in temperature of about 3750 °C ( 6780 °F )
Windscale fire
Windscale fire
Wigner effect