As described in Section 2.1, crystals are hardly perfect even when there is no radiation damage. The deviations from the ideal crystal structure are instrumen­tal in influencing various structure-sensitive properties of crystals. There could be several types of these defects, and collectively they are called crystal or lattice defects. Interestingly, a perfect crystal could be composed of atoms at rest with only zero-point oscillation at the absolute zero temperature. However, as the temperature increases, the amplitude of the lattice vibration also increases. This lattice vibration basically manifests itself as elastic waves and can influence some very important physical properties (such as thermal conductivity). This type of lattice vibration is called phonon because of its similarity in behavior with the light photons (mainly because of the relationship between their fre­quency expressions). Electrons can jump to higher orbits creating electron holes. This can also affect electronic properties (recall the semiconductor theo­ries). However, electronic properties are not of pressing importance in the con­text of nuclear reactor materials. Henceforth, in this section, our focus would be to give the readers an introduction to various types of crystal defects like point defects (vacancies, self-interstitials, substitutional, or interstitial impurity atoms), line defects (dislocations), surface defects (grain boundaries), and vol­ume defects (voids, cavities, and precipitates).

2.2.1