Single vacancies and other vacancy-type defects, such as, SFTs and dislocation loops, have been con­sidered quite extensively since the 1930s because it was recognized that they define many properties of solids under equilibrium conditions. Extensive infor­mation on defect properties was collected before material behavior in irradiation environments became a problem of practical importance. Qualitatively new crystal defects, SIAs and SIA clusters, were required to describe the phenomena in solids under irradiation conditions. This has been studied compre­hensively during the last ^40 years. The properties of these defects and their interaction with other defects are quite different compared to those of the vacancy — type. Correspondingly, the crystal behavior under irradiation is also qualitatively different from that under equilibrium conditions. The basic properties of vacancy — and SIA-type defects are summarized below.

1.13.3.2.1 Point defects

The basic properties of PDs are as follows:

1. Both vacancies and SIAs are highly mobile at tem­peratures of practical interest, and the diffusion coefficient of SIAs, Di, is much higher than that of vacancies, Dv : Д ^ Dv.

2. The relaxation volume of an SIA is much larger than that of a vacancy, resulting in higher interaction energy with edge dislocations and other defects.

3. Vacancies and SIAs are defects of opposite type, and their interaction leads to mutual recombination.

4. SIAs, in contrast to vacancies, may exist in several different configurations providing differ­ent mechanisms of their migration.

5. PDs of both types are eliminated at fixed sinks, such as voids and dislocations.

The first property leads to a specific temperature dependence of the damage accumulation: only limited number of defects can be accumulated at irradiation temperature below the recovery stage III, when vacan­cies are immobile. At higher temperature, when both PDs are mobile, the defect accumulation is practically unlimited. The second property is the origin of the so-called ‘dislocation bias’ (see Section 1.13.5.2) and, as proposed by Greenwood et a/.,47 is the reason for void swelling. A similar mechanism, but induced by external stress, was proposed in the so-called ‘SIPA’ (stress-induced preferential absorption) model of irradiation creep.48-53 The third property provides a decrease of the number of defects accumulated in a crystal under irradiation. The last property, which is quite different compared to that of vacancies leads to a variety of specific phenomena and will be consid­ered in the following sections.