houses are made of stones, so is science made of is not a house and a collection of facts is not neces-

—Henri Poincare

The dislocation concept has already been introduced in Section 2.2 dealing with crystal defects. Now we need to develop the concept further. The importance of dislocations in plastic deformation (i. e., permanent deformation) is well docu­mented. But the question arises as to why we should be concerned about them in a textbook on nuclear materials. We will see in a later chapter how dislocation loops can form from the primary radiation damage; the dislocation loops can either stay as loops or join the overall dislocation networks in the irradiated materials. Indeed, dislocations are the major microscopic defects that are created during irradiation. Hence, this chapter serves as a prelude to understanding these different aspects of dislocations and their significance.

4.1