5. Prevention of progressive deformation damage

5.1 Definitions

Progressive deformation damage can be caused by variations in the loading applied to the device, and in particular by cyclical changes resulting from variations in nuclear reactor power and start-up and shutdown operations.

First of all, let's stress the importance of stress classification. It has already been shown that, in the case of excessive deformation, only primary stresses should be limited, since secondary stresses resulting from imposed deformations are likely to be redistributed following moderate plastic deformation. The same cannot be said for cyclic loading, where variations in secondary stresses must be limited.

Generally speaking, when a first loading cycle is applied under a combination of primary and...