Appendix 1: Material ageing mechanisms
Long Term Operation of Nuclear Power Plants

The term "ageing" is used here to refer to time-related mechanisms which cause materials to lose their initial characteristics and eventually lead to macroscopically observable degradation. This definition excludes quasi-instantaneous failure mechanisms such as fracture or buckling. Aging is not in itself something that can be avoided. Nevertheless, measures can be taken to monitor its effects, limit or delay them, or replace them with less undesirable ones.

This appendix describes the main ageing mechanisms that can affect the components of a nuclear unit.