This section gives comments on the application of each of the elements of the systematic ageing management process, shown in Figure 12.

II.2.1. UNDERSTANDING SSC AGEING

Adequate understanding of SSC ageing is the basis of the systematic ageing management process and the key to successful proactive ageing management. The level of understanding of SSC ageing depends to a large extent on the degree of technical/scientific understanding of relevant ageing mechanisms and the quality and quantity of relevant data from operating experience. Understanding ageing enables predicting future SSC ageing degradation, and the predictability enables optimizing and coordinating SSC operation/use, inspection and maintenance.

In practice, predictability requires modelling of the relevant ageing mechanism and SSC degradation, and measuring the progress of SSC degradation “along the curve” (i. e. condition monitoring).

Irradiation embrittlement is an ageing mechanism leading to changes in bulk material properties that has been successfully modelled; its future effects can be accurately predicted

and measured. Fretting wear is also a reasonably predictable degradation mechanism; at least in sense that it can be easily detected with current inspection technology and, with repeat inspections, the growth rate can quite confidently be determined. Hence it is a relatively low risk degradation mechanism to manage, provided one is looking for it. The predictability of thermal degradation of polymers (used in cable insulation and seals), which also produce changes in bulk material properties, is also generally adequate.

On the other hand, the predictability of stress corrosion cracking or high cycle fatigue, which produce changes in material surfaces or interfaces (loss of material or formation of small cracks), is generally low. The resulting uncertainty has caused significant NPP unavailability and O&M costs.

Operating experience has revealed SSC degradation and failures caused by previously unrecognized ageing mechanisms. So, in addition to improving the understanding and predictability of known ageing phenomena, there is a need to provide for early detection of new ageing mechanisms.