Managing Fatigue-Challenged Components in SLR

Subsequent License Renewal (SLR) will require a shift in the approach for managing plant components for thermal fatigue. The components are older and will have experienced more fatigue damage. As time goes on, more components will become fatigue-challenged, meaning that they will require more management to demonstrate serviceability.

There are several approaches that can be taken to manage fatigue-challenged components in SLR.

Refining the design fatigue analyses is one approach that has been widely used in License Renewal (LR), and will remain useful in SLR. Components that were previously managed through cycle counting alone may still be managed through cycle counting if a refined analysis results in fatigue and environmentally-assisted fatigue (EAF) cumulative fatigue usage values below 1.0.

Another useful approach is to revisit assumptions made about plant operation earlier in life. Conservative assumptions were made about early plant operation for many components. This was often done for expediency and may have been sufficient for LR, but as the components age, those assumptions may prove too conservative. Revisiting these assumptions can help lower the overall fatigue usage for components.

Fatigue monitoring is another approach that has been widely used for LR and will prove even more useful in SLR. As more components become fatigue challenged, expanded monitoring will be important. For those components already monitored, converting to more refined monitoring methods can help remove excess conservatism and lower the overall fatigue usage.

When it becomes impossible to demonstrate that the fatigue or EAF usage for a component will remain below 1.0, component inspection and flaw tolerance is the next step to demonstrate serviceability.

When an inspection is performed and no flaws are identified, ASME Code Section XI, non-mandatory Appendix L contains guidance for performing a flaw tolerance evaluation to accompany the inspection. The Appendix L evaluation sets the inspection interval by calculating the allowable flaw size and crack growth rate of a postulated flaw in the component.

SI:FatiguePro 4.0 (FP4) contains a Fatigue Crack Growth (FCG) module to support the inspection and flaw tolerance approach. The FCG module uses either plant data or simulated design transient data to calculate crack growth for a real or postulated crack. It can be used both to perform the Appendix L evaluation to determine the inspection interval and to monitor crack growth over time to confirm the Appendix L results. If the inspection interval is less than 10 years, the confirmatory monitoring may even be used as a basis for increasing the inspection interval.

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