Fatigue Management
SIB-96-178, Rev. 1

Fatigue related failures cost the nuclear industry millions of dollars annually in repair costs and lost plant availability. Fatigue failures and related problems tend to fall into one of three general classifications: vibrational fatigue failures, thermal fatigue failures, and issues related to plant license and design basis. Each of these problems have different individual cost consequences. Vibrational fatigue is the most prevalent cause of all fatigue failures and occurs mainly in small bore, socket-welded piping. Thermal fatigue failures, though less frequent, occur in both small and large bore piping, and as such, involve larger per-incident costs. While plant license and design basis fatigue issues do not involve component repair costs or lost operating time, a large amount of engineering and licensing time is spent annually to resolve these issues and keep plants operating.

The implementation of an effective fatigue management program can save a substantial portion of these costs. A fatigue management program, enables plant personnel to understand and identify locations that are susceptible to various fatigue failure mechanisms, and to proactively implement cost effective corrective actions, including fatigue monitoring, to prevent their occurrence. The objective of a fatigue management program is to allow for feedback to plant operators and maintenance personnel to avoid the reoccurrence of fatigue-related problems. This allows for rapid response, in the form of run, repair, or replace actions, when fatigue issues or failures are encountered.

The foundation of a fatigue management program involves the development of a screening procedure to help identify vibrational and thermal fatigue related failures and problems.

Screening criteria are then used to determine if future actions are required for fatigue-sensitive locations.

Structural Integrity Associates (SI) has played an important role in the development of fatigue management programs for utilities, as well as in the understanding and mitigation of mechanisms that lead to fatigue. SI has worked independently and with the Electric Power Research Institute (EPRI) to develop several tools that can be used in fatigue management of nuclear power plants. The following includes a discussion of these fatigue management tools.

Fatigue Management Handbook
SI, under the sponsorship of EPRI, developed a comprehensive handbook that helps utilities organize and implement fatigue management programs at operating nuclear power plants. The "EPRI Fatigue Management Handbook" (EPRI TR 104534) consists of four volumes, providing introductory information and theory on metal fatigue; screening criteria; actions that should be taken after fatigue-sensitive areas have been identified; and a comprehensive bibliography of major reference documents crucial to the implementation of fatigue management programs in operating nuclear power plants. As part of this effort, SI is assisting EPRI in developing vibrational fatigue evaluation and remedial procedures for socket welds and is currently conducting testing to fine-tune the analytical approach.

A number of utilities have instituted programs to systematically identify and mitigate socket weld vibration fatigue concerns in their plants in accordance with the EPRI handbook. Such programs can be implemented economically, especially if limited to potential plant shutdown locations, such as ASME Code Class 1 piping.

Code Support
SI has performed numerous evaluations on various plant components (vessels, pipes, pumps, valves, etc.) to determine fatigue usage per the ASME Code Section III requirements. In some cases, SI has been able to use its experience to reduce conservatisms in the fatigue calculations.

SI's employees have been members of various ASME Code Section XI Task Groups which examined the ASME Code requirements for fatigue. One such group developed a Section XI approach for managing fatigue in operating nuclear power plants, resulting in the document "Operating Nuclear Power Plant Fatigue Assessments" (EPRI TR-104691). As part of this effort, a flaw tolerance evaluation approach was developed which provides an alternative for assessing a component's fitness for continued service.

Fatigue Monitoring
Plant cycle counting and fatigue monitoring in nuclear power plants is extremely useful for tracking design life and minimizing fatigue damage to ASME (American Society of Mechanical Engineers) Class 1 components. FatiguePro. developed by SI and licensed by EPRI, is a software system that performs on-line fatigue monitoring of critical nuclear power plant components. FatiguePro monitors fatigue with existing plant instrumentation. employing various software modules to count plant transients, compute fatigue usage, and compute crack growth. FatiguePro has been installed at over two dozen nuclear power plants worldwide.

License Renewal Support
SI has conducted extensive research in the area of license renewal support. With EPRI and Sandia National Laboratories, SI performed a study of conservatisms in ASME Code Section III, Class 1 component fatigue evaluations and the effects of Light Water Reactor (LWR) water environments on fatigue margins (SAND94-0187). In this study, it was concluded that with little effort, existing evaluations could be modified to reduce the overall predicted fatigue usage and that the use of real plant data, in lieu of design transients, can reduce predicted fatigue usage significantly.

In a related effort, SI demonstrated that plants designed to the ANSI B31.1 Piping Code have proven as resistant to fatigue damage as components in later plants designed to the ASME Code Section III, Class 1 rules ("Fatigue Comparison of Piping Designed to ANSI B31.1 and ASME Section III, Class 1 Rules" (EPRI TR-102901s)). More recently, we have assisted utilities and EPRI in providing assessments of the environmental effects on fatigue through use of actual plant data.

TASCS
Most thermal fatigue failures can be attributed to thermal cycling mechanisms that were not anticipated during the plant design process. Such mechanisms include thermal stratification, striping, and thermal cycling (TASCS). Another cause of cycling occurs when stratified flow (such as from valve leakage) enters a region in which turbulence exists.

SI has served on the advisory committee to the TASCS program which has established evaluation tools needed to evaluate parameters critical to TASCS mechanisms (EPRI TR-103581s). The program developed screening methodology, analytical approaches, testing and data acquisition analysis, correlations and models, guidance and evaluation tools to provide plant operators with a working knowledge of the design for affected systems.

Training
Training provides plant personnel with one of the key tools necessary for the implementation and maintenance of a fatigue management program.

SI's Metal Fatigue and ASME Code courses provide plant personnel with fatigue fundamentals and ASME Code design requirements as they relate to nuclear power plants, as well as their cost impact to the utility.

SI's Metal Fatigue seminar introduces plant personnel to modern fatigue design principles, fatigue monitoring, and fatigue management programs. This knowledge helps nuclear power plants achieve their design lives and provides the basis for plant life extension.

SI's ASME Code courses in ASME Code Section III and Section XI provides plant personnel with an understanding of Code criteria and rules for the design of plant components. This has proven invaluable for in-house decisions of plant components and the evaluation of flaws in operating plants.

If you would like more information regarding fatigue management, fatigue, or SI's capabilities in this area, please contact SI.