EPRI is coordinating efforts to assess the impact of environmental fatigue on piping and component design, and to inform potential regulatory or code actions.

The aqueous environment of a nuclear power reactor is widely believed to adversely affect the fatigue life of piping and components. There is significant technical debate, however, regarding how to account for such environmental effects in design and life extension decisions. Engineering design codes for piping and components rely on "fatigue curves" generated from cyclical stress tests, but the American Society of Mechanical Engineers has not been able to reach consensus on a comprehensive code revision that adequately accounts for environmental factors.

Further, although the Nuclear Regulatory Commission has published regulatory guidance for applying an environmental factor to fatigue calculations, the lack of code certainty could lead to difficulties in demonstrating acceptable fatigue usage for both new plant designs and license renewal activities.

Because environmentally assisted fatigue affects both operating and new plants, EPRI is coordinating efforts among its materials and advanced nuclear technology programs to combine expertise and address this issue. Efforts in 2011 are divided into three areas. Together, these research activities are intended to inform potential regulatory and code activities and assess the impact of any new procedures and acceptance criteria.

• In code space, EPRI has engaged ASME Code working groups to develop new procedures that provide guidance for consistent application of environmental factors for fatigue where required. The strategy is to develop code cases for nearer-term use, followed by code revisions.
• The second focus area is identifying postulated break locations for high-energy piping. If the current criterion - based on fatigue - is applied in combination with an environmental factor, many more break locations may have to be considered. Recent EPRI research work indicates that there is no correlation between fatigue usage and leak probability. Research continues in this area to more fully delineate postulated break locations.
• The final area involves coordinating and collaborating with global research entities to investigate fatigue usage (cycling prior to crack initiation) and fatigue crack growth (crack extension after a fatigue crack initiates).

For more information, contact Letitia Midmore at 650-855-8576 or lmidmore@epri.com.