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Report on Assessment of Environmentally-­Assisted Fatigue for LWR Extended Service Conditions

This report provides an update on the assessment of environmentally-assisted fatigue for light water reactor (LWR) extended service conditions. The report is a deliverable in FY11 under the work package for LWRS under the Advanced Reactor Concepts.

Most of the current fleet of aging LWRs were designed using the 1970s version of the ASME Boiler and Pressure Vessels Code, Section III, and are reaching their design lifetime of 30-40 yrs. For economic reasons, the utilities have great interest in extending the operating life of the plants via the Nuclear Regulatory Commission (NRC) licensing renewal application (LRA) process. Based on issues learned from the reviews of LRAs and public comments, NRC has published NUREG-1801, “Generic Aging Lessons Learned (GALL) Report”. The GALL Report lists generic aging management reviews (AMRs) of systems, structures, and components (SSCs) that may be in the scope of license renewal applications (LRAs) and identifies aging management programs (AMPs) that are determined to be acceptable to manage aging effects of SSCs in the scope of license renewal. One of the critical aging issues identified in the GALL report is environmental fatigue.

The ASME Boiler and Pressure Vessel Code Section III fatigue design curves, developed in the late 1960s and early 1970s, are based on tests conducted in laboratory air environments at ambient temperatures. In the Code, adjustments are made to strain and cyclic life to account for variations in material properties, surface finish, data scatter, and unknown effects. The Code does not explicitly account for potential degradation in the fatigue properties attributable to exposure to LWR coolant environments. Recent fatigue test data and analyses have demonstrated conclusively that LWR environments have a significant impact on the fatigue life of reactor structural materials. To address the environmental fatigue issue, NRC has issued Regulatory Guide 1.207 (DG-1144), “GUIDELINES FOR EVALUATING FATIGUE ANALYSES INCORPORATING THE LIFE REDUCTION OF METAL COMPONENTS DUE TO THE EFFECTS OF THE LIGHT-WATER REACTOR ENVIRONMENT FOR NEW REACTORS”.

The overall objective of the current LWRS project is to assess the current state of knowledge in environmentally assisted fatigue of materials in light water reactors under extended service conditions. The report highlights the issues concerning the long-term fatigue of materials in LWR environments, presents a brief review of laboratory and field observations, assesses the ASME Code requirements for the nuclear components, and recommends areas of research and development for improving the reliability of database that enable life prediction for LWR components.