Fatigue Cycle Monitoring

The past and/or current approach to cycle count is a deviation from the current license basis (CLB)/design stress report, CENC-1150 Analytical Report, as applied in the fatigue evaluations for pressure and thermal cycles. These departures are relative to design projection versus actual operating experience in relation to “Cyclic Life” and the intent of ASME III, Section N-415 component suitability requirements for power operations of 40 years and subsequent life extension beyond to 60 years. In general, the practice of counting scram events does not accurately represent the “Cyclic Life” and is overly conservative, resulting in inaccurate projections of component reliability. The Nuclear Regulatory Commission (NRC) requires licensees to keep an account of the number of transient occurrences to ensure that design assumptions used in the fatigue evaluation required by ASME Section III are not exceeded. In the mid 1970s, typical plant technical specifications required “the number of the transients which are comparable to or more severe than the transients evaluated in the stress report Code fatigue usage calculations will be recorded in a log book.” In the mid 1980s this transitioned into counting of transients, “this program provides controls to track the Final Safety Analysis Report, Section III-3.5, cyclic and transient occurrences to ensure that components are maintained within the design limits” (NUREG-1434, Section 5.5.5). In addition, the evaluation of Time-Limiting Age Analysis per Title 10 Code of Federal Regulation Part 54, Section 54.21(c). Cycle counting is used to summarize lengthy, irregular load-versus-time histories by providing the number of times cycles of various amplitudes occur. The definition of a cycle varies with the method of cycle counting. In fatigue analysis, a cycle is the load variation from valley-to-peak-to-valley. Cycle counts can be made for time histories of force, stress, strain, torque, acceleration, deflection or other loading parameters of interest. In general, the operational transients plants experience do not approach the magnitude or severity of the bounding events used in these analysis. Thus counting every transient as equal to a design event is unnecessarily conservative. However, no clear specific guidance for Owners on how to perform cycle counting has been promulgated. This paper provides guidance for formulating a fatigue management program that takes advantage of the original analytical design stress reports.