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Fatigue Usage Life and Gradient Factors for ASME Class 1 Piping Fatigue Analyses

The intent of the ASME Section III (Code) Design by Analysis procedures has not changed since its inception; “…to address the requirements for new construction while providing reasonable assurance of reliable operation.” [Cooper 1992]. In this regard the Code procedures introduced design practices that, when coupled with inservice inspections, provided confidence that reactor coolant pressure boundary system components would provide reliable service. The excellent performance of our operating plants is a testimonial to these procedures; however, today NSSS suppliers and nuclear utilities are faced with challenges not anticipated in the late 1960’s (e.g. reactor water effects, load following operations, and plant life extension to 80 years).

The past 20 years Industry and NRC fatigue related research efforts significantly increased the extent and quality of fatigue test data in both air and reactor water environments. These efforts resulted improved statistically based fatigue life predictions and the development of environmental factors (Fen) that corrected ASME Code fatigue usage factors for reactor water affects.

These environmental factors were successfully applied at fatigue sensitive locations in domestic operating plants to confirm that projected 60 year cumulative usage factors (CUF) remain below the Code allowable (CUF <1). However, at some locations it was necessary to apply more rigorous analyses and modeling efforts. In some cases it was necessary to reduce conservatisms in plant original design transients and original design report computations. Utilities are now considering operation beyond 60 years. In light of these challenges, the ASME Codes and Standards Committees and Electric Power Research Institute (EPRI) are investigating refinements to the existing fatigue design by analysis procedures and practices.

LPI examined conservatisms associated two aspects of piping fatigue life as applied in NB 3600 fatigue calculations. First, piping component allowable cyclic life is defined by stress-life (S N) curves based on constant displacement push-pull test results of small diameter (=0.375 inch (=9.5 mm)) smooth test specimens. These life estimates are subsequently applied to all Class 1 piping components regardless of their actual size/thickness. Secondly, all component cyclic stresses are treated as uniform through-thickness membrane stresses when most transients induce stresses are vary across the thickness of the pipe. LPI’s work resulted in fatigue usage life (LF) and gradient (GF) correction factors that account for: 1) increased cyclic life associated with the growth of engineering size fatigue cracks in thicker components and 2) the presence of actual through-thickness stress gradients.

Example calculations for a Schedule 80 10” X 12” reducing elbow in a BWR 4 Low Pressure Core Spray (LPCS) were investigated. The original fatigue usage calculation was performed according to stress analysis procedures specified in ASME III NB-3650. The corrected 60 year and 80 year usage factors were applied. These corrections resulted in a 23% reduction in fatigue usage.

A technical paper has been prepared and will be presented at the Pressure Vessel and Piping Conference in Vancouver, B.C. in July 2016.

For additional information contact Steve Gosselin at 509-420-7685 or sgosselin@lpiny.com


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