Dispelling the Myths Behind Regulatory Issue Summary 2008-30 (Nuclear Plant Component Fatigue Sensitivity Analyses)

In April of 2008, the U.S. Nuclear Regulatory Commission’s Office of Nuclear Regulation released a draft Regulatory Issue Summary (RIS) regarding fatigue analysis of nuclear power plant components. The final RIS was published in December of 2008 with little modification. At issue in the RIS was the use of a single stress term for use with Green’s Functions, as opposed to the ASME Code, Section III method of combining all six stress components, which could potentially lead to non-conservative results in a fatigue usage evaluation. Single stress term Green’s Functions have been commonly deployed in on-line fatigue monitoring systems, and they have been used to quickly generate thermal transient stress histories in place of more rigorous finite element analysis of transients. The conclusions of the RIS were based on a comparison of results from a single-stress term Green’s Function analysis and a follow-on six-stress component confirmatory analysis performed on a BWR-4 feedwater nozzle. The confirmatory analysis considered all six stress components in accordance with ASME Code, Section III, Subsection NB, Subarticle NB-3200 methodology, and yielded higher fatigue usage in the nozzle corner region, thus calling into question the adequacy of the Green’s Function analysis and its associated judgments. Further concerns were raised because of the use of the single stress term Green’s Function approach in some on-line fatigue monitoring systems. However, the RIS did not address the many differences in inputs that were used in the confirmatory analysis that also contributed to the increase in fatigue usage. This paper re-investigates the BWR-4 feedwater nozzle analysis that originally motivated the promulgation of the subject RIS. The paper parametrically investigates the different inputs that were used in each analysis to further understand the differences between the single stress term Green’s Function and follow-on confirmatory analyses, and to clarify some of the misunderstandings about single stress term Green’s functions that may have been created by the publication of RIS 2008-30 [1].