Rules for assessment of flaws found as a result of periodic in-service examinations of nuclear components, first appeared in Section XI of the ASME Boiler and Pressure Vessel Code in 1974. Since that time, literally hundreds of evaluations have been completed, which have allowed flaws that met the required margins to be accepted for continued service without repair. The flaw evaluation process of Section XI involves prediction of the future growth of such flaws, and then comparison with a calculated allowable flaw size for the location of interest. The crack growth models to be used for such evaluations are generally contained in the ASME Code itself, in Section XI Nonmandatory Appendices A or C, or the new Nonmandatory Appendix Y for Crack Growth Rate Curves (2021 Code Edition), depending on the application and material type. Occasionally, Code Cases are also used to provide recommended crack growth models. The philosophy that has been followed by the Code since its inception has been to model crack growth as accurately as possible, so as to make an accurate prediction of future flaw growth, and then compare the predicted final flaw size with the allowable flaw size for the area of interest, after a margin has been applied. Therefore, the margin is applied only once, so it is easily identified, and is not “double-counted”.

The goal of this paper is to summarize the background and basis behind the ASME Section XI general philosophy of the use of a best estimate, or mean, treatment of the crack growth models in Section XI of the ASME Code. This paper will discuss the various approaches that are available to characterize crack growth, and then discuss the approaches which have been used in ASME Code Section XI flaw evaluation rules over the years.

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