Throughout this report, pipe sizes are referenced by nominal pipe size (NPS), which indicates standard pipe size without an inch symbol. The smallest pipe size considered in this study is NPS3-% (Plant A. b). All references to specific material types are made according to designations by the American Society for Testing and Materials (ASTM). The term “weld failure” is used to indicate a rejectable (non-through-wall or through — wall) flaw.

During the NRC LOCA Elicitation Kick-off Meeting [D.1], a LOCA was defined as “a breach of the reactor coolant pressure boundary which results in a leak rate greater than 380 lpm (100 gpm).” Instead of using the traditional (or historical) LOCA size classes (small — medium — large) that are based on break size, this study uses LOCA sizes that are based on leak rate threshold values as indicated in Table D. 1 (adapted from [D. 1 ]) and Table D.2.

Table D.1 LOCA Size Classification Threshold Values

The estimation of weld failure rates uses Bayesian reliability analysis methodology, and involves the development of prior and posterior failure rate distribution. In this study the term ‘prior’ refers to piping reliability characteristics before the implementation of industry programs to mitigate or eliminate susceptibilities to certain degradation mechanisms. The term ‘posterior’ refers to observed or expected reliability characteristics after reliability improvement actions have been implemented.