Inspection Optimization Justification for PWR Main Steam and Feedwater Nozzles Using Probabilistic Fracture Mechanics

Pressurized water reactor (PWR) steam generator (SG) main steam and feedwater nozzles are classified as ASME Code, Section XI, Class 2, Category C-B, pressure retaining welds in pressure vessels. Current ASME Code requirements specify that the nozzle-to-shell welds (Item No. C2.21 & C2.32) and nozzle inner radius sections (Item C2.22) are to be examined very 10 years. An evaluation was performed to establish a technical basis for optimized inspection frequencies for these items. The work included a review of inspection history and results, a survey of components in the PWR fleet (which included both U.S. and overseas plants), selection of representative main steam and feedwater nozzle configurations and operating transients for stress analysis, evaluation of potential degradation mechanisms, and flaw tolerance evaluations consisting of probabilistic and deterministic fracture mechanics analyses. The results of multiple inspection scenarios and sensitivity studies were compared to the U.S. Nuclear Regulatory Commission (NRC) safety goal of 10⁻⁶ failures per year.