Review of industry efforts to manage pressurized water reactor feedwater nozzle, piping, and feedring cracking and wall thinning

Abstract

Attached for your information and use is a copy of the report, "Review of Industry Efforts t o Manage Pressurized Water Reactor Feedwater Nozzle, Piping, and Feedring Cracking and Wall Thinning," NUREG/CR-6456. The study objective was t o provide a comprehensive overview document discussing pressurized-water reactor (PWR) feedwater nozzle, piping, and feedring cracking and wall thinning; safety aspects; and industry actions taken t o manage these issues. The report should be useful in identifying, assessing, and evaluating program options t o manage these issues. The time frame spans from initial discovery of feedwater nozzle cracking in 1979 through 1996.The review and assessment effort covers relevant field experience with PWR feedwater systems, the factors causing the damage, design modifications, operating procedure changes, augmented inspection programs, and repair and replacement activities carried out because of the degradation that occurred. The effort focused on the feedwater system adjacent t o the feedwater nozzle where fatigue cracking and wall thinning had been reported. This includes the main and auxiliary feedwater piping adjacent t o the feedwater nozzle, and the thermal sleeve, feedring, and J-tubes. The principal areas reviewed were (1 ) feedwater system design, (2) safety significance of feedwater line rupture, (3) fatigue cracking experience, (4) f low-accelerated corrosion-induced wall thinning experience, (5) steam generator water hammer damage experience, (6) degradation mechanisms, (7) inservice inspection methods, and (8) mitigation, monitoring, and replacement activities.This effort emphasized understanding the technical aspects for each area in order t o assess the impact on managing (establish confidence that limits for safe operation are maintained) degradation of these components. In this context, the study concentrates on causes, mechanisms, conditions (temperature, pressure, environment, etc.), inspections, procedures, and corrective actions from the perspective of capability t o assess a specific aspect such as crack characterization. Thus, the report is directed toward determining whether current technology is sufficient t o "manage the problem".The operating experience review addressed feedwater nozzle cracking caused by thermal fatigue; flow-accelerated corrosion wall thinning of carbon steel J-tubes, feedrings, and thermal sleeves in top-feed steam generators, and auxiliary feedwater lines in preheat steam generators; and water hammer damage in both types of steam generators.Feedwater nozzle cracking was detected in 18 PWRs from 1979 through 1983. These were found as a result of IE Bulletin 79-13, "Cracking in Feedwater System Piping," 1979. -2-There was approximately one event per year from 1983 until bulletin closure in 1991. The nozzle cracking event frequency increased t o six per year for 1992 and 1993 but no additional events were detected from then through 1996. It appears that licensee action has been sufficient t o minimize flow-accelerated wall thinning in J-tubes a...