Small Bore Pipe Branch Connections Fatigue

Socket welds are used extensively for small bore pipe branch connections (up to a two-inch nominal diameter). For more than 15 years, numerous failures due to fatigue cracking occured in French NPPs (EDF operates 58 PWRs), mainly on safeguard systems as regards nuclear island safety-related systems. Such failures have significant impacts, on costs, due to repairs or In-Service-Inspections and sometimes, on plants availability. Destructive examination of cracked small bore pipe branch connections have led to a better understanding of failure mechanisms and have allowed to predict crack initiation locations depending on specific features such as main and small bore pipes thicknesses. Modifications implemented, based on empirical approaches, have turned out to be much or less efficient, depending on nuclear units. A three-stage methodology was devised in the early 2000’s and applied on the 8 most important safety-related systems. Based namely on vibration measurements, this methodology has allowed to identify sensitive small bore pipe branch connections, define a relevant In-Service Inspection program, determine vibrations origins and possible solutions. New modifications are currently underway on the 2 most affected systems. So far, they have shown to be effective. Solutions for the other systems should be decided at the end of 2011 and implemented on the whole French nuclear fleet.Copyright © 2011 by ASME