Thermal Fatigue in Mixing Tees: Status and Justification of French Assessment Method

Following the CIVAUX 1 incident of a leak on RHR system, EDF has developed a step by step procedure to screen and analyse similar locations: mixing tees with long duration at high ΔT between the 2 fluids. The paper presents the procedure, the background of the methodology and few R&D work that support this procedure. The procedure is based on: - screening criteria on maximum DT and minimum duration; - screening criteria without any duration consideration, only DT and material; - a simplified and conservative estimation of a usage factor; - a detailed analysis of usage factor and crack growth rate, based on specific data collection of operating transients. Around that procedure EDF launched an R&D program on fatigue curves and fatigue reduction factors for high cycle fatigue. The procedure is compared with field experience and recent R&D fatigue tests.