Thermal fatigue in mixing tees: status and justification of French assessment method

Following the CIVAUX-1 incident of a leak on the reactor heat removal system (RHR), 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 development works that support this procedure. The procedure is based on: -) screening criteria on maximum ΔT and minimum duration, -) screening criteria without any duration consideration, only ΔT and material, -) a simplified and conservative estimation of a usage factor, and -) a detailed analysis of usage factor and crack growth rate, based on specific data collection of operating transients. Around that procedure EDF launched a research and development program on fatigue curves and fatigue reduction factors for high cycle fatigue. The procedure is compared with field experience and recent fatigue tests. (author)