Thermal Fatigue Experience in French Piping: Influence of Surface Condition and Weld Local Geometry

Abstract

In May 1998, a leak occurred in the reactor heat removal system (RHRS) of the French Civaux 1 nuclear power plant. A 180-mm through-wall crack was found on an elbow in the mixing area of high and low temperature fluids. After metallurgical examinations of austenitic stainless steel (AISI 304L) components and an analytical damage evaluation, the major root cause for cracking was identified as high cycle thermal fatigue. The cracks were found in the tee and at the root of welds in mixing areas. The links between the damage and manufacturing parameters are surface condition (roughness), high residual stresses, high level of mean stress and geometrical discontinuities (weld root and counterbore). Decision was taken to suppress welds or locate them away from mixing areas and to improve the surface condition (remove the weld root singularity, avoid striations due to machining, reduce residual stresses).