CIRCUIT RRA N4 ― INCIDENT DE CIVAUX 1 ENDOMMAGEMENT PAR FATIGUE THERMIQUE DE TUYAUTERIES SITUEES DANS DES ZONES DE MELANGE

摘要

In May 1998, a leak occurred in the reactor heat removal system (RHRS) of the Civaux 1 plant. A 180 mm through-wall crack was found on an elbow in mixing area of high and low temperature fluids and was located at the longitudinal weld root on the extrados. After metallurgical examinations of austenitic stainless steel 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 area and to improve the surface condition of mixing area.