Investigations of Mixing Zones subject to Thermal Fatigue

Abstract

Further the through wall crack in an elbow of the Residual Heat Removal (RHR) system of Civaux 1 in 1998, a large investigation program has been completed on removed parts of pipes to better assess the factors responsible for thermal fatigue cracking in pipe components where hot and cold fluids mix together. This article presents the results of destructive investigations performed on numerous parts of the RHR system of EDF plants, on a nozzle of the primary system and on a nozzle of the secondary system and their contribution to understanding and assessing harmfulness of the phenomenon.The damage concerns only certain RHR mixing zones: the mixing tee at intersection between downstream the exchanger and the by-pass line and the mixing tee at location of intersection of two portions of minimum flow line. Deep cracks are always associated with high temperature differences and long times of operation. The deepest cracks are always located close to the welds, in areas showing geometrical singularities and minimal thickness. Crack networks in running parts concern only the very first millimeters of the inner wall.Other mixing zones have been investigated: destructive examinations of a charging line primary nozzle of the Chemical and Volume Control System (CVCS), of two tees between the main line and the minimum flow line of the RHR system and of one tee connecting the Feed Flow Control System (FFCS) to the Auxiliary Feedwater System (AFS) did not show any thermal fatigue crack.EDF has developed a method for identification of fluid mixing zones subject to thermal fatigue damage. This method is based on the assessment of a usage factor, using the knowledge acquired during a large R&D program. This program investigated the thermal loadings in mixing tees acquired through several thermohydraulic mockup tests and the austenitic steel behavior under high cycle thermo-mechanical loadings.The results of the destructive examinations have been used to strengthen the identification methodology of mixing zones potentially affected by thermal fatigue. Damage found in the destructive examinations are always correlated with usage factors a lot greater than 1.The methodology has been applied to all mixing zones of safety related circuits of plants operated by EDF. It has lead to identify the ones with a usage factor greater than unity and to an appropriate in-service inspection program based on volumic non-destructive examinations.