High cycle thermal fatigue issues in PWR nuclear power plants, life time improvement of some austenitic stainless steel components

摘要

Complex thermal loading phenomena as stratification with large AT, mixing of cold and hot water, turbulent penetration of vortex in dead legs or in piping system tee-connections were not always identified at the design stage for oldest plants. In relation with these thermal fatigue issues and in order to have a better knowledge of basic material properties for austenitic stainless steels (304L stainless steel), fatigue test programs were carried out by the AREVA NP Engineering Division and Technical Center of Le Creusot in collaboration with ENSMA Poitiers. These programs concern the following specific needs which were investigated for an austenitic stainless steel type 304L: Cyclic stress strain behavior of 304L stainless steel between 20 deg C and 150 deg C, Effect of loading conditions (stress or strain control) on cyclic stress strain behavior and fatigue life, Effect of mean stress and surface finish on high cycle fatigue strength, Effect of LCF pre-damage and surface state on HCF strength (N approx = 10~7 cycles). A reasonable understanding of root causes of thermal fatigue cracking in austenitic stainless steel components has been obtained from metallurgical investigations. Failure analyses have revealed effects of aggravating factors like the presence of weld roots, high surface roughness and high level of mean stress.