The materials chosen for the main systems and componentsof the pressure-retaining boundary have to grantthe integrity to avoid catastrophic failure under all operationaland emergency conditions during the design life ofa light water reactor nuclear power plant. The quality byproduction according to the basic safety concept used forGerman LWRs provides absence of critical defects at thestart of operation and high safety margins by good mechanicalproperties especially including high toughnessand thus defect tolerance.Ageing mechanisms of the primary systems are neutronirradiation induced reduction of toughness, fatigueand different types of corrosion. They are managed basedon appropriate design, materials selection, manufactureand operational conditions including tracking the relevantoperational parameters.Concerning corrosion, the materials used in lightwater reactor plants exhibit a high general resistance toall relevant mechanisms of corrosion that might occurunder specified service conditions, material conditionsand mechanical loads. On the other hand, as all structuralmetals used for mechanical components are in principlethermodynamically instable under the given environmentalconditions of concern, the actual degree of resistanceto corrosion is always based on the stability ofthe generally protective oxide layer that forms underoperational conditions in high temperature water. Therefore,absolute corrosion resistance can never be achievedin principle. Granting a sufficient corrosion resistance forthe full operational regime is thus always the result ofsuccessful optimization of the interaction between componentdesign, selection of appropriate materials, fabricationand joining (welding) of parts and components andappropriate mild operating conditions. Consequently,when such optimization is not fully achieved, degradationor damage due to corrosion mechanisms may occur undercertain abnormal operating conditions.Among all the relevant corrosion mechanisms to beconsidered, especially the formation of localized cracksby the promotion of corrosion mechanisms related to thegeneral term "Environmentally Assisted Cracking (EAC)"are of particular significance for component integrity. Aprerequisite for the occurrence of EAC is the presence ofcritical values for the determining parameters of the corrosionsystem: material including material condition,environment (medium) and mechanical load. EAC encompassesnot only all forms of corrosion which may leadto crack initiation and growth in aqueous media, but alsopossible combinations of several mechanisms as well asany mutual influence and synergistic effects between thesetypes of corrosion, the occurrence of which always requiresthe presence of mechanical loads. The greatestsignificance and interests in terms of nuclear plant operationare focused on stress corrosion cracking (SCC),environmentally assisted fatigue (EAF) and straininducedcorrosion cracking (SICC). Aspects associatedwith fast neutron irradiation and its effects on corrosionare not considered here. The materials concerned inGerman LWR main systems comprise low-alloy steels,austenitic stainless steels and nickel-base alloys, whichare exposed to the reactor coolant environment of eitherboiling water reactors (BWR) or pressurized water reactors(PWR).This paper gives a comprehensive overview of mainlesson learned about effective corrosion induced ageingand its management in German LWRs as part of the ageingmanagement practiced since start of operation.
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