Stress corrosion life experience of 182 and 82 welds in French PWRs

Large Components of Pressurized Water Reactors (PWR) include many parts fabricated from alloy 600 welded with 182 and 82 alloys. Long-term operating experience shows that the alloy 600 is susceptible to Stress Corrosion Cracking in primary water at high temperature (PWSCC). In the 1980's the first indications were detected in steam generator tubes, and then in pressurizer nozzles. In the 1990's, the vessel head penetrations were in turn affected. In such components, stress corrosion cracks initiated in the base metal and, in some cases propagated in the weld metal. For this reason, much attention has been paid to the behavior of the base metal. In comparison, relatively few studies have been carried out on the weld metal, essentially because of the better service experience and experimental difficulties. Alloy 182 is susceptible to stress corrosion cracking in PWR primary water only if the applied stress exceeds the yield stress. Most of the welds in alloys 182 and 82 of EDF plants are ground and stress relieved with the exception of vessel head penetrations and the weld between the divider plate and the partition stub of the steam generator channel head. The stress-relief at 600 deg. C, primarily used for stress-relieving low alloy components, has a favorable effect in reducing the residual stresses but does not modify the intrinsic PWSCC resistance of the welds. Up to now, the service behavior of 182 welds in EDF plants appears to be better than the components in alloy 600. It can be explained by the following causes -the SCC threshold is generally higher for alloy 182 than for alloy 600, - in alloy 182, the manufacturing processes (final welding, grinding) rarely induce applied stresses higher than yield stress, whereas in alloy 600, various manufacturing processes (expansion, deformations caused by welding, machining) sometimes induce applied stresses higher than yield stress. Due to the recent incidents reported on CRDM nozzles and safe-end welds, significant inspection program and research and development studies on the SCC behavior 182/82 welds, are maintained. (authors)