A number of large heavy wall reactors showed severe stress corrosion cracking in the high strength low alloy steel shells. The stress corrosion was mainly caused by the fact that no PWHT was performed which resulted in very high hardness of the Heat Affected Zone; this made the HAZ extremely sensitive for stress corrosion cracking in the feed water environment. The extent of the cracking was such that replacement of all reactors was unavoidable. The redesign and fabrication of the new reactors would take over a year. Fitness for Service methodologies were used keep the reactors running safely until the replacements were installed with minimized down time. This resulted in large economic benefits over several years. The paper covers the FFS assessments performed, the special NDT methods required to get the necessary crack size information at operating conditions as well as failure mode and repair options. It demonstrates an application and the benefits of Fitness for Service assessments on key equipment.

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