NRC Information No. 89-79: Degraded coatings and corrosion of steel containment vessels

On August 24, 1989, Duke Power Company reported significant coating damage and base metal corrosion on the outer surface of the steel shell of the McGuire Unit 2 containment which was discovered during a pre-integrated leak rate test inspection. Subsequently, Duke Power identified similar degradation of the McGuire Unit 1 containment, which is essentially identical to the Unit 2 structure. Both units have ice condenser-type containments consisting of a freestanding steel shell surrounded by a concrete shield building. The steel shells have a nominal thickness near the annulus floor of 1 inch. The degraded area on the shells of both units is limited to 30-foot circumferential sections no higher than 1 1/2 inches above the annulus floors. The average depth of the corrosion is 0.1 inch with pits of up to 0.125 inch. Corrosion that is up to 0.03 inch deep was also found in areas below the level of the annulus floor on the Unit 2 shell, where concrete was removed to expose the shell surface. The probable cause of the degradation is attack by condensed boric acid coolant leaking from instrument line compression fittings. Drains are provided in the annulus floor, but they are widely separated and the floor is not sufficiently graded to prevent pooling of the condensate between the drain locations. On September 21, 1989, Duke Power Company discovered coating damage and base metal corrosion during inspections of the steel shells of its two Catawba containments. The cause is also believed to be attack by boric acid coolant, which had leaded from instrument line compression fittings, condensed, and collected on the annulus floor. Duke Power Company plans to repair the steel shells of all four units during their next respective refueling outages