In-service monitoring and fractographic inspection of ultrasonic indications on a feed water suction pipe of the Gundremmingen nuclear power plant

Radiographic and ultrasonic examinations of three pipes of the Gundremmingen nuclear power plant revealed cracks with a maximum depth of about 2 mm on the inside of the pipes with a wall thickness of 7.1 mm. For continued service until replacement two representative areas were monitored for crack growth by means of the potential drop method. As crack growth during the last year of service can be excluded by metallographic examinations after replacement the result of monitoring by means of potential probes – in relation to its resolution capacity no crack growth in the period under monitoring – is confirmed. A final evaluation based on the actual crack depths results in minimum resolution of 0.2 and 0.5 mm in crack depth. The suitability of the potential drop method for in-service monitoring of existing indications for crack growth – even until replacement of the component, if necessary – could thus be demonstrated convincingly.