Analyses of the Failed Impulsion Lines of NPP Dukovany Steam Generator

Over the years the cracks have been detected on the impulsion lines of all steam generators of the NPP Dukovany. The tubing is made from the austenitic stainless steel. These lines are designed for purposes of measurement and also containment of possible leaks, e.g. between the primary and secondary steam generator seals. Their integrity is periodically tested during each outage and if the leak is detected the tube segment must be cut out and the new one is welded in place. Most of the time the tubes are “dry”, i.e. no medium is flowing inside. Due to confined space the movement of persons around the steam generator is difficult and often the relatively subtle impulsion lines have been subjected to stresses leading to their bending. This deformation has then led to evolution of the failure. To address the issue the original manufacturing, test and control procedures for the impulsion tubing have been studied. Several cracked tubes have been analyzed thoroughly to find the root cause and offer possible remedies.