Although the extent of cracking was rather surprising for a bolt-loaded specimen, the average stress corrosion crack growth rate of 0.5 mm/year over the five-year testing period does not represent an immediate concern. The overwhelming part of crack growth can have occurred during a 20 day chloride transient during the third year of exposure because of a condenser leakage after an outage. The test results fit well to the experimental background knowledge and have basically confirmed the excellent service record of properly fabricated carbon and low alloy steel primary pressure-boundary components, where to date, no cases of stress corrosion cracking have been observed.
There is therefore a limited risk that an interdenderitic stress corrosion crack in the Inconel 182 weld metal of an attachment weld or in regions with Inconel 182 cladding might propagate to the adjacent reactor pressure vessel steel. Even in this rare case, only very limited stress corrosion cracking or strain induced stress corrosion cracking in the thickwalled reactor pressure vessel is expected as long as prolonged and severe chloride excursions are avoided and the number of plant transients is limited.
Although there is no direct evidence from the field of stress corrosion cracking risks in reactor pressure vessels, the effect of chloride excursions on stress corrosion cracking crack growth in reactor pressure vessel steels under BWR normal water chemistry conditions, and in particular the possibility of long-term effects after severe and prolonged transients, should, in the opinion of PSI be evaluated further.