Pitting of alloy 600 steam generator tubes under heat transfer conditions

Severe pitting has occurred in the NiCrFe Alloy 600 steam generator tubes at several operating PWR stations since 1981. In some of the affected units, pitting has been sufficiently severe so as to result in several thousand plugged or sleeved tubes. Results of destructive examinations of tubes removed from service, analysis of non-destructive examinations of tubes removed from service, analysis of non-destructive test data from affected units and literature data were not sufficient to define the causes of the pitting or define corrective actions to prevent further pitting. This paper describes a test program directed toward reproducing in the laboratory the pitting that has occurred in field units and identifying remedial actions. The program consisted of operating two pot boiler test units designed to have thermal-hydraulic characteristics prototypical of field units experiencing pitting under various environmental conditions. The internals of each unit consisted of four Alloy 600 tubes with concentrating devices attached to provide steam blanketed areas where contaminants could concentrate