Microstructure Analysis on Alloy 600 Steam Generator Drain Nozzle of PWR

Primary water stress corrosion(PWSCC) of alloy 600 in a PWR has been a big concern. A PWSCC has been reported in a control rod drive mechanism (CRDM), pressurizer instrumentation nozzle, and a pressurizer heater sleeve. Recently some cracks and precipitation of boric acid were found in a steam generator (SG) drain nozzle in a Korea nuclear power plant. PWSCC cracks were reported first in the SG drain nozzle at Shearon Harris in 1988. These drain nozzles had been roll expanded into the SG head and then seal welded on the inside surface of the head. The cracking was evaluated as a PWSCC like in the EdF pressurizer nozzles which were roll expanded into the shell. The objective of the present work is to evaluate the microstructure of a alloy 600 sample removed from a Korean plant, and to find the causes of a PWSCC