An Experience in Low Temperature Stress Corrosion Cracking at Steam Generator Coldleg Drain Nozzle for YGN 3 Unit

摘要

Throughout performing Boric Acid Corrosion program for Yonggwang Nuclear Unit 3 at its 10th refueling outage, an evidence of boron crystallization was found at one of the coldleg drain nozzles on Steam Generator-B. The material specification for the pressure body of Steam Generator is SA-508 and for two drain nozzles, located at hotleg and coldleg for allowing chamber drain, Inconel-690 for hotleg and Inconel-600 for coldleg respectively. Crystallized boron on the leaking drain nozzle was removed from the system and non-destructive examination (NDE) was performed. The result of NDE reveals that there 're some linear defects inside the nozzle base metal. Several instances of Primary Water Stress Corrosion Cracking (PWSCC) of nickel based alloys have been reported in the reactor pressure boundaries. Since there has been no such indications ever reported for low-temperature (less then 321 °C) alloy nozzle as Steam Generator coldleg drain nozzles, and the normal operating temperature for YGN 3 Steam Generator coldleg has been maintained to be low enough as 293 V (560 °F) which is well within its design operation tolerance, we believe that there should be no such a possibility of PWSCC on the coldleg drain nozzles. Based on the preliminary evaluation, it is assumed that the remaining complex stresses such as mechanical manufacturing stress, welding heat stress, and operational environmental stress were contributing to stress corrosion cracking of alloy nozzle. As a corrective measure, the cracked alloy nozzle was removed and a metal plug was installed in a drain hole for temporally block until subsequent outage cycle, and the remaining alloy 600 nozzles were also verified to be no flaws by doing relevant NDE. In addition, YGN unit 3 and 4 intends to replace all the alloy 600 based nozzles on the coldleg loop at coming refueling outages including the temporary metal plug at one of Steam Generator coldleg in YGN unit 3.