Darlington NGS A SGECS Condensation Induced Waterhammer Analysis using the TUF Code

Abstract

In the event of an accident such as the main steam line break, the Reheater Drain pump and the Feedwater pump could fail. As a result, feedwater to the steam generators (SG-1 and SG-3) will stop, and the inventory in the SGs flash. This would cause depressurization of the SGs and formation of steam void in the piping of the Steam Generator Emergency Cooling System (SGECS). The subsequent low SG pressure will initiate the SGECS injection into the SGs. Upon the injection of the cold SGECS fluid to the steam filled SGECS piping, condensation would take place, and condensation induced waterhammer in the SGECS may occur. As an interim measure to protect the SGECS piping, the Second Stage Reheater Drain flow to the SGs was suspended. This resulted in a 3% loss of power, a significant economic penalty to the heat cycle of the turbine. To remove the economic penalty, the original design of the SGECS was revised. To ensure the adequacy of the revised design, condensation induced waterhammer analysis were performed for 6 design basis events. The predicted pressure transients were submitted for stress analysis and have passed the allowable level of stress limits. The analysis was presented to the AECB. Although the AECB has accepted the analytical results, the AECB requested a 'hot' commissioning test at site to demonstrate that the revised design does meet the operation requirement, The test was successfully performed. Subsequently, the AECB has allowed OH to re-instate the Second Stage Reheater Drain flow to the SGs.