Automatic isolation of auxiliary feedwater

Generic Issue 125.II.7 was evaluated to determine if the disadvantages of the automatic auxiliary feedwater system (AFW) isolation system, such as reduced AFW system reliability, outweighed the benefits provided by automatic AFW isolation to a steam generator during a steam or feed line break accident. Reactor plants with automatic AFW isolation systems from each pressurized water reactor (PWR) vendor were evaluated by determining the automatic isolation system's contribution to the core damage frequency (CDF) and determining any CDF increase or decrease that would occur if the system were removed. The CDF changes were used to estimate the change to the consequences or public risk due to removing the isolation system. Recent probabilistic risk assessments (PRAs) for a reactor plant from each PWR vendor were used to quantify the changes to CDFs and risks. The findings of this study indicate that the automatic AFW isolation system does not contribute significantly to the CDF or public risk for any of the plants evaluated. Removing the AFW isolation system resulted in a slight decrease in CDF and risk in some plants and caused a slight increase in CDF and risk in others