The team reviewed the pertinent design calculations concerning the automatic swapover of the suction source for the auxilary
feedwater (
AFW) pumps from the condensate storage tank (
CST) to the emergency
service water (
ESW) system. In responding to a loss of offsite power (
LOOP), the team noted that the
TDAFW pump and
EDGs are started immediately on an undervoltage signal, and safety loads are then automatically sequenced after the
EDG is ready to accept load. The team questioned the condition if the
CST was not available and noted the
ESW system would be automatically aligned by a low
AFW suction pressure signal to provide suction to the
AFW system. Considering the 12 second maximum start time for the diesel, and the subsequent load sequence times, the team noted that the first
ESW pump might not be at full flow until about 32 seconds after a
LOOP, and the second
ESW pump at about 37 seconds. Therefore, if the
CST were not available and
ESW were the
AFW source, the water initially available to the
TDAFW pump suction would be limited to the useable volume in the suction piping. The team questioned whether the system had the capability to break a vacuum in the suction piping with sufficient inflow of air. In addition, the team questioned when the
ESW pumps are eventually started, if vacuums formed in the piping, whether there would be potential for
water hammer in both trains of the
ESW system. The licensee responded with their design assumptions and calculations for the systems. The team reviewed the licensees response and questioned whether the licensees assumptions were within the design and licensing bases of the plant. Pending further inspection of the licensees adherence to these bases, this issue was considered an unresolved ite
