05000280/LER-2006-001
| Surry Power Station | |
| Event date: | 05-04-2006 |
|---|---|
| Report date: | 07-03-2006 |
| 2802006001R00 - NRC Website | |
1.0 DESCRIPTION OF THE EVENT On May 4, 2006, with Unit 1 in refueling shutdown/defueled and Unit 2 operating at 100% power, while evaluating the use of the auxiliary feedwater (AFW) system (versus the main feedwater system) for heat removal during plant startup, an AFW system design issue was discovered. The design concern is that a motor driven AFW pump [EIIS-BA, ID] could be subject to an unanalyzed high flow condition while supplying all three steam generators with the reactor coolant system (RCS) temperature above 350°F and prior to the point at which the steam generator pressure is high enough to operate the turbine driven AFW pump at or near its design capacity and/or to provide sufficient backpressure to limit AFW flow.
The time frame during which this high flow condition could occur is limited to when the RCS temperature is between 350°F and 500°F. Specific postulated scenarios with certain initiating events, along with an assumed single failure, could result in no operating AFW pumps on a unit.
A Unit 1 scenario that illustrates the issue is as follows:
Unit 115 at 350°F.
Prior to exceeding 350°F in the RCS, the AFW system is required to be operable in accordance with Technical Specification (TS) 3.6.
On a Loss of Offsite Power (LOOP) the #3 emergency diesel generator (EDG) preferentially powers Unit 2, which renders motor driven AFW pump 1-FW-P-3B without power.
The LOOP results in an AFW initiation signal (on both units) when the main feedwater pumps trip. The AFW initiation provides a signal to open the 1-FW-MOV-151A - F motor operated valves (MOVs).
- Motor driven AFW pump 1-FW-P-3A starts following the #1 EDG startup/bus reenergization and flows to all three steam generators through the normally open 1-FW-MOV-151A - F valves.
Because the steam generator pressure corresponding to 350°F in the RCS is approximately 112 psig and the cavitating venturis may not limit flow sufficiently, 1-FW-P-3A could reach unanalyzed high flow conditions with the potential for inadequate net positive suction head (NPSH) available.
With 112 psig steam pressure, turbine driven AFW pump 1-FW-P-2 starts and accelerates but does not reach full speed. Because the head of 1-FW-P-3A (even at high flows) is greater than the head of 1-FW-P-2 operating on 112 psig steam, the discharge check valve for 1-FW-P-2 remains closed.
- At some point, the possible unanalyzed high flow condition on 1-FW-P-3A could damage or disable the pump such that 1-FW-P-2 becomes the only available pump.
Further, if a single failure is assumed for 1-FW-P-2, Unit 1 will not have any operable AFW pumps.