05000370/LER-2005-001
| Docket Number | |
| Event date: | |
|---|---|
| Report date: | |
| 3702005001R00 - NRC Website | |
BACKGROUND
Applicable Energy Industry Identification (EIIS) system and component codes are enclosed within brackets. McGuire unique system and component identifiers are contained within parentheses.
Main Feedwater System [SJ](CF):
The purpose of the CF system is to take treated Condensate [KA](CM) system water, heat it to improve the plant's thermal efficiency and deliver it at a required flow rate, pressure and temperature to the steam generators. The CF system is designed with two 50% capacity dual admission type steam turbine driven pumps which discharge to three parallel trains of "A" and "B" feedwater heaters. Discharge from the "A" and "B" feedwater heaters enters the steam generators.
Auxiliary Feedwater System [BA](CA):
The purpose of the CA system is to provide an emergency feedwater supply to the steam generators [SG](SG) if the respective Unit's Condensate and Feedwater System [SJ](CF) is not available. The CA system is capable of transferring fission product decay heat and other residual heat loads from the reactor coolant system [AB](NC) to a heat sink during both normal operation and accident conditions. Each Unit's CA system contains an "A" and "B" train motor driven pump [P] (MDCAP) and a "C" train turbine driven pump [P](TDCAP) configured into three trains. The normal suction source of water for the MDCAPs and TDCAP is the CA storage tank [TK](CAST). The Nuclear Service Water System [BI](RN) provides the assured suction source of water for the CA pumps when the normal suction supply from the CAST is not available. Technical Specification (TS) 3.7.5, "Auxiliary Feedwater (AFW) System," requires three CA system trains to be operable in MODES 1, 2 and 3. Furthermore, the TS requires only one CA train, which includes a motor driven pump, to be operable in MODE 4 when the steam generators are relied upon for heat removal.
EVENT DESCRIPTION
On March 1, 2005 the CF system 2B feedwater pump was tripped by procedure.
In addition, the 2A feedwater pump was running in the recirculation mode.
Steam generator levels were being maintained by a condensate booster pump for heat removal. The unit had been held in Mode 3 (Hot Standby) for testing of the Main Steam Isolation Valves (MSIVs). Testing had been completed and the plant was being cooled to Mode 4. Other activities were in progress to support the Unit 2 refueling outage.
On March 2, 2005, Unit 2 was in Mode 3 and Operations was performing the restoration portion of PT/2/B/4250/024, "Feedwater Heater Tube Integrity Test." The purpose of this test is to verify heater tube bundle integrity.
The test is accomplished by isolating and draining the tube side of the feedwater heaters, and aligning the shell side of the heaters to the condenser. Heater tube integrity is verified by a lack of vacuum on the heater tube side. Upon conclusion of the test, high pressure heater strings are backfilled through each feedwater heater strings outlet isolation.
During the backfill of the 2A3 and 2B3 feedwater heater string, the feed pump suction pressure unexpectedly dropped to 184 psig (feed pump trip at 230 psig suction pressure). The introduction of additional flow caused the pressure drop which resulted in a trip of the 2A feedwater pump. With both 2A and 2B feedwater pumps tripped, the logic for automatic start of the 2A and 2B motor driven CA (MDCA) pumps was satisfied and the pumps started as designed.
CAUSAL FACTORS
The cause of this event is attributed to a procedural deficiency.
PT/2/B/4250/024 did not prohibit the restoration portion of the procedure from being performed in Mode 3 or above. The restoration portion has historically been performed in Mode 4 or below with the CA autostart blocked and the CF feedwater pumps shutdown. The performance of this test was rescheduled to Mode 3 during the outage to allow for breaking vacuum earlier. In Mode 3, the MDCA pumps are required to be operable and the autostart function cannot be procedurally blocked. Therefore, with a main feedwater pump running, (although not needed to maintain steam generator levels) and the MDCA pumps unblocked, the unexpected pressure anomaly resulted in the main feedwater pump trip and the automatic start of the MDCA pumps.
CORRECTIVE ACTIONS
Immediate 1. Feedwater Heater Tube Integrity Testing via PT/2/B/4250/024 was suspended.
2. The CA pumps were shutdown and system realignments completed.
Subsequent:
1. Procedures PT/1(2)B/4250/024 were placed on technical hold.
Planned:
1. Procedure PT/1(2)/B/4250/024 will be revised to ensure blocking of the CA autostart signal (in accordance with the Technical Specifications) has been completed prior to performing the restoration section of the test.
SAFETY ANALYSIS
The loss of both Main Feedwater pumps initiated an automatic start of the MDCA pumps as designed. All steam generators levels were maintained at adequate levels to ensure an adequate heat sink for the reactor coolant system.
The risk significance of this event was evaluated using the current version of the McGuire PRA. The conditional core damage probability and conditional large early release probability have been estimated to be approximately 4E 07 and 2E-08 respectively.
Based on this analysis, this event is not considered to be significant. At no time were the health and safety of the public or plant personnel affected as result of this event.
ADDITIONAL INFORMATION
A three year search of the McGuire corrective action database (PIP) and LER database revealed no other automatic starts of the MDCA pumps during testing; therefore, this event is not recurring.