05000389/LER-2003-001
| St. Lucie Unit 2 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3892003001R00 - NRC Website | |
Description of the Event
On April 1, 2003, St. Lucie Unit 2 was in Mode 1 operation at 100 percent power. At approximately 1500 hours0.0174 days <br />0.417 hours <br />0.00248 weeks <br />5.7075e-4 months <br />, control room operators noted that the backpressure in the St. Lucie Unit 2 main condenser was slowly increasing. The off-normal operating procedure for loss of condenser vacuum was implemented and operators attempted to recover condenser vacuum by placing the 2A hogging ejector [EIIS:SH:EJR](hogger) in service. However, the 2A hogger did not appear to be operating properly, and the 2B hogger was placed in service. While securing the 2A hogger, the operators performed a manual reactor trip at approximately 1603 hours0.0186 days <br />0.445 hours <br />0.00265 weeks <br />6.099415e-4 months <br /> when condenser backpressure reached the procedurally defined limit.
The plant was stabilized in Mode 3 with feedwater supplied by the main feedwater system. All safety related equipment responded to the reactor trip as designed with the exception of the steam-driven 2C auxiliary feedwater (AFW) pump [EIIS:BA:P:TUR].
The pump tripped on an invalid mechanical overspeed trip.
Cause of the Event
The cause for the manual reactor trip was the reduction of main condenser vacuum.
The vacuum reduction was caused by a combination of increased air in-leakage to the condenser via a through-wall hole in the steam jet air ejector piping and the degraded function of the 2A condenser hogger. The through-wall hole was caused by corrosion.
The dissolved oxygen levels on Unit 2 had been elevated for several days prior to the event, and FPL had an on-going investigation into the cause of the increased dissolved oxygen. Shortly prior to the event, insulation had been removed from a section of piping near the loop seal between the steam jet air ejector intercondenser and the main condenser and a significant corrosion cell was observed in this area.
Chemistry personnel were notified to investigate the corrosion cell as a potential source of air in-leakage. It was subsequently determined that a through-wall hole had been exposed in this piping.
Shortly after discovery of a through-wall hole in the air ejector system line, operators observed a slowly increasing trend in condenser backpressure. The operators attempted to recover vacuum in the condenser by placing the hoggers in service per procedural direction, but condenser backpressure continued to increase.
The manual reactor trip was implemented when the administrative limit was reached.
Following the plant trip, FPL determined that the hole in the loop seal piping between the steam jet air ejector intercondenser and the main condenser was approximately 1" x 1/2" in size. Additionally, the event investigation revealed that a local steam pressure gauge, PI-12-48A [EIIS:SH:PI] used to manually adjust the steam pressure supply to the 2A hogger, was out of calibration by 80-85 psig. This resulted in an actual steam pressure of approximately 115 psig at the ejector when the gauge was indicating 200 psig. At pressures below 200 psig, hogger performance rapidly declines. Therefore, instead of drawing vacuum from the main condenser, the 2A hogger acted as a vacuum breaker on the main condenser. Placing the 2B hogger was not effective because the suction piping was cross connected with the degraded 2A hogger. This lineup allowed the degraded 2A hogger to act as a vacuum breaker on the 2B hogger. Therefore, the 2B hogger was unable to draw a vacuum on the main condenser.
Based on a review of the probable causes and inspection of the 2C AFW pump mechanical trip devices, the most likely cause of the mechanical overspeed trip was excessive wear of the overspeed trip mechanism. The overspeed mechanism was reworked.
Analysis of the Event
This condition is reportable under 10 CFR 50.73(a)(2)(iv)(A) as an event that resulted in manual actuation of the reactor protection system (RPS).
Analysis of Safety Significance With the exception of the 2C AFW pump, all safety related equipment responded to the manual reactor trip as designed. The electric-driven AFW pumps were available if required. Steam generator water levels were maintained with the main feedwater system post-trip. Therefore, the manual reactor trip had no adverse impact on the health and safety of the public.
Corrective Actions
1. The loop seal piping between the steam jet air ejector intercondenser and the main condenser was replaced during the Spring 2003 St. Lucie Unit 2 refueling outage under work order (WO) 33006353.
2. The 2A pressure gage was replaced under WO 33006364.
3. The St. Lucie Unit 2 hogging ejectors were inspected during the Spring 2003 St.
Lucie Unit 2 refueling outage under WO 33006358 with no significant items noted.
4. St. Lucie is evaluating the current planned/preventative maintenance practices for the hogging ejectors.
5. The 2C AFW overspeed mechanism was repaired under WO 33006356.
Additional Information
Failed Components Identified Component:
� Local steam pressure gage PI-12-48A Manufacturer: Helicoid Gage Co.
Model: � 440R Component:
� Auxiliary feedwater pump turbine Manufacturer: Dresser Industries Model: � GS-2N (103323E) Similar Events None