05000250/LER-2003-002-01, Manual Reactor Trip Due to Low Steam Generator Level
| ML032390035 | |
| Person / Time | |
|---|---|
| Site: | Turkey Point  |
| Issue date: | 08/15/2003 |
| From: | Jones T Florida Power & Light Co |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| L-2003-174 LER 03-002-01 | |
| Download: ML032390035 (7) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2502003002R01 - NRC Website | |
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FPL AUG 1 5 2003 L-2003-174 10 CFR § 50.73 U. S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, D. C. 20555 Re:
Turkey Point Unit 3 Docket No. 50-250 Reportable Event: 2003-002-01 Date of Event: January 27, 2003 Manual Reactor Trip due to Low Steam Generator Level The attached supplemental Licensee Event Report 250/2003-002-01 is being submitted pursuant to the requirements of 10 CFR § 50.73(a)(2)(iv)(A) to document the root cause of the event and related corrective actions.
If there are any questions, please call Olga Hanek at (305) 246-6607.
Very truly yours, Terry
. oes Vice President Turkey Point Nuclear Plant OH Attachment cc:
Regional Administrator, USNRC, Region II Senior Resident Inspector, USNRC, Turkey Point Nuclear Plant an FPL Group company
Abstract
On January 27, 2003, with the unit operating at approximately 100% reactor power, Unit 3 was manually tripped due to low steam generator level on the 3C steam generator. The low steam generator level was caused by a momentary loss of running instrument air compressors. The degraded instrument air pressure affected the air-operated main feedwater control valves, causing them to drift closed. The Control Room Operator took prompt procedurally driven actions to reduce power in response to the transient and to manually load the diesel driven instrument air compressors. Steam generator levels decreased, with the 3C steam generator level reaching approximately 12 percent, when the Unit 3 Control Room Operator initiated the manual trip of the unit. Following the manual reactor trip, all three turbine-driven auxiliary feedwater pumps started as expected and instrument air was restored. All control rods fully inserted. The plant was stabilized in Mode 3. This event is reported per the requirements of 10CFR50.73(a)(2)(iv)(A).
The cause of this event was a failure of the 4CM motor driven instrument air compressor, with the 3CM compressor out of service for maintenance. A contributing cause was the failure of the 3CD and 4CD diesel-driven compressors to autostart. Corrective actions included the installation of temporary instrument air compressors. Evaluation of the 4CM motor driven compressor failure and the 3CD and 4CD diesel-driven instrument air compressors failure to autostart was completed and corrective actions were implemented.
NRC FORM 366 (12001)
It-I (If more space Is required use additional copies of (f more space Is required use addlionol copies of NRC Form 36AM) (7) manual turbine power reduction. The 4B steam generator level reached a minimum of 24%. Unit 4 was stabilized at approximately 89% reactor power after IA was recovered. Unit 4 was returned to 100% reactor power at 1400 hours0.0162 days <br />0.389 hours <br />0.00231 weeks <br />5.327e-4 months <br /> on January 27, 2003.
This event is reported per the requirements of 10CFR50.73(a)(2)(iv)(A) due to a manual reactor trip.
Causes of Event 4CM Motor Driven Instrument Air Compressor Failure The cause of this event was a failure of the 4CM motor driven instrument air compressor. The 4CM IA compressor motor grounded. The cause of the failure was a loose sheet metal internal baffle in the motor rubbing against the aluminum rotor fan blades. Constant rubbing created aluminum dust and caused pieces of the baffle to break off.
This led to the winding insulation being abraded to the point that a short circuit developed in the motor when bare copper was exposed. The baffle became loose due to the loss of two of the four #10-24 screws used to secure the baffle. The 4CM IA compressor motor had never been disassembled for maintenance; therefore, the cause of the loose screws is attributed to a manufacturing problem. A review of industry failure history for the 4CM GE motor did not identify any failures. A review of historical plant records revealed that the 4CM motor had operated without any failures since installation in 1996, therefore, the 4CM motor failure is considered a random isolated incident.
Generic Implications The 3CM motor driven instrument air compressor is equipped with a Westinghouse electric motor whose design is not susceptible to the problem experienced by the GE motor. The Westinghouse electric motor was installed in March 2003.
3CD and 4CD Diesel-Driven Compressors Failure to Autostart A contributing cause of the event was the failure of the 3CD and 4CD diesel-driven compressors (CDs) to autostart as designed. These compressors are the installed backup to the 3CM and 4CM IA motor driven compressors. Both compressors received the start signal but failed to autostart. The local annunciator panel indicated compressor low oil pressure.
The 3CD and 4CD diesel-driven IA compressors were assembled by Tide-Air Inc. using Atlas Copco. compressors and Detroit diesels. The autostart failure of the 3CD and 4CD diesel-driven instrument air compressors is attributed to the deficiencies in the design of the control logic scheme, specifically the starting and loading logic.
Three non-standard logic schemes in the start and control logic design were identified. This logic design is intolerant of deviations from ideal starting conditions.
- 1. The start logic of the CDs is a single 15-second crank signal which provides power to the starter when the CDs receive a start signal. During the 15 seconds, the starter will crank the engine until the engine oil pressure reaches the engine low oil pressure reset of 26 PSI. Simulated testing performed as part of the root cause evaluation showed that on several occasions the engine oil pressure reached the 26 PSI reset setpoint and disengaged the starter before the engine was ready to run. The standard signal for standby diesel (if more space s required use addbndl copies of NRC Form 3664) (1 7) engines is a speed-based crank-disconnect switch set at 400 to 600 RPM to ensure the engine is running prior to disengaging the starter.
- 2. The single 15-second crank signal was identified as a logic scheme that can lead to minor engine problems preventing the diesel from starting. The normal setup for cranking controls should provide four to six 15-second crank signals divided by 15-second rest periods. This setup provides the engine with additional crank attempts that would allow a sluggish starting diesel to start. During nine simulated start attempts, the engine cranked for a total of 57 seconds before starting. The normal standard logic design would provide 60 to 90 seconds of cranking with one auto start signal, allowing the diesel engine to start successfully.
- 3. The design of the CDs load permissive allows the compressor to load 5 seconds after engine oil pressure reaches 26 PSI. This design does not insure that the diesel is in a condition where it is ready to accept a load prior to allowing the compressor to load. Simulated testing showed that the engine started and ran sluggishly at low RPMs and loaded while at low RPMs causing the engine to shutdown shortly after due to overload at low RPMs. The normal setup for a load permissive logic is an RPM signal set to the engine speed where the engine is expected to load successfully.
Some contributing causes identified include deficiencies in the CDs diesel-driven fuel system design:
- 1. The fuel supply check valves had minor back leakage causing sluggish engine starts.
- 2. The fuel supply system did not have fuel return relief check valves installed to ensure the system stayed primed.
- 3. The fuel tank design did not include a suction strainer to prevent debris from entering the check valve area.
Debris in the seating area caused the fuel supply check valve back leakage.
Generic Implications The 3CD and 4CD IA compressors were assembled by Tide-Air Inc. using Atlas Copco. compressors and Detroit diesels, and are unique to Turkey Point. The auto-start and load design logic deficiencies of the 3CD and 4CD based on engine oil pressure is unique to the Turkey Point IA system, therefore, there are no generic implications related to this failure.
Turbine Generator Load Limiter A contributing factor to the 3C steam generator reaching 12% was the sluggish response of the turbine generator load limiter which may have delayed the power reduction. The load limiter was inspected and found to have a slight misalignment of load limit motor to load limit controllers, which caused the sluggish response.
Analysis of Safety Significance In the January 27, 2003 event, the momentary loss of the running instrument air compressor affected the main feedwater control valves causing them to drift closed. A manual turbine load reduction was initiated to reduce the steam generator feedwater flow/steam flow mismatch caused by the drift of the steam generator main feedwater control valves. The event was a reduction of normal feedwater flow and is bounded by the total loss of normal feedwater flow event analyzed in the Updated Final Safety Analysis Report (UFSAR)(Section 14.1.11.1).
(7f more space Is requhe4 use additoa copies of NRC Form 36)
(7)
The UFSAR analysis assumes a total loss of normal feedwater to all steam generators, due to the loss of the non-safety feedwater pumps or valve malfunction. A total loss of normal feedwater results in a reduction in capability of the secondary system to remove heat generated in the reactor core. In the January 27 event, feedwater flow was not lost completely; therefore, an automatic turbine runback due to loss of a main feedwater pump did not occur. A manual turbine load reduction was initiated by the reactor operator in accordance with Off-Normal Operating Procedures. All steam generators were affected by the reduction in feedwater flow. In the analysis, the reactor trip is expected to occur due to a Low-Low Level in any steam generator or Steam/Feedwater Flow Mismatch Coincident with Low Level in any steam generator. In the January 27, 2003 event, neither trip occurred, since operator actions manually tripped the reactor prior to reaching the steam generator Low-Low Level setpoint or the Steam/Feedwater Flow Mismatch Coincident with Low Level setpoint in any steam generator. The analysis does not credit turbine runback as a result of the loss of a feedwater pumps or valve malfunction. The analysis shows that following a loss of normal feedwater, the Auxiliary Feedwater (AFW) System is capable of removing the stored and residual heat, thus preventing either overpressurization of the reactor coolant system or loss of water from the reactor core. The analysis also assumes only one AFW pump is available due to a single active failure. In this event the AFW system automatically actuated on steam generator Low-Low level and all three AFW pumps started as designed. The plant's response was bounded by the UFSAR analysis. Thus, this event did not compromise the health or safety of plant personnel or the general public.
Risk Significance A scoping risk assessment assuming loss of instrument air system without restoration yielded a conditional core damage probability of less than 1.OE-6. A more realistic estimate would yield a smaller value as instrument air could be restored. The risk associated with the event was not significant.
Corrective Actions
The following corrective actions have been completed:
- 1. Temporary instrument air compressors, consisting of two diesel driven portable air compressors for each unit, were installed under the Temporary System Alteration process.
- 2. A GE replacement motor was installed in the 4CM motor driven compressor and returned to service.
Before installation, the baffle screws in the motor were secured with Loc-Tite and RTV.
- 3. The main turbine load limiter was inspected, aligned and returned to service.
- 4. Design changes were implemented modifying the cranking and loading logic for the 3CD and 4CD to:
a) insure starter is not disengaged until the engine reaches a certain RPM, eliminating the engine oil pressure permissive for starter engagement from the logic, b) change the single 15 second crank attempt to six 15 second crank attempts divided by 15 second periods of inactivity.
(If mome space Is required use additional copies of NRC Forrn 366A) 1 7)
- 5. Fuel return check valves were installed on the 3CD and 4CD to ensure positive pressure is maintained on the fuel system when the engine is shutdown.
- 6. A fuel supply strainer was installed on the 3CD and 4CD to prevent debris from entering the fuel supply check valve.
- 7. The fuel supply check valves installed in the 3CD and 4CD were replaced to prevent back leakage when the engine is not running.
Additional Information
There are no other events reported related to degraded instrument air pressure.
EIIS Codes are shown in the format EIS SYSTEM: iEE component function identifier, second component function identifier (if appropriate)]. There have been no previous similar events at Turkey Point Units 3 and 4.