ML17221A570

LER 87-007-00:on 871125,reactor Tripped on Loss of Load Due to Turbine Trip Caused by Failure of Main Generator Exciter Bearing.Root Cause Is Personnel Error.Electrical Maint Personnel Trained & Procedures revised.W/871221 Ltr

ML17221A570
Person / Time
Site:	Saint Lucie
Issue date:	12/21/1987
From:	Andrea Johnson, Woody C FLORIDA POWER & LIGHT CO.
To:	NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM)
References
L-87-528, LER-87-007-02, LER-87-7-2, NUDOCS 8712310052
Download: ML17221A570 (6)

Text

REGULATORY INFORMATION DISTRIBUTION SYSTEM (RIDS)

ACCESSION NBR: 8712310052 DOC. DATE: 87/12/21 NOTARIZED: NO DOCKET FACIL:50-389 St. Lucie Plant, Unit 2, Florida Power & Light Co. 05000389 AUTH. NAME AUTHOR AFFILIATION JOHNSON,A.B. Florida Power & Light Co.

WOODY,C.O. Florida Power & Light Co.

RECIP.NAME RECIPIENT AFFILIATION

SUBJECT:

LER 87-007-00:on 871125,reactor trip on loss of load caused b main enerator exciter bearing failure. R W/8 ltr.

DISTRIBUTION CODE: IE22D COPIES RECEIVED:LTR J ENCL SIZE:

TITLE: 50.73 Licensee Event Report (LER), Incident Rpt, etc.

NOTES:

RECIPIENT COPIES RECIPIENT COPIES ID CODE/NAME LTTR ENCL ID CODE/NAME LTTR ENCL PD2-2 LA 1 1 PD2-2 PD 1 1 TOURIGNY,E 1 1 A INTERNAL: ACRS MICHELSON 1 1 ACRS MOELLER 2 2 D AEOD/DOA 1 1 AEOD/DSP/NAS 1 1 AEOD/DS P/ROAB 2 2 AEOD/DSP/TPAB 1 1 ARM/DCTS/DAB 1 1 DEDRO 1 1 NRR/DES T/ADS 1 0 NRR/DEST/CEB 1 1 NRR/DEST/ELB 1 1 NRR/DEST/ICSB 1 1 8 NRR/DEST/MEB 1 1 NRR/DEST/MTB 1 1 NRR/DEST/PSB NRR/DEST/SGB NRR/DLPQ/QAB 1

1 1

1 1

1 NRR/DEST/RSB NRR/DLPQ/HFB NRR/DOEA/EAB 1,L 1

1 1 L

NRR/DREP/RAB 1 1 NRR/DREP/RPB 2 2

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NRR~RXS/+ I B 02 1

1 1

1 NRR/PMAS/ILRB RES DEPY GI 1

1 1

1 RES TELFORD,J 1 1 RES/DE/EIB 1 1 RGN2 FILE 01 1 1 EXTERNAL EG&G GROH g M 5 5 FORD BLDG HOY,A 1 1 H ST LOBBY WARD 1 1 LPDR 1 1 NRC PDR 1 1 NSIC HARRIS,J 1 1 NSIC MAYS,G 1 1 S

A TOTAL NUMBER OF COPIES REQUIRED: LTTR 46 ENCL 45

NRC Form 355 US. NUCLEAR REGULATORY COMMISSION (943)

APPROVED DMS NO. 3)600104 LICENSEE EVENT REPORT (LER) EXPIRES: BI31ISS FACILITY NAME (Il DOCKET NUMBER (2) PA 5 3 St. Lucie, Unit

REACTOR 2 o s o o o 389 ioFO TRIP ON LOSS OF LOAD CAUSED BY MAIN GENERATOR EXCITER BEARING FAILURE DUE TO PERSONNEL ERROR EVENT DATE (51 L'ER NUMBER (6) REPORT DATE I7) OTHER FACILITIES INVOLVED (5)

SEQVSNrrAL MONTH OAY YEAR YEAR rg NUMBER NvxrsEtr MONTH DAY YEAR FACILITYNAMES DOCKET NUMBER(SI N/A o s o o o 11 258 787 007 0 0 1 2 2 1 8 7 0 5 0 0 0 THIS REPORT IS SUBMITTED PURSUANT T 0 THE REOUIREMENTS OF 10 CFR g: IChech oni or mori Of thi,loiiowinpl (11 OPERATING MODE (9) 20.402(bl 20.405(c) 50.734) (2)(lr) 7$ .71BII POWER 20.405 (~ I (1)(0 50M(c)(I), 50.734) (2) (r) 73.7 1(c)

LEVEL (10) 0 5 20AOS(e)(ll(5) 50M(cl(2) 50.7$ (c)(2)(rB) OTHER ISpicify In Ahrtrict INrow enrf in rixt, flite Form 20.405(e I (I I(iti I 50.73(el(2)(II 50.73(e) (2) (rBII(A) 3FSAI 20.405(e)(I ) (lrl 50.73(el l1)(S) 50.73(el(2)4(5)(BI 20.405(e) (1)(r) 50.734)(2) (IB) 50.73(cl(2)lx)

LICENSEE CONTACT FOR THIS LER l12)

NAME TELEPHONE NUMBER AREA CODE A. B. Johnson, Shift Tech. Advisor COMPLETE ONE LINE FOll EACH COMPONENT FAILURE DESCRI ~ ED IN THIS REPORT (13) 3 54 65-3 550 CAUSE SYSTEM COMPONENT MANUFAC EPORTABLE MANUFAC. EPORTABLE TURER TO NPROS CAUSE SYSTEM COMPONENT TURER TO NPRDS A TB EXC 120 N SUPPLEMENTAL REPORT EXPECTED (14) MONTH DAY YEAR EXPECTED SUBMISSION YES IifyrL compiite EXPECTED SUBhriSSIDht DATEI DATE HS)

NO ABSTRACT ILimir to 1400 rprcrL I i., ipproximitiiy fifteen rtnprccpecc typiwritmn iiniri (IS)

On 25 November 1987, St. Lucie Unit Two was operating at 50 percent power steady state in Mode l. At 2331 hours0.027 days <br />0.648 hours <br />0.00385 weeks <br />8.869455e-4 months <br />, the rea'ctor tripped on loss of load due to a turbine trip. The turbine tripped on a main generator lockout. The failure of the main generator exciter bearing caused the armature of the Permanent Magnet Generator (PMG) to grind into its stator. When this occurred, the PMG discontinued supplying voltage to the exciter of the main generator. The trip was uncomplicated and the unit was quickly stabilized in Mode 3, Hot Standby.

The root cause of the event was a personnel error during the performance of the weekly generator exciter ground check on which a prolonged ground resulted in the failure,.of the generator exciter bearing.

The following corrective actions have been implemented: replaced the PMG and the exciter bearing, removed the exciter bearing thermocouple to prevent future grounding occurrence of this type, procedures have been revised to ensure any future grounds are recognized, and training was given to all Electrical Maintenance personnel for better understanding of the exciter ground check.

8712310052 871221 PDR ADOCK 05000389

.DCD NRC Form 355 (9 53 I

RC Form 388A U.S. NUCLEAR REGULATORY COMMISSION (94)3)

LICENSEE EVENT REPORT (LER) TEXT CONTINUATION APPROVEO OMB NO. 3)50&)04 EXPIRES: 8/31/88 FACILITY NAME (I) DOCKET NUMBER (2) LER NUMBER (8) PAGE (3)

YEAR SEQUENTIAL REVISION NUMBER NUMSER St. Lucie, Unit 2 0 5 0 0 0 3 8 9 007 p OF TEXT /lfmore e>>oe /s rer)o/red, ose sddldor>>/NRC Form 36SA's/ ()7)

DESCRIPTION OF EVENT:

On 25 November, 1987, St. Lucie Unit Two was operating at 50 percent power steady state in Mode 1. The unit was returned to service on 23 November following a refueling outage and was holding at 50 percent power while repairs were in progress on the 2A Steam Generator Feed Pump (SGFP) (EIIS:SJ) due to a small water leak that developed on the vent line at the pump casing. The reactor was being maintained in a steady state condition with all automatic controllers

• placed in automatic with the exception of the Control Rod Drive System (EIIS:JD) which was in OFF.

At 2331 hours0.027 days <br />0.648 hours <br />0.00385 weeks <br />8.869455e-4 months <br />, the Reactor Protective System (EIIS:JC) initiated a reactor trip on loss of load due to a turbine trip. The turbine tripped on a main generator (EIIS:TB) lockout. The control room was alerted with information that smoke was seen coming out of the Main Generator Exciter (EIIS:TL). The Assistant Nuclear Plant Supervisor (ANPS) instructed the Turbine Operator (TO) to check for rubs on the rotating components of the turbine-generator. The TO reported evidence of turbine-generator rubs to the control room and the ANPS instructed the Reactor Control Room Operators (RCOs) to open the vacuum breakers on the main condenser (EIIS:SG) to aid in slowing down the turbine.

A fire team was assembled and stationed with fire fighting equipment at the vicinity of the main generator exciter. There were no reports of an actual fire.

The trip was an uncomplicated trip and all systems functioned normally. The Steam Bypass Control System (SBCS) (EIIS:JI) operated to reduce primary average temperature (T-avg) to the zero percent power setpoint of 532 degrees F. Auxiliary feedwater (EIIS:BA) was initiated manually to control the steam generator (S/G) level for Reactor Coolant System (RCS) (EIIS:AB) heat removal. The standard post trip actions were completed and the unit was immediately stabilized in Hot Standby, Mode 3.

NRC FORM 388A (94)31

NRC Form 388A U.S. NUCLEAR REGULATORY COMMISSION (BS)3)

LICENSEE EVENT REPORT (LER) TEXT CONTINUATION APPROVED OMB NO. 3)50-0104 EXPIRES: 8/31/88 FACILITY NAME (11 DOCKET NUMBER (2) LER NUMBER (8) PAGE (3)

SEQUENTIAL REVISION YEAR @o NUMBER NUMBER St. Lucie, Unit 2 O S O 0 O 8 7 0 7 0 0 3 OF 0 4 TEXT /I/ more sosseis rer/Ir/red, Iree edd/dorM///RC Form 3//SA's/ (17)

CAUSE OF THE EVENT:

The immediate cause of the main generator lockout was loss of the generator field. The loss of field was caused by destruction of the exciter Permanent Magnet Generator (PMG) that occurred when the exciter bearing failed allowing the PMG armature to rub on the PMG stator.

Further investigation into the failure of the exciter bearing indicated that a ground was caused by a completed electrical circuit between the exciter bearing and the exciter base. This completed electrical circuit bypassed the insulation between the bearing pedestal and the exciter base. The ground was caused by an improper installation of an instrument cable (EIIS:IT). The instrument cable provides the terminal connection from the thermocouple of the I)9 exciter bearing housing to its terminal box, which provides remote temperature indication in the control room. The as-found condition of the instrument cable indicated that the braided wire cable shield was not properly positioned in the clamp; therefore, a ground developed at the exciter bearing. Examination of the exciter bearing revealed pitting marks on the bearing surface, characteristic of current flowing across the bearing journal to the bearing pad, which resulted in the bearing failure.

The improper arrangement of the instrument cable cannot be conclusively identified as a wiring error during the installation of the new main generator exciter.

The PMG and the instrument cable were meggered to detect for possible grounds and the results were acceptable. It has been postulated that one of two circumstances may have occurred: 1) the instrument cable was installed correctly with the braided wire shield in the clamp when it the instrument cable was accidently stepped on causing the cable shield to was meggered; then at some later date, become separated from the clamp; or 2) the instrument cable, being clamped incorrectly to the pedestal, had not yet been connected to the terminal box

. when the megger test was performed.

After placing the main generator back in service, the ground should have been detected during the performance of the weekly generator exciter ground check.

The preventative maintenance Plant Work Order (PWO) requires voltages to be read across the O9 bearing shaft to ground and across the I)9 bearing pedestal to ground. The data is to be evaluated to ensure no grounds have developed in the main generator exciter. Early recognition of the ground would have prompted immediate action to remove the main generator from service for the necessary repairs.

The root cause of the event was a cognitive personnel error by a utility main-tenance supervisor in misinterpretation of the main generator exciter ground voltage readings. The appropriate procedure was properly followed. However, lack of understanding of the significance of the generator exciter ground check and inadequate acceptance criteria in the PWO were contributing factors in the personnel error. "There were no unusual characteristics 'of the work location that directly contributed to the personnel error.

NRC FORM 3BBA (9831

NRC Form 366A U.S. NUCLEAR REGULATORY COMMISSION (9S)3)

LICENSEE EVENT REPORT (LER) TEXT CONTINUATION APPROVED OMB NO. 3(SOW104 EXPIRES: 8/31/88 FACILITY NAME (1) DOCKET NUMBER (3) LER NUMBER (6) PAGE (3)

SSOUSNTIAL gC)r REVISION St. Lucie, Unit 2 YEAR go NUMBER NUMSSR o s o o o 3 8 7 0 0 7 0 0 0 4 OF TEXT /// mors spsso/s>>r/u/rsd, u>> sdd/oooo/ HRC Form SFs/ (17)

ANALYSIS OF THE EVENT:

The event is reportable under 10 CFR 50.73 (a)(2)(iv), "any event or condition that resulted in manual or automatic actuation of any Engineered Safety Feature, including the Reactor Protection System."

This event was observed to be a routine reactor trip on loss of load. The resulting transient was well enveloped by the St. Lucie Unit I2 Final Updated Safety Analysis Report section 15.2.1.2 "Limiting Reactor Coolant System Pressure Event-Isolation of Turbine (100% power)." All Plant Safety Functions were met and there were no additional complications. Consequently, the health and safety of the public were not affected by this event.

CORRECTIVE ACTIONS:

1. The Permanent Magnet Generator was replaced.

2. The Exciter Bearing was replaced.

3. An evaluation was made and the Exciter Bearing thermocouple was determined to be unnecessary and it was removed to prevent future incidents of this type.

4. The preventative maintenance plant work order was revised to adequately define the acceptance criteria and the proper actions when the acceptance criteria are not met.

5. Training was given to all Electrical Maintenance personnel to elaborate on the importance of properly evaluating the data during performance of the weekly generator exciter ground check.

6. The plant training group will evaluate this item to determine appropriate training requirements and methods.

NRC FORM 366A (943)

P. O f4000, JUNO BEACH, FL 33408 0420 OEGD(IBER 2 1 L-87-528 10 CFR 50.73 U.S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, D.C. 20555 Gentlemen:

Re: St. Lucie Unit 2 Docket No. 50-389 Reportable Event: 87-07 Date of Event: November 25, 1987 Reactor Trip on Loss of Load Caused by Hain Generator Exciter Bearin Failure. due to Personnel Error The attached Licensee Event Report (LER) is being submitted pursuant to the requirements of 10 CFR 50.73a to provide notification on the subject event.

Yery truly yours, cg C. 0. dy Executive Yice President COW/GRH/cn: H/011 Attachment cc: Dr. J. Nelson Grace, Regional Administrator, Region II, USNRC Senior Resident Inspector, USNRC, St. Lucie Plant r@ g~

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