ML20029C868

LER 94-003-00:on 940329,automatic Reactor Shutdown Occurred Due to Trip of Main Turbine.Caused by Loss of Main Generator Field Excitation.Damaged Main Generator Exciter Rotor replaced.W/940425 Ltr

ML20029C868
Person / Time
Site:	Hatch
Issue date:	04/25/1994
From:	Beckham J, Tipps S GEORGIA POWER CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
HL-4572, LER-94-003-02, LER-94-3-2, NUDOCS 9405020227
Download: ML20029C868 (6)

Text

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ooorgia Power Company 40 inverness Center Parkwa,

. Post Otfice Dox 1295

,o lumingham, Alat>ama 35201 l Telephono 205 877 7279 m

J. T Deckham,Jr.

Vice Prendent Nuclear Georgia' Power Hatch Projwt (H> % utwin<M:rn w tem April 25, 1994 Docket No. 50-321 IIL-4572 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, D.C. 20555 Edwin I. Hatch Nuclear Plant - Unit 1 Licensee Event Report Arcing in Main Generator Exciter Results in Automalig Reaclor Shutdp_w_a Gentlemen:

In accordance with the requirements of 10CFR50.73 (a)(2)(iv), Georgia Power Company is submitting the enclosed Licensee Event Report concerning arcing in the main generator exciter which resulted in an automatic reactor shutdown. This event o .urred at Plant IIatch - Unit 1.

Sincerely, 9

J. T. Beckham, Jr.

JKB/cr

Enclosure:

LER l-94-003 cc: Georgia Power Company Mr. L, II. Sumner, General Manager - Nuclear Plant NORMS U.S. Nuclea_r]hgulatory Commission. Washingk1I).C.

Mr. K. Jabbour, Licensing Project Manager - Ilatch U.S. Nuclear Reen!atory Commission. Region H Mr. S. D. Ebneter, Regional Administrator Mr. L. D. Wert, Senior Resident inspector - Ilatch nn ,

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NMC FORJ1364 U.S. NUCLEAR REGULATORY COMMi&blON APPROVED OMS No. S150 0104 (5 92) EXPIRES' 6t3119$

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Edwin I. Ilatch Nuclear Plant - Unit 1 0l5l0l0l0l3l2l1 1 OF 5 TITLE Arc (4)ing in Main Generator Exciter Results in Automatic Reactor Shutdown EVENT DATE (Sl LER NUMBER (6) l REPORT DATE (7) OTHER F ACIUTIES INVOLVED (8)

MONTH oAv YEAR YF a ; sey4pt .aavg'l MOiGH DAY YEAR F ACiUTY NAME DOCisET NUMBER (S) ol5lololol l'I F ACluiY NAME 0 13 2 19 9 14 9 14 0 l013 O l0 014 2 l5 9 14 olslololol l l THIS REPORT IS SUOMIITED PURSUANT TO THE REQU4MEMENTS OF 10 CFR 7 :(Check one or more of she Awowmp)(11)

OPERAflNG MODE (9) } 20 402(b) 20 405(c) X SO 1Ma)Q(w) 7311(b)

POWER 20 405(a)(1}{i) 50 96(c)(1) 50 73(a)QM 73 71(c)

LEVEL (10) 1 10 l0 20 Amiu mam)m w 73mm(-) 01HER (3,.c, m A3 ,ec, .eu,,

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NAME IELEPHONE NUMBER (mclude area code)

ARE A COOL Steven B. Tipps, Nuclear Safety and Compliance Manager 91112 31617 l- 1718 I 5 l1 COMPLE TE ONE UNE FOR EACH COMPONENT F ALLURE DESCRISED IN THl$ REPOHi(13)

CAUSE SYSTE M COMfCNENT MANUF ACTURER REPORTABLE +

CAUSE SYSTEM COMPONENT MANUFACTURER REPORTABLE TO NPRDS s. TO NPROS X TIL I II Ul01315 Yes , %; l i l l l l l l 1 II II I I I I I I I SUPPLEMENT AL REPORT EXPECTED (14) EXFtCIED MONTH DAY YEAR SUBM $$10N YES (F yos, compMo fMYCTE D SUBM/$$tCW D4 TO X NO DATE (15) l l l ASS 1HAGY (Lomst u 14tc spaces, o e, approermetely 13 smgWapace tysewntren knes)(16)

On 3/29/94 at 0101 EST, Unit I was in the Run mode at a power level of 2436 CMWT (100 percent rated thermal power). At that time a full Reactor Protection System (RPS) actuation and an automatic reactor shutdown occurred due to a trip of the Main Turbine. When the Main Turbine tripped, reactor pressure increased as expected to a maximum of1085 psig and was relieved by the artomatic opening of the Main Turbine Bypass Valves and seven safety relief valves (SRVs) per-design. The SRVs closed as required and reactor pressure was then controlled by the Main Turbine Bypass Valves. Reactor water level decreased to a minimum of 14 inches above instrument zero

~(172 inches above the top of active fuel), resulting in closure of the inboard Group 2 Primary Containment Isolation System valves. Normal reactor water level was restored with the Feedwater System. The cause of this event was a loss of Main Generator field excitation leading to a trip of the Main Turbine and an automatic reactor shutdown. Field excitation was lost when arcing occurred between the Main Generator Exciter (MGE) brush rigging and one of the collector rings on the MGE rotor. Engineering personnel believe the primary factor in the arcing was sticking of a brush in the brush rigging.

Corrective actions for this event included replacing the damaged MGE rotor with a rotor from the.

Unit 2 MGE, sending the Unit 1 MGE rotor off site for repair, and increasing the frequency of MGE brush inspections to daily.

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YEAR SEQJENYlAL ,: REY $2 Edwin I. Hatch Nuclear Plant - Unit 1 m1e-.. n a,a.-.aw n a mco-,,,m m, 015 l0 l0 l0 l3 l2 l1 9 l4 0 l0 l3 0l0 l2 oF l5 PLANT AND SYSTEM IDENTIFICATION I General Electric - Boiling Water Reactor Energy Industry identification System codes appear in the text as (Ells Code XX). I I

DESCRIPTION OF EVENT 4

On 3/29/94 at 0101 EST, Unit I was in the Run mode at a power level of 2436 CMWT (100 percent rated thermal riower). At that time Unit I experienced a full Reactor Protection System (RPS, EIIS Code JC) act sation and an automatic reactor shutdown following a trip of the Main Turbine (Ells Code TA) on a Main Generator (EIIS Code TB) sensed loss of field excitation. When the Main Turbine tripped, reactor pressure increased as expected resulting in the Main Turbine Bypass Valves automatically opening, seven of the eleven Safety Relief Valves (SRVs) opening, and Low-Low Set (LLS, Ells Code JE) actuating. The SRVs closed as required afler relieving the initial pressure transient and reactor pressure was controlled thereafler by the Electrohydraulic Control (EHC, Ells Code TG) system and the Main Turbine Bypass Valves. Reactor pressure reached a maximum during the transient of 1085 psig. Following the reactor shutdown, reactor water level decreased as expected due to steam void collapse resulting from the pressure increase and the rapid power reduction produced by control rod (Ells Code AA) insertion and trip of the Reactor Recirculation (Ells Code AD) Systern pumps. Water level was recovered by use of the Reactor Feedwater Pumps (RFPs, EIIS Code SK). The minimum water level reached was 14 inches above instrument zero (172 inches above the top of active fuel) which is very close to the setpoint of the Level 3 instrumentation for the Group 2 Primary Containment Isolation System (PCIS, EIIS Code JM) valves. Thus the inboard PCIS valves received a closure signal and responded as required.. The outboard Group 2 PCIS valves did not close because not all their isolation logic was tripped during the event. Normal variations in sensed level arising from calibration tolerances account for the fact that not all the Level 3 instruments for inboard and outboard PCIS valves actuated simu'taneously.

When the transient occurred, licensed shift personnel confirmed all expected actuations occurred as designed per procedure 34AB-C71-001-1S, " SCRAM PROCEDURE " At 0114 EST the 'A' loop of the Reactor Recirculation System was restarted to prevent thermal stratification of the reactor coolant, and at 0121 EST the 'B' loop was started also. Following other necessary actions to ensure plant conditions had stabilized, shift personnel entered procedure 34GO-OPS-013-1S, " NORMAL PLANT SHUTDOWN" at 0150 EST and continued placing plant systems in lineups appropriate for the Hot Shutdown condition.

NRC FORJ 366A U S. huCLEAR e SOULATORY COMMISSION APPROVED OMB NO 3160 0104

. (5 92) EXPIRES: 5(31/95 ESTIMATED BURDEN PER REF.PONSE TO COMPLY WITH THIS It4 FORMATION COLLECTION REQUEST 50 0 HRS FORWARD

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0001. AND TO THE PAPERWORK REDUCTION PROJECT (31500104).

Of FICE OF MANAGEMENT AND BUDGET. WASHINGTON DC 20503 PACILITY NAME (1) DOCFET NUMt1ER (2) LER NUMBER (6) PAGE (3)

YEAR NU B R Edwin I. Ilatch Nuclear Plant - Unit 1 1kXl (R thwh specAF se f9quvost use Orkithonal Cope $ Of NHC ham 360AX17) 0 l5 l0 l0 l0 l3 l211 9 l4 0 l0 l3 0l0 l3 oF l$

CAUSE OF EVENT The cause of this event was a loss of Main Generator field excitation leading to a trip of the Main Turbine and an automatic reactor shutdown. Main Generator field excitation was lost when arcing occurred between the exciter brush rigging and one of the collector rings on the rotor shan of the Main Generator Exciter (MGE, Ells Code TL). Engineering personnel who investigated the event believe the primary reason for the arcing was a carbon brush sticking in its rigging. Normally, brushes must be free to move to maintain electrical contact while accommodating motion due to normal machine vibration, slight collector ring eccentricity, and brush wear. It is believed that when the brush became lodged in its rigging, an air gap developed between the brush and the collector ring, leading to a small amount ofinitial arcing. This arcing roughened the surface of the collector ring which damaged all the other brush contact surfaces on the involved ring and led to a phenomenon of much more intense arcing known as "Dashover." When Dashover occurred in the MGE, excitation was lost which actuated the Main Generator loss of excitation relay, resulting in a Main Turbine trip and a full RPS actuation.

BEPQRTABILITY ANALYSIS AND SAFETY ASSESSMENT This event is reportable per 10 CFR 50.73 (a)(2)(iv) because unplanned actuations of engineered safety features (ESFs) occurred, including an unplanned automatic reactor shutdown following a trip of the Main Tmbine on a loss of Main Generator field excitation.

Loss of field excitation in the Main Generator is sensed by a device which signals the Main Turbine to trip and the Main Generator Power Circuit Breakers (Ells Code EL) to open. The Main Turbine stop valves and control valves close rapidly to prevent overspeed of the Main Turbine-Generator.

Rapid closure of these valves causes a sudden reduction in steam flow which results in the nuclear system pressure increasing and the Main Turbine Bypass Valves fully opening. To limit the transient on the nuclear system, the Main Turbine trip signalis an input to the Reactor Protection System which initiates an automatic reactor shutdown. Rapid power reduction is further ensured by an automatic trip of the Reactor Recirculation System pump motor breakers whenever a Main Turbine trip occurs at reactor power levels of 30 percent or greater. The Final Safety Analysis Report (FS AR) assume 3 that an event involving a Main Turbine trip at rated power will result in nuclear system pressures sufficient to lin some or all of the SRVs. Following an initial relief of steam through the SRVs to the suppression pool, reactor pressure is controlled by discharging steam through the Main Turbine Bypass Valves to the Main Condenser (EIIS Code SG).

NRC DORQ 3664 U.S. NUCLEAR REQULATORY COMMi&SION LPPROVED OMS NO. 31b104 (5 62) EAPIRES: SI31/93 INF M TON LEC REQU ST 0 $ W LICENSEE EVENT REPORT (LER) CouggRgARg9ggEsmAgTO HE,, wari u TEXT CONTINUATION gugE,A R COu gNyG. De OFFICE OF MANAGEMENT ANO BUDGET, WASHINGTON, DC 20503 FACfLl1Y NAME (1) Ducp.ET NUMBER (2) LER NUMBER (6) PAGE (3)

VEAR y SEQU NTLAL , R St N Edwin I. Hatch Nuclear Plant - Unit 1

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0l0 l4 OF l5 In this event, all plant systems responded as designed to a Main Generator loss of excitation, including the RPS, the Recirculation System Pump trip (or, RPT), the SRVs and LLS logic, the EHC System, and the Bypass Valves. The closure of some Group 2 PCIS valves is explained by the reactor water level transient in which water level decreased to the point where some, but not all, Level 3 instrumentation tripped. The High Pressure Coolant Injection (HPCI, EIIS Code BJ) system did not inject to the vessel during this event, nor was it required to since reactor water level remained well above the Level 2 initiation setpoint at all times during the transient. Water level was controlled by the RFPs and normal Feedwater Level Control. There were no unusual, unexpected, or unexplained occurrences associated with this transient. The minimum reactor water level observed during the event was 172 inches above the top of active fuel, and the maximum reactor pressure was 1085 psig. Both of these are well within the limits of current safety analyses. Had a design basis accident occurred coincident with this event, all systems assumed by current accident analyses to function would have responded per design.

Based on this analysis, it is concluded that this event had no adverse impact on nuclear safety. This analysis is applicable to all power levels.

i CORRECTIVE ACTIONS s

l

1. The MGE rotor from Unit 2, which is currently in a refueling outage, was installed in Unit 1,

and Unit I was returned to full power operation.

1

2. The rotor from the involved Unit i exciter has been sent off site for repair.

3. The frequency of brush inspections has been changed from monthly to daily on both units' Main Generator Exciters.

ADDITIONAL INFORMATION

1. Other Systems AtTected: No systems were afTected by this event othe than those already mentioned in this report.

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'e' 48 NRC PC.Q 396A U.S. NUCLEJJt AEOULATORY COMMIS2 WON APPROVED OMJ NO.31604104 m ($47) EXP1RES: 6t31196 ESTIMATED BURDEN PER RESPONSE TO COMPLY %. .TH THIS INFORMATION COLLECTION RFQUEST 50 0 HR$ FORWARD CouMENTs RecARDiNo BURDEN EsTiuArE ToinE iesORMATiON LICENSEE EVENT REPORT (LER) AND RECORDS MANAGEMENT BRANCH (MNS87714) US TEXT CONTINUATION NUCLEAR REGULATORY COMMISSIOtt WASHINGTON. DC 20$$$.

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F ACIUTY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3)

VEAR ^ SFQUENTIAL REvlssON YEAR NUMBER Edwin I. Hatch Nuclear Plant - Unit 1 1LX1 (M true space en requeens, use a:kMounet cupee c4 hrHC fam 366AX1!)

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2. Failed Equipment Information:

Master Parts List Number: IN51-S001 EIIS System Code: TL Type: Carbon Brush EIIS Component Code: None Manufacturer: Union Carbide Root Cause Code: X Model Number: 634 Reportable to NPRDS: Yes Manufacturer Code: UO35

3. Previous Similar Events: No events have been reported in the past two years in which automatic reactor shutdowns resulted from failures in the Main Generator or switchyard.

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