ML18038B112

LER 94-013-01:on 941202,Unit 2 Received Full Automatic Scram from 54% Reactor Power.Caused by Mechanical Degradation of Stator Cooling Water Temp Switch.Affected Systems Restored to Operable Status & Temp Switch Replaced Before Restart

ML18038B112
Person / Time
Site:	Browns Ferry
Issue date:	01/19/1995
From:	Austin S TENNESSEE VALLEY AUTHORITY
To:
Shared Package
ML18038B111	List: ML18038B110 ML18038B112
References
LER-94-013, LER-94-13, NUDOCS 9502010030
Download: ML18038B112 (12)

Text

NRC FORM 366 .S NUCLEAR REGULATORY C(HNIISSI ON OVED BY ae NO. 3150-0104 (5-92) EXPIRES 5/31/95 ESTIMATED BURDEN PER RESPONSE TO COMPLY WITH THIS INFORMATION COLLECTION REQUEST: 50.0 HRS ~

LICENSEE EVENT REPORT (LER) FORWARD COMMENTS REGARDING BURDEN ESTIMATE TO THE INFORMATION AHD RECORDS MANAGEMENT BRANCH (MHBB 7714), U.S. HUCLEAR REGULATORY COMMISSIONS (See reverse for required nunber of digits/characters for each block) WASHINGTON, DC 20555-0001 AND TO THE PAPERWORK REDUCTION PROJECT (3140-0104), OFFICE OF MANAGEMENT AND BUDGET WASHINGTON DC 20503.

FACILITY NAME (1) DOCKET NWBER (2) PAGE (3)

Browns Ferry Nuclear Plant (BFN) Unit 2 05000260 1OF6 TITLE (4) Unit 2 Scram From 54 Percent Power Caused By Balance Of Plant Equipment Failure EVENT'ATE 5 LER.NINBER 6 REPORT DATE 7 OTHER FACILITIES INVOLVED 8 SEQUENTIAL REVISION FACILITY NAME DOCKET NUMBER

'EAR DAY YEAR

'2 MONTH DAY YEAR MONTH , NA NUMBER HUMBER, FACILITY NAME DOCKET NUMBER 12 94 94 013 01 01 19 NA OPERATING THIS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10 CFR: Check one or more 11 NSE (9) N 20.402(b) 20.405(c) 50.73(a)(2)(iv) 73.71(b)

POWER 20.405(a)(1)(i) 50.36(c)(1) 50.73(a)(2)(v) 73.71(c)

LEVEL. (10) 54 20.405(a)(1)(ii) 50.36(c)(2) 50.73(a)(2)(vii) OTHER 20.405(a)(1)(iii) 50.73(a)(2)(I) 50.73(a)(2)(viii)(A) (Specify in 20.405(a)(1)(iv) 50.73(a)(2)(ii) 50.73(a)(2)(viii)(B) Abstract below and in Text, 20.405(a)(1)(v) 50.73(a)(2)(iii) 50,73(a)(2)(x) NRC Form 366A LICENSEE CONTACT FOR THIS'LER 12 NAME TELEPHONE NUMBER (Include Area Code)

Steven W Austin, Compliance Licensing Engineer (205)729-2070 C(NIPLETE ONE LINE FOR EACH CQ0%NENT FAILURE DESCRIBED IN THIS REPORT 13 SYS 'REPORTABLE REPORTABLE CAUSE COMPONENT MANUFACTURER CAUSE SYSTEM'OMPOHEHT MANUFACTURER TO NPRDS TEM TO HPRDS X JJ TIS M235 N SUPPLEMENTAL REPORT EXPECTED 14 EXPECTED MONTH DAY YEAR YES SUBM I SS I ON (If yes, complete EXPECTED SUBMISSIOH DATE) ~

X No DATE (15)

ABSTRACT (Limit to 1400 spaces,, i.e., approximately 15 single-spaced typeMritten lines) (16)

On December 2, 1994 at 0717 hours0.0083 days <br />0.199 hours <br />0.00119 weeks <br />2.728185e-4 months <br />, Unit 2 received a full automatic scram from 54 percent reactor power. The scram was generated by a main generator turbine trip with the first stage turbine pressure greater than 154 psig. The turbine trip was caused by an invalid loss of main generator stator cooling, water signal. The scram resulted in a low reactor water level which caused the isolation/actuation engineered safeguards (ESF) and reactor protection (RPS) systems. The affected ESF and RPS systems were returned to standby readiness by 0729 hours0.00844 days <br />0.203 hours <br />0.00121 weeks <br />2.773845e-4 months <br />. The root cause of the event was mechanical degradation of the Main Generator Stator Cooling Water system temperature switch. The switch experienced setpoint drift which caused it to prematurely actuate. TVA will issue a procedure for calibration of the stator cooling water temperature switch. Additionally, TVA will evaluate balance of plant devices capable of causing turbine generator trips and ensure that the procedural guidance for calibration is adequate. Finally, TVA will evaluate other balance of plant devices to see if due to its age, replacement is required. TVA is reporting this event in accordance -with 10 CFR 50.73 (a)(2)(iv), as any, event or condition that resulted in manual or automatic actuation of any ESF including the reactor protection system.

HRC FORM 366 S'DR 9502010030 950119 PDR (5-92)

ADOCK 05000260

il S

NRC FORM 366A U.S. NUCLEAR REGULATORY CQIISSION APPROVED BY (NGI NO. 3150-0104 (5-92) EXPIRES 5/3'I/95

, ESTIMATED BURDEN PER RESPONSE TO COMPLY WITH'HIS INFORMATION COLLECT ION REQUEST: 50.0 HRS. FOR'WARD COMMENTS REGARDIHG BURDEN ESTIMATE TO THE INFORMATION. AND RECORDS MANAGEMENT BRANCH (MNBB 7714), U.S ~ NUCLEAR REGULATORY COMMISSION, LICENSEE EVENT REPORT WASHINGTON, DC 20555-0001, AND TO THE PAPERWORK REDUCTION TEXT CONTINUATION PROJECT (3150-0104), OFFICE OF MANAGEMENT AND BUDGET, WASHINGTON DC 20503 FACILITY NAME (1) DOCKET HINBER (2) LER IRNBER (6) PAGE (3)

YEAR, SEQUENTIAL REVISION NUMBER NUMBER Browne Ferry Unit 2 05000260 94 013 01 2 of 6 TEXT If more s ce is r uired use additional co ies of NRC Form 366A (17)

Zo PLANT CONDITIONS Unit 2 was at approximately 54 percent power (1763 megawatts thermal).

Units 1 and 3 were shutdown and defueled.

IZo DESCRIPTION OF EVENT Ao Event On December 2, 1994 at 0717 hours0.0083 days <br />0.199 hours <br />0.00119 weeks <br />2.728185e-4 months <br />, Unit 2 received a full automatic scram from 54 percent reactor power. The scram was generated by a main generator turbine [TB] trip with the first stage turbine pressure greater than 154 psig. The turbine trip was caused by an invalid loss of main generator stator cooling water signal [JJ]. The scram resulted in a low reactor water level. signal of which caused actuation or isolation of the following Primary Containment Isolation System [JE](PCIS) systems/components.

~ PCIS group 2, Shutdown cool'ing mode of Residual Heat Removal

[BO) system; Drywell floor drain isolation valve, Drywell equipment drain sump isolation valve [WP]

~ PCIS group 3, Reactor Water Cleanup [CE)

~ PCIS group 6, Primary Containment Purge and Ventilation

[JM); Unit 2 Reactor Zone Ventilation [VB]; Refuel Zone Ventilation [VA]: Standby Gas Treatment (SGT) [BH) system; Control Room Emergency Ventilation (CREV)[VI]

~ PCIS group 8, Transverse Incore Probe [IG] withdrawal On December 2, 1994, at approximately 0716 hours0.00829 days <br />0.199 hours <br />0.00118 weeks <br />2.72438e-4 months <br />, the stator cooling system [TJ] alarm was received in the Unit 2 Main Control Room. Approximately 10 seconds later a stator cooling failure alarm was received. Control room indication did not identify any stator cooling water problems. The Unit Operator (UO), utility licensed, initiated a load reduction on the stator.

Additionally, an Assistant Shift Operations Supervisor (ASOS),

utility licensed,investigate.

was dispatched to the stator cooling water alarm panel to Approximately 70'econds after the initial alarm the main generator [TA) and main turbine tripped, followed immediately by a reactor scram.

The affected systems were returned to standby readiness by 0729 hours0.00844 days <br />0.203 hours <br />0.00121 weeks <br />2.773845e-4 months <br />. All systems responded as expected during the reactor scram.

0 FL; C,

NRC FORM 366A U.S. NUCLEAR REGULATORY APPROVED BY (%8 NO. 3150-0104 (5-92) EXPIRES 5/31/95 CQtlISSION'ICENSEE ESTIHATED BURDEN'PER.RESPONSE TO COHPLY WITH THIS INFORMATION COLLECTION REQUEST: 50.0 NRS. FORWARD COHHENTS REGARDING BURDEN EST IKATE TO THE INFORHAT ION AND RECORDS HANAGEKENT BRANCH (HNBB 7714), U.S. NUCLEAR REGULATORY COHHISSION, EVENT REPORT WASHINGTON, DC 20555.0001, AND TO THE PAPERWORK REDUCTION TEXT CONTINUATION PROJECT (3150-0104), OFFICE OF HANAGEMENT AND BUDGET, WASHINGTON DC 20503 FACILITY NAME (1) DOCKET NWBER (2) LER NINBER (6) PAGE (3)

YEAR SEQUENTIAL REVISION NUHBER NUMBER Browns Ferry Unit 2 05000260 '94 013 01 3 of 6 TEXT If more s ace is r ired use additional co ies of NRC Form 366A (17)

This event is reportable in accordance with 10 CFR 50.'73 (a)(2)(iv), as any event or condition that resulted in manual or automatic actuation of any engineered safety feature including the reactor protection system.

B~ Ino erable Structures Com onents or S stems that Contributed to the Event:

None.

C~ Dates and A roximate Times of Ma'or Occurrences:

December 2, 1994 at 0717 CST 'The Unit 2 Reactor received a full scram from a main turbine trip.

December 2, 1994 at 0729 CST , The PCIS actuations were reset.

SGT and CREV systems are returned to standby readiness.

December 2, 1994 at 1001 CST TVA made a 4 hour nonemergency notification to NRC in accordance with 10 CFR 50.72 (b)(2)(ii).

D. Other S stems or Seconda Functions Affected:

None.

E. Method of Discove The Unit 2 Operator received a Stator Cooling System Abnormal alarm followed by a Stator Cool'ing Water Failure alarm in the Unit 2 Main Control Room. These alarms were followed by alarms associated with the full reactor scram.

0 erator Actions:

Operator actions taken during this event were as expected. At the onset of the event an ASOS was dispatched to the Stator Cooling Water panel to investigate the alarms. Additionally, the UO began to reduce main generator stator load to reduce stator current and thus lower the stator cooling heat load.

Upon receiving the reactor scram, the reactor operator performed the actions described by Abnormal Operating Instruction "Reactor Scram," bringing the reactor to hot standby condition. The plant responded to the scram as expected.

NRC FORM 366A U.S. NUCLEAR REGULATORY COSNIISSION APPROVED BY (NB NO. 3150-0104 (5-92) EXP I RES 5/31/95 ESTIHATED BURDEN PER RESPONSE TO COMPLY llITN THIS INFORHATION COLLECTION REQUEST: 50.0 NRS. FORUARD COMMENTS REGARDING BURDEN ESTIHATE TO THE INFORHATION AND RECORDS HANAGEHENT BRANCH (HNBB 7714), U.S. NUCLEAR REGULATORY COHHISSION, LICENSEE EVENT REPORT l!ASHINGTON, DC 20555-0001, AND TO TNE PAPERNORK REDUCTION

'TEXT CONTINUATION PROJECT (3150-0104), OFFICE OF HANAGEHENI'ND BUDGET, IIASNINGTON DC 20503 FACILITY NAME (1) DOCKET NINIBER (2) LER NOSER (6) PAGE (3)

YEAR SEQUENTIAL REVISION NUMBER NUHBER Browns Ferry Unit 2 05000260 94 013 01 4 of 6 TEXT If more s ce is r ired use sdditionel co ies of NRC Form 366A (17)

Go Safet S stem Res onsest The safety systems listed in section ZIA of this report responded to the reactor scram as designed.

ZZZ. CAUSE OF THE EVENT A Immediate Cause:

The immediate cause of the generator and main turbine trip was a premature operation of the stator cooling water high temperature switch [TIS). This was followed by a reactor scram.

B~ Root Cause:

The, root cause of the event was a mechanical degradation of the stator cooling water temperature switch. The degradation caused a setpoint drift in the conservative direction thus premature operation of the temperature switch. At the time of the trip the indicated cooling water temperature was approximately 112 degrees F while the normal trip setpoint is 179 degrees F. The initial post event field verification of the setpoint indicated that the switch would actuate at approximately 126 degrees F.

Subsequent additional testing indicated that the actual setpoi.nt had drifted to approximately 115 degrees F.

C. Contributin Factors:

Contributing in the event was that during a routine calibration on October 6, 1994, the trip setpoint was, verified at the required 179 degrees F. However, the observed reset dead band was unusually high. Post trip calibration found that at 174 degrees F the reset dead band was approximately 94 degrees F.

This dead band exceeds the normal dead band of 10-12 degrees.

The large dead band gave indication that the switch actuation mechanism may have been unreliable. There is no specific gui.'dance for the calibration of'his instrument with regard to the dead band. Had guidance been available that specified a dead band the instrument would have been replaced during the previous calibration.

The single-failure vulnerability of the turbine generator control and protection system components, places the plant at risk for single failures that can cause undesirable trips similar to the event described in this report.

'NRC FOHN 366A U.S. NUCLEAR REGULATORY CCSBIISSION APPROVED BY'MB NO. 3150-0104 (5-92) EXPIRES 5/31/95 ESTIHATED BURDEN,PER RESPONSE TO COHPLY WITH THIS INFORHATION COLLECTIOH REQUEST: 50.0 HRS. FORWARD COHHEHTS REGARDING BURDEN ESTIHATE TO THE INFORHATIOH AND RECORDS HANAGEHEHT BRANCH (NNBB 7714), U.S. NUCLEAR REGULATORY COHHISSION, LICENSEE EVENT REPORT WASHINGTON, DC 20555-0001, AND TO THE PAPERWORK REDUCTION TEXT CONTINUATION PROJECT (3150-0104), OFFICE OF HANAGENEHT AND BUDGET, WASHINGTON DC 20503 FACILITY NANE (1) DOCKET NQIBER (2) LER IRBIBER (6) PAGE (3)

YEAR SEQUENT IAL REVISION NUHBER NUHBER Browns Ferry Unit 2 05000260 94 013 01 5 of 6 TEXT If more s ace is r uired use additional co ies of NRC Form 366A (17)

IV, I ANALYS S OF THE EVENT BFN is analyzed in Chapter 14 of the Updated Finala Safety Analysis starting point of Report for a turbine trip/reactor scram assuming greater than 100% reactor power and associated steam flow. The most severe transient for a full power generator trip occurs if the turbine bypass valves fail to open. In this event, the trip occurred at 54 percent power. Additionally, affected components (including the turbine bypass valves) functioned as designed. Thus this event is bounded by the plant safety analysis.

V. CORRECTIVE ACTIONS A. Immediate Corrective Actions!

The affected systems were restored to 'to operable status. The temperature switch was replaced prior the restart of the unit.

B~ Corrective Actions to Prevent Recurrencet

1. TVA will issue a specific instruction for the calibration of the stator cooling water temperature switch.'This instruction is scheduled for issuance by April 11, 1995.

The below corrective action describes actions being taken for other similar type procedures.

2. TVA will evaluate balance of, plant devices capable of causing turbine generator trips and determine if procedural guidance for calibration is adequate.'his evaluation is scheduled for completion by May 29, 1995.

3. TVA will evaluate other balance of plant devices capable of causing .turbine generator trips or equipment operation to determine if the equipment will need replacement due to its age.'his evaluation is scheduled for completion by April 24, 1995.

TVA does not consider these actions to be Regulatory Comnitments. That is, they are not actions required to restore compliance Mith obligations. Obligation means an action that is a legally binding, requirement imposed through applicable rules, regulations, orders, and licenses.

The above actions are not necessary to preserve compliance with obligations. They are measures taken to minimize recurrence of reactor scrams.

~I ~I NRC FORM 366A U.S. NUCLEAR REGULATORY CQHISSION APPROVED BY INS NO. 3150-0104 (5-92) EXPIRES 5/31/95 ESTIMATED BURDEN PER RESPOHSE TO COMPLY MITH THIS IHFORHATION COLLECT ION REQUEST: 50. 0 HRS FORllARD COHHENTS REGARD IHG BURDEN ESTIMATE TO THE INFORHATION AND RECORDS HAHAGEHENT

, BRANCH (HNBB 7714), U.S. NUCLEAR REGULATORY COMHI SS I ON, LICENSEE EVENT REPORT NASH IHGTOH, DC 20555-0001, AND TO THE PAPERlJORK REDUCTION TEXT CONTINUATION PROJECT (3150 0104), OFFICE OF HANAGEMENT AND BUDGET, MASHIHGTOH DC 20503 FACILITY NAME (1) DOCKET NQIBER (2) LER NIHIBER (6) PAGE (3)

YEAR SEQUENT IAL REVI SION NUMBER NUHBER Browns Ferry Unit 2 05000260. 94 013 01 6 of 6 TEXT If more s ace is r uired use additional co ies of NRC Form 366A (17)

VI ~ ADDITIONAL INFORMATION Ao Failed'om onentst A Mercoid model DA 38-3 wet fluid filled capillary design temperature switch failed. The temperature switch experienced a setpoint drift which caused the switch to prematurely actuate.

B~ Previous LERs on Similar Events:

No previous BFN events were identified in which the stator cooling water temperature switch was the cause of a unit trip and reactor scram.

VII. Commitments None.

Energy Industry Identification System (EIIS) system and component codes are identified in the text with brackets (e.g., [XX]).

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