ML062640230

Vermont Yankee Hearing - Entergy Exhibit 17, LER 1991-005-00, Vermont Yankee, Reactor Scram Due to Mechanical Failure of 345KV Switchyard Bus Caused by Broken High Voltage Insulator Stack

ML062640230
Person / Time
Site:	Vermont Yankee
Issue date:	04/12/1991
From:	Reid D Vermont Yankee
To:	Document Control Desk, NRC/SECY
Byrdsong A T
References
50-271-OLA, Entergy-Licensee-17, RAS 12267 LER 91-005-00, NUDOCS 9104180244
Download: ML062640230 (6)

Text

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April 12, 1991 C.

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REFERENCE:

Operating License DFR-28 Docket No. 50-271 Reportable Occurrence No. LER f 91-05

Dear Sirs:

As defined by 10 CFR 50.73, we are reporting the attached Reportable Occurrence as LER t 91-05.

Very truly yours, VEM14ONT YANKEE NUCLEAR POWER CORPORATION Polaln a.-qed Plant ;Hanaqer cc: Regional Administrator VSNRC Region I 475 Allendale Road King of Prussia, FA 19406 "MNFAM MMOMUM__

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Wc Form 3" U.S. NUCLEAR .-GULATORY COMMISSION APPROVED 0HS NO.3160-0104 EXPIRES 4/30/92 ESTIMATED BURDEN PER RESPONSE TO COMPLY WITH THIS INFORMATION COLLECTION REQUEST:

50.0 HRS. FORWARD COMMENTS REGARDING LICENSEE EVENT REPORT (LER) BURDEN ESTIMATE TO THE RECORDS AND REPORTS MANAGEMENT BRANCH (P-630), U.S. NUCLEAR REOU.ATORY COMMISSION, WASHINGTON, DC 20555, AND TO THE PAPERWORK REDUCTION PROJECT (3160-0104). OFFICE OF MANAOEMENT AND 8UD ET. WASHINGTON. DC 20603.

FACILITY NAME (' -OCT NO. - PAGE (

VEMRONT YANIKEE NUCLEAR POWER STATION 10 Is 10 10 10 12 17 I 10 IIZ 1OF 0I14

ýTITLE (4)

Reactor Scram due to Mechanical Failure of 345KV Switchyard Bus caused by Broken High Voltage Insulator Stack EVENT DATE IL LEA NNL3SER (6) IREPORT DATEJ(1)I OTHER FACILITIES INVOLVED_1!')

"OAD YEARf SE0. 9 REV# MON DAY YEAR FACILITY NAMES DOCKET NO.(S) 010 I dSdd IfIOf

&)13 1113 9i I 3Y0 5 000 261 .9j 01_01 OPERATING THIS REPORT IS SUBMITTED PURSUANT TO RE0'TS OF 10CFR I:.- ONE OR MORE }

NODE (9) N 20.402(b) 1J 2o.40s(c) 60.73(a)(2)(lv) 73.71(b)

A*J J 73.71(c) fl P 20.405(a)(1)(i) 60.36(c)(1) 60 50.73(a)4(2)v) 4 ?L(191 20.405(a)(1)(44-) 50.36(c)(2) 50.73(a)(2)(vii) OTHER:

...... 20.405(a)(l)(iii) 60.73(a)(2)(i) 50.13(a)i2)(viii)(A) 20.405(a)(1)(iv) 60.73(a)(2)(ii) 50.13(a)(2)(viii)(B)

.. ............ 20.405(a)(l)(v0 I I-.13(a)(2)(iii) 50.13(a0121(x) I LICENSEE CONTACT FOR THIS LER (r1" NAME TELEPHONE NO.

AREA CODE DONAD A. REID, PLANT MANAGER d- it4 71Ai CAUSE SYST COMPHY I

COMPLETE ONE LINE FOR EACH COMPONENT FAILURE DESCRIBED IN THIS REPORT (201 MFR I

IREPORTABLE TO.*PRODS N

..... ~ CAUSE

..... I

.. r E.... SYST COMMW MFR REPORTABLE T.O NPRSL..__

SUPPLEMENTAL REPORT EXPECTED (14) EXPECTED MO DA YR SUBMISSION ES(If - yes, complete EXPECTED SUBMISSION DATE 31h O DATE ( "

ABSTRACT (Limit to 14vO spaces. i.e., approx. fifteen single-space typewritten lines) (1')

On 3/13/91 at 2228 hours0.0258 days <br />0.619 hours <br />0.00368 weeks <br />8.47754e-4 months <br />, with reactor power at lO0o a Reactor scram occurred due to a generator/turbine trip as a result of the failure of an 80 ft. vertical section of 345KV Switchyard Bus (B Phase) between the Main Transformer aerial Ti disconnect switch and the horizcMatl bus bar spanning the IT-11 and 81-1T-2 disconnect switches. The cause of the bus failure is attributed to a broken insulator stacL which secured the bus to the tower. The plant was subsequently stablized by resetting Primary Containment isolations, restarting Reactor Water Cleanup and establishing level control using the 10t Feedwater Regulator valve. Shutdown Cooling was later employed at 0504 hours0.00583 days <br />0.14 hours <br />8.333333e-4 weeks <br />1.91772e-4 months <br /> on 3/14/91 and maintained until the necessary repairs and testing were completed. The reactor was returned to critical on 3/18/91 at 0056 hours6.481481e-4 days <br />0.0156 hours <br />9.259259e-5 weeks <br />2.1308e-5 months <br />. The need to expand present Switchyard system maintenance is being evaluated.

Form 366 (6-89)

APPROVED OHS NO.3150-0104 MC Form WU 4S-I NUCLEAR REGULATORY U.S. )EXPIRES COMMISSION 4/30/92 ESTIMATED BURDEN PER RESPONSE TO COPLYi WITH THIS INFORMATION COLLECTION REQUEST:

50.0 HRS. FORWARD COMMENTS REGARDING LICENSEE EVENT REPORT (LER) BURDEN ESTIMATE TO THE RECORDS AND REPORTS TEXT CONTINUATION MANAGEMENT BRANCH (P-530), U.S. NUCLEAR REGULATORY COMMISSION, WASHINGTON, DC 20555, AND TO THE PAPERWORK REDUCTION PROJECT (3160-0104), OFFICE OF MANAGEMENT

________________________________ I ANDBUDGET,._WASHINGTON._VC_20603.

UTILITY MNE ("I) DOCKET NO. (8) LER NUMBER (6) PAGE (8)

YEAR SEO. 0 REV#;

MItM ITYANKEENIUCLEARPOWER STATIOiC dd d 21111 M9 - 0olols -Ioo d1O5d TEXT (If wea space is required, use additinal NRC Form 366A) (")

DESOIPTION OF EVENT On 3/13/91 at 2228 hours0.0258 days <br />0.619 hours <br />0.00368 weeks <br />8.47754e-4 months <br />, during normal operation with Reactor power at 100. a Reactor stru occurred as a result of a turbine trip on Generator Load Reject due to a 345KV bitchyard Tie Line Differential Fault. During the first 14 seconds of the event, the following-automatic system responses occurred without Operator intervention:

a. Trip of Tie Line breakers IT and 81-IT.

b. Fast Transfer of 4KV Buses and 1 and 2 to the Startup transformers.

C. Reactor scram on Turbine Control Valve Fast Closure signal.

d. Primary Contairment Isolation System (PCIS)(JM) Initiation, Groups 2, and 3 on Reactor Vessel "Lo" water level.

operations personnel responded to the scram by iapleaenting the required steps delineated in Emergency Operating Procedure OE-3100 OScram Procedure" which governs reactor operation in a post-crm envwioiment.

Automatic system responses a) thru c) were anticipated as a result of the 345KV Tie Line Fault. The Primary Containment Isolation System (PCIS) initiations experienced subsequert to the turbine trip were in response to the characteristic drop in Reactor water level f om vesel void collapse. Vessel level. which initially dropped to a 120 inch level from the void collapse, quicily recovered with the "Am and "C" Reactor Feedeater pumps running.

In an effort to control the increasing level, the "CO Reactor Feedwater pump was secured by Operations personnel. At 2230 hours0.0258 days <br />0.619 hours <br />0.00369 weeks <br />8.48515e-4 months <br /> (2 minutes into the event), the 'A" Reactor Feedwater pum tripped on High Reactor water level (I? inches).

At 2231 hours0.0258 days <br />0.62 hours <br />0.00369 weeks <br />8.488955e-4 months <br />, the Reactor scram was reset and the plant subsequently stabilized in Hot Standby by: restarting Reactor Water Cleanup; resetting PCIS Group 2, 3, and 5 isolations and establishing level control using the 10% Feedwater Regulator valve.

At 2235 hours0.0259 days <br />0.621 hours <br />0.0037 weeks <br />8.504175e-4 months <br />, operators received a report from Security that a large flash had been observed in the Suitchyard just prior to the Reactor scram. The local Fire Department was notified, but no fire ensued. The flash that had been observed was an electrical arc resulting from the connection break of the 0B6 phase.

At 2)66 hours7.638889e-4 days <br />0.0183 hours <br />1.09127e-4 weeks <br />2.5113e-5 months <br />, Reactor depressurization and cooldown began using the Main Condenser and the Bypass Opening Jack. At 0504 hours0.00583 days <br />0.14 hours <br />8.333333e-4 weeks <br />1.91772e-4 months <br /> on 3/14/91. RHR Shutdown Cooling was established on the '5 tIw loop.

• Eneray Information Identification System (EIIS) Comonent Identifier WCorm 36aa (6-39)

NOWrom age^ U.95. WGUR.H EILATURT CI5SSION APPROVIED NS W.3160-0104 EXPIRES 4/30/92 ESTIMATED BURDEN PER RESPONSE 10 CTOPLY WITH THIS INFORMATION COLLECTION REQUEST:

60.0 HRS. FORWARD CO~IENTS REGAMDING LICENSEE EVENT REPORT (LER) BURDEN ESTIMATE TO THE RECORDS AND REPORT11 TEXT CONTINUATION MANAGEMENT BRANCH (P-630). U.S. NUCLEAR REGULATORY COIHISSION, WASHINGTON, DC 2055S. AND TO THE PAPERIORK REDUCTION PROJECT (3160-0104). OFFICE OF MANAGEMENT

_ _ _ _ _ _ __ AND BUDGET. WASHINGTON, DC 20603.

UTILITY NAME ' DOCET NO. C' EA NUMBER ( )1-PACE 10 YEA sOTo. 0 1 IREV#Ti ]

VE]ONT YANIEE NULA POWER STATION Cj! 1111n~1(Ojj di TEXT (If more space is required. use additional MaC Form 366A) 1")

DESCRIPTION OF EVENT (Contd.)

The reactor was returned to critical on 3/18/91 at 0OSS hours.

Ouwing the course of the event, the following additional anomalies occurred:

a) Turbine Pressure Control switched from Electrical regulation to Mechanical regulation which-emainedn -effect-during -Reactor cooldom.

b) ANG A" and 8" Train Recomblners tripped and isolated. The 08" Recomner was reset and returned to service.

C) RPS Alternate Pouer Supply breakers from MCC 68 tripped. The breakers were sub sequvntly manually reset.

4) Spurious Reactor and Turbine Area Radiation alaram woere r ived during the event.

The alarms were subsequently cleared and did not return.

e) The PCIS group 2A, 3A, SA and SB (RWCU) isolation signals occurred within one second of the trip. These isolations were expected to occur after the low water level trip 8.6 seconds into the event.

An analysis of the above events was performed. Recorded data confirmed that the above equipment/circuitry responses occurred coincident with the Switchyard Fault. A review of recorded bus voltage data for buses supplying the above equipment and circuitry revealed that 4 separate voltage dips on the buses had occurred during the fault. These voltage dips were concluded significant enough to cause the equipment responses experienced, which in each case, the equipment had Undervoltage features or Seal-In circuitry.

An inspection of the Switchyard was performed immediately after the event which revealed the lower section of 80" Phase bus bar to be broken off at the lower horizontal bus bar attachent point. (Reference attached pictorial.) The upper insulator stack and I connec tor which served as a tie point for the lower and uWr bus bar sections was observed broken between the third and fourth inslators with the fourth insulator and T connector still

-attached to the busmork. During the course of inspections the next owning (on 3/14/91). a gust of mind caused the hanging bus work to break off at the T-1 dis-onnect switch jaw and fall to the ground. No additional Switchyard damge occurred from the falling bus.

CAUSE OF EVENT The root cause of the Switchyard bus failure is attributed to a failed insulator support betwen the bus and the tower. The lower insulator stack, which is comprised of four insula.

tors coupled together, broke away from the tower at the base of the first insulator. This caused a swinging moment arm developing a force on the bus connector at the cpposite end of the insulator. The excessive foace snapped the vertical bar out of the welded socket on the horizontal bus bar. This resulted in an open circuit in "B" Phase and a *am to *Cm Phase flashover as the bus swung past the "C Phase vertical bus bar. The combination of these two events initiated the Tie Line Differential Protective Relavina.

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u.s* rwi 0 *V0* Veo kW WnKULA6UMT %ANMJUlfRW AFryucu7 %mm I.a* tVWU41U (4 1) EXPIRES 4/30/92 ESTIMATED BURDEN PER RESPONSE TO COMPLY WITH THIS INFORMATION COLLECTION REQUEST:

50.0 WRS. FORWARD COMMENTS REGARDING LICENSEE EVENT REPORT (LER) BUROEN ESTIMATE TO THE RECORDS AND REPORTI TEXT CONTINUATION MANAGEMENT 'BRANCH (P-630), U.S. NUCLEAR REGULATORY COMHISSION, WASHINGTON, OC 20566, AND TO THE PAPERWORK REDIJCTION PROJECT (3160-0104), OFFICE OF MANAGEMENT AND BUOGET, WASHINGTON, DC 20603.

UTILITY NME DOCKET N ') LR-WMER PAGE (8)

VERMONT YANKEE NUCLEAR POWER STAT. 9 Jl]9o.1 c 10-Io0 i0A OFI of, TEXT (If more space is required, use additional NRC Form 366A) ("J ANALYSIS OF EENT The events detailed in this report did not have adverse safety Implications.

1. The Tie Line Differential Protective Relaying operated as designed which initiated the generator trip and Fast Transfer of plant buses to the Startup transformers.

2. The Reactorzprotectlve System operated as desgftd ands ethe reactor after rIeiving a Turbine Control Valve fast closure signal.

3. All other safety system responded as expected.

CO&MRCTIVE ACTIONS IMEDIATE CORRECTIVE ACTIONS

1. Immediate corrective actions included recovering from the Reactor scram utilizing appropriate plant procedures.

2. Efforts were imediately initiated to repair the "B" and "C" phase vertical bus work. A visual and therimgraphy inspection was conducted of the entire Switchyard to identify any additional trouble spots. An additional insulator on the "A" Phase

-s found with arc damage and subsequently replaced.

3. The Main and Auxiliary transformers were Doble tested and oil samples were taken to assess any damage which might have been caused by the Switchyard fault. No anoma lies or degradation were found. The fault effects on the transformers were analyzed and determined to be bounded by the design.

LONG TM CORRECTIVE ACTIONS

1. The plant will meet with VELCO (Vemont Electric Power Co., Inc.) and evaluate the adequacy of the Switchyard Maintenance Program.

2. The failed insulator has been returned to the manufacturer for anialysis and recommendations.

3. A detailed engineering analysis of the Switchyard vertical busmrk will be performed to determine the adequacy of the present mounting configuration.

The above long term corrective actions are expected to be completed by 12/31/91. Based upon analysis results and findings, additional corrective actions will be initiated as appropriate.

ADDITIONAL INFORMATION There have been no similar events of this type reported to the Comission in the past five years.

NRC Form 366A (6-89)

LER 91-05