05000313/LER-1991-001, :on 910110,automatic Reactor Trip Occurred Due to Main Turbine Trip.Caused by Failure of Turbine Generator Exciter.Strengthened Controls Implemented During Permanent Exciter Installation

:on 910110,automatic Reactor Trip Occurred Due to Main Turbine Trip.Caused by Failure of Turbine Generator Exciter.Strengthened Controls Implemented During Permanent Exciter Installation

ML20073G333
Person / Time
Site:	Arkansas Nuclear
Issue date:	04/29/1991
From:	James Fisicaro, Scheide R ENTERGY OPERATIONS, INC.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
1CAN049106, 1CAN49106, LER-91-001, LER-91-1, NUDOCS 9105030252
Download: ML20073G333 (5)

LER-1991-001, on 910110,automatic Reactor Trip Occurred Due to Main Turbine Trip.Caused by Failure of Turbine Generator Exciter.Strengthened Controls Implemented During Permanent Exciter Installation

Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(iv), System Actuation 10 CFR 50.73(a)(2)(v), Loss of Safety Function 10 CFR 50.73(a)(2) 10 CFR 50.73(a)(2)(1)
3131991001R00 - NRC Website

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Operations April 29, 1991 1CAN.'49106 D. S. Nuclear Regulatory Commission Document Control Desk Mail Station P1-137 Washington, D. C. 205$5 SUIUECT: Arkansas Nucicar One - Unit 1 Docket No. 50-313 License No. DPR-51 Liconuco Event Report 50-313/91-001-01 Gentlemen:

In accordance with 10CFR50.73(a)(2)(iv), attached is a supplement. to the subject report concerning an automatic reactor t rip due to a main turbine trip which was caused by failure of the turbine generator excitor.

This supp?cmont is being submitted to provide the results of the root cause evaluation of the exciter failure.

Very t.ruly yours,

. f..Jw James [.Fisicato Director, Licens.ing JJF/RilS/mmg Attachment cc:

Regional Administrator Region IV U. S. Nuclear Regulatory Commission 611 Ryan Plaza Drive, Suite 1000 Arlington, TX 76011 INPO Records Center Suite 1500 1100 Circle, 75 Parkway

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AIFTRACT (I.imit to 1400 sinces, i.e., arpmxinntely fiftean single-rpace typwrittm linm) (16) 6 January 10, 1991, at app.oximately 2326, with the plant at 100 percent of rated power, a reactor trip occurrad as a renuit of the main turbino tripping due to loss of field excitation to the main generator.

An anticipatory Reactor Protection System (RPS) trip was initintad, as designed, when the main turbine tripped while reactor power was greater that 43 percent.

Plant response to the trip was-as expected. Reactor Coolant System (RCS) pressure decreased to 1828 psig and was quickly recovered into the post trip window.

Minimum post trip RCS temperature was 553 degrees. A temporary exciter was installed while the plant remained in the hot shutdown condition and the reactor was returned to power on January-17, 1991. The temporary exciter was replaced with a permanent exciter during mid cycle outage 1M91, which began on April 7, 1991. The cause of the exciter failure was inadequate cleanliness / material accountability controls.

Strengthened controls were implemented during the permanent exciter installation.

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A.

Plant Status l

At the time of this event, Arkansas Nuclear Ono, tinit 1 (ANO 1) was operating at approximately 100 percent of rated power.

Reactor Coolant System (RCS) [ All) average temperature was 579 degrees and RCS pressuro was approximately 2153 psig.

No major equipment was out of service.

11.

Event Description

On January 10, 1991, at approximately 2326, an automatic reactor trip occurred as a result of the main turbinc [TA) tripping due to loss of flold oxcitation to the main generator (r,L]. An anticipatory Reactor Protection System (RPS) [JC) trip was initiated, as designed, when the main turbine tripped while reactor power was greater than 43 percent.

Plant response to the trip was as expected.

RCS pressurn decreased to 1828 psig and was quickly recovered into the post trip window.

Minimum post trip RCS temperature was 553 degrees.

The Integrated Control System (ICS) [JA] initiated a runback of the 'A' and 'B' main feedwater pumps (HrPs).

Fifteen of the sixteen main steam safety valves (MSSVs) opened to rollove the excess steam pressure /RCS energy. All MSSVs which opened rescated properly.

Forced flow was maintained with all four (4) reactor coolant pumps operat.ing.

The ability to removo heat from the RCS was anintained by the use of the turbine bypass valves.

Approximatelv 30 minutos after the trip, the

'D' MFP was secured and the

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l auxiliary foodwater pump (P-75) was placed in service.

After placing P-75 in servico, the 'A' MFp was secured.

The plant was maintained at hot shutdown conditions while an investigation was conducted to determino the cause of the excitor problem, No equipment malfunctions or failures were noted that complicated the plant i

recovery effort.

Additionally, no procedural or operator inadequacios were found as transient response was in a timely and professional manner.

C.

Root Causo An ovaluation was conducted by the vendor (Westinghouse) which concluded that the most likely cause of the exciter failure was the presence of a forolgn object in the excitor which created a short circuit between the diodo heat sink (AC potential) and the diode whool fuso rim (DC potential).

The resultant short circuit current generated sufficient heating to melt the aluminum heat sinks causing the loss of the excitor field relay and a subsequent main generator / reactor trip.

It is not known when the foreign object was introduced into the excitor.

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D.

Corrective Actions

A temporary exciter was installed and the reactor was returned to power on January 17, 1991.

The temporary exciter was repinced with a permanent exciter during mid cycle outage it191, which began on April 7, 1991.

During the Installation, additional controls were implemented to ensure that cleanliness and material accountability were maintained.

These controls included:

Around the clock AND System Engineering supervisory interface.

Strengthened material accountability controls which included the establishment of a security fence around the exciter work area.

This fence had only one entrance at which all materials entering and leaving thn area were logged and accounted for.

Work was accomplished using a detailed job order package which included Quality Control (QC) hold points and required periodic inspections and signoffs by vendor as well as ANO System Engineering and QC personnel.

E.

Safety Significance

The RPS initiated a reactor trip due to the main turbino tripping while reactor power was greater than 43 percent, as designed.

The plant response to the transient was as expected, with no major complications, and normal post trip parameters were maintained.

Thorofore, there was no safety significanco associated with this event.

F.

Itasis for Reportability This event is reportable pursuant to 10CFR50.73(a)(2)(iv) sinco an automatic reactor trip was initiated by the RPS.

This event was also reported in accerdanen with 10CFR50.72 at 0147 on January 11, 1991.

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Additional Information

A nimilar event in which a reactor trip was caused by a main generator exciter failure was reported in LER 50-313/89-002-00.

Energy Indus.try Information System (EIIS) codes are Indicated in the text as [XX).

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