ML20108E534

Special Rept:On 850114,CRD Dc Breaker Failed Slow During Channel D Reactor Protective Sys on-line Test.Caused by Sticking Trip Latch Roller Bearings & Excessive Trip Latch Operating Torque.Breaker Front Frame Replaced

ML20108E534
Person / Time
Site:	Oconee
Issue date:	02/14/1985
From:	Tucker H DUKE POWER CO.
To:	Grace J NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION II)
References
NUDOCS 8503120218
Download: ML20108E534 (2)

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DUKE POWER GOMPANY P.O. nox 3310D CHARLOTTE, N.C. 20242 HALB. TUCKER

. 85 Fe r g h A /,19 5 (7o4) ora-4sai Dr. J. Nelson Grace, Regional Administrator U. S. Nuclear Regulatory Commission Region II 101 Marietta Street, NW, Suite 2900 Atlanta, Georgia 30323 Re: Oconee Nucl tation Docket No. 0-26

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Dear Dr. Grace:

Please find attached a report concerning the response time failure of a Control Rod Drive (CRD) DC breaker during a Channel D Reactor Protective System (RPS) on-line testing on January 14, 1985. The breaker, which should have opened within 80 milliseconds, did not open for 168 milli-seconds. This report is submitted per our commitment to inform NRC of unacceptable performance of CRD breakers.

Very truly yours, fh. Yg l Hal B. Tucker MAH: sib Attachment cc: American Nuclear Insurers INPO Records Center c/o Dottie Sherman, ANI Library Suite 1500 The Exchange, Suite 245 1100 circle 75 Parkway 270 Farmington Avenue Atlanta, Georgia 30339 Farmington, CT 06032 Ms. Helen Nicolaras Mr. J. C. Bryant Office of Nuclear Reactor Regulation NRC Resident Inspector U. S. Nuclear Regulatory Commission Oconee Nuclear Station Washington, D. C. 20555 Document Control Desk U. S. Nuclear Regulatory Commission Washington, D. C. 20555 M&M Nuclear Consultants 1221 Avenue of the Americas New York, New York 10020 5 oh69 PDR OAC COP.Y.

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l Duke Power Company Oconee Nuclear Station Special Report Response Time Failure of Control Rod Drive DC Breaker

Introduction:

On January 14, 1985 at 1532 hours0.0177 days <br />0.426 hours <br />0.00253 weeks <br />5.82926e-4 months <br /> with Unit 1 operating' t 100% full power a Control Rod Drive (CRD) DC Breaker failed a. Channel D Reactor Protective System (RPS) on-lina test. The breaker which should have

-opened within 80 milliseconds (msec), did not open for 168 milliseconds.

-The immediate corrective action was to replace the slow front frame from the affected CRD DC breaker with a spare front frame which had been tested and.had preventive maintenance performed on it. All Unit 1 CRD breakers were tested and they all passed the on-line. test.

The apparent causes of the failure were a sticking trip shaft bearing and a high trip latch operating torque. There were no releases of radioactivity and the health and safety of the public were not affected.

Description of Occurrence:

On January 14, 1985 at 1332 hours0.0154 days <br />0.37 hours <br />0.0022 weeks <br />5.06826e-4 months <br /> a CRD DC breaker (#CB-3) failed'a monthly Unit 1 Channel D on-line functional test. On-line test of the CRD DC breaker #CB-3 showed a response time of 168 msee which was greater than 80 msec delay time for CRD breaker assumed in the safety.

analyses' calculation of overall RPS delay times. The slow front frame of the failed CRD breaker was replaced with a spare front frame which had been tested and had preventive maintenance performed on it. At.

1728 hours0.02 days <br />0.48 hours <br />0.00286 weeks <br />6.57504e-4 months <br />, CRD breaker #CB-3 was retested and tripped in 28 maec. - At this time, all Unit 1 CRD breakers were on-line tested and all tripped ,

in less than the required 80 msee time limit.

Cause of Occurrence:

The slow front frame from CRD breaker #CB-3 was bench' tested and indicated a trip response time of 30 msec. This indicated similar results seen in past slow front frames which were found to have sticking trip latch roller bearings. Inspection of-the breaker showed only'one abnormality.

The trip latch operating torque measured 2 inch pounds which was greater than the 1.5. inch pound limit. Apparently the combination of a sticking-roller latch bearing and the excessive trip latch operating torque-caused the' slow trip time. The slow front frame was cleaned and success-fully bench tested.

In the event that an~ actual reactor trip had been called during this period of time,'and the CRD DC breaker #CB-3 had' delayed in tripping, all safety rod groups 1, 2, 3 and 4 and regulating rod groups 5, 6 7 would have dropped immediately as designed. This would have been caused by the interruption of power to the CRD mechanisms by the other

-CRD AC and DC breakers and the silicon control rectifiers (SCRs).

Insertion of these rod groups would have shut down'tne reactor.

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The SCRs function independently of the CRD breakers and act to interrupt power to and trip regulating rod groups 5, 6 and 7 upon receipt of a trip signal. Thus, the SCRs would have provided a diverse and independent means of shutting down the reactor.

The health and safety of the public were not affected by this event.

Corrective Action:

The immediate corrective action taken was to replace Unit 1 CRD DC breaker

1. CB-3 front frame with a spare front frame. The CRD DC breaker #CB-3 was then retested and met trip time requirement of 80 msec. All Unit 1 CRD breakers were retested and all had response time within the acceptable limit. The faulty front frame from Unit 1 CRD DC. breaker- #CB-3 was inspected and successfully tested.

The immediate and supplemental corrective actions ensured that all other CRD breakers on Unit I were operable and would have tripped within the acceptance' criteria if a trip signal had been received.

The subject breaker is an old General Electric (GE) Type AK 2 breaker.

On the GE Type AK 2 breaker, the trip latch roller is part of the front frame.

Similar failures of CRD breakers have previously occurred and reported to NRC by Reportable Occurrence Report R0-269/83-20, dated December 30, 1983 and Special Reports dated August 17, 1984 and January 16, 1985.

Presently a program is being implemented to replace the front frame of all GE Type AK 2 breakers in an attempt to correct these recurring problems. These new front frame assemblies are being installed as a result of the Salem ATWS event. Finally, in an amended response to Generic Letter 83-28, Items 4.1 and 4.2 submitted to NRC on August 10, 1984, Duke's activities regarding reactor trip system reliability, preventive maintenance, and surveillance program have been documented.