05000317/LER-1986-001, :on 860123,automatic Reactor Trip Occurred During Monthly Testing of Reactor Protective Sys.Caused by Mechanical Failure of Reactor Trip Circuit Breaker 2. Breaker Replaced

:on 860123,automatic Reactor Trip Occurred During Monthly Testing of Reactor Protective Sys.Caused by Mechanical Failure of Reactor Trip Circuit Breaker 2. Breaker Replaced

ML20153C583
Person / Time
Site:	Calvert Cliffs
Issue date:	02/14/1986
From:	Brian Holian, Lemons J BALTIMORE GAS & ELECTRIC CO.
To:	NRC OFFICE OF ADMINISTRATION (ADM)
References
LER-86-001, LER-86-1, NUDOCS 8602210133
Download: ML20153C583 (3)

LER-1986-001, on 860123,automatic Reactor Trip Occurred During Monthly Testing of Reactor Protective Sys.Caused by Mechanical Failure of Reactor Trip Circuit Breaker 2. Breaker Replaced

Event date:
Report date:
3171986001R00 - NRC Website

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At-1056 on January 23,1986, while operating in MODE 1 at 100% power, an automatic trip of Calvert Cliffs Unit 1 Reactor occurred during monthly Reactor Protective System Surveillance testing. The normal surveillance test sequence causes one pair of trip circuit breakers to trip open at a time. This action does not normally cause a unit trip because redundant trip breakers remain closed.

However, in this case, one redundant breaker (#2) remained partially open after reset. Technicians did not realize the breaker had not fully reset because it indicated shut. The unit immediately tripped when the next set of breakers was opened per the Surveillance Test Procedure. Investigation reve.aled the root cause of the trip to be a mechanical failure of #2 trip circuit breaker (GE AK-2-25).

The breaker was found to have one of its two mechanism operating springs displaced, resulting in inadequate main contact wipe. The breaker was replaced and functionally tested by 1500, January 23,1986. Unit I reentered MODE 1 at 1710 on January 24,1986.

All Unit 1 and Unit 2 trip circuit breakers, (including nine front frame assemblies in stock), were inspected for indications of a similar defect. There was no evidence of loose or displaced operating springs as was found on Unit 1 trip circuit breaker #2. Additional corrective actions planned include: 1) returning the failed breaker to the vendor for repair and testing; and 2) evaluating additional methods of ensuring trip circuit breaker main contact closure.

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At 1056 on January 23,1986, while operating in MODE 1 at 100% power, an automatic trip of Calvert Cliffs Unit 1 Reactor occurred due to a mechanical failure of #2 Reactor Trip Circuit Breaker (TCB)(EIIS JC-52). Monthly Reactor Protective System (Ells JC) testing was being conducted at the time. The Surveillance Test Procedure required technicians to trip, and then subsequently reset one pair of TCBs at a time. Prior to the trip, each set of TCBs had been successfully cycled several times. At 1056, immediately following the opening of TCBs #3 and #7 (per the Surveillance Test Procedure), the unit tripped. Investigation following the trip revealed that TCB #2, which was cycled just prior to the trip, did not close, although by indication it appeared reset. Further investigation revealed the root cause of the trip to be a mechanical failure of TCD #2.

The breaker was found to have one of its two mechanism operating springs displaced, resulting in inadequate main contact wipe (C phase was not making contact). This combination of TCB " opens" resulted in Unit l's trip.

Following the trip, the control room operators properly evaluated the event and carried out the applicable procedures for this reactor trip - Emergency Operating Procedures #0 and #1. An Auxiliary Feedwater (AFW) Signal (Ells BA), apparently triggered by a momentary low spike on the steam generator water level instrument, caused 13 AFW pump to start immediately after the trip. The pump was secured at 1124. All safety systems functioned as expected.

Calvert Cliffs uses General Electric AK-2-25 trip circuit breakers. The failed breaker, TCB #2, has a relatively new operating mechanism (front frame assembly). The front frame assembly was delivered on 2-28-85 and installed 4-4-85. Following the unit trip TCB #2 was cycled again. Once more, the breaker indicated shut, yet remained " cocked open". The breaker was replaced and functionally tested by 1500, January 23,1986. Unit i reentered MODE 1 at 1710 on January 24,1986.

All Unit 1 and Unit 2 trip circuit breakers, (including nine front frame assemblies in stock), were inspected for indications of a similar defect. There was no evidence of loose or displaced operating springs as was found on Unit 1 TCB #2. There was no indication of defects or problems with any of the springs inspected (or with the metal pins that retain the springs). We are returning the failed breaker to the vendor for repair and testing. If any generic concerns are substantiated through the vendor examination, we will notify the industry. Also, we are evaluating additional methods of ensuring trip circuit breaker main contact closure.

The Reactor Protective System functioned properly in performing its design function; i.e., shutting down the reactor. "Ihis event would not have been more severe under credible, reasonable alternative circumstances. The overall safety significance is considered minimal.

A review of previous reportable events at Calvert Cliffs revealed no similar occurrences. The contact for further discussion of this event is B. E. IIollan, (301) 260-4384.

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L B ALTIMORE G AS AND ELECTRIC COMPANY I

P.O. B O X 14 7 'J l

B ALTIMOR E. M AR YL AN D 21203 NUCLEAR POWER DEPARTMENT COLVERT CLIFFS NUCLEAR POWER PLANT LU58Y. MARYLANC 20657 February 14, 1986 U. S. Nuclear Regulatory Comnission Docket No. 50-317 Document Control Desk License No. 53 Washington, D. C.

20555

Dear Sirs:

The attached LER 86-01 is being sent to you as required by 10 CFR 50.73.

Should you have any questions regarding this report, we would be pleased to discuss them with you.

Very truly yours, sec.< V J. R. 'Immons Manager Nuclear Operations Department h

s JRL:BDi:pah cc: Dr. Thomas E. bhrley Director, Office of thnagement Information.

and Program Control Messrs:

A. E. Lundvall, Jr.

J. A. Tiernan W. J. Lippold IE L2-

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