ML20127K043

Forwards,For Consideration,Draft IE Info Notice Re 850605 Failure of Reactor Trip Breaker at Facility.Safety Significance of Event Warrants Issuance of Notice.Failure Due to Excessive Clearance Between Roller Rivet & Armature

ML20127K043
Person / Time
Site:	Rancho Seco
Issue date:	06/20/1985
From:	Thompson H Office of Nuclear Reactor Regulation
To:	Jordan E NRC OFFICE OF INSPECTION & ENFORCEMENT (IE)
References
NUDOCS 8506270238
Download: ML20127K043 (4)

Text

S0-312.

JUN 2 01985 MEMORANDUM FOR:

Edward L. Jordan, Director Division of Emergency Preparedness &

Engineering Response FROM:

Hugh L. Thmpson, Jr., Director Division of Licensing

SUBJECT:

FAILURE OF A REACTOR TRIP BREAKER AT THE RANCHO SECO STATION On June 5, 1985, one of the d.c. reactor trip breakers (RTBs) at the B&W-designed Rancho Seco Station failed to trip open when its undervoltage trip attachment (UVTA) was actuated during a test. This failure appears to have been caused by excessive clearance between the roller rivet and armature within the UVTA, which was sufficient to allow the trip paddle of the RTB trip shaft to become jammed against the armature of the UVTA. All the other RTBs at this plant may also have had excessive clearances. This event is discussed further in the enclosure.

At this time, we believe we have sufficient information to provide a useful IE Notice to other licensees and have prepared the enclosed draft for your consideration. We believe that the safety significance of this matter warrants the issuance of an Information Notice in an expeditious manner. This matter has been discussed between E. Rossi of IE/EAB and J. T. Beard of NRR/0RAB.

GrizM B@ b?

Hugh"1".*ThoShion~,'dr. Director Division of Licensing

Enclosure:

DISTRIBUTION As stated Central File Nf3C PDR cc w/ enclosure:

ORAB Rdg H. Denton, NRR E. Rossi, IE GHolahan R. Bernero, DSI G. Zech, IE DCrutchfield F. Rosa, ICSB B. Faulkenberry, R-V JTBeard D. Kirsch, R-V RWessman WSwenson DLasher CONTACT:

RSingh

-J. T. Beard, NRR RHernan x27465 RWright NRomney SMiner

• PREVIOUS CONCURRENCE SEE DATE ORAB:DL*

SL:0RAB:0L*

C:0RAB:DL*

AD/SA:DL*

D/DLk JTBeard:dm RWessman GHolahan DCrutchfield HThompson 6/10/85 6/11/85 6/12/85 6/13/85 6/Jb/85

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0506270238 850620 PDR ADOCK 05000312 G

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ENCLOSURE FAILURE OF A GENERAL ELECTRIC TYPE AK-2-25 REACTOR TRIP BREAKER Description of Circumstances:

The Rancho Seco Nuclear Power Generating Station was completing a refueling outage and preparing to restart. During the outage, the licensee installed refurbished reactor trip breakers. On June 5, 1985, one of the d.c. reactor trip breakers (RTBs) failed to trip open when its undervoltage trip attachment (UVTA) was actuated during a test. Although the UVTA had de-energized, its armature had not moved out of the energized position.

(A RTB with all,

components in their normal positions is shown in Figure 1).

Investigation revealed that the trip paddle, which is the mechanical interface between the UVTA and the main trip shaft of the RTB, had jammed against the armature of the UVTA. When the shunt trip coil was actuated subsequently, the trip paddle associated with the UVTA rotated about 45* clockwise as viewed in the diagram to a position above the th/TA, such that the UVTA became effectively disconnected from the RTB trip shaft. Further investigation revealed that within the UVTA the clearance between the roller rivet and armature was significantly greater than the specified allowable range. This increased downward displacement was sufficient to allow the trip paddle to interfere with the UVTA armature. The UVTA trip paddle may also have been of a slightly different shape and length.

.Pteliminary information indicates that the UVTA rivet-armature clearances may "have been excessive for all five of the other RTBs installed at the plant.

The RTBs used at B&W-designed PWRs and at CE-designed PWRs are the General Electric type AK-2-25 breaker. The licensee had sent the Rancho Seco RTBs to GE-Atlanta for refurbishment, which included installing a new lubricant in critical bearings in the front-frame of the RTB. Subsequent to their refurbishment, the RTBs were tested at B&W-Lynchburg and certified as acceptable for service as safety-related reactor trip breakers.

Incoming receipt inspection of the RTBs at Rancho Seco consisted of only a visual review; no functional test nor verification of critical parameters was conducted prior to installation.

At Rancho Seco, the licensee has now developed procedures to perform checks of the critical parameters of the breakers, as required for safety-related equipment. These are based on guidance recently provided by B&W to B&W owners.

It is our understanding that CE has not issued similar guidance. The UVTA rivet-armature clearance is a difficult measurement to perform properly and may require a special tool.

Further, although not mentioned in previous vendor information, the measurement snould be made with the UVTA armature down in the energized position. The manufacturer's representative is providing on-site assistance to the licensee. A B&W engineer is also assisting at the site.

The licensee has identified the failure mechanism, completed appropriate corrective action, and satisfactorily tested all RTBs prior to withdrawing control rods and resuming power operations.

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0 2-The refurbishment of the RTBs is one of the major items of the long-term program developed by the B&W-owners group and the CE-owners group to assure that the RTBs will function in a highly reliable manner. The failure experienced at Rancho Seco is the first reported failure of a refurbished RTB and is of a different failure mechanism than previously experienced. The NRC is currently assessing the generic applicability of this failure. Recipients of this Information Notice should consider reviewing the adequacy of incoming receipt inspections and other testing of RTBs to be assured that all critical parameters are properly measured and within specifications and that installed RTBs are capable of performing their safety-related function in a highly reliable manner.

No specific action or written response is required by this information notice.

If you have any questions about this matter, please contact the Regional Administrator of the appropriate regional office or this office.

Edward L. Jordan, Director Division of Emergency Preparedness &

Enginecring Response Technical

Contact:

J. T. Beard, NRR (301) 492-7465 R. Singh, IE (301)492-8985

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