United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 85-58: Failure of a General Electric Type AK-2-25 Reactor Trip Breaker

                                                            SSINS NO.: 6835 
                                                            IN 85-58       

                                UNITED STATES
                        NUCLEAR REGULATORY COMMISSION
                    OFFICE OF INSPECTION AND ENFORCEMENT
                           WASHINGTON, D.C. 20555

                                July 17, 1985

Information Notice No. 85-58:   FAILURE OF A GENERAL ELECTRIC TYPE 
                                   AK-2-25 REACTOR TRIP BREAKER 

Addressees: 

All nuclear power reactor facilities designed by Babcock and Wilcox Company 
(B&W) and Combustion Engineering (CE) and holding an operating license (OL) 
or construction permit (CP). 

Purpose: 

This information notice is to alert recipients of a potentially significant 
problem pertaining to the failure of a General Electric (GE) - type AK-2-25 
reactor trip breaker. It is expected that recipients will review the 
information for applicability to their facilities and consider actions, if 
appropriate, to preclude a similar problem occurring at their facilities. 
However, suggestions contained in this information notice do not constitute 
NRC requirements; therefore, no specific action or written response is 
required. 

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 (RTBs). On June 5, 1985, one of the dc 
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 sketch of an RTB with all 
components in their normal positions is shown in Attachment 1). 
Investigation revealed that the trip paddle, which is the mechanical 
interface between the armature and the trip shaft of the RTB, had jammed 
against the armature, and as a result the RTB would not trip. Subsequently, 
when the shunt trip coil was actuated, the trip paddle associated with the 
UVTA rotated about 450 clockwise to a position above the armature (See 
Attachment 1). In this position the armature cannot engage the trip paddle 
when the UVTA is de-energized, and the RTB would not trip. Further 
investigation revealed that the clearance between the roller rivet and 
armature within the UVTA was significantly greater than the specified 
allowable range. This increased downward displacement was sufficient to 
allow the trip paddle to interfere with the armature. Preliminary 
information also 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- and CE-designed reactor facilities are the GE-type 
AK-2-25 breaker. The licensee had sent the Rancho Seco RTBs to GE-Atlanta 
for 

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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 before 
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 procedures are based on guidance recently provided by B&W 
to its customers. 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 should be made 
with the UVTA armature down in the energized position. The manufacturer's 
representative is providing onsite assistance to the licensee. A B&W 
engineer also is assisting at the site. The licensee has identified the 
failure mechanism, completed appropriate corrective action, and 
satisfactorily tested all RTBs before withdrawing control rods and resuming 
power operations. 

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. 

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 
                                     and Engineering Response 
                                   Office of Inspection and Enforcement 

Technical Contact:  J. T. Beard, NRR
                    (301) 492-7465

                    R. N. Singh, IE
                    (301) 492-8985

Attachments:
1.   Undervoltage Trip Device Coil De-engerized
2.   List of Recently Issued Information Notices
Page Last Reviewed/Updated Tuesday, November 12, 2013