Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
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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IN 85-58
July 17, 1985
Page 2 of 2
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