United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 85-58, Supplement 1: Failure of a General Electric Type AK-2-25 Reactor Trip Breaker

                                                     SSINS No.: 6835       
                                                     IN 85-58, Supplement 1 

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

                              November 19, 1985

Information Notice No. 85-58, SUPPLEMENT 1:    FAILURE OF A GENERAL 
                                                  ELECTRIC TYPE AK-2-25 
                                                  REACTOR TRIP BREAKER 

Addressees: 

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

Purpose: 

Information Notice No. 85-58, "Failure of a General Electric Type AK-2-25 
Reactor Trip Breaker," described the failure of a refurbished reactor trip 
breaker (RTB) at the Rancho Seco Nuclear Power Generating Station. This 
supplement to the notice provides information on other recent failures of 
General Electric (GE) AK-2-25 type RTBs. It is expected that recipients will
review the information for applicability to their facilities and consider 
actions, if appropriate, to preclude similar problems at their facilities. 
However, suggestions contained in this supplement do not constitute NRC 
requirements; therefore, no specific action or written response is required.

Description of Circumstances and Discussion: 

Calvert Cliffs Nuclear Power Plant 

Recently, there have been two failures of undervoltage (UV) trip devices in 
the GE AK-2-25 type RTBs at Calvert Cliffs. The first failure was discovered
in February 1985 when the response time of an UV trip device measured 628 
milliseconds, well above the licensee's acceptance criteria. This UV trip 
device was installed in October 1984. The analysis of the failure revealed 
that several laminated sections that are part of the armature had slipped 
down and effectively eliminated the air gap between the movable armature and
the pole face. By design, there must be an air gap between the laminations 
and the pole face. The physical contact between the laminations and pole 
face allowed the armature to be held down by residual magnetism after dc 
power was removed, resulting in the slow response time. 

The second failure was discovered during preventive maintenance of an RTB in
July 1985. In this case, laminations had moved down only slightly to make 
the air gap below tolerance. This did not affect the response time. However,
it did affect the pickup and dropout voltages of the UV trip device causing 
them to be low. 


8511150089 
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                                                    IN 85-58, Supplement 1 
                                                    Page 2 of 3           
                                                    November 19, 1985     

The licensee's corrective actions included replacing the UV trip devices and
instituting a program to measure the air gap between the laminations and 
pole face on a yearly basis. It was pointed out by the licensee that the air 
gap measurement is not part of the checks recommended in the GE Service 
Advice Letter. Therefore, it is likely that this is not being performed in 
the industry. 

Oconee Nuclear Station 

On July 22, 1985, at the Oconee Nuclear Station Unit 1, one of the GE 
AK-2-25 dc RTBs failed its trip response time during on-line testing of a 
reactor protection system channel while the unit was operating at 100% 
power. The trip response time of the breaker was 1738 milliseconds, well 
over the licensee's acceptance criteria. 

On July 23, 1985, the licensee exercised the failed breaker numerous times, 
but no failure occurred. However, a detailed inspection of the breaker 
showed a meta burr on the head of one of the mounting studs for the UV trip 
device. The licensee concluded that the probable cause of the failure was 
the armature of the UV device touching the stud as it moved toward the trip 
position. 

The failed UV trip device was a new device installed on the breaker. It was 
discovered that the mounting stud heads of the new devices had square edges 
rather than round ones like the old devices. According to the licensee, the 
possible reduced clearance between the armature and the heads of the 
mounting bracket studs could have caused the contact and, thus, the slow RTB 
response time. The licensee's corrective actions included the replacement of 
the failed RTB with an operable spare RTB, and the on-line testing and 
operability verification of all RTBs. 

The licensee's review of previous RTB failures has indicated that a new UV 
device installed in the same breaker had failed once before on April 29, 
1985 because of mechanical binding. The cause of the previous failure, which 
also resulted in a slow trip response time, was thought to be some 
particles, possibly paint chips or meta shavings, stuck in the pivot point 
of the UV device. The licensee's corrective actions included the replacement 
of the failed RTB with an operable spare RTB, and the revision of the RTB 
inspection and maintenance procedure to include detailed inspections of the 
UV and shunt devices and other binding points. 

In addition to the RTB failures discussed above, GE notified the NRC and 
affected facilities on September 13, 1985 of certain defects in the UV trip 
devices supplied for use on AK and AKR type low-voltage power circuit 
breakers. Subsequently, GE issued Service Advice Letter No. 300 on September
26, 1985 that outlines the actions to be taken with respect to those 
defects. One of the defects addressed by GE involved insufficient clearance 
between the armature and mounting stud, similar to the Oconee problem. The 
other defect involved improper painting of the mating surfaces of the 
armature and pole pieces in ac powered UV devices. 
.

                                                    IN 85-58, Supplement 1 
                                                    Page 3 of 3           
                                                    November 19, 1985     

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:  R. N. Singh, IE
                    (301) 492-4149

Attachments: List of Recently Issued Information Notices 
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