United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 87-62: Mechanical Failure of Indicating-Type Fuses

                                                      IN 87-62

                                  UNITED STATES
                          NUCLEAR REGULATORY COMMISSION
                      OFFICE OF NUCLEAR REACTOR REGULATION
                             WASHINGTON, D.C.  20555

                                December 8, 1987


Information Notice No. 87-62:  MECHANICAL FAILURE OF INDICATING-TYPE
                                   FUSES


Addressees:

All holders of operating licenses or construction permits for nuclear power 
reactors.

Purpose:

This information notice is being provided to alert addressees to potential 
problems resulting from the mechanical failure of indicating-type fuses.  It 
is expected that recipients will review the information for applicability to 
their facilities and consider actions, as appropriate, to avoid similar 
problems.  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 Nuclear Regulatory Commission has been notified of four separate events in
the past two years related to the mechanical failure of indicating-type fuses.  
The events occurred at McGuire Nuclear Station, Unit 1; Catawba, Unit 2; and 
Sequoyah Nuclear Plant and are described herein.  

On March 25, 1986, Duke Power Company's McGuire Nuclear Station, Unit 1, ex-
perienced a reactor trip on a steam generator low-low level signal when the 
mechanical failure of a Bussman FNA-type fuse caused a main feedwater contain-
ment isolation valve to close.  McGuire personnel determined that the failure 
was the result of the fuse element having pulled loose from the solder joint 
inside the fuse.  The solder joint was found unbroken; the element wire had 
pulled out of the solder joint.  The licensee found that 8% of the spare fuses 
in stock also had failed mechanically.  Previously, in December 1981 and 
December 1985, the licensee had reported to the NRC the mechanical failures of 
FNA-type fuses [Licensee Event Reports (LERs) 369-81-179 and 369-85-036].  

On July 3, 1986, the NRC issued a Confirmation of Action Letter (CAL) to Duke 
Power Company as a followup to an event at Catawba, Unit 2.  In that event one 
of the auxiliary feedwater trains failed to start during testing because of a 
mechanically failed FNA-type fuse.  On July 7, 1986, Duke Power notified the 
NRC of the preliminary results of a review of all safety-related circuits 
where FNA-type fuses were used.  The review included the inspection of 
approximately 



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2500 fuses.  The inspection found 14 failed FNA-type fuses, 9 of which were 
determined to have failed mechanically.  The inspection of the spare fuses in 
the warehouse stock found an additional 11 mechanically failed fuses.  

On October 29, 1986, the Tennessee Valley Authority (TVA) submitted a 
notification on Bussman MIS-5-type fuses to the NRC pursuant to the 
requirements of 10 CFR Part 21.  The Bussman MIS-5 actuating fuse consists of 
two very thin wires in a sand-like filler.  One of the wires, which is 
approximately 96% silver, acts as a fuse link; the other, a nichrome alloy, 
acts as a retaining wire for a spring-loaded actuator/indicator rod that is 
located at one end of the fuse assembly.  In the notification TVA indicated 
that fuses at the Sequoyah Nuclear Plant had exhibited partial actuation, not 
detectable in all cases by visual examination, that was the result of the 
elongation of very thin wires.  The elongation of these wires could 
significantly change the characteristics of the fuse and its current-carrying 
characteristics.  This is particularly true if the silver wire breaks but 
remains in contact with the elongated and unbroken nichrome wire.  TVA 
contacted Bussman and established the resistance and current values that could 
be used to conclusively test the operability of the remaining fuses.  

On July 20, 1987, TVA submitted an LER on Littlefuse Incorporated FLAS-5 type 
fuses to the NRC (LER 327-87-030).  The LER noted that there had been two 
separate Engineered Safety Feature actuations of the Sequoyah Nuclear Plant's 
onsite emergency diesel generators as a result of blown FLAS-5-type fuses in 
the emergency diesel generator start logic circuitry.  The FLAS-5 fuse 
consists of a fuse wire in parallel with a 560-ohm resistor, a spring-loaded 
indicator pin, and sand-like filler.  The indicator pin is mechanically 
attached to the spring.  At the end of the spring, the resistor and the fuse 
wire are soldered together.  The solder material used is a eutectic alloy that 
has a low melting point.  During normal operating conditions, the fuse wire 
carries the operating current.  During a fault condition the solder material 
rapidly melts.  During overcurrent conditions, the resistor heats up with 
increasing current and serves as the heat source that melts the solder 
material.  When the solder joint melts it interrupts the circuit and releases 
the indicator pin.  The indicator pin itself causes annunciation only and does 
not trigger any safety features.  Because 69 out of 3200 installed FLAS-5-type 
fuses have failed to date, TVA perceives that a mechanical weakness, such as a 
defect in the solder joint, is the main cause of the blown fuses in at least 
two FLAS-5-type fuse lots.  The vendor believes that the problem has been 
corrected by modification of the solder material and processes.  

Discussion:

The fuses involved in the events described above are of the pin indicating 
type.  These fuses have an internal spring-loaded indicating pin that 
protrudes from the end of the fuse when the fuse links separate.  These fuse 
links are designed to melt when the current exceeds the design load; however, 
in the cases described above, the fuses apparently failed as a result of 
either a cold solder joint, creep, or fatigue induced by the internal spring 
tension.  Bussman and Littlefuse supply other indicating-type fuses, and other 
fuse suppliers also make indicating-type fuses.  The fuses that have failed 
mechanically are of the same type that have successfully undergone seismic 
testing.  

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The NRC staff reviewed 575 fuse-related LERs for the period 1981-1986.  They 
showed no additional cases of specific mechanical failure; however, 183 
reports indicated that the licensee had determined the failure to be from 
unknown causes, and many of the reports identified a blown fuse as the cause 
of the associated circuit failure.  Fuse replacement was the usual corrective 
action taken.  Because of the large number of fuses involved, the total number 
of fuse failures may not be abnormal.  However, the experience of Duke Power 
Company and TVA shows that the safety significance evaluation is dependent on 
an accurate root-cause determination.  In the event of an indicating fuse 
failure, additional investigation, including internal examination of the fuse, 
may be warranted if an electrical fault cannot be found.  

No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact the technical 
contact listed below or the Regional Administrator of the appropriate regional 
office.  




                              Charles E. Rossi, Director
                              Division of Operational Events Assessment
                              Office of Nuclear Reactor Regulation


Technical Contacts:  James C. Stewart, NRR
                     (301) 492-4644

                     Joseph J. Petrosino, NRR
                     (301) 492-4316


Attachment:  List of Recently Issued NRC Information Notices
Page Last Reviewed/Updated Tuesday, November 12, 2013