United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 96-44: Failure of Reactor Trip Breaker from Cracking of Phenolic Material in Secondary Contact Assembly

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

                                August 5, 1996


NRC INFORMATION NOTICE 96-44:  FAILURE OF REACTOR TRIP BREAKER FROM CRACKING
                               OF PHENOLIC MATERIAL IN SECONDARY CONTACT
                               ASSEMBLY


Addressees

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

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to the possible failure of reactor trip breakers to
properly function because of cracking or breakage of the secondary
disconnecting contact assemblies.  The disconnect assemblies provide circuit
connections between the control and monitoring devices on the breaker and
external control circuits.  The housing of the electrical contacts in the
disconnect assemblies consists of a phenolic material.  Breakage or partial
cracking of these assemblies may prevent the breaker from performing its
design function or other secondary functions provided by the status of the
breaker position.  It is expected that recipients will review this information
for applicability to their facilities and consider actions, as appropriate, to
avoid similar problems.  However, suggestions contained in this information
notice are not NRC requirements; therefore, no specific action or written
response is required.

Description of Circumstances

On June 12, 1996, during reactor trip breaker testing at McGuire Nuclear
Station, Unit 2, the licensee for McGuire (Duke Power Company) found that one
of the bypass breakers failed to open electrically when the local shunt trip
push button was depressed.  The breaker was later opened mechanically. 
McGuire Unit 2 was in cold shutdown (MODE 5) at the time.  Main reactor trip
breakers and bypass reactor trip breakers of the McGuire Units 1 and 2 are
480 volt Westinghouse Model DS-416 equipped with four secondary disconnecting
contact assemblies, each containing eight spring-loaded contacts, mounted on
the upper rear portion of the breaker.  The shunt trip, the undervoltage trip,
and the open/closed monitoring circuits for the breakers are wired through
these assemblies.  The assemblies are made of a molded, cellulose-filled,
phenolic material that appears to have low impact strength and may be highly
susceptible to chipping or cracking.  During subsequent inspection of the
breaker, a small piece of the assembly was found lodged in the secondary
disconnecting contact assembly, which may have prevented reliable electrical
continuity for the local shunt trip push button circuitry for the manual trip
function.


9608020246  .                                                            IN 96-44
                                                            August 5, 1996
                                                            Page 2 of 3


On July 1, 1996, while inspecting the remaining breakers of Units 1 and 2, the
licensee discovered that an entire secondary disconnecting contact assembly on
a Unit 1 breaker was broken in half and one of the spring-loaded finger
contacts held together by a phenolic block had fallen out in the breaker
cubicle.  Unit 1 was operating at 100 percent at the time of discovery. 
According to the licensee, the broken disconnecting assembly was held in place
by the breaker cubicle stops and the wiring harness, and this breaker had
successfully passed its in situ surveillance test.  The licensee replaced the
failed breaker with an available bypass breaker.
 
On the basis of the situation identified at McGuire, the licensee performed
additional inspections of breaker assemblies at Catawba, and found cracks in
12 of 32 secondary disconnecting contact assemblies.  All reactor trip
breakers at Catawba were tested in situ and successfully passed their
surveillance tests.  Most of the cracks discovered at Catawba were found in
the top disconnecting assemblies.

The postulated root cause of the chipped disconnecting assembly of the breaker
in McGuire Unit 2 and the cracked assembly in Unit 1 was determined to be
stress induced from mishandling or overtorquing of the disconnecting assembly
mounting bolts.

Discussion

In a typical control-rod power-supply system, two series-connected main
breakers in each train transmit power from the rod control motor-generator
sets to the control rod mechanism.  A loss of power to the control rod
mechanism causes rods to drop into the core.  A bypass breaker is installed in
parallel with each main breaker to allow on-line testing of the main breakers. 
The main breaker is tested in its latched (closed) position whereas the bypass
breaker is tested slightly drawn out to disengage main contacts.  The bypass
breaker is normally in its fully disconnected position and when required for
service is moved to its test position and tested before it is inserted into
its fully latched position.  The bypass breaker is subjected to more handling
and thus is more susceptible to mishandling compared to the main breaker. 
Most of the cracks were on the top contact blocks which are susceptible to
damage because they may be used as convenient points for lifting the breaker
in and out of the cubicle.
 
In 1993, Westinghouse issued a revised technical manual for a variety of
breakers, including the DS-416 model.  The revised manual specifies a maximum
torque value for the mounting bolts of the secondary disconnecting contact
assemblies.  Ensuring that maximum torque values have been specified in
maintenance procedures may prevent overtorquing and resultant cracking of the
assemblies.  Inspection of secondary disconnecting contact assemblies may
identify abnormalities prior to the breaker being rendered inoperable.

Information Notice 95-19, "Failure of Reactor Trip Breaker to Open Because of
Cutoff Switch Material Lodged in the Trip Latch Mechanism," was issued on
March 22, 1995, to alert licensees to a related problem involving breakage of
phenolic material in the breaker (General Electric Model AK 2-25 circuit
breaker) subcomponents..                                                            IN 96-44
                                                            August 5, 1996
                                                            Page 3 of 3


This information notice requires no specific action or written response.  If
you have any questions about the information in this notice, please contact
one of the technical contacts listed below or the appropriate Office of
Nuclear Reactor Regulation (NRR) project manager.


                                          signed by

                                    Brian K. Grimes, Acting Director
                                    Division of Reactor Program Management
                                    Office of Nuclear Reactor Regulation

Technical contacts:  S. Shaeffer, Region II           M. Sykes, Region II
                     (704) 875-1681                   (704) 875-1681
                     Email:  sms@nrc.gov              Email:  mds1@nrc.gov

                     E. J. Benner, NRR                S. K. Mitra, NRR
                     (301) 415-1171                   (301) 415-2783
                     Email:  ejb1@nrc.gov             Email:  skm1@nrc.gov


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