Information Notice No. 96-44, Supplement 1:Failure of Reactor Trip Breaker from Cracking of Phenolic Material in Secondary Contact Assembly
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
July 2, 1997
NRC INFORMATION NOTICE 96-44, SUPPLEMENT 1: FAILURE OF REACTOR TRIP BREAKER
FROM CRACKING OF PHENOLIC
MATERIAL IN SECONDARY CONTACT
All holders of operating license permits for nuclear power reactors.
The U.S. Nuclear Regulatory Commission (NRC) issued Information Notice (IN)
96-44 to alert addressees to the possible failure of reactor trip breakers
(RTBs) to properly function because of cracking or breaking 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. Breaking or partial
cracking of these assemblies may prevent the breaker from performing its
design function or other secondary functions that are controlled by the status
of the breaker position.
The NRC is issuing this supplement to IN 96-44 to notify the addressees of the
findings of the survey conducted by the Westinghouse Owners Group (WOG) on the
RTB breaker failures from cracking of phenolic material in secondary contact
assemblies. This survey [Attachment 1(a)] highlights typical causes of damage
and ways to minimize damage to the secondary contact assembly. 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 RTB testing at McGuire Nuclear Station, Unit 2, the
licensee found that one of the bypass breakers failed to open electrically
when the local shunt trip pushbutton was depressed. During subsequent
inspection of the breaker, a small piece of the assembly was found in the
secondary contact block, which prevented electrical continuity for the local
shunt trip pushbutton circuitry for the manual trip function. Following this
incident, the licensee for Catawba Nuclear Station conducted an inspection of
the RTBs and
9706260303. IN 96-44, Supp. 1
July 2, 1997
Page 2 of 3
found cracks in several secondary contact blocks. The NRC staff issued IN
96-44 to alert the licensees to possible failure of properly functioning
reactor trip breakers because of cracking or breaking of the secondary contact
blocks. As a result of the information notice follow-up on this issue, Farley
Nuclear Plant reported cracks similar to those found at McGuire and Catawba.
Although the RTBs are functional with minor cracks, this failure mode may lead
to eventual failure of the RTB to perform its safety function.
After preliminary assessment of this problem, the staff contacted the WOG to
assess the scope of the breaker problem by surveying member utilities to
determine the generic implication of the RTB failures. The survey findings
indicated that 53 secondary contact assemblies have been replaced during the
last 10 years out of an estimated 9,000 secondary contact assemblies used on
3,000 DS series breakers. Seven failures occurred when the breaker was cycled
during startup and periodic testing. Five operational failures occurred,
involving three plants, and these failures were considered to be of no safety
significance. The typical causes of damage to secondary contact assemblies
were from the lifting of breakers by the secondary contact assemblies, over-
torquing of secondary contact assembly bolts, aging, and cycle fatigue.
In 1993, Westinghouse issued the MPM-DS breaker manual. This manual was
provided as an attachment to Technical Bulletin NSD-TB-93-05-R0, dated
January 10, 1994. The Westinghouse guidance in the manual included the
1. "Inspect the secondary contacts closely to ensure they are
firmly mounted and there are no cracks in the insulating
material. The conductive surfaces should be free of
abnormal or uneven wear. Check for evidence of
overheating. Depress each contact of the assembly and
observe its spring return to normal."
2. "The secondary contact assembly mounting hardware
including the contact covers should not exceed 25 to 35
inch-pounds." [Found under specific torque requirements.]
3. Proper circuit breaker handling guidance.
The survey findings indicated that adequate procedures describing testing,
inspection, specific torque requirements, and proper handling of circuit
breakers are available to the licensees and are being used in procedures for
maintenance and surveillance purposes.
IN 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) subcomponents.
. IN 96-44, Supp. 1
July 2, 1997
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 S.H. Weiss for
Marylee M. Slosson, Acting Director
Division of Reactor Program Management
Office of Nuclear Reactor Regulation
Technical contacts: Amar Pal, NRR
1. Letter (OG-97-033) from R. A. Newton, Chairman,
Regulatory Response Group/Issues Review
Group/PI Core Team, Westinghouse Owners Group
(WOG) to WOG Representatives, dated March 18, 1997
(not included in this electronic file)
2. List of Recently Issued NRC Information Notices
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