Information Notice 2002-04, Wire Degradation at Breaker Cubicle Door Hinges
| ML013510387 | |
| Person / Time | |
|---|---|
| Site: | Diablo Canyon  |
| Issue date: | 01/10/2002 |
| From: | Beckner W Division of Regulatory Improvement Programs |
| To: | |
| Thomas Koshy, NRR-DRIP, 301-415-1176 | |
| References | |
| TAC MB3369 IN-02-004 | |
| Download: ML013510387 (7) | |
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555-0001
January 10, 2002
NRC INFORMATION NOTICE 2002-04:
WIRE DEGRADATION AT BREAKER CUBICLE
DOOR HINGES
Addressees
All holders of licenses for nuclear power reactors.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to inform
addressees of degraded breaker cubicle wires found at the Diablo Canyon nuclear power plant.
These wires connect electrical equipment mounted on cubicle doors to equipment inside the
breaker cubicles. Over time, the wires degraded due to cold-working and aging. 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 are not NRC requirements; therefore, no specific actions or written response is required.
Description of Circumstances
On March 14, 2001, at Diablo Canyon, Unit 2, functional testing of relays associated with
containment spray (CS) pump 2-2 indicated that the overcurrent relay would not trip the pump
breaker. Troubleshooting revealed a broken wire between the CS pump 2-2 breaker cubicle
swingout door and the main 4kV cubicle. The pump was returned to operable status after the
broken wire was replaced and other cubicle wires were inspected.
The broken wire was analyzed, and it was determined that a number of wire strands had been
broken for some period of time based on the presence of corrosion at the end of the broken
strands. After reviewing the finding for potential generic implications, the licensee determined
that a 10% sample of 4kV breaker cubicles would be inspected for wire degradation. On
March 21-22, 2001, the first three 4kV breaker cubicles from the sample were inspected. Two
of the breaker cubicles showed no wire damage, but the third breaker cubicle contained two
degraded, but not broken, wires. This breaker cubicle is associated with Unit 1, CS pump 1-1.
On March 23, 2001, it was decided that an inspection of all 4kV breaker cubicles was needed, except where an inspection would be risky with the reactors at power (e.g., 4kV-to-480V feeder
breakers and startup crosstie breakers). Subsequent inspections during March 23-28, 2001, revealed four more cubicles with degraded wires, resulting in a total of six cubicles containing
degraded wires. Table 1, identifies the cubicle, the type of wire damage, and the effect on the
vital 4kV electrical system.
TABLE 1. DESCRIPTION OF BREAKER CUBICLE WIRE DEGRADATION
Breaker Cubicle
Equipment
Description
Containment spray
pump 2-2 The overcurrent relay had one completely broken wire.
Containment spray
pump 1-1 One wire used for overcurrent protection had exposed and broken
strands. A second spare wire had damaged insulation.
Safety injection
pump 2-2 A wire associated with overcurrent protection for one of the three
phases had broken strands.
Unit 2, bus H
auxiliary feeder
breaker
Two wires associated with load shedding of residual heat removal
(RHR) pump 2-2 and CS pump 2-2 had broken strands.
Unit 1, bus F
auxiliary feeder
breaker
A wire connecting the ammeter select switch and test cut-out switch
had damage to the outer cloth jacket of the insulation.
Diesel generator 1-
3 feeder breaker
Two wires associated with the ground sensor auxiliary relay and alarm
had broken strands. A third abandoned wire also had broken strands.
Component cooling
water pump 2-1 A wire providing power to an indicating lamp had damaged outer
insulation, although the inner insulation was still intact.
Of the degraded wires found in the six cubicles, only the broken wire associated with CS pump
2-2 was found to impact the 4kV electrical distribution system. If an overcurrent condition
occurred on CS pump 2-2, the broken wire would prevent the overcurrent relay from opening
the breaker, resulting in potential damage to the pump motor and/or loss of the associated vital
4kV bus. Submitting the other degraded wires to a 5-ampere current for one minute proved
them able to carry out their respective functions. A 5-ampere current is the maximum current
expected for any of the degraded wires.
Discussion
Breaker Cubicle Arrangement and Wire Type
The broken wire removed from CS pump 2-2 breaker cubicle was AWG #12, Type TA
insulation, National Electrical Manufacturers Association (NEMA) Class K (high strand) copper
wire. This wire has an inner rubber-type jacket and an outer fibrous jacket. The degraded
wires found in the other breaker cubicles were of similar type, but some were of a different
gauge. The breaker cubicles and the components inside them, including the wires, were
bought from General Electric and installed circa early 1970s during plant construction. The vital Class 1E 4kV breaker cubicles are General Electric Type M-26 breaker cubicles that
have two swingout doors. The bottom swingout door accesses the breaker and the top
swingout door accesses the breaker controls. Various relays, controls, and indications are
mounted on the top swing-out door. The number and type of equipment mounted on the top
swingout door depend on the type of plant equipment powered through the breaker. The
number of pieces and type of equipment mounted on the top swingout door also determines the
number of wires connecting the door-mounted equipment with terminals inside the cubicle
(typically 40 to 100 wires.)
The wires connecting the door-mounted equipment with terminals inside the cubicle are
supported by two vertical wire braces (also called wire looms) as shown in Attachment 1. The
purpose of the wire looms is to prevent the wires from becoming pinched in the door hinge
when the breaker is opened and closed. One wire loom is mounted on the swingout door
approximately 6 inches off the door surface and 2 inches from the door hinge. The other wire
loom is mounted inside the cubicle approximately 10 inches from the door hinge and
approximately 2 inches off the inner cubicle wall. The figure shows the construction and the
location of the wire loom in relation to the door, door hinge, and other breaker components.
When the door is fully opened, there is approximately 2 to 10 inches of wire slack between the
two wire braces.
Root Cause
The licensee has performed a formal evaluation to determine the cause of the breaker cubicle
wire degradation. The evaluation indicates that cold working and aging of the wires are the
causes.
(1)
Cold Working
In a wire loom layout, in certain cases the wires are forced into the side of the breaker
cubicle when the cubicle door is closed. This can force the wire on the inside of the
bend to exceed the minimum bend radius for a dynamic bend, resulting in cold-working
of the strands. Subsequent cycling of the door eventually results in wire failure.
(2)
Age-related degradation of the wires
Over time, the plasticizer leaches out of the PVC insulation, resulting in embrittlement.
Flexing of the wire causes the insulation to break. The loss of the mechanical support
provided by the insulation focuses the bending at the break, speeding the cold working
of the wire strands and causing eventual failure.
Corrective Actions
All degraded wires that have been identified have been replaced, and a continuity test has been
performed to ensure that the wires are connected to the correct terminals and functional. The
licensee is currently planning to inspect other wires in the plant that are susceptible to cold
working and age-related degradation using a risk-informed work schedule. Specifically, the
licensee is carrying out the following modifications. (1)
Redesign the breaker cubicle wire looms and/or wire loom supports to eliminate
excessive bending at the door hinge area.
(2)
Replace the wires that span the breaker cubicle hinge areas with standing class K or S
wire that uses contemporary insulation (non-PVC).
This information notice requires no specific action or written response. If you have any
questions regarding the information notice, please contact the technical contacts listed below or
the appropriate Office of Nuclear Reactor Regulation (NRR) project manager.
/RA/
William D. Beckner, Program Director
Operating Reactor Improvements Program
Division of Regulatory Improvement Programs
Office of Nuclear Reactor Regulation
Technical contacts:
T. Jackson, Region IV
Paul Shemanski, NRR
(805) 595-2354
301-415-1377 E-mail: twj@nrc.gov
E-mail: pcs@nrc.gov
Attachments:
1. Breaker Cubicle Interior
2. List of Recently Issued NRC Information Notices
ML013510387 Template: NRR-052
- See previous concurrence
OFFICE
RORP
TECH ED
DE:EEIB
(A) SC:RORP
PD:RORP
NAME
TKoshy
PKleene*
JCalvo*
TKoshy*
WBeckner*
DATE
01/09/02
12/04/01
01/10/02
01/10/02
01/10/02
Attachment 1 Breaker Cubicle Interior
______________________________________________________________________________________
OL = Operating License
CP = Construction Permit
Attachment 2 LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
______________________________________________________________________________________
Information
Date of
Notice No.
Subject
Issuance
Issued to
______________________________________________________________________________________
2002-03
Highly Radioactive Particle
Control Problems During Spent
Fuel Pool Cleanout
1/10/2002
All holders of operating licenses
for nuclear power reactors, holders of licenses for
permanently shutdown facilities
with fuel onsite, and holders of
licenses for non-power reactors.
2002-02
Recent Experience with
Plugged Steam Generator
Tubes
01/08/2002
All holders of operating licenses
for pressurized-water reactors
(PWRs), except those who have
permanently ceased operations
and have certified that fuel has
been permanently removed from
the reactor.
2002-01
Metalclad Switchgear Failures
and Consequent Losses of
Offsite Power
01/08/2002
All holders of licenses for nuclear
power reactors.
2001-19
Improper Maintenance and
Reassembly of Automatic Oil
Bubblers
12/17/2001
All holders of operating licenses
for nuclear power reactors, except those who have
permanently ceased operations
and have certified that fuel has
been permanently removed from
the reactor vessel.
2001-18
Degraded or Failed Automated
Electronic Monitoring, Control,
Alarming, Response, and
Communications Needed for
Safety and/or Safeguards
12/14/2001 All uranium fuel conversion, enrichment, and fabrication
licensees and certificate holders
authorized to receive safeguards
information. Information notice is
not available to the public
because it contains safeguards
information.