Information Notice 2008-18, Loss of Safety-Related Motor Control Center Due to Bus Fault
| ML082540130 | |
| Person / Time | |
|---|---|
| Issue date: | 12/01/2008 |
| From: | Vijay Goel Generic Communications Projects Branch |
| To: | |
| dls10 | |
| References | |
| IN-08-18 | |
| Download: ML082540130 (5) | |
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, DC 20555-0001 December 1, 2008 NRC INFORMATION NOTICE 2008-18: LOSS OF A SAFETY-RELATED MOTOR
CONTROL CENTER CAUSED BY A BUS FAULT
ADDRESSEES
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, as well as all holders of operating licenses or certificates of fuel cycle facilities.
PURPOSE
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to inform
addressees of a recent event involving an electrical fire caused by a bus fault at the Arkansas
Nuclear One Unit 2 (ANO 2) power plant, which resulted in the loss of a safety-related motor
control center (MCC) and the associated loss of one division of some safety-related loads. The
NRC expects recipients to review the information for applicability to their facilities and to consider
actions, as appropriate, to avoid similar problems. The suggestions contained in this IN are not
NRC requirements; therefore, no specific action or written response is required.
DESCRIPTION OF CIRCUMSTANCES
An electrical fire caused by a bus fault occurred at ANO 2 on October 23, 2007, and resulted in
the loss of a safety-related MCC and the associated loss of one division of some safety-related
loads. On this occasion, centrifugal charging pump A was undergoing routine surveillance
testing following mechanical maintenance. While operators were starting the pump remotely
from the control room, a fire occurred in MCC Cubicle 2B-52A5, which caused the loss of power
to the MCC 2B-52 because its upstream load center breaker tripped. As a result, the licensee
declared the associated safety-related Division A loads inoperable, including the low-pressure
and high-pressure safety injection pumps, the control room emergency chiller, and the
containment spray pump. The licensee declared an Alert on ANO 2 because the onsite fire
affected one power train of engineered safety feature systems. Operators realigned the
electrical equipment in accordance with plant procedures, and the plant exited the Alert 11/2 hours after it was declared.
The licensees root-cause analysis indicated that the fire originated in MCC Cubicle 2B-52A5, which supplies centrifugal charging pump A. This cubicle had a large starter, which was
located at the bottom of the MCC in the A column. The licensee found that all three vertical
aluminum bus bars in A column had melted (from bottom up) to the first horizontal bus brace.
Videoscope photographs behind the bus work showed molten metal slag at the outer C and B
phases where the 2B-52A5 stab fingers engage the bus bars. Although the stab fingers were
aligned with the bus bars, the visual inspection following the removal of the starter cubicle and
bus safety barrier indicated that the engagement and contact of the stab fingers on all the three
phase bus bars were questionable. MCC Breaker 2B-52A5 did not trip during the event
because the fault was on the source side of the breaker (bus bars) at the MCC. The fault event
at the MCC occurred in the following sequence:
(1) Service-related conditions (i.e., large loads, frequent starts, repetitive starts, equipment
age, and environmental conditions) caused a high-resistance connection to develop at
the MCC bus/stab interface, leading to arcing at MCC Cubicle 2B-52A5.
(2) The arcing at the connection caused a phase-to-ground fault at the stab/bus connection, which moved to the bottom of the MCC. As this fault progressed to a phase-to-phase
fault, it vaporized the bottom of each bus bar.
(3) The heat from the fault caused the MCC plastic base pan and space heater wires in the
MCC to catch on fire.
(4) The pressure from the fault caused the side wireway door to open.
(5) The upstream 480-volt feeder breaker then tripped open and interrupted the fault. The
fire in the base of the MCC extinguished itself, ending the event sequence.
The MCC bus bars consisted of tin-plated aluminum, whereas the cubicle bus stabs consisted of
tin-plated copper alloy. Any loss of the tin plating could result in the oxidation of bare aluminum
and the development of a high-resistance connection at the stab/bus bar interface. The fault
and the resulting fire were primarily caused by the high-resistance connection at the stab/bus
interface, combined with the high starting current associated with large load.
The licensee identified the following two factors as contributing to high resistance at the stab/bus
bar interface:
(1) Preventive maintenance was inadequate. The existing procedures did not require
lubrication, which conflicted with the industry practice and cubicle installation instructions
contained in the MCC technical manual. Lubrication protects against aluminum oxidation
and plate damage. The preventive maintenance procedure did not require bus
maintenance (i.e., visual inspection, cleaning and lubrication of the bus/stab contact
surface), allowing degradation to continue unrecognized.
(2) High resistance developed in the connection because of the limited physical engagement
of the stab on the bus. The cubicle design allows each bucket some ability to flex.
However, this design made proper engagement depth difficult to achieve, especially on
large cubicles. A fully engaged stab would have only a 1/8-inch to 3/16-inch overlap on
the bus.
The MCC 2B-52 is an ITE 5600 Series MCC that was installed in 1975 at ANO 2 as original
plant equipment. The MCC model installed at ANO 2 had a history of problems, including brief
fires in the MCC, from 1984 to 2007. The preventive maintenance of the MCC was not
adequate. As a result of the October 23, 2007, fire in MCC 2B-52, the licensee took or planned
a number of corrective actions, including the following: * Revise the preventive maintenance procedures to provide guidance for inspecting, cleaning, and lubricating the bus/stab connections, specifically regarding the following
actions:
- Visually inspect the plating on the stab fingers and on the vertical bus at the point
where the stab fingers engage the vertical bus. If enough plating has worn off
such that bare metal is exposed (copper or aluminum), replace both the stab
assembly and the vertical bus.
- Perform a breaker stab tension inspection of each cubicle.
- Apply a thin film of NO-OX-ID grease to the cubicle stabs and contact surface of
the bus.
- For the high-risk ITE MCC cubicles, replace the existing stabs with longer ones or install
double stabs.
MCC columns with large loads.
Additional information related to the October 23, 2007, event is available in NRC Special
Inspection Reports 05000313/2007009 and 05000368/2007009, dated February 27, 2008, (Agencywide Documents Access and Management System (ADAMS) Accession
No. ML080590142).
The licensees root-cause analysis report also discussed another recent fault related to the ITE
5600 Series MCC, which occurred on January 31, 2007, at Calvert Cliffs Nuclear Power Plant. A
fault occurred on an ITE 5600 Series MCC because of a high-resistance stab connection on a
50-horsepower motor load caused by degradation of the bus-to-breaker stab connection. The
corrective actions taken by the Calvert Cliffs licensee included the installation of double stabs for
loads of 50 horsepower and larger and the periodic replacement of stabs.
DISCUSSION
The electrical fire at ANO 2 resulted in the partial loss of one power train of a Class 1E system;
however, the opposite power train was still available to provide power to the redundant loads, which would allow for the safe shutdown of the plant. Although the safety significance of this
event was not considered high because it impacted only one Class 1E division MCC, the
potential for a common-cause failure still existed without the implementation of the proposed
corrective actions.
NRC Office of Inspection and Enforcement Circular 77-03, Fire in a Motor Control Center, dated February 28, 1977, documented two events associated with the ITE 5600 Series MCC.
One event happened at Three Mile Island, Unit 2, where an MCC cubicle was not fully inserted
and therefore provided only partial engagement of the stabs. The resulting high-resistance
connection eventually led to a fire and loss of power from the MCC. The second event, which
occurred at the Trojan Nuclear Power Plant, involved misalignment of the stabs. Licensee Event Report 50-368/2006-001-00, dated December 21, 2006 (ADAMS Accession
No. ML070030511), discusses a previous event at ANO 2 involving a small MCC fire. The root
cause of this event was determined to be an inadequate MCC design that made it possible to
misalign the stabs such that both stab fingers end up on the same side of bus bar resulting in a
high resistance connection.
The high resistance of stab connections at the MCC could lead to a fire when large loads are
started. The following factors can cause high resistance in these connections:
- oxidation of the aluminum bus caused when the protective coating is worn off over a
period of time from the repeated action of the stabs
- poor maintenance practices, including a lack of lubrication at the stabbing contact area
- design deficiencies, such as misalignment of the stabs and insufficient stab contact
surfaces
In general, aluminum buses are more prone to oxidation as compared copper buses and
therefore to developing high-resistance in stab-type connections. Licensees are encouraged to
evaluate the design and anticipated frequency of operations of the loads within MCCs within
their facilities and develop preventive maintenance procedures that are appropriate for the
application conditions.
CONTACT
This IN requires no specific action or written response. Please direct any questions about this
matter to the technical contact listed below.
Timothy J. McGinty, Director Daniel H. Dorman, Director
Division of Policy and Rulemaking Division of Fuel Cycle Safety and Safeguards
Office of Nuclear Reactor Regulation Office of Nuclear Material Safety and Safeguards
Technical Contact:
Vijay Goel, NRR
(301) 415-3730
e-mail: vijay.goel@nrc.gov
NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov.
To access this information, select Electronic Reading Room and then Document Collections. Licensee Event Report 50-368/2006-001-00, dated December 21, 2006 (ADAMS Accession
No. ML070030511), discusses a previous event at ANO 2 involving a small MCC fire. The root
cause of this event was determined to be an inadequate MCC design that made it possible to
misalign the stabs such that both stab fingers end up on the same side of bus bar resulting in a
high resistance connection.
The high resistance of stab connections at the MCC could lead to a fire when large loads are
started. The following factors can cause high resistance in these connections:
- oxidation of the aluminum bus caused when the protective coating is worn off over a
period of time from the repeated action of the stabs
- poor maintenance practices, including a lack of lubrication at the stabbing contact area
- design deficiencies, such as misalignment of the stabs and insufficient stab contact
surfaces
In general, aluminum buses are more prone to oxidation as compared copper buses and
therefore to developing high-resistance in stab-type connections. Licensees are encouraged to
evaluate the design and anticipated frequency of operations of the loads within MCCs within
their facilities and develop preventive maintenance procedures that are appropriate for the
application conditions.
CONTACT
This IN requires no specific action or written response. Please direct any questions about this
matter to the technical contact listed below.
Timothy J. McGinty, Director Daniel H. Dorman, Director
Division of Policy and Rulemaking Division of Fuel Cycle Safety and Safeguards
Office of Nuclear Reactor Regulation Office of Nuclear Material Safety and Safeguards
Technical Contact:
Vijay Goel, NRR
(301) 415-3730
e-mail: vijay.goel@nrc.gov
NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov.
To access this information, select Electronic Reading Room and then Document Collections.
Distribution: IN Reading File
ADAMS Accession Number: ML082540130 *Concurrence by e-mail
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