Information Notice 2017-05, Potential Binding of Schneider Electric/Square-D Masterpact Nt and Nw 480-VAC Circuit Breaker Anti-Pump Feature
| ML17100B278 | |
| Person / Time | |
|---|---|
| Issue date: | 09/01/2017 |
| From: | Louise Lund, Mcginty T Division of Construction Inspection and Operational Programs, Division of Policy and Rulemaking |
| To: | |
| Govan T | |
| References | |
| TAC MF9367 IN 2017-05 | |
| Download: ML17100B278 (6) | |
ML17100B278 UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, DC 20555-0001
September 1, 2017
NRC INFORMATION NOTICE 2017-05:
POTENTIAL BINDING OF SCHNEIDER
ELECTRIC/SQUARE-D MASTERPACT NT AND
NW 480-VAC CIRCUIT BREAKER ANTI-PUMP
FEATURE
ADDRESSEES
All holders of an operating license or construction permit for a nuclear power reactor under
Title 10 of the Code of Federal Regulations (10 CFR) Part 50, Domestic Licensing of
Production and Utilization Facilities, except those that have permanently ceased operations
and have certified that fuel has been permanently removed from the reactor vessel.
All holders of and applicants for a power reactor early site permit, combined license, standard
design approval, or manufacturing license under 10 CFR Part 52, Licenses, Certifications, and
Approvals for Nuclear Power Plants. All applicants for a standard design certification, including
such applicants after initial issuance of a design certification rule.
PURPOSE
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to inform
addressees about recent issues related to the operation of Schneider Electric/Square-D
Masterpact 480-volt NT and NW circuit breakers. The design of the breaker results in a
susceptibility to internal binding in certain circumstances that can prevent the breaker from
closing on demand. The NRC expects that recipients will review the information for applicability
to their facilities and consider actions, as appropriate, to avoid similar problems. Suggestions
contained in this IN are not NRC requirements. Therefore, no specific action or written
response is required.
DESCRIPTION OF CIRCUMSTANCES
On March 9, 2015, during Division I emergency core cooling system and loss-of-coolant
accident testing at River Bend Station, control building chiller 1C shed from the electrical bus as
expected, but then failed to restart and sequence onto the emergency diesel generator.
Because of unrelated issues that prevented the other three chillers from starting, this resulted in
a loss of control room cooling. Control room ventilation duct air temperatures rose from 18 degrees Celsius (64.5 degrees Fahrenheit) to 23.9 degrees Celsius (75 degrees Fahrenheit)
before compensatory measures were implemented; the technical specification limit is
104 degrees Fahrenheit. The failure of chiller 1C to restart was the result of mechanical internal
binding of a 480-volt alternating current (Vac) Masterpact NT circuit breaker. The licensee later
determined that this same condition had been responsible for nine breaker failures at the site
from 2007-2015 and may have been a factor in six additional breaker failures. All of these breakers had been subject to a standing close signal, where the closing coil of the
breaker remained energized while the breaker was in its normal, closed position. This circuit
configuration set up a situation where any open signal, if received while the breaker was also
receiving a close signal, would activate the mechanical anti-pump interlocka feature
designed to prevent the circuit breaker from cycling between closing and opening. Testing
performed by the breaker vendor, AZZ/Nuclear Logistics, Inc. (AZZ/NLI), found that the
anti-pump mechanism was susceptible to mechanical internal binding of the closing coil plunger, which would prevent the breaker from closing until manual action was taken to operate the
breaker locally. The licensee identified susceptible breakers and reconfigured the circuitry so
the breakers would no longer be subject to a standing close signal.
Subsequently, the licensee reviewed notifications from AZZ/NLI of additional circuit alignments
that could lead to activation of the anti-pump interlock and potentially introduce the same failure
mechanism. Specifically, all Masterpact NT and NW style electrically/remotely-operated circuit
breakers are susceptible to the mechanical internal binding of the anti-pump lever and the
closing coil plunger if the breaker receives a start signal longer than 200 milliseconds during the
approximately 4 seconds it takes for the spring charging motor to recharge the closing springs.
The licensee identified additional impacted breakers that had not been included in the original
extent of condition checks because they were not subject to a standing close signal. They
included breakers for the emergency ventilation fans in the Division 1 and 2 emergency diesel
generator rooms, and breakers supplying the Division 1 and 2 containment unit coolers and the
Division 1 and 2 auxiliary building general area unit coolers. Subsequent notification from
AZZ/NLI alerted the licensee to further potential problems with breakers that had already been
modified to address the issue with the standing close signal. The affected breakers could be
manually operated to start or stop their associated equipment, if necessary for operation. The
licensee identified compensatory measures for each impacted breaker to restore system
operability until further modifications could be made. This included placing Division 1 systems
in continuous run when possible to avoid susceptibility to the failure mechanism and
implementing a standing order with dedicated operators to press the push to open button on
the breaker after any remote opening during power operations or hot-shutdown conditions. This
manual action would clear the binding condition if it occurred and allow the breaker to close if a
subsequent close signal was received.
The NRC chartered a special inspection to review the events surrounding the loss of control
room cooling. The results of the inspection are available in NRC Special Inspection Report 05000458/2015010, dated February 16, 2016 (Agencywide Documents Access and
Management System (ADAMS) Accession No. ML16047A268). Additional information is
available from the River Bend Licensee Event Reports 05000458/2016-005, dated
April 25, 2016 (ADAMS Accession No. ML16126A229), and 05000458/2016-006, dated
July 12, 2016 (ADAMS Accession No. ML16208A056), and from the 10 CFR Part 21, Reporting
of Defects and Noncompliance, report 2016-20-03 submitted by AZZ/NLI on
September 22, 2016 (ADAMS Accession No. ML16278A471).
BACKGROUND
The NRC IN 1988-75, Disabling of Diesel Generator Output Circuit Breakers by Anti-Pump
Circuitry, dated September 16, 1988 (ADAMS Accession No. ML031150110), and its
Supplement 1, dated April 17, 1989 (ADAMS Accession No. ML082970437), discuss the
circumstances in which simultaneous open and close signals for safety-related equipment
actuated anti-pump circuitry on the breakers that disabled the affected equipment until the
anti-pump circuit was manually reset.
DISCUSSION
Appendix B, Quality Assurance Criteria for Nuclear Power Plants and Fuel Reprocessing
Plants, to 10 CFR Part 50, Criterion III, Design Control, requires, in part, to subject design
changes to control measures commensurate with those applied to the original design. The
Masterpact breakers used at River Bend and at several other sites are a modification from the
original General Electric AKR electrically-operated breakers. The original breakers used an
electrical anti-pump interlock feature. The change from the electrical anti-pump interlock to a
mechanical anti-pump interlock feature introduces mechanical binding as a potential failure
mode.
Additional breaker testing by Schneider Electric identified the following five scenarios in which
the circuit breaker could be more susceptible to the mechanical binding condition that would
prevent the breaker from being able to reclose on command:
(1)
The closing circuit is continually energized during charge and/or open operations.
(2)
An anti-pump condition is present.
(3)
The breaker receives a command to open electrically before or at the same time as the
close command is initiated.
(4)
The operator initiates a local or remote electrical closing action that may hold the close
signal for longer than 200 milliseconds, which would extend into the closing spring
charging cycle.1
(5)
The logic scheme has a component controlling the close circuit that would apply the
voltage to the close coil for longer than 200 milliseconds, extending into the closing
spring charging cycle.1
In Technical Bulletin TB-12-007, Revision 3, which is attached to the referenced 10 CFR Part 21 report (ADAMS Accession No. ML16278A471), AZZ/NLI provided a proposed modification to
the breaker and an updated circuit diagram, shown in Figure 1. This modification replaces the
normal XF coil in the breaker with an XFCOM coil. The XFCOM coil acts as a oneshot and
releases the close coil plunger immediately after the close signal is applied to the breaker and
will not reactivate the close coil plunger unless power is first removed from the operating order
signal of the XFCOM and then re-applied. With the closing coil plunger retracted, this
modification is designed to eliminate the potential for mechanical binding from the anti-pump
feature.
1 The closing springs are electrically recharged automatically each time the breaker closes. Figure 1 The XF coil in the original logic is replaced by an XFCOM coil, which releases the close coil plunger as soon as the close signal is
received, eliminating the potential for binding
The time periods involved, on the order of a few seconds, make it unlikely that a breaker not
wired with a standing close signal will receive a close and an open signal in quick enough
succession to expose the breaker to this vulnerability. However, this situation could occur
during a design-basis scenario involving a loss of offsite power concurrent with a loss-of-coolant
accident. Compliance with the analysis of design-basis scenarios in a stations updated final
safety analysis report may require that the breakers be operable during these design-basis
accident scenarios.
CONTACT
This IN requires no specific action or written response. Please direct any questions about this
matter to the technical contact(s) listed below or the appropriate Office of Nuclear Reactor
Regulation or Office of New Reactors project manager.
/ra/ (Paul G. Krohn for)
/ra/ (Gregory Bowman for)
Timothy J. McGinty, Director
Louise Lund, Director
Division of Construction Inspection
Division of Policy and Rulemaking
and Operational Programs
Office of Nuclear Reactor Regulation
Office of New Reactors
Technical Contacts: Rebecca Sigmon, NRR/DIRS
301-415-0895
E-mail: Rebecca.Sigmon@nrc.gov
Samuel Graves, RIV/DRS
817-200-1102
E-mail: Samuel.Graves @nrc.gov
Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under NRC Library.
- concurred via e-mail
TAC No. MF9367 OFFICE
NRR/DPR/PGCB/LA*
Tech Editor*
NRR/DIRS/IOEB* RIV/DRS/EB2*
NRR/DE/EEEB/BC*
NAME
ELee
JDougherty
RSigmon
JWatkins
JQuichocho
DATE
04/17/17
04/24/17
05/01/17
05/24/17
07/03/17 OFFICE
NRO/DCIP/QVIBI/BC*
NRR/DIRS/IOEB/BC*
NRR/DIRS/D*
NRR/DPR/PGCB/LA* NRR/DPR/PGCB/PM
NAME
TJackson
HChernoff (EThomas for) CMiller (MKing for) ELee
TGovan
DATE
07/05 /17
07/27/17
08/22/17
08/23/17
08/25/17 OFFICE
NRR/DPR/PGCB/BC
NRO/DCIP/D
NRR/DPR/D
NAME
AGarmoe
TMcGinty (PKrohn for)
LLund (GBowman
for)
OFFICE
08/25/17
08/29/17
09/ 01 /17