Information Notice 2017-05, Potential Binding of Schneider Electric/Square-D Masterpact Nt and Nw 480-VAC Circuit Breaker Anti-Pump Feature

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.

ML17100B278

• 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