Information Notice 2014-10, Unfused DC Ammeter Circuits and Potential for Fire Events
| ML14169A264 | |
| Person / Time | |
|---|---|
| Issue date: | 09/16/2014 |
| From: | Michael Cheok, Kokajko L Division of Construction Inspection and Operational Programs, Division of Policy and Rulemaking |
| To: | |
| References | |
| TAC MF4189 IN-14-010 | |
| Download: ML14169A264 (5) | |
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
OFFICE OF NEW REACTORS
WASHINGTON, DC 20555-0001 September 16, 2014 NRC INFORMATION NOTICE 2014-10: POTENTIAL CIRCUIT FAILURE-INDUCED
SECONDARY FIRES OR EQUIPMENT DAMAGE
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 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 of recent operating experience related to a potential for circuit failure-induced
secondary fire events or equipment damage. The NRC expects that recipients will review the
information for applicability to their facilities and consider actions, as appropriate. Suggestions
contained in this IN are not NRC requirements; therefore, no specific action or written response
is required.
DESCRIPTION OF CIRCUMSTANCES
This section describes events caused by a possible unanalyzed condition involving a potential
for circuit failure-induced secondary fire events or equipment damage. This unanalyzed
condition has been reported in event notifications (ENs) and licensee event reports (LERs) to
the NRC. Three examples are discussed below.
Browns Ferry Nuclear Plant
On October 15, 2011, the Browns Ferry Nuclear (BFN) Plant reported this issue via EN 47374 and subsequently on December 27, 2011, per BFN LER 2011-010 DC Ammeter Cables Not
Adequately Isolated (Agencywide Documents Access and Management System (ADAMS)
Accession No. ML12003A256). The LER documents that during reviews to transition to the
National Fire Protection Association (NFPA) Standard 805, "Performance-Based Standard for
ML14169A264 Fire Protection for Light-Water Reactor Electric Generating Plants" (NFPA 805), the licensee
identified that the ammeters located in the main control room (MCR) for Battery Boards 1, 2, and 3 were electrically connected to the safety-related 250-volt dc bus at the battery boards
without electrical isolation. This condition may have the potential to cause secondary fires if two
ungrounded common power supply circuit faults occur during a fire that impacts the ammeter.
This condition impacted all three nuclear units, as the electrical isolation requirements for the
MCR ammeters were not maintained. This constituted a fire protection program deficiency that
could adversely affect the ability to achieve and maintain safe shutdown of the plant in the
unlikely event of a MCR fire. At the time of discovery, fire watches were already in place and
will remain in place until the identified condition is resolved. Corrective actions included the
addition of fuses to the ammeter circuits.
Clinton Power Station
During a Clinton Power Station (CPS) review of the event notification at the BFN Plant (EN 47374), CPS determined that the original plant design for the station battery ammeter circuits
contains a shunt in the current flow from each station battery. Attached to the shunt bar are two
leads to an ammeter in the MCR. The ammeter circuits attached to the shunt does not have
fuses, and if one of the ammeter circuits shorts to ground due to a fire at the same time another
dc circuit from the opposite polarity on the same battery also shorts to ground, a ground loop
through the unfused ammeter circuit could occur. The circuit was bounded by testing showing
that self-ignition of a secondary fire was not possible. However, the potential exists that
overloaded ammeter circuit could result in thermal damage to safe shutdown circuits in direct
physical contact with the ammeter circuit, resulting in a loss of the associated safe shutdown
function or capability. CPS reported this information to the NRC on January 27, 2012, in LER
2011-007-00 (ADAMS Accession No. ML12053A099).
Nine Mile Point Nuclear Station, Unit 1
During a review of industry operating experience regarding the potential for secondary fires
being caused by dc motor control circuits, Nine Mile Point Nuclear Station determined that the
described condition could be applicable to Nine Mile Point, Unit 1. This resulted in a potentially
unanalyzed condition with respect to 10 CFR Part 50, Appendix R analysis requirements. The
original plant design and configuration for a containment isolation valve included only over
current protection for the power circuits. There was no separate, properly-sized fuse protection
for the control circuits. The only protection for the control circuits was provided by the power
circuit fuses.
In the postulated event, a fire in the reactor building at the dc motor operated valve could cause
one of the control circuits to short and potentially over heat since the current draw from the short
circuit would not be large enough to open the power circuit fuse, creating the potential for a
secondary fire in a different fire zone. The secondary fire could adversely affect safe shutdown
equipment and potentially affect the ability to achieve safe shutdown as required by 10 CFR
Part 50, Appendix R. This event was entered into the Nine Mile Point corrective action program and reported via EN 50098 and subsequently on July 8, 2014, in LER 2014-002-00 Unanalyzed Condition Due to
Unfused Motor Operated Valve Control Circuit (ADAMS Accession No. ML14205A279).
DISCUSSION
In the course of licensees performing self-assessments of their fire protection programs, several
facilities have reported issues involving a potential for circuit failure-induced secondary fire
events or thermal damage to equipment. In many cases, the issue is related to a lack of circuit
isolation on ammeter circuits and, therefore, these circuits present a potential fire source in
equipment if the ammeter circuit becomes grounded coincident with another circuit fault. In
some other cases, the issue is related to a lack of circuit isolation for power and control circuits
that can result in overheated cabling due to an electrical short and cause a secondary fire or
thermal damage. In some cases, safety-related equipment could be impacted due to thermal
damage. Note that there have been no fires reported to the NRC related to these issues.
However, licensees are reporting the potential for this to occur and impact associated safe
shutdown functions or capabilities.
Based on additional industry operating experience reviews, licensees have determined that
unanalyzed conditions may exist with respect to their existing deterministic fire protection
program requirements and analysis requirements involving dc ammeter circuits. Postulated
events could cause an unfused dc ammeter circuit to be grounded. If the event also causes
another dc circuit of the opposite polarity on the same component to become grounded, a
ground loop through the unfused dc ammeter circuit could then exist. This ground loop could
draw excessive current and the electrical conductor could overheat and damage adjacent
cables, or may result in a secondary fire or damage depending on the cable size, insulation, and
whether the type of cable would support self-ignition. NRC guidance contained in Generic
Letter (GL) 81-12 - Fire Protection Rule (45 FR 76602, 11/19/80) concerning associated circuits
of common enclosures, aims to prevent secondary fires and equipment damage from adversely
affecting safe shutdown. Licensee responses to GL 81-12 typically indicate that all circuits are
provided with appropriately-sized circuit protection devices.
As noted in the LERs, compensatory measures (i.e., fire watches or other appropriate
compensatory measures) for this type of issue were instituted for the affected areas of the plant.
Some licensees have reported taking corrective actions for the circuits that might be susceptible
to this unanalyzed condition in order to remove the possibility of grounded fault-induced damage
in these circuits. At least one licensees corrective actions included planned modifications to
install fusing in the susceptible circuits to eliminate the potential for such events or damage.
Licensees are encouraged to review the information provided through this IN and review similar
industry internal operating experience, as appropriate. Lessons learned during NFPA 805 transition evaluations may also be useful for other sites to consider. Licensee extent of
condition reviews may be appropriate for other similar potential hot short concerns to help
ensure plant safety.
CONTACT
This IN requires no specific action or written response. Please direct any questions about this
matter to one of the technical contacts listed below or to the appropriate Office of Nuclear
Reactor Regulation or Office of New Reactors project manager.
/RA/ /RA/
Michael C. Cheok, Director Lawrence E. Kokajko, Director
Division of Construction Inspection Division of Policy and Rulemaking
and Operational Programs Office of Nuclear Reactor Regulation
Office of New Reactors
Technical Contact:
Brian Metzger, NRR
301-415-3972 E-mail: brian.metzger@nrc.gov
Note: NRC generic communications can be found on the NRC public Web site, http://www.nrc.gov, under Document Collections in the NRC Library.
- via e-mail TAC MF4189 OFFICE NRR/DRA/AFPB Tech Editor QTE NRR/DRA NRR/DRA BC: RES/FRB
NAME Metzger JDougherty * DFrumkin AKlein MSalley*
DATE 06 / 26 /14 06 / 23 /14 08 / 14 /14 08 / 19 /14 08 / 18 /14 OFFICE NRO/DSRA/BPFP NRO/DSRA BC: LA:PGCB:NRR PM:PGCB:NRR BC:PGCB:NRR
NAME RVettori ADias CHawes MBanic SStuchell
DATE 08 / 12 /14 08/13/14 08/27/14 08/27/14 9 / 2 /14 OFFICE D: DCIP: NRO DD:DPR:NRR D:DPR:NRR
NAME MCheok AMohseni LKokajko
DATE 9 / 5 /14 9 / 9 / 14 9 / 16 / 14