[Federal Register Volume 70, Number 146 (Monday, August 1, 2005)]
[Notices]
[Pages 44127-44130]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 05-15124]
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NUCLEAR REGULATORY COMMISSION
Proposed Generic Communication Inaccessible or Underground Cable
Failures That Disable Accident Mitigation Systems
AGENCY: Nuclear Regulatory Commission.
ACTION: Notice of opportunity for public comment.
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SUMMARY: The U.S. Nuclear Regulatory Commission (NRC) is proposing to
issue a generic letter (GL) to:
Alert the licensees on the potential susceptibility of certain
cables to affect the operability of multiple accident-mitigation
systems;
Request that addressees provide information regarding the
monitoring of the inaccessible or underground electrical cables in
light of the information provided in this letter. Adequate monitoring
will ensure that cables will not fail abruptly and cause plant
transients or disable accident mitigation systems when they are needed;
Require addressees, to submit a written response to this generic
letter pursuant to 10 CFR 50.54(f).
This Federal Register notice is available through the NRC's
Agencywide Documents Access and Management System (ADAMS) under
accession number ML050880448.
DATES: Comment period expires September 30, 2005. Comments submitted
after this date will be considered if it is practical to do so, but
assurance of consideration cannot be given except for comments received
on or before this date.
ADDRESSES: Submit written comments to the Chief, Rules and Directives
Branch, Division of Administrative Services, Office of Administration,
U.S. Nuclear Regulatory Commission, Mail Stop T6-D59, Washington, DC
20555-0001, and cite the publication date and page number of this
Federal Register notice. Written comments may also be delivered to NRC
Headquarters, 11545 Rockville Pike (Room T-6D59), Rockville, Maryland,
between 7:30 a.m. and 4:15 p.m. on Federal workdays.
FOR FURTHER INFORMATION, CONTACT: Thomas Koshy at 301-415-1176 or by e-
mail [email protected].
SUPPLEMENTARY INFORMATION: NRC Generic Letter 2005-XX, Inaccessible or
Underground Cable Failures that Disable Accident Mitigation Systems.
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.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this
generic letter to:
(1) Alert the licensees on the potential susceptibility of certain
cables to affect the operability of multiple accident-mitigation
systems.
(2) Request that addressees provide information regarding the
monitoring of the inaccessible or underground electrical cables in
light of the information provided in this letter. Adequate monitoring
will ensure that cables will not fail abruptly and cause plant
transients or disable accident mitigation systems when they are needed.
Pursuant to 10 CFR 50.54(f), addressees are required to submit a
written response to this generic letter.
Background
Cable failures have a variety of causes: Manufacturing defects,
damage caused by shipping and installation, and exposure to electrical
transients or abnormal environmental conditions during operation. Most
of these defects worsen gradually over time as insulation degradation
leads to cable failure.
Electrical cables in nuclear power plants are usually located in
dry environments. However, some cables are exposed to moisture from
condensation and wetting in inaccessible locations such as buried
conduits, cable trenches, cable troughs, duct banks, underground vaults
and direct buried installations. Cables in these environments can fail
due to various failure mechanisms such as water treeing (physical
degradation), electrical treeing or other mechanisms of insulation
degradation over varying voltage levels that decrease the dielectric
strength of the conductor insulation.
Information Notice (IN) 2002-12 described medium-voltage cable
failures at Oyster Creek and Davis-Besse and several other plants which
experienced long-term flooding problems in manholes and duct banks in
which safety related cables were submerged. In response to the concern
identified in IN 2002-12, several plants began manhole restoration
projects to replace faulty dewatering equipment and cable supports and
made other modifications. Several other plants have reported water
removal problems but have not yet
[[Page 44128]]
reported any program for the early detection of potential failures.
The rugged design of the electrical cables may prevent early
failures even when they have been immersed in water for extended
periods. When the staff observed that some of the cables qualified for
40 years through the equipment qualification program were also failing
at several nuclear stations, a detailed review was conducted. Even
though there are only about a dozen cables susceptible for moisture-
induced damage in a nuclear station, the staff identified 23 Licensee
Event Reports (LERs) and morning reports since 1988 on failures of
buried medium-voltage cables from insulation failure. These reported
events are believed to be only a very small fraction of the failures
since not all cable failures are reportable. In most of the reported
cases, the failed cables were in service for 10 years or more and none
of these cables were identified as designed or qualified for long-term
wetting or submergence.
Applicable Regulatory Requirements
NRC regulations in title 10 of the Code of Federal Regulations
(CFR) part 50, Appendix A, General Design Criterion (GDC) 4 states
that, ``Structures, systems, and components important to safety shall
be designed to accommodate the effects of and to be compatible with the
environmental conditions associated with normal operation[.]''
10 CFR, part 50, Appendix A, GDC 17 states that, ``Provisions shall
be included to minimize the probability of losing electric power from
any of the remaining [power] supplies, * * * loss of power from the
transmission network, or the loss of power from the onsite electric
power supplies.''
10 CFR, part 50, Appendix A, GDC 18 states that, ``Electric power
systems important to safety shall be designed to permit appropriate
periodic inspection and testing of important * * * features, such as
wiring, insulation, * * * the operability of the systems as a whole
and, * * * the transfer of power among the nuclear power unit, the
offsite power system, and the onsite power system.''
10 CFR 50.65(a)(1) states that, ``Each holder of a license to
operate a nuclear power plant * * * shall monitor the performance or
condition of structures, systems, or components, * * * in a manner
sufficient to provide reasonable assurance that such structures,
systems, and components, * * * are capable of fulfilling their intended
functions.''
10 CFR, part 50, Appendix B, Criterion XI, requires, ``A test
program shall be established to assure that all testing required to
demonstrate that * * * components will perform satisfactorily in
service is identified and performed[.]''
These design criteria require that cables which are routed
underground be capable of performing their function when subjected to
anticipated environmental conditions such as moisture or flooding.
Further, the design should minimize the probability of power
interruption when transferring power between sources. The cable
failures that could disable risk-significant equipment are expected to
have monitoring programs to demonstrate that the cables can perform
their safety function when called on. However, the recent industry
cable failure data indicates a trend in unanticipated failures of
underground/inaccessible cables that are important to safety.
Discussion
Although nuclear plant systems are designed against single
failures, undetected degradation of cables due to pre-existing
manufacturing defects or wetted environments of buried or inaccessible
cables could result in multiple equipment failures. The following are
examples of risk-significant cable failures:
The failure of power cables that connect the offsite power
to the safety bus could result in an inability to recover offsite power
far beyond the coping time considered for station blackout conditions.
The incipient failures of these cables can go undetected because these
cables generally remain de-energized when the plant is generating
power.
The failure of the power cables from an emergency diesel
generator (EDG) to the respective safety bus (where the EDGs are
located in separate buildings) would prevent recovery of standby power
from the respective EDG and result in the unavailability of a full
train of accident mitigation systems during a loss-of-offsite-power
event (LOOP).
The failure of the power cables to an emergency service
water (ESW) or component cooling water pump can disable one train of
emergency core cooling systems for long-term service unless the headers
can be cross-connected and the redundant pump(s) can be lined up to
supply sufficient cooling for both trains. If the EDGs are cooled from
ESW or service water, the cable failure could disable the EDG and lose
one train of standby power.
At the Davis-Besse nuclear station, an underground cable insulation
failure resulted in the trip of the 13.8kV circulating water pump
breaker and loss of power to two other 4kV substations. The cable
showed signs of insulation degradation caused by moisture intrusion
(Inspection Report No: 05000346/2004017, ADAMS Accession No:
ML050310426, issued on January 30, 2005). Generally, cable failure
results in fault currents several orders of magnitude over the normal
current. Until isolated by a breaker, the fault current or transient
voltages travel on the immediate power systems, trip breakers that
operate near their trip setpoint and fail other degraded insulation
systems.
As cables that are not qualified for wet environments are exposed
to wet environments, they will continue to degrade with an increasing
possibility that more than one cable will fail on demand from a cable
fault or a switching transient. While a single failure may be
manageable, multiple failures of this kind would pose undue challenges
for the plant operators.
Certain plants have reported failures in other safety systems such
as auxiliary feedwater and containment spray systems with AC and DC
power and control cables routed underground or along other inaccessible
paths. Those degraded cables that are normally energized may fail to
reveal their degraded condition, and the potential failure of the de-
energized safety systems might only be revealed during a demand for the
mitigation capability.
Certain licensees have attempted to periodically drain the
accumulated water from the cable surroundings to avoid cable failures.
In areas where the water table is relatively close to the cable, the
water refills the cavity soon after the draining. In other cases, the
water accumulates seasonally during snow fall or rain, filling the
conduit or raceways, and cables may dry out whenever the humidity
drops. In both cases, periodic draining may decrease the rate of
insulation degradation but it does not prevent cable failures.
Potential cable failures can be detected through state-of-the-art
techniques for measuring and trending the condition of cable
insulation. The cables that are susceptible to moisture-induced
failures may vary from plant to plant, and they are generally routed in
underground conduits, concrete duct banks, cable trenches, cable
troughs, underground vaults or direct buried installations. Selective
use of testing techniques, such as the partial discharge test, time
domain reflectometry, dissipation factor testing, very low frequency AC
testing, and broadband impedance spectroscopy, have helped licensees
assess the condition of cable insulation with reasonable confidence,
[[Page 44129]]
such that cables can be replaced in a planned way during refueling
outages. The Oconee Nuclear Station relied on the partial discharge
test to monitor the condition of the emergency power supply cable
insulation and replaced the cable during a scheduled outage (Inspection
Report 50-269/99-12, 50-270/99-12, ADAMS Accession No: ML0036767490
issued on September 21, 1999).
A diagnostic cable test program provides reasonable confidence that
the cable will perform its intended function. The frequency of the test
should be commensurate with the observed cable test results. To avoid
unplanned outages and unanticipated failures, certain licensees have
adopted a baseline frequency of 5 years for new cables or more frequent
testing when insulation degradation is observed.
Requested Information
Within 90 days of the date of this generic letter, addressees are
requested to provide the following information to the NRC:
(1) Provide a history of inaccessible or underground cable
failures, that are within the scope of 10 CFR 50.65 (the Maintenance
Rule), for all voltage levels indicating the type, voltage class, years
of service and the root causes for the failure.
(2) Provide a description and frequency of all inspection, testing
and monitoring programs, including surveillance programs, to detect
degradation of inaccessible or underground cables used to support EDGs,
offsite power, emergency service water, service water, component
cooling water and other systems that are within the scope of 10 CFR
50.65 (the Maintenance Rule).
(3) If a program as described in (2) is not in place, explain why
you believe such a program is not necessary.
The required written response should be addressed to the U.S.
Nuclear Regulatory Commission, ATTN: Document Control Desk, 11555
Rockville Pike, Rockville, Maryland 20852, under oath or affirmation
under the provisions of Section 182a of the Atomic Energy Act of 1954,
as amended, and 10 CFR 50.54(f). In addition, a copy of the response
should be sent to the appropriate regional administrator.
Required Response
In accordance with 10 CFR 50.54(f), addressees are required to
submit written responses to this generic letter. There are two options:
(a) Addressees may choose to submit written responses providing the
information requested above within the requested time period.
(b) Addressees who cannot meet the requested completion date or who
choose an alternate course of action are required to notify the NRC of
these circumstances in writing as soon as possible but no later than 60
days from the date of this generic letter. The response must address
any alternative course of action proposed, and the basis for the
acceptability of the proposed alternative course of action.
Reasons for Requested Information
This generic letter requests addressees to submit information. The
requested information will enable the NRC staff to determine whether
applicable requirements (10 CFR part 50, Appendix A, General Design
Criteria 4, 17 and 18; 10 CFR 50.65, and 10 CFR part 50, Appendix B,
Criterion XI) are being met in regard to the operational readiness of
the power system and accident mitigation systems and whether additional
action is necessary on those topics. The staff considers 40 hours of
information collection burden to be reasonable in light of the benefit
gained to identify and correct unanticipated failures of accident
mitigation systems.
Backfit Discussion
Under the provisions of section 182a of the Atomic Energy Act of
1954, as amended, and 10 CFR 50.54(f), this generic letter transmits an
information request for the purpose of verifying compliance with
applicable existing requirements. Specifically, the requested
information will enable the NRC staff to determine whether applicable
requirements (plant Technical Specification in conjunction with 10 CFR
part 50, Appendix A, General Design Criteria 4, 17 and 18; 10 CFR
50.65, and 10 CFR part 50, Appendix B Criterion XI) are being met in
regard to the operation readiness of the power system. No backfit is
either intended or approved in the context of issuance of this generic
letter. Therefore, the staff has not performed a backfit analysis.
Federal Register Notification
A notice of opportunity for public comment on this generic letter
was published in the Federal Register on (xx Frxxxxx) on {date{time} .
Comments were received from {indicate no of commentors by type{time} .
The staff considered all comments that were received. The staff's
evaluation of the comments is publicly available through the NRC's
ADAMS under Accession No. ML052020036.
Paperwork Reduction Act Statement
This generic letter contains information collection requirements
that are subject to the Paperwork Reduction Act of 1995 (44 U.S.C. 3501
et seq.). These information collections were approved by the Office of
Management and Budget, approval No: 3150-0011, which expires on
February 28, 2007.
The burden to the public for these mandatory information
collections is estimated to average 40 hours per response, including
the time for reviewing instructions, searching existing data sources,
gathering and maintaining the data needed, and completing and reviewing
the information collection. The U.S. Nuclear Regulatory Commission is
seeking public comment on the potential impact of the information
collection contained in the generic letter and on the following issues:
1. Is the proposed information collection necessary for the proper
performance of the functions of the NRC, including whether the
information will have practical utility?
2. Is the estimate of burden accurate?
3. Is there a way to enhance the quality, utility, and clarity of
the information to be collected?
4. How can the burden of the information collection be minimized,
including the use of automated collection techniques?
Send comments regarding this burden estimate or any other aspect of
these information collections, including suggestions for reducing the
burden, to the Records and FOIA/Privacy Services Branch (T-5 F52), U.S.
Nuclear Regulatory Commission, Washington, DC 20555-0001, or by
Internet electronic mail to [email protected]; and to the Desk
Officer, Office of Information and Regulatory Affairs, NEOB-10202,
(3150-0011), Office of Management and Budget, Washington, DC 20503.
Public Protection Notification
The NRC may not conduct or sponsor, and a person is not required to
respond to, a request for information or an information collection
requirement unless the requesting document displays a currently valid
OMB control number.
Contacts
Please direct any questions about this matter to the technical
contact listed below or the appropriate Office of Nuclear Reactor
Regulation (NRR) project manager. Bruce A. Boger, Director, Division of
Inspection Program Management, Office of Nuclear Reactor Regulation.
[[Page 44130]]
Technical Contact: Thomas Koshy, NRR, 301-415-1176. E-mail:
[email protected].
End of Draft Generic Letter
Documents may be examined, and/or copied for a fee, at the NRC's
Public Document Room at One White Flint North, 11555 Rockville Pike
(first floor), Rockville, Maryland. Publicly available records will be
accessible electronically from the Agencywide Documents Access and
Management System (ADAMS) Public Electronic Reading Room on the
Internet at the NRC Web site, http://www.nrc.gov/NRC/ADAMS/index.html.
If you do not have access to ADAMS or if you have problems in accessing
the documents in ADAMS, contact the NRC Public Document Room (PDR)
reference staff at 1-800-397-4209 or 301-415-4737 or by e-mail to
[email protected].
Dated in Rockville, Maryland, this 22nd day of July 2005.
For the Nuclear Regulatory Commission.
Patrick L. Hiland,
Chief, Reactor Operations Branch, Division of Inspection Program
Management, Office of Nuclear Reactor Regulation.
[FR Doc. 05-15124 Filed 7-29-05; 8:45 am]
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