[Federal Register Volume 69, Number 246 (Thursday, December 23, 2004)]
[Notices]
[Pages 76960-76962]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 04-28067]
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NUCLEAR REGULATORY COMMISSION
Proposed Interim Staff Guidance (ISG)-17 on; Periodic Inspection
of Bus Ducts for License Renewal Solicitation of Public Comment
AGENCY: Nuclear Regulatory Commission (NRC).
ACTION: Solicitation of public comment.
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SUMMARY: The NRC is soliciting public comment on its proposed interim
staff guidance (ISG) for license renewal. This ISG proposes an
acceptable aging management program (AMP), ``Periodic Inspection of Bus
Ducts,'' to manage the effects of aging on bus ducts during the period
of extended operation. The NRC staff issues ISGs to facilitate timely
implementation of the license renewal rule and to review activities
associated with a license renewal application. Upon receiving public
comments, the NRC staff will evaluate the comments and make a
determination to incorporate the comments, as appropriate. Once the NRC
staff completes the ISG, it will issue the ISG for NRC and industry
use. The NRC staff will also incorporate the approved ISG into the next
revision of the license renewal guidance documents.
DATES: Comments may be submitted by February 22, 2005. Comments
received after this date will be considered, if it is practical to do
so, but the Commission is able to ensure consideration only for
comments received on or before this date.
ADDRESSES: Comments may be submitted to: Chief Rules and Directives
Branch, Office of Administration, U.S. Nuclear Regulatory Commission,
Washington, DC 20555-0001.
Comments should be delivered to: 11545 Rockville Pike, Rockville,
Maryland, Room T-6D59, between 7:30 a.m. and 4:15 p.m. on Federal
workdays. Persons may also provide comments via e-mail at
[email protected]. The NRC maintains an Agencywide Documents Access and
Management System (ADAMS), which provides text and image files of NRC's
public documents. These documents may be accessed through the NRC's
Public Electronic Reading Room on the Internet at http://www.nrc.gov/reading-rm/adams.html. Persons who do not have access to ADAMS or who
encounter problems in accessing the documents located in ADAMS should
contact the NRC Public Document Room (PDR) reference staff at 1-800-
397-4209, (301) 415-4737, or by e-mail at [email protected].
FOR FURTHER INFORMATION CONTACT: Mr. Mark Lintz, License Renewal
Project Manager, Office of Nuclear Reactor Regulation, U.S. Nuclear
Regulatory Commission, Washington, DC 20555-0001; telephone (301) 415-
4051 or e-mail [email protected].
SUPPLEMENTARY INFORMATION: Attachment 1 to this Federal Register
notice, entitled Staff Position and Rationale for the Interim Staff
Guidance (ISG)-17: Proposed Aging Management Program (AMP) XI.E4,
``Periodic Inspection of Bus Ducts'' contains the NRC staff's rationale
for publishing ISG-17. Attachment 2 to this Federal Register notice,
entitled Proposed Aging Management Program: Periodic Inspection of Bus
Ducts, contains the proposed AMP required to implement ISG-17.
NRC Information Notice 89-64, entitled ``Electrical Bus Bar
Failures,'' and NRC Information Notice 98-36, entitled ``Inadequate or
Poorly Controlled, Non-Safety-Related Maintenance Activities
Unnecessarily Challenged Safety Systems,'' provide examples that
illustrate the importance of periodic inspection of bus ducts and the
potential problems that can arise from age-related bus duct failures.
Instances of the loosening of the bus bar connecting bolts at several
plants due to thermal cycling have been reported in Sandia National
Laboratory Report entitled Aging Management Guideline for Commercial
Nuclear Power Plants (Sandia 96-0344; September 1996) and in NRC
Information Notice 2000-14, entitled ``Non-Vital Bus Fault Leads to
Fire and Loss of Offsite Power.'' The last report identified torque
relaxation of splice plate connecting bolts as one potential cause of a
bus duct failure.
Operating experience has shown that electrical buses in bus ducts
have failed due to cracked insulation in the presence of moisture,
debris buildup, and loosening of bus connecting bolts. These failures
could lead to loss of power to electrical loads connected to the buses
and could cause unnecessary challenges to plant safety systems. To
prevent such failures, NRC has developed ISG-17 to ensure that:
(1) Internal portions of bus duct assemblies are free of corrosion,
debris, excessive dust buildup, and moisture intrusion;
(2) Electrical buses and their supports are free of insulation
cracking; and
(3) Bolted connections of the buses are secure.
Additionally, the external portions of bus ducts and structural
supports will also be inspected in accordance with a plant-specific
structural monitoring program.
Dated at Rockville, Maryland, this 16th day of December 2004.
For the Nuclear Regulatory Commission.
Pao-Tsin Kuo,
Program Director, License Renewal and Environmental Impacts Program,
Division of Regulatory Improvement Programs, Office of Nuclear Reactor
Regulation.
Attachment 1: Staff Position and Rationale for the Interim Staff
Guidance (ISG)-17: Proposed Aging Management Program (AMP) XI.E4,
``Periodic Inspection of Bus Ducts''
Staff Position
Consistent with the requirements specified in 10 CFR 54.4(a), bus
ducts (non-segregated phase bus and isolated phase buses) support
safety-related and non-safety-related functions in that the failure of
the bus ducts precludes a safety function from being accomplished [10
CFR 54.4(a)(1) and (a)(2)].
Thermal cycling of bus ducts can result in torque relaxation of
connecting bolts, causing loose connections that lead to arcing,
overheating, and explosive damage. Bus insulation material may
experience a significant temperature rise during operation that may
cause age-related degradation during the period of extended operation.
Insulation failure, along with the presence of moisture or debris, may
provide phase-to-phase or phase-to-ground electrical tracking paths
that eventually result in catastrophic failure of the bus ducts. These
bus ducts, therefore, need to be inspected periodically during the
period of extended operation to preclude their failure.
In order to prevent such occurrences, the staff has developed an
aging management program for periodic inspection of bus ducts for
license renewal to ensure that;
(1) Internal portions of bus duct assemblies are free of corrosion,
debris, excessive dust buildup, and moisture intrusion; and
(2) Electrical buses and their supports are free of insulation
cracking; and
(3) Bolted connections of the buses are secure.
Additionally, the external portions of bus ducts and structural
supports will also be inspected in accordance with a plant-specific
structural monitoring program.
Rationale
An electrical bus is an assembly of bus conductors with the
associated connections, joints, and insulating supports. Bus ducts are
electrical buses installed on electrically insulated supports and are
constructed with all
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phase conductors enclosed in either a separate metal enclosure or a
common metal enclosure. The conductors are separated and insulated from
each other and from the ground by insulating bus supports. Also, the
conductors in the non-segregated bus are insulated throughout to reduce
corona and electrical tracking. The bus ducts are used in power systems
that connect various elements of electric power circuits, such as
switchgears, transformers, main generators, and diesel generators.
Industry operating experience indicates that the failure of bus
ducts is caused by the cracking of bus bar insulation (bus sleeving)
combined with the accumulation of moisture or debris. Cracked
insulation results from high ambient temperatures and contamination
from bus bar joint compound. Cracked insulation in the presence of
moisture or debris provides phase-to-phase or phase-to-ground
electrical tracking paths, which results in catastrophic failure of the
buses. Bus failure leads to loss of power to electrical loads connected
to the buses, causes subsequent reactor trips, and initiates
unnecessary challenges to plant safety systems.
Bus ducts exposed to appreciable ohmic heating during operation may
experience loosening of bolted connections because of the repeated
cycling of connected loads. This phenomenon can occur in heavily loaded
circuits, i.e., those exposed to appreciable ohmic heating. A Sandia
National Laboratory Report entitled, Aging Management Guideline for
Commercial Nuclear Power Plants (Sandia 96-0344; September 1996)
documents instances of bolted connection loosening at several plants
due to thermal cycling. NRC Information Notice 2000-14, entitled Non-
Vital Bus Fault Leads to Fire and Loss of Offsite Power, identified
torque relaxation of splice plate connecting bolts as one potential
cause of a bus duct fault.
In addition to NRC Information Notice 2004-14, NRC Information
Notice 89-64, entitled Electrical Bus Bar Failures, and NRC Information
Notice 98-36, entitled Inadequate or Poorly Controlled, Non-Safety-
Related Maintenance Activities Unnecessary Challenged Safety Systems,
also provide examples that underscore the safety significance of bus
ducts and the potential problems that can arise from age-related bus
duct failures.
Attachment 2: Proposed Aging Management Program: Periodic Inspection of
Bus Ducts
XI.E4 Periodic Inspection of Bus Ducts
Program Description
An electrical bus is an assembly of bus conductors with the
associated connections, joints, and insulating supports. Bus ducts are
electrical buses installed on electrically insulated supports and are
constructed with all phase conductors enclosed in either a separate
metal enclosure or a common metal enclosure. The conductors are
separated and insulated from each other and from the ground by
insulating supports. Also, the conductors in the non-segregated bus are
insulated throughout to reduce corona and electrical tracking. The bus
ducts are used in power systems that connect various elements of
electric power circuits, such as switchgears, transformers, main
generators, and diesel generators.
Industry operating experience indicates that the failure of bus
ducts is caused by the cracking of bus bar insulation (bus sleeving)
combined with the accumulation of moisture or debris. Cracked
insulation results from high ambient temperatures and contamination
from bus bar joint compound. Cracked insulation in the presence of
moisture or debris provides phase-to-phase or phase-to-ground
electrical tracking paths, which results in catastrophic failure of the
buses. Bus failure leads to loss of power to electrical loads connected
to the buses, causes subsequent reactor trips, and initiates
unnecessary challenges to plant systems.
Bus ducts exposed to appreciable ohmic heating during operation may
experience loosening of bolted connections because of the repeated
cycling of connected loads. This phenomenon can occur in heavily loaded
circuits, i.e., those exposed to appreciable ohmic heating. Sandia 96-
0344 identified instances of bolted connection loosening at several
plants due to thermal cycling. NRC Information Notice 2000-14
identified torque relaxation of splice plate connecting bolts as one
potential cause of a bus duct fault.
One objective of the aging management program is to provide an
inspection of bus ducts. In managing this aspect of the aging
management program, bolted connections at sample sections of the buses
in the bus ducts will be checked for proper torque, or the bolted
joints will be checked for low resistance. This activity will include
visual inspection of interior portions of bus ducts to identify aging
degradation of insulating and metallic components and water/debris
intrusion. The external portions of bus ducts and structural supports
will be inspected in accordance with a plant-specific structural
monitoring program.
Evaluation and Technical Basis
1. Scope of Program: This program applies to all bus ducts within
the scope of license renewal.
2. Preventive Actions: This is an inspection program and no actions
are taken as part of this program to prevent or mitigate aging
degradation.
3. Parameters Monitored/Inspected: A sample of accessible bolted
connections (bus joints and ending devices) will be checked for proper
torque, or the resistance of bolted joints will be checked using a
micro-ohm meter of sufficient current capacity that is suitable for
checking bus bar connections. This program will also inspect the
internal portions of accessible bus ducts for cracks, corrosion,
foreign debris, dust buildup, and moisture intrusion. The bus
insulating system will be inspected for signs of embrittlement,
cracking, melting, swelling, or discoloration, which may indicate
overheating or aging degradation. The bus supports will be inspected
for structural integrity and cracking.
4. Detection of Aging Effects: Visual inspection of internal
portions of bus ducts detects cracks, corrosion, debris, dust and
moisture. Visual inspection of the bus insulating system detects
embrittlement, cracking, melting, swelling and discoloration. Visual
inspection of bus supports detects cracking and lack of structural
integrity. Internal portions of bus ducts, the bus insulating system,
and the bus supports are visually inspected at least once every 10
years.
A torque test or a resistance test of a sample of accessible bolted
connections is performed at least once every 10 years. This program
will be completed before the end of the initial 40-year license term
and every 10 years thereafter. This is an adequate period to identify
failures of the bus ducts since experience has shown that aging
degradation is a slow process. A 10-year inspection frequency will
provide two data points during a 20-year period, which can be used to
characterize the degradation rate.
5. Monitoring and Trending: Trending actions are not included as
part of this program because the ability to trend inspection results is
limited. Although not a requirement, trending would
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provide additional information on the rate of degradation.
6. Acceptance Criteria: Bolted connections must meet the
manufacturer's minimum torque specifications, or the resistance of
bolted joints must meet required specifications. Bus ducts are to be
free from any surface anomalies that suggest that conductor insulation
degradation exists. An additional acceptance criterion includes no
indication of unacceptable corrosion, cracking, foreign debris, dust
buildup, or moisture intrusion. Any condition or situation that, if not
corrected, could lead to a loss of intended function is considered
unacceptable.
7. Corrective Actions: Further investigation and evaluation is
performed when the acceptance criterion is not met. Corrective actions
may include but are not limited to sample expansion, increased
inspection frequency, and replacement or repair of the affected bus
duct insulation components. When an unacceptable or situation is
identified, a determination shall be made as to whether the same
condition or situation is applicable to other areas, and sample
expansion shall include those areas. As discussed in the appendix to
this report, the staff finds the requirements of 10 CFR part 50,
appendix B, acceptable to address corrective actions.
8. Confirmation Process: As discussed in the appendix to this
report, the staff finds the requirements of 10 CFR part 50, appendix B,
acceptable to address the confirmation process.
9. Administrative Controls: As discussed in the appendix to this
report, the staff finds the requirements of 10 CFR part 50, appendix B,
acceptable to address administrative controls.
10. Operating experience: Industry operating experience has
demonstrated that the failures of bus ducts are caused by cracked
insulation of the bus combined with moisture or debris buildup internal
to the bus ducts. It has also been shown that bus duct internals
exposed to appreciable ohmic heating during operation may experience
loosening of bolted connections related to repeated cycling of
connected loads.
References
1. IEEE Std. P1205-2000, IEEE Guide for Assessing, Monitoring
and Mitigating Aging Effects on Class 1E Equipment Used in Nuclear
Power Generating Stations.
2. SAND 96-0344, Aging Management Guideline for Commercial
Nuclear Power Plants--Electrical Cable and Terminations, prepared by
Sandia National Laboratories for the U.S. Department of Energy,
September 1996.
3. EPRI TR-109619, Guideline for the Management of Adverse
Localized Equipment Environments, Electric Power Research Institute,
Palo Alto, CA, June 1999.
4. EPRI TR-104213, Bolted Joint Maintenance & Application Guide,
Electric Power Research Institute, Palo Alto, CA, December 1995.
5. NRC Information Notice 89-64, ``Electrical Bus Bar
Failures.''
6. NRC Information Notice 98-36, ``Inadequate or Poorly
Controlled, Non-Safety-Related Maintenance Activities Unnecessary
Challenged Safety Systems.''
7. NRC Information Notice 2000-14, ``Non-Vital Bus Fault Leads
to Fire and Loss of Offsite Power.''
[FR Doc. 04-28067 Filed 12-22-04; 8:45 am]
BILLING CODE 7590-01-P