[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