XI.E4 (NUREG-2191 R0)
XI.E4 METAL ENCLOSED BUS
Program Description
The purpose of this aging management program (AMP) is to provide an internal and external inspection of metal enclosed buses (MEBs) within the scope of subsequent license renewal (SLR) to identify age-related degradation of electrical insulating material (i.e., porcelain, xenoy, thermoplastic organic polymers), and metallic and elastomer components (e.g., gaskets, boots, and sealants). This AMP provides reasonable assurance that in-scope MEBs will be maintained consistent with the current licensing basis (CLB) through the subsequent period of extended operation.
MEBs are electrical buses installed on electrically insulated supports that are constructed with each phase conductor enclosed in a separate metal enclosure (isolated phase bus), all conductors enclosed in a common metal enclosure (nonsegregated bus), or all phase conductors in a common metal enclosure, but separated by metal barriers between phases (segregated bus). The conductors are adequately separated and insulated from ground by insulating supports or bus electrical insulation. The MEBs are used in power systems to connect various elements in electric power circuits, such as switchgear, transformers, main generators, and diesel generators.
Industry operating experience (OE) indicates that the primary failure modes of MEBs have been caused by cracked electrical insulation, moisture, debris, loose connections, corrosion, or excessive dust buildup internal to the bus housing. Cracked insulation has resulted from high ambient temperature and contamination from bus bar joint compounds. Cracked electrical insulation in the presence of moisture or debris has caused phase-to-phase or phase-to-ground electrical paths, which has resulted in catastrophic failure of the buses. Significant ohmic heating of the bus may result in loosening of bolted connections associated with repeated cycling of connected loads. Bus failure has led to loss of power to electrical loads connected to the buses, causing subsequent reactor trips and initiating unnecessary challenges to plant systems and operators.
MEBs may experience increased resistance of connection due to loosening of bolted bus duct connections caused by repeated thermal cycling of connected loads. This phenomenon can occur in heavily loaded circuits (i.e., those exposed to appreciable ohmic heating). For example, SAND96-0344 identified instances of termination loosening at several plants due to thermal cycling and U.S. Nuclear Regulatory Commission Information Notice 2000-14 identified torque relaxation of splice plate connecting bolts as one potential cause of MEB failures.
This AMP includes the inspection of accessible bus ducts and a sample of MEB bolted connections within the scope of license renewal for increased resistance of connections.
Evaluation and Technical Basis
- 1. Scope of Program: This AMP manages the age-related degradation effects for electrical bus bar bolted connections, bus bar electrical insulation, bus bar insulating supports, bus enclosure assemblies (internal and external), and elastomers. This program does not manage the aging effects on external bus structural supports, which are managed under Generic Aging Lessons Learned for Subsequent License Renewal (GALL-SLR) Report AMP XI.S6, “Structures Monitoring.” Alternatively, the aging effects on elastomers can be managed under GALL-SLR Report AMP XI.M38, “Inspection of Internal Surfaces in Miscellaneous Piping and Ducting Components,” and the external surfaces of MEB enclosure assemblies can be managed under GALL-SLR Report AMP XI.S6, “Structures Monitoring.” Cable bus arrangements as described in GALL-SLR Chapter VI Table A, “Electrical Components – Equipment Not Subject to 10 CFR 50.49 Environmental Qualification Requirements” are excluded from this AMP and are evaluated as a site-specific further evaluation item per Section 3.6.2.2.2 of the Standard Review Plan for Review of Subsequent License Renewal Applications for Nuclear Power Plants.
- 2. Preventive Actions: This is a condition monitoring program and no actions are taken as part of this program to prevent or mitigate aging degradation.
- 3. Parameters Monitored or Inspected: This AMP provides for the inspection of the internal and external portions of the MEB. Internal portions (bus enclosure assemblies) of the MEB are inspected for cracks, corrosion, foreign debris, excessive dust buildup, and evidence of water intrusion. The bus electrical insulation material is inspected for signs of reduced insulation resistance due to thermal/thermoxidative degradation of organics/thermoplastics, radiation-induced oxidation, moisture/debris intrusion, or ohmic heating, as indicated by embrittlement, cracking, chipping, melting, discoloration, or swelling, which may indicate overheating or aging degradation. The internal bus insulating supports are inspected for structural integrity and signs of cracks. A sample of bolted connections is inspected for increased resistance of connection (e.g., loose or corroded MEB bolted connections and hardware including cracked or split washers). Alternatively, a sample of bolted connections covered with heat shrink tape, sleeving, insulating boots, etc., may be visually inspected for electrical insulation material surface abnormalities. The external portions of the MEB, including accessible gaskets, boots, and sealants, are inspected for hardening or loss of strength due to elastomer degradation that could permit water or foreign debris to enter the bus. MEB external surfaces are inspected for loss of material due to general, pitting, and crevice corrosion.
- MEBs are generally accessible structures and as such are inspected and tested in their entirety. However, depending on particular plant configurations, some segments of the MEB may be considered inaccessible due to close proximity to other permanent structures (e.g., nearby walls, ducts, cable trays, equipment or other structural elements). For inaccessible MEB internal or external segments, the applicant demonstrates (e.g., through alternative analysis, inspection, test or plant OE) that the inaccessible MEB segments evaluation, together with the accessible MEB inspection and test program, will continue to maintain the MEB consistent with the current licensing basis during the subsequent period of extended operation.
- 4. Detection of Aging Effects: MEB internal surfaces are visually inspected for aging degradation including cracks, corrosion, foreign materials debris, excessive dust buildup, and evidence of moisture intrusion. MEB insulating material is visually inspected for signs of embrittlement, cracking, chipping, melting, discoloration, swelling, or surface contamination. Internal bus insulating supports are visually inspected for structural integrity and signs of cracks. MEB external surfaces are visually inspected for loss of material due to general, pitting, and crevice corrosion. Accessible elastomers (e.g., gaskets, boots, and sealants) are inspected for degradation including surface cracking, crazing, scuffing, dimensional change (e.g., “ballooning” and “necking”), shrinkage, discoloration, hardening or loss of strength.
- A sample of accessible bolted connections is inspected for increased resistance of connection by using thermography or by measuring connection resistance using a micro ohmmeter. Twenty percent of the population with a maximum sample size of 25 constitutes a representative sample size. When thermography is employed by the applicant, the applicant demonstrates with a documented evaluation that thermography is effective in identifying MEB increased resistance of connection (e.g., infrared viewing windows installed, or demonstrated test equipment capability). In addition to thermography or resistance measurement, bolted connections not covered with heat shrink tape or boots are visually inspected for increased resistance of connection (e.g., loose or corroded bolted connections and hardware including cracked or split washers).
- The first inspection for measuring connection resistance or thermography is completed prior to the subsequent period of extended operation and every 10 years thereafter. This is an adequate period to preclude failures of the MEBs since experience has shown that MEB aging degradation is a slow process.
- As an alternative to thermography or measuring connection resistance of bolted connections, for accessible bolted connections covered with heat shrink tape, sleeving, insulating boots, etc., the applicant may use visual inspection of insulation material to detect surface anomalies, such as embrittlement, cracking, chipping, melting, discoloration, swelling, or surface contamination. When an alternative visual inspection is used to check MEB bolted connections, the first inspection is completed prior to the subsequent period of extended operation and every 5 years thereafter.
- 5. Monitoring and Trending: Trending actions are not included as part of this AMP because the ability to trend inspection results is limited. However, results that are trendable provide additional information on the rate of degradation.
- 6. Acceptance Criteria: An unacceptable condition is defined as a noted condition or situation that, if left unmanaged, could potentially lead to a loss of the intended function.
- MEB electrical insulation materials are free from unacceptable regional indications of surface anomalies such as embrittlement, cracking, chipping, melting, discoloration, swelling, or surface contamination. MEB internal surfaces show no indications of unacceptable corrosion, cracks, foreign debris, excessive dust buildup, or evidence of moisture intrusion. Accessible elastomers (e.g., gaskets, boots, and sealants) show no indications of unacceptable surface cracking, crazing, scuffing, dimensional change (e.g., “ballooning” and “necking”), shrinkage, discoloration, hardening, and loss of strength. MEB external surfaces are free from unacceptable loss of material due to general, pitting, and crevice corrosion.
- MEB bolted connections are below the maximum allowed temperature (e.g., comparison of compartment temperatures, trending of temperature over time, or comparison to a baseline thermography signature) for the application when thermography is used or a low resistance value appropriate for the application when resistance measurement is used.
- When the visual inspection alternative for MEB bolted connections is used, the absence of embrittlement, cracking, chipping, melting, discoloration, swelling, surface contamination of the electrical insulation material provides positive indication that the bolted connections are not loose. Visual inspection of bolted connections not covered with heat shrink tape, sleeving, insulating boots, etc. are free from corrosion, loose connections and hardware including cracked or split washers.
- 7. Corrective Actions: Results that do not meet the acceptance criteria are addressed in the applicant’s corrective action program under those specific portions of the quality assurance (QA) program that are used to meet Criterion XVI, “Corrective Action,” of Title 10 of the Code of Federal Regulations (10 CFR) Part 50, Appendix B. Appendix A of the GALL-SLR Report describes how an applicant may apply its 10 CFR Part 50, Appendix B, QA program to fulfill the corrective actions element of this AMP for both safety-related and nonsafety-related structures and components (SCs) within the scope of this program.
- Corrective actions are taken and an engineering evaluation is performed when the acceptance criteria are not met. Corrective actions may include, but are not limited, to cleaning, drying, increased inspection frequency, replacement, or repair of the affected MEB components. An engineering evaluation is performed when the acceptance criteria are not met to demonstrate that the MEB intended function can be maintained consistent with the CLB. The engineering evaluation considers the significance of the surveillance, inspection or test results on the performance of intended functions, the extent of the concern, the potential root causes for not meeting the acceptance criteria, the corrective actions required, and the likelihood of recurrence. If an unacceptable condition or situation is identified, (e.g., internal surface degradation including cracks, corrosion, foreign debris, excessive dust buildup, moisture intrusion, insulating material embrittlement, cracking, chipping, melting, discoloration, swelling, or surface contamination) a determination is made as to whether the same condition or situation is applicable to MEB bolted connections not inspected or tested. Further, when acceptance criteria are not met, a determination is made as to whether the surveillance, inspection, or test, including frequency intervals, needs to be modified.
- 8. Confirmation Process: The confirmation process is addressed through those specific portions of the QA program that are used to meet Criterion XVI, “Corrective Action,” of 10 CFR Part 50, Appendix B. Appendix A of the GALL-SLR Report describes how an applicant may apply its 10 CFR Part 50, Appendix B, QA program to fulfill the confirmation process element of this AMP for both safety-related and nonsafety-related structures and components (SCs) within the scope of this program.
- 9. Administrative Controls: Administrative controls are addressed through the QA program that is used to meet the requirements of 10 CFR Part 50, Appendix B, associated with managing the effects of aging. Appendix A of the GALL-SLR Report describes how an applicant may apply its 10 CFR Part 50, Appendix B, QA program to fulfill the administrative controls element of this AMP for both safety-related and nonsafety-related structures and components (SCs) within the scope of this program.
- 10. Operating Experience: Industry experience has shown that failures have occurred on MEBs caused by cracked electrical insulation and moisture or debris buildup internal to the MEB. Experience also has shown that bus connections in the MEBs exposed to appreciable ohmic heating during operation may experience loosening due to repeated cycling of connected loads.
- The program is informed and enhanced when necessary through the systematic and ongoing review of both plant-specific and industry OE including research and development such that the effectiveness of the AMP is evaluated consistent with the discussion in Appendix B of the GALL-SLR Report.
References
10 CFR Part 50, Appendix B, “Quality Assurance Criteria for Nuclear Power Plants and Fuel Reprocessing Plants.” Washington, DC: U.S. Nuclear Regulatory Commission. 2016.
EPRI. “Switchgear and Bus Maintenance Guide.” Palo Alto, California: Electric Power Research Institute, Nuclear Maintenance Application Center. December 2006.
_____. “Cable System Management.”(Archived) Palo Alto, California: Electric Power Research Institute. 2002.
_____. “Electrical Connector Application Guidelines.” Palo Alto, California: Electric Power Research Institute. December 2002.
_____. “Infrared Thermography Guide.”(revision referenced is archived) Palo Alto, California: Electric Power Research Institute. 2002.
_____. “Plant Support Engineering: License Renewal Electrical Handbook.”(revision referenced is archived) Palo Alto, California: Electric Power Research Institute. 2001.
IAEA. Safety Guide No. NS-G-2.12, “Ageing Management for Nuclear Power Plants.” IAEA. Vienna: International Atomic Energy Agency. February 2009.
IEEE. IEEE Standard 1205-2014, “IEEE Guide for Assessing, Monitoring and Mitigating Aging Effects on Electrical Equipment Used in Nuclear Power Generating Stations and Other Nuclear Facilities.” New York, New York: Institute of Electrical and Electronics Engineers. 2014.
US NRC. Information Notice 89-64, “Electrical Bus Bar Failures.” Agencywide Documents Access and Management System (ADAMS) Accession No. ML013180735. Washington, DC: U.S. Nuclear Regulatory Commission. September 7, 1989.
_____. Information Notice 98-36, “Inadequate or Poorly Controlled, Non-Safety-Related Maintenance Activities Unnecessary Challenged Safety Systems.” ADAMS Accession No. ML031040558. Washington, DC: U.S. Nuclear Regulatory Commission. September 18, 1998.
_____. Information Notice 2000-14, “Non-Vital Bus Fault Leads to Fire and Loss of Offsite Power.” ADAMS Accession No. ML003748744. Washington, DC: U.S. Nuclear Regulatory Commission. September 27, 2000.
_____. Information Notice 2010-25, “Inadequate Electrical Connections.” ADAMS Accession No. ML102530012. Washington, DC: U.S. Nuclear Regulatory Commission. November 17, 2010.
_____. NUREG/CR–5461, “Aging of Cables, Connections, and Electrical Penetration Assemblies Used in Nuclear Power Plants.” ADAMS Accession No. ML041280192. Washington, DC: U.S. Nuclear Regulatory Commission. July 31, 1990.
SNL. SAND96-0344, “Aging Management Guideline for Commercial Nuclear Power Plants-Electrical Cable and Terminations.” Albuquerque, New Mexico: Sandia National Laboratories. September 1996.