XI.E5 (NUREG-2191 R0)
XI.E5 FUSE HOLDERS
Program Description
The purpose of this aging management program (AMP) is to provide reasonable assurance that the intended functions of fuse holders within the scope of subsequent license renewal (SLR) and subject to aging management are maintained consistent with the current licensing basis. The fuse holder program was developed specifically to address aging management of fuse holder insulation material and fuse holder metallic clamp aging mechanisms and effects. This AMP utilizes visual inspection and testing to identify age-related degradation for both fuse holder electrical insulation material and fuse holder metallic clamps. Visual inspection and testing provides reasonable assurance that the applicable aging effects are identified and fuse holder insulators and metallic clamps are age managed.
Fuse holders (fuse blocks) are classified as a specialized type of terminal block because of the similarity in fuse holder design and construction to that of a terminal block. Fuse holders are typically constructed of blocks of rigid insulating material, such as phenolic resins. Metallic clamps (clips) are attached to the blocks to hold each end of the fuse. The clamps, which are typically made of copper, can be spring-loaded clips or bolt lugs to which the fuse ends are connected.
Industry operating experience (OE) has shown that repetitive removal and reinsertion of fuses during maintenance or surveillance activities can lead to degradation of the fuse holders. Fuse holders, located outside of active equipment, where fuses are removed and replaced frequently for maintenance or surveillance activities are also included in this AMP to manage these repetitive activities.
The metallic portion of fuse holders that are within the scope of SLR and subject to aging management are tested for the following aging stressors: increased resistance of connection due to chemical contamination, corrosion, and oxidation or fatigue caused by ohmic heating, thermal cycling, electrical transients, frequent removal and insertion, or vibration. The specific type of test is determined prior to the initial test and detects increased resistance of fuse holder metallic clamp connections. Tests may include thermography, contact resistance testing, or other appropriate testing justified in the application.
Fuse holders within the scope of SLR and subject to aging management are visually inspected to provide an indication of the condition of the electrical insulation portion of the fuse holders. Fuse holders are visually inspected for electrical insulation surface anomalies indicating signs of reduced insulation resistance due to thermal/thermoxidative degradation of organics, radiolysis and photolysis [ultraviolet (UV) sensitive materials only] of organics, radiation-induced oxidation, and moisture intrusion as indicated by signs of embrittlement, discoloration, cracking, melting, swelling, or surface contamination.
As stated in NUREG-1760, “Aging Assessment of Safety-Related Fuses Used in Low and Medium-Voltage Applications in Nuclear Power Plants,” licensees have experienced a number of age-related failures. The major concern is that failures of a deteriorated cable system (cables, connections including fuse holders, and penetrations) might be induced during accident conditions. Since they are not subject to the environmental qualification requirements of Title 10 of the Code of Federal Regulations (10 CFR) 50.49, an AMP is required to manage the aging effects. This AMP demonstrates that fuse holders, including both the insulation and metallic components will maintain the ability to perform their intended function for the subsequent period of extended operation.
Evaluation and Technical Basis
- 1. Scope of Program: This AMP manages in-scope fuse holders outside of active devices that are considered susceptible to the following aging effects: increased resistance of connection due to chemical contamination, corrosion, and oxidation or fatigue caused by ohmic heating, thermal cycling, electrical transients, frequent removal and replacement, or vibration. It also manages degradation of electrical insulation for the fuse holders with metallic clamps susceptible to the aging effects identified. Fuse holders inside an active device (e.g. switchgears, power supplies, inverters, battery chargers, and circuit boards) and not subject to the aging effects identified, are not within the scope of this AMP.
- 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: The metallic clamp portion of the fuse holder is tested to detect increased resistance of connection due to chemical contamination, corrosion, and oxidation or fatigue caused by ohmic heating, thermal cycling, electrical transients, frequent removal and replacement or vibration. The electrical insulation material portion of the fuse holder is visually inspected to identify insulation surface anomalies indicating signs of reduced insulation resistance due to thermal/thermoxidative degradation of organics, radiolysis and photolysis (UV sensitive materials only) of organics, radiation-induced oxidation, and moisture intrusion as indicated by signs of embrittlement, discoloration, cracking, melting, swelling, or surface contamination.
- 4. Detection of Aging Effects: Fuse holders within the scope of this AMP are visually inspected and tested at least once every 10 years to provide an indication of the condition of the metallic clamp of the fuse holder. Testing may include thermography, contact resistance testing, or other appropriate testing methods. Visual inspection includes inspection for electrical insulation surface anomalies indicating signs of reduced insulation resistance. Visual inspection and testing at least once every 10 years is an adequate period to preclude failures of the fuse holders since experience has shown that aging degradation is a slow process. The first visual inspections and tests for SLR are to be completed prior to the subsequent period of extended operation.
- 5. Monitoring and Trending: Trending actions are not included as part of this AMP because the ability to trend visual inspection and test results is dependent on the inspection and specific type of test chosen. However, results that are trendable provide additional information on the rate of degradation.
- 6. Acceptance Criteria: An unacceptable indication is defined as a noted condition or situation that, if left unmanaged, could potentially lead to a loss of intended function.
- The acceptance criteria for each visual inspection and test are defined by the specific type of inspection or test performed and the specific type of fuse holder tested. When thermography is used, the metallic clamp of the fuse holder needs to be below the maximum allowed temperature for the application; otherwise, a low resistance value appropriate for the application is applicable when resistance measurement is used. Test acceptance criteria show that fuse holders are free from the unacceptable aging effects of increased resistance of connection due to chemical contamination, corrosion, and oxidation or fatigue caused by ohmic heating, thermal cycling, electrical transients, frequent removal and replacement, or vibration. Visual inspection acceptance criteria show that fuse holders are free from unacceptable electrical insulation surface anomalies indicating signs of reduced insulation resistance due to thermal/thermoxidative degradation of organics, radiolysis and photolysis (UV sensitive materials only) of organics; radiation-induced oxidation, and moisture intrusion as indicated by signs of embrittlement, discoloration, cracking, melting, swelling, or surface contamination.
- 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 10 CFR 50, Appendix B. Appendix A of the Generic Aging Lessons Learned for Subsequent License Renewal (GALL-SLR) Report describes how an applicant may apply its 10 CFR 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.
- 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 50, Appendix B. Appendix A of the GALL-SLR Report describes how an applicant may apply its 10 CFR 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 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 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: Operating experience has shown that loosening of fuse holder metallic clamps due to chemical contamination, corrosion, oxidation or fatigue caused by ohmic heating, thermal cycling, electrical transients, frequent removal and replacement, vibration, and electrical insulation surface (i.e., fuse blocks) abnormalities, are aging mechanisms indicating signs of reduced insulation resistance. If left unmanaged, these aging mechanisms can lead to a loss of function. NUREG-1760 documents fuse holder failures due to fatigue and recommends the review of maintenance procedures (e.g., fuse control programs) to minimize removal and reinsertion of fuses to de-energize components (as this can lead to degradation of the fuse holder assembly).
- 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.
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 86-87, “Loss of Offsite Power Upon an Automatic Bus Transfer.” Agencywide Documents Access and Management System (ADAMS) Accession No. ML031250328. Washington, DC: U.S. Nuclear Regulatory Commission. October 10, 1986.
_____. Information Notice 87-42, “Diesel Generator Fuse Contacts.” ADAMS Accession No. ML031130353. Washington, DC: U.S. Nuclear Regulatory Commission. September 4, 1987.
_____. Information Notice 91-78, “Status Indication of Control Power for Circuit Breakers Used in Safety-Related Applications.” ADAMS Accession No. ML082380373. Washington, DC: U.S. Nuclear Regulatory Commission. November 28, 1991.
_____. NUREG–1760, “Aging Assessment of Safety-Related Fuses Used in Low- and Medium-Voltage Applications in Nuclear Power Plants.” ADAMS Accession No. ML021360517. Washington, DC: U.S. Nuclear Regulatory Commission. May 31, 2002.