XI.M22 (NUREG-2191 R0)
XI.M22 BORAFLEX MONITORING
Program Description
Many neutron‐absorbing materials, such as Boraflex, Boral, Metamic, boron steel, and carborundum, are used in spent fuel pools. This aging management program (AMP) addresses aging management of spent fuel pools using Boraflex as the neutron‐absorbing material. Generic Aging Lessons Learned for Subsequent License Renewal (GALL-SLR) Report AMP XI.M40, “Monitoring of Neutron‐Absorbing Material Other Than Boraflex,” addresses aging management of spent fuel pools using neutron‐absorbing materials other than Boraflex, such as Boral, Metamic, boron steel, and carborundum. When a spent fuel pool criticality analysis credits Boraflex and materials other than Boraflex, the guidance in both GALL-SLR Report AMPs XI.M22 and XI.M40 applies.
For Boraflex panels in spent fuel storage racks, gamma irradiation and long-term exposure to the wet fuel pool environment causes shrinkage resulting in gap formation, gradual degradation of the polymer matrix, and the release of silica to the spent fuel storage pool water. This results in the loss of boron carbide in the neutron absorber sheets. A monitoring program for the Boraflex panels in the spent fuel storage racks is implemented to assure that no unexpected degradation of the Boraflex material compromises the criticality analysis in support of the design of spent fuel storage racks. This AMP relies on periodic inspection, testing, monitoring, and analysis of the criticality design to assure that the required 5 percent subcriticality margin is maintained. Therefore, this AMP includes: (a) completing sampling and analysis for silica levels in the spent fuel pool water on a regular basis, such as monthly, quarterly, or annually (depending on Boraflex panel condition), and trending the results by using the Electric Power Research Institute (EPRI) RACKLIFE predictive code or its equivalent; and (b) performing neutron attenuation testing to determine gap formation in Boraflex panels or measuring boron-10 areal density by techniques such as the BADGER device.
Evaluation and Technical Basis
- 1. Scope of Program: This program manages the effect of reduction in neutron-absorbing capacity due to degradation in sheets of neutron-absorbing material made of Boraflex affixed to spent fuel racks.
- 2. Preventive Actions: This program is a performance monitoring program and does not include preventive actions.
- 3. Parameters Monitored or Inspected: The parameters monitored include physical conditions of the Boraflex panels, such as gap formation and decreased boron-10 areal density, and the concentration of the silica in the spent fuel pool. These are conditions directly related to degradation of the Boraflex material. When Boraflex is subjected to gamma radiation and long-term exposure to the spent fuel pool environment, the silicon polymer matrix becomes degraded and silica filler and boron carbide are released into the spent fuel pool water. As indicated in the U.S. Nuclear Regulatory Commission (US NRC) Information Notice (IN) 95-38 and US NRC Generic Letter (GL) 96-04, the loss of boron carbide (washout) from Boraflex is characterized by slow dissolution of silica from the surface of the Boraflex and a gradual thinning of the material. Because Boraflex contains about 25 percent silica, 25 percent polydimethyl siloxane polymer, and 50 percent boron carbide, sampling and analysis for the presence of silica in the spent fuel pool provide an indication of depletion of boron carbide from Boraflex; however, the degree to which Boraflex has degraded is ascertained through measurement of the boron-10 areal density.
- 4. Detection of Aging Effects: Aging effects on Boraflex panels are detected by monitoring silica levels in the spent fuel storage pool on a regular basis, such as monthly, quarterly, or annually (depending on Boraflex panel condition); by measuring boron-10 areal density on a frequency determined by the material condition of the Boraflex panels, with a minimum frequency of once every 5 years; and by applying predictive methods to the measured results. The amount of boron-10 carbide present in the Boraflex panels is determined through direct measurement of boron-10 areal density by periodic verification of boron-10 loss through areal density measurement techniques, such as the BADGER device. Frequent Boraflex testing is sufficient to verify that Boraflex panel degradation does not compromise criticality analysis for the spent fuel pool storage racks. Additionally, changes in the level of silica present in the spent fuel pool water provide an indication of changes in the rate of degradation of Boraflex panels.
- 5. Monitoring and Trending: The periodic inspection measurements and analysis are compared to values of previous measurements and analysis providing a continuing level of data for trend analysis. Sampling and analysis for silica levels in the spent fuel pool water is performed on a regular basis, such as monthly, quarterly, or annually (depending on Boraflex panel condition), and results are trended using the EPRI RACKLIFE predictive code or its equivalent. Silica concentration is monitored against time to trend degradation. Rapid increases of silica concentration may indicate accelerated Boraflex degradation. The frequency to perform boron-10 areal density testing will be determined by the material condition of the Boraflex panels, with an interval not to exceed 5 years.
- 6. Acceptance Criteria: The 5 percent subcriticality margin of the spent fuel racks is maintained for the subsequent period of extended operation.
- 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 initiated if the test results find that the 5 percent subcriticality margin cannot be maintained because of the current or projected future degradation. Corrective actions consist of providing additional neutron-absorbing capacity by Boral® or boron steel inserts or other options which are available to maintain a subcriticality margin of 5 percent.
- 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 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 SCs within the scope of this program.
- 10. Operating Experience: US NRC IN 87-43 addresses the problems of development of tears and gaps (average 1-2 inches, with the largest 4 inches) in Boraflex sheets due to gamma radiation-induced shrinkage of the material. US NRC IN 93-70, US NRC IN 95-38 and US NRC GL 96-04 address several cases of significant degradation of Boraflex test coupons due to accelerated dissolution of Boraflex caused by pool water flow through panel enclosures and high accumulated gamma dose. In such cases, the Boraflex may be replaced by boron steel inserts or by a completely new rack system using Boral®. Experience with boron steel is limited; however, the application of Boral for use in the spent fuel storage racks predates the manufacturing and use of Boraflex. The experience with Boraflex panels indicates that coupon surveillance programs are not reliable. Therefore, during the subsequent period of extended operation, the measurement of boron-10 areal density correlated, through a predictive code, with silica levels in the pool water, is verified. These monitoring programs provide assurance that degradation of Boraflex sheets is monitored so that appropriate actions can be taken in a timely manner if significant loss of neutron-absorbing capability is occurring. These monitoring programs provide reasonable assurance that the Boraflex sheets maintain their integrity and are effective in performing their intended function.
- The program is informed and enhanced when necessary through the systematic and ongoing review of both plant-specific and industry operating experience 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. EPRI 1003413, “Guidance and Recommended Procedure for Maintaining and Using RACKLIFE Version 1.10.” Palo Alto, California: Electric Power Research Institute. April 2002.
_____. EPRI NP-6159, “An Assessment of Boraflex Performance in Spent-Nuclear-Fuel Storage Racks.” Palo Alto, California: Electric Power Research Institute. December 1988.
_____. EPRI TR–101986, “Boraflex Test Results and Evaluation," Electric Power Research Institute.” Palo Alto, California: Electric Power Research Institute. March 1993.
_____. EPRI TR–103300, “Guidelines for Boraflex Use in Spent-Fuel Storage Racks.” Palo Alto, California: Electric Power Research Institute. December 1993.
US NRC. BNL–NUREG–25582, “Corrosion Considerations in the Use of Boral in Spent Fuel Storage Pool Racks.” Washington, DC: U.S. Nuclear Regulatory Commission. January 1979.
_____. Generic Letter 96-04, “Boraflex Degradation in Spent Fuel Pool Storage Racks.” Agencywide Documents Access and Management System (ADAMS) Accession No. ML031110008. Washington, DC: U.S. Nuclear Regulatory Commission. June 26, 1996.
_____. Information Notice 87-43, “Gaps in Neutron Absorbing Material in High Density Spent Fuel Storage Racks.” ADAMS Accession No. ML031130349. Washington, DC: U.S. Nuclear Regulatory Commission. September 8, 1987.
_____. Information Notice 93-70, “Degradation of Boraflex Neutron Absorber Coupons.” ADAMS Accession No. ML031070107. Washington, DC: U.S. Nuclear Regulatory Commission. September 10, 1993.
_____. Information Notice 95-38, “Degradation of Boraflex Neutron Absorber in Spent Fuel Storage Racks.” ADAMS Accession No. ML031060277. Washington, DC: U.S. Nuclear Regulatory Commission. September 8, 1995.
_____. Regulatory Guide 1.26, Revision 3, “Quality Group Classifications and Standards for Water, Steam, and Radioactive-Waste-Containing Components of Nuclear Power Plants (for Comment).” ADAMS Accession No. ML003739964. Washington, DC: U.S. Nuclear Regulatory Commission. February 29, 1979.