XI.M22 (NUREG-1801 R2)
XI.M22 BORAFLEX MONITORING
Program Description
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 aging management program (AMP) relies on periodic inspection, testing, monitoring, and analysis of the criticality design to assure that the required 5% 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 EPRI RACKLIFE predictive code or its equivalent; and (b) performing neutron attenuation testing or blackness testing to determine gap formation in Boraflex panels or measuring boron 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/Inspected: The parameters monitored include physical conditions of the Boraflex panels, such as gap formation and decreased boron 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% silica, 25% polydimethyl siloxane polymer, and 50% boron carbide, sampling and analysis of 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 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 performing blackness testing to measure gap formation or measuring boron 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 carbide present in the Boraflex panels is determined through direct measurement of boron areal density by blackness testing or by periodic verification of boron loss through areal density measurement techniques, such as the BADGER device. Frequent Boraflex testing is sufficient to ensure 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. The frequency to perform blackness testing will be determined by the material condition of the Boraflex panels, with a maximum of 5 years.
- 6. Acceptance Criteria: The 5% subcriticality margin of the spent fuel racks is maintained for the period of extended operation.
- 7. Corrective Actions: Corrective actions are initiated if the test results find that the 5% 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%. As discussed in the Appendix for GALL, the staff finds the requirements of 10 CFR Part 50, Appendix B, acceptable to address the corrective actions.
- 8. Confirmation Process: Site quality assurance procedures, site review and approval processes, and administrative controls are implemented in accordance with the requirements of 10 CFR Part 50, Appendix B. As discussed in the Appendix for GALL, the staff finds the requirements of 10 CFR Part 50, Appendix B, acceptable to address the confirmation process and administrative controls.
- 9. Administrative Controls: As discussed in the Appendix for GALL, the staff finds the requirements of 10 CFR Part 50, Appendix B, acceptable to address the administrative controls.
- 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. Two spent fuel rack cells with about 12 years of service have only 40% of the Boraflex remaining. 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 period of extended operation, the measurement of boron 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.
References
10 CFR Part 50, Appendix B, Quality Assurance Criteria for Nuclear Power Plants, Office of the Federal Register, National Archives and Records Administration, 2009.
BNL-NUREG-25582, Corrosion Considerations in the Use of Boral in Spent Fuel Storage Pool Racks, January 1979.
EPRI NP-6159, An Assessment of Boraflex Performance in Spent-Nuclear-Fuel Storage Racks, Electric Power Research Institute, Palo Alto, CA, December 14, 1988.
EPRI 1003413, Guidance and Recommended Procedure for Maintaining and Using RACKLIFE Version 1.10, Electric Power Research Institute, Palo Alto, CA, April 2002.
EPRI TR-101986, Boraflex Test Results and Evaluation, Electric Power Research Institute, Palo Alto, CA, March 1, 1993.
EPRI TR-103300, Guidelines for Boraflex Use in Spent-Fuel Storage Racks, Electric Power Research Institute, Palo Alto, CA, December 1, 1993.
US NRC Generic Letter 96-04, Boraflex Degradation in Spent Fuel Pool Storage Racks, U.S. Nuclear Regulatory Commission, June 26, 1996.
US NRC Information Notice 87-43, Gaps in Neutron Absorbing Material in High Density Spent Fuel Storage Racks, U.S. Nuclear Regulatory Commission, September 8, 1987.
US NRC Information Notice 93-70, Degradation of Boraflex Neutron Absorber Coupons, U.S. Nuclear Regulatory Commission, September 10, 1993.
US NRC Information Notice 95-38, Degradation of Boraflex Neutron Absorber in Spent Fuel Storage Racks, U.S. Nuclear Regulatory Commission, September 8, 1995.
US NRC Regulatory Guide 1.26, Rev. 3, Quality Group Classifications and Standards for Water, Steam, and Radioactive-Waste-Containing Components of Nuclear Power Plants (for Comment), U.S. Nuclear Regulatory Commission, February 1976.