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Information Notice No. 87-43: Gaps in Neutron-Absorbing Material in High-Density Spent Fuel Storage Racks
SSINS No.: 6835 IN 87-43 UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON D.C. 20555 September 8, 1987 Information Notice No. 87-43: GAPS IN NEUTRON-ABSORBING MATERIAL IN HIGH-DENSITY SPENT FUEL STORAGE RACKS Addressees: All nuclear power reactor facilities holding an operating license or a con- struction permit. Purpose: This notice is to alert recipients to a potentially significant problem per- taining to gaps identified in the neutron absorber component of the high- density spent fuel storage racks at Quad Cities Unit 1. The safety concern is that certain gaps might excessively reduce the margin of nuclear subcriticality in the fuel pool. The NRC expects that recipients will review this notice for applicability to their facilities and consider actions, if appropriate, to preclude a similar problem occurring at their facilities. However, suggestions in this notice do not constitute NRC requirements; there- fore, no specific action or written response is required. Description of Circumstances: On May 1, 1987, Commonwealth Edison Company (CECO), the licensee at Quad Cities 1 and 2, presented data to the NRC regarding gaps measured in Boraflex, a neutron-absorbing material used in the high-density fuel storage racks manufactured by the Joseph Oat Corporation (OAT). Boraflex is a trade name for a boron carbide dispersion in an elastomeric silicone matrix manufactured by Bisco Products, Inc. (BISCO). Data pertaining to the gap size and distribution had been obtained by National Nuclear Corporation (NNC) under contract to CECO. The licensee had retained Northeast Technology Corporation (NETCO) to interpret the data. NETCO prefaced their assessment as preliminary, noting that available data was limited, but concluded that the gap formation mechanism may be related to large local stresses in the Boraflex from fabrication-induced restraint within the rack and to tearing and shrinkage of the material. The average gap size is 1-1/2 inches, with the largest 4 inches. The gaps occur in the upper two-thirds of the cell length. 8709010085 . IN 87-43 September 8, 1987 Page 2 of 3 These gaps are inferred from anomalies in "blackness" testing results by NNC. The existence of a gap in the Quad Cities neutron absorber panel has been confirmed by underwater neutron radiography conducted by Nusurtec, Inc. CECO also discussed the effects these gaps might have on the approved safety analysis for the spent fuel storage racks. CECO used conservative assumptions for gap size, gap location, and fuel burnup. Considerable margin in k-eff appears to be available before the licensing limit of 0.95 would be approached. In July 1986, Wisconsin Electric Company, the licensee at Point Beach 1 and 2, reported to the NRC that test coupons of Boraflex material had shown consider- able degradation under high radiation. However, the licensee asserted that this result did not represent the actual condition of Boraflex used in its spent fuel storage racks because of differences in methods of encapsulation, sample geometry, and handling frequency. Additionally, the coupons had been subjected to about 5 times more radiation than is associated with the average fuel rack position. Subsequent examination of full-length panels disclosed two results: in one panel examined for effects of the water environment but exposed to negligible gamma radiation, there was no degradation of the Boraflex. In another panel exposed to significant gamma radiation, 1-2 percent of the surface showed a gray discoloration at the edges, similar to the degradation of the coupons. Discussion: The concern is that separation of the neutron-absorbing material used in high density fuel storage racks might compromise safety. Although Quad Cities reports that its racks, even with gaps in the Boraflex as large as 4 inches, can meet the criticality criterion of k-eff less than or equal to 0.95, this may not be the case for larger gaps or for other plants. A list of the 31 sites using Boraflex is given in Attachment 1. Related information is given in "Behavior of High-Density Spent-Fuel Storage Racks," EPRI NP-4724, Electric Power Research Institute, August 1986. Efforts to understand the gap formation have revealed several topics on which information is needed. Accordingly, the material supplier (BISCO) and the Electric Power Research Institute (EPRI) have undertaken research programs to collect this information. Some of their objectives are described below. The BISCO program aims to establish with increased accuracy the relationship between radiation dose and size changes. The program also evaluates the potential effects of handling and restraint, during and subsequent to the fuel rack fabrication, on gap formation. The EPRI program will correlate data from utilities' neutron absorber coupon surveillance programs. EPRI will further examine data obtained from CECO, as well as from BISCO and other sources, to improve the understanding of possible or actual gap formation models, including the effects of rack fabrication methods and irradiation damage mechanisms. The EPRI Program will also attempt . IN 87-43 September 8, 1987 Page 3 of 3 to model the specific Quad Cities experience considering absorbed gamma dose as a function of axial elevation, neutron absorbing sheet restraint, and fractional change in length. The effect of rack design and manufacturing methods on the consequences of stress, temperature, and chemical environment to irradiated Boraflex is uncer- tain. Recent blackness test results at Turkey Point, who uses a Westinghouse spent fuel storage rack, did not indicate the presence of gaps in the Boraflex. The research programs are designed to evaluate each consequence and, in particular, to improve the understanding of stress caused by method of attachment of the Boraflex panel to the stainless steel wall of the cell. Together, these programs are designed to improve the industry understanding of the safety implications of the observed gaps in the Boraflex neutron absorber component of the OAT high-density spent-fuel storage racks at Quad Cities. No specific action or written response is required by this information notice. If you have any questions about this matter, please contact the Regional Administrator of the appropriate regional office or this office. Charles E. Rossi, Director Division of Operational Events Assessment Office of Nuclear Reactor Regulation Technical Contacts: Vern Hodge (301) 492-8196 Albert D. Morrongiello (309) 654-2227 Attachments: 1. List of Plants Using Boraflex Structures in the Spent Fuel Pool 2. List of Recently Issued NRC Information Notices . Attachment 1 IN 87-43 September 8, 1987 Page 1 of 1 LIST OF PLANTS WITH BORAFLEX STRUCTURES IN THE SPENT FUEL POOL 1. Arkansas 1,2 16. Peach Bottom 2,3 2. Beaver Valley 1 17. Pilgrim 3. Diablo Canyon 1,2 18. Pt. Beach 1,2 4. Calvert Cliffs 2 19. Pr. Island 1,2 5. Farley 1,2 20. Quad Cities 1,2* 6. Fermi 2* 21. Rancho Seco* 7. Ft. Calhoun 22. River Bend 8. Ginna 23. Robinson 2 9. Grand Gulf 1,2* 24. Summer* 10. McGuire 1,2 25. Trojan 11. Millstone 1,2,3 26. Turkey Pt. 3,4 12. Nine Mi. Pt. 1,2 27. Waterford 3 13. North Anna 1,2 28. Seabrook 1,2 14. Oconee 1,2,3 29. Watts Bar 1,2 15. Oyster Creek* 30. Comanche Peak 1,2 31. Harris *Plants having spent fuel storage racks fabricated by Joseph Oat Corporation.
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