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
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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
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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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