United States Nuclear Regulatory Commission - Protecting People and the Environment

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.  





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                                                            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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