ML20195H236
| ML20195H236 | |
| Person / Time | |
|---|---|
| Site: | Grand Gulf  |
| Issue date: | 11/21/1988 |
| From: | Cesare J SYSTEM ENERGY RESOURCES, INC. |
| To: | NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM) |
| References | |
| AECM-88-0233, AECM-88-233, IEIN-87-043, IEIN-87-43, NUDOCS 8811300283 | |
| Download: ML20195H236 (5) | |
Text
___ __ ________ _ ______
.L.<n, cwa o I$Eoma November 21, 1988
'J. S. Nuclear Regulatory Cranission Mail Station P1-137 Washington, D. C.
20555 Attention:
Document Control Desk Gentlemen:
SUBJECT:
Grand Gulf Nuclear Station Unit 1 Docket No. 50-416 License No. NPF-29 Request for Additional Information Criticality Analysis for Cycla 4 AECH-88/0233 By letters dated October 27,1988'and Neverrber 15, 1988 (AECM-88/0206 and 0228 ressectively). System Energy Resources, Inc. (SERI) requested the NRC to review tie criticality analysis for Cycle 4 fuel to be stored in the Grand Gulf
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huclear Station Unit 1 spent fuel pool storage racks prior te loading in the l
reactor.
I Based on its review, the NRC Staff requested additional information (MAEC-88/0336, dated November 8, 1988) regarding the Boraflex material contained in the racks and the associated Boraflex monitoring program.
SERI's response to the NRC request fcr additional infonnation is attached, j
If you have additional questions please advise.
Yours truly.
0 l
JGC:swb l
Attachment 1
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AECM-88/0233 Page 2 cc: Mr. W. T. Cottle (w/a)
Mr. T. H. Cloninger (w/a)
Mr.R.B.McGehee(w/a)
Mr. N. S. Reynolds (w/a) i Mr. H. L. Thomas (w/o)
Mr.H.O.Christensen(w/a)
Mr. Malcolm L. Ernst (w/a)
Acting Regional Administrator U. S. Nuclear Regulatory Conrnission Region !!
l 101 Marietta St., N.
W., Suite 2900 Atlanta, Georgia 30323 Mr. L. L. Kintner, Project Manager (w/a)
Office of Nuclear Reactor Regulation
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U. S. Nuclear Regulatory Commission r
Mail Stop 14B20 t
Washington. 0.C.
20555 I
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Attachment to AECM-88/0233 Response to NRC Question Regarding i
GGNS Boraflex Gap Surveillance Program Introduction As described in NRC Informatian Notice No. 87-43 Commonwealth Edison Company l
(CECO) has observed gaps in the neutron-absorbing material used in the Quad l
Cities high-density spent fuel storage racks. These racks were manufactured by the Joseph Oat Corporation (OAT). The neutron-absorbing material is a boron carbide dispersion in an elastomeric silicone matrix which is manufactured by Bisco Products, Inc. (BISCO) sinder the trade name Boraflex.
On May 5, 1987, CECO presented to the NRC under Dockets No. 50-254 and 50-265 the results of measureNnts and criticality analysis relating to the cbserved presence of gaps in Boraflex panels used in the Quad Cities spent fuel pool racks. Additionally EPRI has evaluated BISCO and Utility data relating to the degradation of Boraflex as reported in the November 1988 presentation of the draft report RP-2813-4 to EPRI Boraflex working group member utilities.
j Gap Fomation As described in these reports, the formation of gaps is postulated to be caused by radiation induced shrinkage in the Boraflex panels due to crosslinking in the Boraflex polper. When this shrinkage occurs in the presence of some structural element (s) which restrict the shrinkage, local stress accumulates l
until the panel tears. The elements of this scenario are present in the high f
density spent fuel racks manufactured by OAT such as those installed at Quad l
Cities and uGNS.
l Gap Measurements i
In respense to NRC Information Notice 87-43, tests for the presence of gaps in t
the GGNS spent fuel racks were conducted during August, 1988.
The test apparatus consisted of a Californium-262 neutron source and 4 sets of I
Boren tri-Flouride themal neutron detectors enclosed in a specially designed logging tool. The detectors are shielded from the fast neutrons produced by the Ca-252 source but detect any themal neutrons which will be reflected from adjacent storage rack cells if gaps in the Bt,raflex are present. A set of detectors is mounted on each wall of the logging tool in order to test all four
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Boraflex panels at once. This design minimizes uncertainties with respect to l
the elevation of gaps. This device is lowered into the storage rack cell to be i
tested and slowly moved up the channel at approximately 1 foot / minute. This process allows detection of gaps greater than 1/2 inch.
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The results of this test were provided to SERI in October, 1988. The tests were perfomed on 101 storage rack cells (tot 41 of 404 Beraflex panels).
These cell locations have typical radiation histories for the GGNS spent fuel storage
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racks. Of these, 48 had not previously been used to store irradiated fuel. No I
gaps were found in the unirradiated cells.
In the irradiated cells 87 gaps were observed in 85 panels with 127 panels showing no evidence of gaps.
The l
average gap size was approxinately 0.8 inch, with a maximum gap size of 1.4 inch.
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l Attachment to AECH-@8/0233
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Gap Impact Assessment 3-D KENO models were developed to assess the irpact of the observed gaps on the
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spent fuel rack criticality analysis. These models included gaps in the mid-plane of all Boraflex panels equal to the maximum observed gap size plus l
the associated measurement uncertainty. An infinite number of cells, in this configuration, was assured to be present in the radial direction.
This is a very conservative treatment since the GGNS and Quad Cities measurements indicate that the gaps are essentially randomly distributed except for a slight bias J
towards the mid-plane.
No storage cells in the GGNS racks were observed to have 4 gaps in the same plane.
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Results of analyses using the 3-D KENO models demonstrated that if the presence of the maximum observed gap size had been explicitly accounted for in the Cycle
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4spentfuelcriticalityanalyses[SERIletterstoNRCserialAECM-88/0206and AECM-88/0228), the reported reactivity would not have increased. An analysis l
of a 3 inch gap in the mid plane of all Boraflex panels demonstrated significant margin to the 0.95 k-effective acceptance criteria. Additionally, i
if a more realistic treatment of the distribution of gap sizes and locations e
j had been included, gaps substantially larger than those observed would not result in an increase in the reported rack reactivity.
Since the fomation and i
1 growth of gaps is slow, no immediate concern for the integrity of the GGNS l
l racks exists, i
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However, the potential for continued growth and gap fomation exists.
In order to assess the long term impact of the degradation of the Boraflex panels upon spent fuel rack c.sticality, a detailed evaluation of the available industry data is currently in progress.
The objective of this evaluation is to f
i establish a maximum credibic gap size and distribution for the GGNS racks.
In
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conjunction with this assessment, a more detailed criticality analysis is
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underway in order to determine the long tenn impact of the femation of gaps on the criticality of the spent fuel racks at GGNS.
These analyses are l
l currently scheduled for completion on or about February 15, 1988.
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Surface Degradation i
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In July of 1986. Wisconsin Electric Company, the licensee of Point Beach 1 and 2 reported significant degradation in test coupons of Boraflex naterial, As reported in NRC Information Notice 87-43, subsequent examinat'on of full length panels disclosed 1-2 percent of the surface showed a gray discoloration at the edges. No evidence of this degradation has been observed in the GGNS 35% Coupons.
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Monitoring program j
I Since little industry data is available concerning the femation of gaps in i
the Boraflex for 0AT racks, SERI intends to continue monitoring the GGNS spent fuel racks until gap sizes and the distribution of gaps are no longer significantly changing. The monitorinq will use methods equivalent to those i
previously described.
Due to the observed slow formation and growth of these gaps, one set of measurements each cycle is considered adequate.
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Attachment to AECM-88/0233 Approximately 50 storage rack cells are planned for monitoring each cycle. The cells will be loaded with discharged fuel during each iefueling outage. After significant additional irradiation, the fuel in the specified cells will be moved and additional measurements will be performed.
This process will maximize the formation of gaps. This process will provide an early indication of any significant changes in the behavior of the Boraflex and a confirmation of any assumptions used in the criticality analyses.
The Boraflex coupon surveillance program described in the original licensing basis for the racks (SERI letter to NRC serial AECH-85/0143) will centinue and provide a basis to monitor for the possible surface degradation of the panels.
Sumary While the presence of small gaps in the GGNS spent fuel rack Boraflex has been detected, they have no impact upon the criticality safety analysis currently under review by the NRC Staff. Since larger gaps have been observed at other sites, a detailed assessment of the long tem impact is on schedule for completion in February 1989.
In order to assure that the basis for this assessment remains valid, periodic measurements of the specified storage rack cells wili ce perfomed until gap sizes and the gap locations are no longer significantly changing.
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