L-96-260, Responds to GL 96-04, Boraflex Degradation in Spent Fuel Pool Storage Racks
| ML17229A099 | |
| Person / Time | |
|---|---|
| Site: | Saint Lucie  |
| Issue date: | 10/22/1996 |
| From: | Stall J FLORIDA POWER & LIGHT CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| GL-96-04, GL-96-4, L-96-260, NUDOCS 9610290004 | |
| Download: ML17229A099 (18) | |
Text
CATEGORY 1 REGULAT INFORMATION DISTRIBUTION'STEM (RIDE) 4 ACCESSION NBR:9610290004 DOC.DATE: 96/10/22 NOTARIZED: YES FACIL:50-$35 St. Lucie Plant, Unit 1, Florida Power 6 Light Co.
50-389 St. Lucie Plant, Unit 2, Florida Power 6 Light Co.
AUTH.NAME AUTHOR AFFILIATION STALL,J.A.
Florida Power 6 Light Co.
RECIP.NAME RECIPIENT AFFILIATION Document Control Branch (Document Control Desk)
DOCKET I 05000335 05000389
SUBJECT:
Responds to GL 96-04, "Boraflex Degradation in Spent Fuel Pool Storage Racks."
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TITLE: Responses to GL-96-04: Boraflex Degradation in Spent Fuel Storage NOTES:
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Florida Power &Light Company, P.O. Box128, Fort Pierce. FL34954-0128 October 22, 1996 L-96-260 10 CFR 50.4 10 CFR 50. 54 (f)
U. S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, D. C. 20555 RE:
St. Lucie Units 1 and 2 Docket Nos. 50-335 and 50-389 Generic Letter 96-04 Res onse The Florida Power and Light Company (FPL) response to Generic Letter (GL) 96-04, Borafiex Degradation in Spent Fuel Pool Storage Racks, for St. Lucie Unit 1 is attached.
This GL is not applicable to St. Lucie Unit 2 since the spent fuel pool design for Unit 2 does not rely on Boraflex.
The NRC issued the generic letter to inform all licensees of issues concerning the use of Boraflex in spent fuel storage racks.
Only those licensees that use Boraflex were required to respond.
Licensees that use Borafiex as a neutron absorber in its spent fuel storage racks were requested to assess the capability of the Borafiex to maintain a five-percent subcriticality margin and submit to the NRC proposed actions if this subcriticality margin cannot be maintained by Boraflex material because of current or projected future Borafiex degradation.
All licensees that use Borafiex in their spent fuel storage racks were required to provide a written response within 120 days of June 26, 1996.
The attached information is provided pursuant to the requirements of Section 182a of the Atomic Energy Act of 1954, as amended, and 10 CFR 50.54(f).
Please contact us ifthere are any questions about this submittal.
Very truly yours, J. A. Stall Vice President St. Lucie Plant JAS/GRM Attachment cc:
Stewart D. Ebneter, Regional Administrator, Region II, USNRC Senior Resident Inspector, USNRC, St. Lucie Plant 9bf0290004 9bi022 PDR ADOCK,05000335, P
St. Lucie Units 1 and 2 Docket Nos. 50-335 and 50-389 L-96-260 STATE OF FLORIDA COUNTY OF ST. LUCIE
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J. A. Stall being first duly sworn, deposes and says:
That he is Vice President, St. Lucie Plant, for the Nuclear Division of Florida Power k Light Company, the Licensee herein; That he has executed the foregoing document; that the statements made in this document are true and correct to the best of his knowledge, information and belief, and that he is authorized to execute the document on behalf of said Licensee.
J. A. Stall STATE OF FLORIDA COUNTYOF S<. L-U<>: @:a=- W COMMISStOV S CC359928 EXPlRES ApRI 18, 1998 NONCE0 THRUTROY FAIN DURANCE, INC.
(Print, type or stamp Commissioned Name of Notary Public)
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St. Lucie Units 1 and 2 Docket Nos. 50-335 and 50-389 L-96-260 Attachment Page 1
NRC Re uested Information'll licensees of power reactors with installed spent fuel pool storage racks containing the neutron absorber Boraflex are requested to provide an assessment of the physical condition of the Boraflex, including any deterioration, on the basis of current accumulated gamma exposure and possible water ingress to the Boraflex and state whether a subcritical margin of five percent can be maintained for the racks in unborated water.
Monitoring programs or calculational models in effect or being developed, or an estimation of anticipated concerns based on the specific rack design, are considered an appropriate basis for this response.
All licensees are further requested to submit to the NRC a description of any proposed actions to monitor or confirm that this five-percent subcriticality margin can be maintained for the lifetime of the storage racks and describe what corrective actions could be taken in the event it cannot be maintained.
Licensees should describe the results from any previous post operational blackness tests and state whether blackness testing, or other in-situ tests or measurements, willbe periodically performed.
Chronological trends of pool reactive silica levels, along with the timing of significant events such as refueling, pool silica cleanups, etc.,
should be provided.
Implications of how these pool silica levels relate to Boraflex performance should be described.
Alllicensees are requested to submit the information to the NRC to ensure that the onsite storage of spent fuel is in compliance with GDC 62 for the prevention of criticality in fuel storage and handling and with the five-percent subcriticality margin position of the NRC staff to assure compliance with GDC 62.
S ent Fuel Rack General Descri tion The St. Lucie Unit 1 spent fuel pool contains two discrete fuel storage Regions; Regions 1 and 2'.
Each Region has its own specially designed high density fuel storage racks.
Region 1 has four (4) high density fuel storage rack modules with storage capacity for 342 fuel assemblies.
Region 2 contains thirteen (13) high density fuel storage rack modules with a capacity of 1364 fuel assemblies.
The. design of the high density storage racks includes use of Boraflex materials to maintain a spent fuel pool 5% subcriticality margin.
The Region 1 fuel storage racks are designed to accommodate fresh fuel assemblies with uranium enrichment up to 4.5% w/o, and discharged fuel assemblies with initial uranium enrichment up to 4.5% w/o that have not accumulated adequate burnup for Region 2 storage.
Region 2 racks are capable of storing spent fuel assemblies with various initial uranium enrichments which have achieved minimum burnup within an acceptable bound.
Region 2 is also capable of storing fresh fuel assemblies of 4.5% w/o in a checkerboard pattern.
St. Lucie Units 1 and 2 Docket Nos. 50-335 and 50-389 L-96-260 Attachment Page 2 Regions 1 and 2 employ different Boraflex panel and fuel storage cell configurations.
In Region 1, each fuel storage cell uses four high Boron-10 (B10) areal loading Boraflex panels for the four side walls. A water gap between two adjacent storage cells forms a flux trap zone which moderates fast neutrons to thermal energies in order to maximize the effectiveness of Boraflex panels. In Region 2, a low B10 areal loading Boraflex panel is sandwiched between the two adjacent walls of consecutive storage cells.
Regardless of region configuration differences, encapsulation of Boraflex panels is common.
Each Boraflex panel was placed against the storage cell wall and retained by a stainless steel cover plate to form a stainless steel jacket.
The cover plate was spot welded to the side-wall of the storage cell. The jacket is not a watertight enclosure to permit wetting and venting of the Boraflex panels.
The Boraflex panels are physically free to move within the jacket while being irradiated'.
FPL Res onse The St. Lucie Unit 1 original Boraflex coupon surveillance program'as established and implemented in June 1989.
The goals of the Boraflex surveillance program were to provide Boraflex coupon test data that could be used for monitoring the performance of the Boraflex panels installed in both the Region 1 and Region 2 storage racks, and to predict problems with Boraflex panels so that the need for remedial actions could be determined and implemented prior to challenging GDC 62.
On the basis of accumulated coupon test data results evaluated in 1989', 1990', and 1993', it was concluded that the initial program did not support the originally intended goals because of difficulties encountered correlating the physical data between the Boraflex coupons and the Boraflex panels.
Therefore, the original Boraflex coupon surveillance program was terminated.
An enhanced Boraflex verification program'as developed to continuously support Boraflex surveillance activity. The goal of the Boraflex verification program is twofold. First, the program confirms the physical presence of the Boraflex panels in the storage racks, and then provides performance data on the Boraflex panels in terms of gap formation, gap distribution, and gap growth. To achieve these goals, the program includes a one-time only Boraflex gamma dose estimate, followed by periodic blackness tests.
I Prior to performing an analytical gamma dose calculation for the Boraflex panels, the validity of the dose model was first demonstrated.
This was accomplished by comparing the calculated and measured gamma doses.
The measured gamma dose data was taken from a 1990 Boraflex spent fuel gamma dose measurement conducted at the spent fuel pool.
Comparison of the results has indicated that the calculation to measurement (C/M) ratio is 1.15.
Therefore, the calculational dose model predicts a conservative gamma dose.
St. Lucie Units 1 and 2 Docket Nos. 50-335 and 50-389 L-96-260 Attachment Page 3 Based on the above dose model and the conservative assumption of no axial gamma dependence, an estimate of the Region 1 Boraflex panel time-integrated gamma dose was made as of August 1996, This time-integrated gamma dose estimate is approximately 9.64E9 rads.
It is reasonable to postulate that the Region 1 estimate of 9.64E9 rads gamma dose is equally representative for a Region 2 Boraflex panel dose.
The reasons for this are: a) the estimate used a representative spent fuel storage arrangement for various fuel enrichments and burnup; and b) the spent fuel gamma penetration range in the spent fuel pool water is much larger than the mechanical difference between Region 1 and Region 2. As indicated above, the Boraflex time-integrated gamma dose is essentially at the EPRI reported'aturated gamma dose range (9E9 - 1.5E10 rads).
Therefore, no significant additional shrinkage is expected to occur due to the cumulative gamma dose beyond this saturation level.
Boraflex panel deterioration (such as gap formation, gap size, and gap distribution) can be measured by using the blackness test technique, Results from the blackness tests will demonstrate that the Boraflex panel performance is acceptable for continued service, and identify the need for any future remedial actions, should the Boraflex panel gaps grow to an unacceptable limit.
The first blackness test measurement campaign was conducted and the results evaluated in 1989". A total of twenty (20) spent fuel storage cells in Region 1 were tested, of which fifteen (15) were designated as test cells intentionally exposed to radiation from freshly discharged fuel during the Suly 1988 cycle 8 refueling outage.
The remaining five (5) cells were tested in an unirradiated area of the Region 1 storage racks to provide reference test data.
Region 2 storage cells were not selected for testing due to low accumulated gamma dose received by these Boraflex panels.
Evaluation of the test data revealed numerous small gaps on the order of 0.25 inches, generally occurring at intervals of about 6 inches or multiples of 6 inches.
Since the 0.25 inch gap size was in the range of the blackness test limiting resolution,(0.25 inches), test data could not reliably be interpreted as indicating that small gaps existed.
This suggested an overall high integrity of the Boraflex panels.
The largest gap observed was approximately 0.4 inches in width. The effect of these observed gaps on the spent fuel pool criticality analysis was negligible and the 5% subcriticality margin was maintained and not negatively affected.
In March 1995, the second blackness test measurement campaign was performed and evaluated according to the schedule specified in the spent fuel pool coupon surveillance program".
The second campaign included a total of 20 cells of which 11 were Region 1 cells and 9 were Region 2 cells.
Nine of 11 Region 1 cells were previously tested during the first blackness tests in 1989 to provide a means for estimating gap growth. Two new cells in Region 1 and nine new cells in Region 2 were tested during the second test campaign.
A total of 20 old/new cells were tested to provide performance data on a total of 78 Boraflex panels.
St. Lucie Units 1 and 2 Docket Nos. 50-335 and 50-389 L-96-260 Attachment Page 4 Evaluation of the test data from the second campaign revealed that among the 78 Boraflex panels tested, 20 panels were found to have measurable gaps, most of which were 0.5 inches or wider. The largest single gap was 2.8 inches wide occurring in Region 2 cell, ID=AH17.
A total of27 measurable gaps were found among the 20 Boraflex panels having gaps with an average gap size of approximately 0.8 inches.
On average, one out of every three Boraflex panels tested contained a measurable gap.
Comparison of test data from the first and second test campaigns has demonstrated that the second campaign has revealed 27 measurable gaps at 0.5 inch or wider, when no gap at 0.5 inch or wider was detected in the first blackness test campaign measurement.
In general, the average gap size increased to 0.8 inch width (resolution 0.25 inch) in the second test campaign from a 0.25 inch in the first test campaign.
To estimate the probable gap growth, the largest observed gap size, 2.8 inches was divided by the number of years between 1989 and 1995 yielding a conservative gap growth rate of 0.5 inch per year.
This growth rate serves as one of several criteria used to establish the schedule for the third blackness measurement campaign in the year 2000.
The increase in gap formation and gap size that occurred in the Boraflex panels, is judged to have a negligible effect on the 5% subcriticality margin.
The 5% margin is still maintained.
The conservative assumption of a gap growth rate of 0.5 inches/year results in the projected maximum gap in the year 2000 of 5.3 inches (.05 "x5years + 2.8"). The allowable maximum gap size is 5.7 inches (4% of 143") which was used in the original criticality analysis".
Since the projected maximum size is less than the allowable maximum gap size, the 5% spent fuel pool subcritcality margin is expected to be maintained in the year 2000, assuring GDC 62
- compliance.
The projected gap size and distribution of these gaps willbe confirmed by blackness testing performed in the year 2000.
Ifthe results of the future blackness tests show that gap size growth and distribution are projected to exceed the conservative assumptions of the design basis criticality analysis, FPL willhave sufficient time to initiate long term remedial action.
One noticeable abnormality observed during the second blackness measurement campaign was the loss of Boraflex material in the top 15 inches of the panel in the north wall of cell ID=AH17. This abnormality has been fully evaluated in a separate report". In summary, the most likely cause of the missing top 15 inches of the Boraflex panel was determined to be a hidden manufacturing defect in the storage rack. The 5% subcriticality margin for Region 2 is still valid. Recommended corrective actions included developing plant administrative procedures to preclude storing fuel in cells AH17/AG17, performing spent fuel pool silica analysis daily when silica level is greater than or equal to 20 PPM and conducting the third blackness measurement campaign in the year 2000.
St. Lucie Units 1 and 2 Dock'et Nos. 50-335 and 50-389 L-96-260 Attachment Page 5 Silica Monitorin A conceivable Boraflex panel degradation at St. Lucie Unit 1 would be a Boraflex panel "thinning" process which can be attributed to the silica releases from the Boraflex panel
- surfaces, Unfortunately, current industry knowledge is unable to quantify precisely the effect of silica release on the "thinning" process.
Qualitatively speaking, spent fuel pool silica monitoring is a good indicator for revealing the relative severity of Boraflex degradation.
A plant chemistry procedure" provides routine spent fuel pool water silica concentration data.
Chronological trends of spent fuel pool silica levels for the years January 1991 - August 1992, 1994, 1995, and January-August 1996, are presented in Appendix A. By examining the silica level data, it can be postulated there is a slowly increasing trend of silica release with time.
Procedurally, the impact is evaluated ifthe silica levels become elevated to 20 PPM or above.
For comparison, Appendix A also presents the silica concentration data for the period January 1991 - August 1992, for the St. Lucie 2 spent fuel pool which does not have Boraflex materials in storage racks.
The higher silica concentration in the Unit 1 spent fuel pool is attributed to the presence of Boraflex materials in storage racks.
At present, several potential remedies for the long-term degradation are available or under consideration:
FPL is following the initiative to credit soluble boron in spent fuel pool, the installation of neutron absorbing rod inserts in fuel or neutron absorbing plates in storage racks, the use of intermediate fuel storage measures such as checkerboard configuration storage, three out of four arrangement storage, on-site dry,storage casks, and the modification or replacement of existing storage racks.
Conclusions:
On the basis of the Boraflex panel blackness testing and other surveillance data for the spent fuel storage racks at St. Lucie Unit 1, FPL has concluded:
1.
The types of Boraflex degradation reported at Palisades, South Texas, and Fort Calhoun have not been found in fuel storage racks at St. Lucie Unit 1.
2.
The effect of Boraflex panel gaps on the spent fuel pool criticality analysis is small, and is not much larger than the statistical variations inherent in the criticality analysis.
The 5% subcritical reactivity margin for 'complying with 10 CFR 50, Appendix A, General
.. Design Criterion 62 has been maintained.
3.
Continued spent fuel pool operation at St. Lucie Unit 1 has not been degraded.
St. Lucie Units 1 and 2 Docket Nos. 50-335 and 50-389 I 96-260 Attachment Page 6 4.
The Boraflex panels in the spent fuel storage racks at St. Lucie Unit 1 are essentially intact and capable of performing their intended function.
Endnotes 2.
3.
4.
5.
6.
7.
8.
9.
10.
NRC Generic Letter 96-04, Boraflex Degradation in Spent Fuel Pool Storage Racks, June 26, 1996.
Florida Power & Light Company, St. Lucie Unit No. I Spent Fuel Storage Facility Modification Safety Analysis Report, Docket No. 50-335.
USNRC Atomic Safety and Licensing Public Hearing in Matter of: Florida Power &Light St. Lucie Unit No. 1, Lbp-89-12, Page 20.
JPN-PSL-SEN$ 047,Revision 0, Justification ofProposed Boraflex Testing Program, June 29, 1989.
Stanley E. Turner, NUSURTEC Project 92810, Examination ofSt. Lucie UnitI Boraflex Surveillance Coupons Number 1,2,3, and 4, NST-89111, July 27, 1989.
Stanley E. Turner, Holtec International, Examination of St. Lucie Unit I Boraflex Surveillance Coupons Number S, AND 6, Holtec Report HI-90444, March 9, 1990.
Stanley E. Turner, Holtec International, Examination of St. Lucie Unit I Boraflex Surveillance Coupons Number 7, AND 8, Holtec Report HI-931113, December 1993.
JPN-PSL-SEFJ-93-014 Revision 3, St. Lucie UnitI Safety Evaluation ofSpent Fuel Pool Coupons Surveillance Program, March 1995.
EPRI NP-6159, An Assessment ofBoraflex Performance in Spent-Nuclear-Fuel Storage Racks, December 1988, pages 5-10.
Stanley E. Turner, Blackness Testing of Boraflex in Selected Cells of the Spent Fuel Storage Racks of St. Lucie Unit I, NST-89112, NUSURTEC Incorporated, December 1989.
11.
Walter E. Bustynowicz, Blackness Testing ofBoraflexin Selected Spent Fuel Storage Rack Cells of St. Lucie Unit I Nuclear Station, Second Measurement
- Campaign, Holtec International; Holtec Report HI-95-1303, April 1995.
12.
FPL Letter to NRC, L-89-246, July 11, 1989.
St. Lucie Units 1 and 2 Docket Nos. 50-335 and 50-389 L-96-260 Attachment Page 7 13..
JPN-PSL-SEFJ-95-023, Revision 3, Safety Evaluation for 1995 Boraflex Blackness Testing Results, March 1996.
14.
St. Lucie Plant Chemistry Procedure No. C-01, Schedule for Periodic Tests, Revision 43.
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