ML20148B956
| ML20148B956 | |
| Person / Time | |
|---|---|
| Site: | Browns Ferry  |
| Issue date: | 03/16/1988 |
| From: | Gridley R TENNESSEE VALLEY AUTHORITY |
| To: | NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM) |
| References | |
| TAC-00316, TAC-00317, TAC-00318, TAC-316, TAC-317, TAC-318, TAC-R00316, TAC-R00317, TAC-R00318, TAC-R316, TAC-R317, TAC-R318, NUDOCS 8803220203 | |
| Download: ML20148B956 (6) | |
Text
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.k TENNESSEE VALLEY AUTHORITY CHAT 1 ANOOGA. TENNESSEE 37401 SN 1578 Lookout Place MAR 161988 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, D.C.
20555 Gentlemen:
In the Matter of
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Docket Nos. 50-259 Tennessee Valley Authority
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50-260 50-296 BROWNS FERRY NUCLEAR PLANT (BFN) - SECONDARY CONTAINMENT PENETRATIONS This letter _ describes the BFN program for resolving a discrepancy between the BFN Final Safety Analysis Report (FSAR) and the as-constructed configuration of secondary containment penetrations with regards to seismic qualification.
This condition was reported to NRC in Licensee Event Reports (LERs) 05000259-024 and 05000259-024, supplement 1, and is discussed in the Browns Ferry Nuclear Performance Plan,Section III.3.11 of Revision 1, that was transmitted by S. A. White's letter dated July 1, 1987.
The enclosure to this letter describes the BFN program for resolving this issue. TVA requests your review of this program and the issuance of a written statement documenting program acceptability.
Please refer any questions in regards to this submittal to M. J. Hay, Manager, BFN Site Licensing at (205) 729-3570.
l Very truly yours, l
/
R. Gridley, trector Nuclear Licensing and l
Regulatory Affairs Enclosure cc:.See page 2 l
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DCD An Equal Ooportunity Employer
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,. U.S. Nuclear Regulatory Comission M4k } g jggg
-cc (Enclosure):
Mr. K. P. Barr, Acting Assistant Director for Inspection Programs TVA Projects Division U.S. Nuclear Regulatory Comission Region II 101 Marietta Street, NW, Suite 2900 Atlanta, Georgia 30323 Mr. G. G. Zech, Assistant Director for Projects TVA Projects Olvision U.S. Nuclear Regulatory Comission One White Flint, North 11555 Rockville P!ke Rockville, Maryland 20852 Browns Ferry Resident Inspector Browns Ferry Nuclear Plant-Route 12, P.O. Box 637 Athens, Alabama 35611 l
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i ENCt.0SURE SECONDARY CONTAINNENT PENETRATIONS
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Background
The BFN secondary containment forms a boundary common to all three units which is basically at the exterior of the Reactor Building and includes the refueling floor. Under those conditions which require secondary containment isolation, the secondary containment is maintained at a negative pressure relative to the i
building's exterior by the Standby Gas Treatment System (SGTS).
Thus, in the unlikely event of a radioactive release inside the building, the pressure difference will tend to contain the radioactivity within the building and prevent'its escape to the environment. Under isolation conditions, the secondary containment atmosphere is filtered by the SGTS before release from
~the plant stack. Upon secondary containment isolation, the SGTS is,'equired.to maintain a negative one-fourth inch of water vacuum with a prescribed design inleakage flow rate. The inleakage flow rate considers the total infiltration including air locks, roof, siding, piping penetration seals, and isolation dampers in addition to the inleakage allowance for seismic and thermal expansion effects.
The total inleakage flow rate and internal pressure consider the atmospheric effects of wind conditions.
Definition of Problem The Browns Ferry Nuclear Plant (BFN) Final Safety Analysis Report (FSAR) states the Reactor Building (secondary containment) and all penetrations through the containment envelope are of Class I seismic design.
The penetrations are so classified regardless of the classification of the systems using such penetrations.
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l Significant Condition Report (SCR) BFNNEB8601, revision 1, stated that the BFN secondary containment may not perform its function of limiting inleakage following a design basis earthquake (DBE) because, even though the building structure itself is Class I seismic, some of the plant features which penetrate secondary containment were not designed and installed as Class I seismic commodities.
Since the post-DBE integrity of the nonselsmic penetration seals i
cannot be ensured, it can be postulated that seal degradation could potentially increase the inleakage flow area.
Thus, if the inleakage area was to be increased sufficiently as a result of a DBE, the SGTS would no longer be capabic of maintaining the negative one-fourth inch of water vacuum within the design basis SGTS flow rate. Hence, radioactivity could potentially bypass the SGTS and be released directly to the environment.
TVA Position The purpose of the statements in the FSAR regarding the secondary containment was to ensure the secondary containment's retention and filtering capability rather than its absolute design standards. Demonstrating that the plant is capable of maintaining the one-fourth inch of water pressure requirement following a DBE will satisfy the regulatory requirement for the secondary containinent.
. TVA intends to demonstrate BFN's ability to satisfy the regulatory requirement by demonstrating that the post-DBE configuration of the SGTS and secondary containment will be capable of maintaining a negative one-fourth inch of water vacuum inside the secondary containment within the design basis SGTS flow rate. Modifications to some penetration seals will be required to ensure this capability. Upon satisfactory completion of the testing and modifications, final resolution of the issue will require an FSAR revision to restate the secondary containment's design in terms of a performance-based commitment to maintain the pressure boundary capability following a DBE.
Description of Program TVA's program for resolution of this issue involves ensuring that the secondary containment boundary will maintain sufficient integrity such that the SGTS will be capable of maintaining a post-DBE one-fourth inch of water negative pressure inside secondary containment.
The issue of the seismic response of the secondary containment boundary seals and hence their post-DBE integrity will involve several steps.
The major programmatic steps are:
(1) evaluate and quantify the potential post-DBE tr. leakage flow rates through the secondary containment, and (2) modify the flow paths, as required, to ensure that the total post-DBE inleakage flow will be within the capability of the SGTS to maintain the required one-fourth inch of water pressure differential within the design basis SGTS flow rate.
The inleakage flow paths through the secondary containment boundary arise from the air locks, refueling floor roof and siding, piping penetrations, HVAC ducting penetrations, and electrical conduit and cable tray penetrations.
These flow paths potentially present two types of paths:
- 1) normal inleakage area, and 2) post-DBE inleckage area increase.
Flow paths such as the air locks and the refueling floor roof and siding are seismically qualified and are, therefore, not expected to experience an increase in post-DBE inleakage.
The interim seismic qualification of the cable trays has been evaluated in the NRC's safety evaluation which was transmitted by the NRC's letter from D. R. Mueller to S. A. White dated February 5, 1987. The long-term evaluation of the cable tray / supports seismic qualification will be covered under the Seismic Qualification of Equipment Program (USI-A46).
The post-DBE area increase can arise from two different failure mechanisms for 1tems such as the HVAC ducting, piping and electrical conduit penetrations.
Inleakage area can be created by a failure of the annular seal around the item where the item passes through the secondary containment boundary or area can be created if the item itself fails such that a leakage path internal to the item is created.
This second failure mechanism requires a throughwall failure of the item on both sides of the secondary containment boundary for piping, conduit, and non-isolated HVAC ducting.
HVAC ducting which contains isolation dampers would require a failure of both isolation dampers in order to degrade secondary containment.
The HVAC ducting and isolation dampers will be seismically qualified and, therefore, will not experience failures leading to an increase in internal post-DBE inleakage area. The ducting is grouted in place and will therefore not experience an increase in inleakage due to failure of the seal.
The specific program to address this issue will be described by a separate submittal.
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Piping penetrating secondary containment can be divided into two categorits:
- 1) seismically qualifted, and 2) nonseismically qualified.
The seismically qualifted pipe and its associated penetration seal, like other seismically qualified equipment, should not experience an increase in inleakage.
Class I seismic large bore piping will be demonstrated to be adequate by the program to resolve IE Bulletins 79-02 and 79-14.
This program was submitted for the NRC staff's review by letter from R. L. Gridley, dated April 8,1987 and has been extensively discussed with the NRC staff during TVA/NRC meetings on September 16, 1987, December 11, 1987, and February 29, 1988.
Class I seismic small bore piping will be demonstrated to be adequate.
The specific program to address this issue will be described by a separate submittal.
For the conduits and nonseismic piping, TVA will use a two-staged approach.
It must first be demonstrated that the conduit or piping itself will survive the effects of a DBE and then, secondly, an earthquake resistant seal must be confirmed at the secondary containment boundary.
The 2onduit qualification program was submitted for the NRC staff's review by letter from R. L. Gridley, dated April 8, 1987.
For the nonseismic pipirig outside the secondary containment boundary, a sample walkdown using experience-based criteria along with limited analysis will be conducted.
The nonseismic piping inside the secondary containment boundary will be evaluated under the Class I/II water spray program.
The purpose of these efforts will be to confirm that a throughwhil failure on both sides of the boundary would be a highly unlikely occurrence.
The remaining issue for the conduit, cable trays, and nonseismic piping is to confirm or establish an earthquake-resistant seal around the item at the secondary containment boundary in order to limit inleakage to be within the capability of the SGTS to maintain the required one-fourth inch differential pressure.
The cable trays are sealed with approved 10 CFR 50, Appendix R type seals. Through comparison to the earthquake experience data base, it has been confirmed that there are some nonselsmic piping penetration seals which are already earthquake resistant (e.g., flexible fiberglass boots and tapered rubber boots) while others are sealed in a manner that will exhibit a small, predictable area increase (i.e., tack-welded plates).
For the conduit and the other nonselsmic piping, TVA intends to seal the annular area using the flexible foam type seal materials currently in use by the 10 CFR 50, Appendix R, Fire Protection Program for fire seals.
The flexibility and sealing characteristics of the materials will permit the expected seismic induced motion of the item without experiencing a significant increase in post-DBE inleakage area.
The integrity of the seals around the conduit should not be challenged since the conduit will be seismically qualified and should not experience an appreciable movement.
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. In order to demonstrate the expected post-DBE performance of the flexible foam seal design for the piping, a serics of special tests will be conducted.
Hockups of piping and penetrations with the seal material in place will be subjected to cyclic axial displacements of the pipe.
The magnitude of the displacements will be stepwise increased until they are in excess of both the rated displacements for the material and the expected seismic induced motion.
Seals will be subjected to a one-fourth inch of water pressure differential.
The seal inleakage rate will be measured before and after the displacement.
Similarly, the test apparatus will be subjected to stepwise lateral displacements and the flow rate measured before and after.
Based on the measured penetration test leakage rates, the expected total post-DBE secondary containment inleakage rate can be approximated by adding the expected post-DBE inleakage flow rate increase to the technical specification surveillance requirement value for SGTS flow rate.
The expected total post-DBE inleakage rate will be compared to the flow characteristics of the SGTS.
The total inleakage rate must not exceed the design basis SGTS flow rate.
Tests will be conducted on various sized piping and penetration configurations in order to bound the entire range existing in the plant. Multiple tests for each configuration will be conducted to demonstrate the repeatability of the data.
Following completion of the above described program, TVA intends to revise the BFN FSAR to clarify the performance and design of the secondary containment in terms of its pressure capability rather than the current commitment to Class I seismic penetrations.
Conclusion This program will ensure that the secondary containment maintains an adequate pressure boundary following a DBE. This program satisfies the intent of the FSAR commitment and provides an adequate basis for the revision of the FSAR.
This program will be completed before restart of BFN unit 2.
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