ML20096A867
| ML20096A867 | |
| Person / Time | |
|---|---|
| Site: | Browns Ferry  |
| Issue date: | 05/05/1992 |
| From: | Zeringue O TENNESSEE VALLEY AUTHORITY |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| NUDOCS 9205110253 | |
| Download: ML20096A867 (11) | |
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k Tennessee van um, Pau eunu m oxo A#m ye May 5, 1992 O J.'lke'Zenngue bee Prescent, Bowns F cny Om fam U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, D.C.
20555 Gentlement In the Matter of
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Docket Nos. 50-259 Tennessee Valley Authe.
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50-260 50-296 BROWNS FERRY NUCLEAR PLANT (BFN) - CONTROL ROOM EMERGENCY VENTILATION SYSTEM (CREVS)
This letter provides a list of corrective actions and an action plan for defining the remaining corrective actions required to resolve previous deficiencies identified with the CREVS. Upon implementation of the corrective actions, BFN will be in full compilance with General Design Criterion (GDC) 19 - Control Room.
In References 1 and 2, TVA requested approval to allow operation of Browns. Ferry Unit 2 during Cycle 6 with the CREVS inoperable because it did not meet its design basis for essentially zero unfiltered inleakage.
Compensatory actions were taken to ensure that GDC limits were-not a
exceeded. This request was approved by Reference 3.
The schedule for providing the long term corrective action plan was modified in Reference
- 4. to this letter summarizes the background of this issue, describes the alternatives investigated, provides a list of corrective actions and an action plan for defining the remaining cort tive actions, discusses the operator dose calculation methodology and its conformance to current regulatory guidance. A description of the re ults of the ntrol bay habitability zone analysis and/or testing, a complete description of the corrective actions, and the results of the control room operator dose calculations will be submitted to NRC by the end of July, 1992.
9205110253 920505 M'
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PDR ADOCK 05000259 I
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U.S.. Nuclear Regulatory Commission May 5, 1992
% summary list of commitments contained in this letter is provided as snelosure-2.
If you have any questions, please contact R. R. Baron, Manager of Site Licensing,'at (205) 729-3570.
Sincerely, V
V
- 0. J. Zeringue Enclosure cc (Enclosure):
NRC Resident Inspector l
Browns Ferry Nuclear Plant-l-
Route 12, Box 637 Athens, Alabama 35611' Mr. Thierry M. Ross,' Project Manager U.S. Nuclear Regulatory Commission One White Flint, North 11555 Rockville Pike Rockville, Maryland 20852
-Mr. B. A. Wilson, Project Chief U.S. Nuclear Regulatory Commission Region II-101 Marietta Street, Ini, Suite 2900
' Atlanta, Georgia 30323
REFERENCES
- 1) TVA-letter, dated February 14, 1989, Unit 1, 2, and 3 Technical Specification No. 265T
- 2) TVA letter, dated July 14, 1989, Technical Specification (TS) No. 265T Control Room Habitability
- 3) NRC letter, dated September 18, 1989, Technical Specification Revisions Concerning Operability of the Control Room Emergency Ventilation System (TAC 72398, 72199, 72200) (TS 2651) Browns Ferry Nuclear Plants, Units 1, 2, and 3
- 4) TVA letter, dated November 21, 1991, Control Room Emergency Ventilation System (CREVS) Corrective Action Plan
ENCLCHSURE 1 BROWNS FERRY NUCLEAR PLANT (BFN)
CONTROL ROOM EMERGENCY VENTILATION SYSTEM BACK0ROUND:
The Control Rocm Emergency Ventilation System (CREVS) is designed to protect the control room operators by automatically starting on receipt of a control room isolation signal and pressurizing the main control bay habitability zone with filtered outdoor air during accident conditions that could rerE.t in radioactive releases.
This filtered air maintains the control room st a positive pressure so that all leakage should be outleakage.
The CREVS uses charcoal admorbers to assure the removal of radioactive iodine from the air and high efficiency particulate absolute IHEPA) filters for removing radioactive particulate matter.
The Contrvl Bay ventilation towers, located on the north wall of the reactor bui.' ding, provide thw outsAde air for the Control Building supply ductwork.
Ventilation ' ens..hich are located in the ventilation towers, pressurite the supply ductwork that traverses the main control bay habitability zone. These fans operate during the accident recovery period (30 days) to suppl, necessary co' sling for essential equipment.
The existing CREVS unitr ' ake auction from those positively pressurized ducts.
Durlag thn Unit 2 Cycle 5 outage, an employee concern identified a specific condiJion that could impact the ability of the CHEVS to provide an environment suitable for personnel occupancy. The Control Duilding air supply ducts are not designe,i or fabricated to be leak tight.
Unfiltered outside air could leak f rom t he seams / joints of the supply air ducts that traverse the control bay habitability zone. This duct leakage could result in make-up air bypassing the CREVS and introducing potentially contamlnated and unfiltered outside air into the control bay habitability zone.
Dact leakage was not cecounted for in the prefivus control room dose calculations. A condition adverse to quality report was initiated and this was determined to be an unanalyzed condition. A survey of ino ducts that pass through the habitability zone was completed and the ducts that contributed to the untilte ad inleakage were identified. A representative section of duct was leak tested and ths results were used to estimate the total leakage of the supply duct work. Duct leakage was estimated to be 2750 CFM.
Following a postulated loss of coolant accident (LOCA), winds from the SSE, S or SSW sectors at speeds greater than thirty six miles per hour could offset the negative prensure maintained in the secondary containment by the standby gas treatment system (SGTS) and produce ex-filtration from the rwactor building.
TVA evaluated the applicable design basis events and determined that a postulated LOCA is the controlling event in terr.s of radioactivity release and dose consequences to the control room oprcators.
Page 2 of 7 ENCLOSURE 1 BROWNS FERRY NUCLEAR PLANT CONTROL ROOM EMERGENCY VENTILATION SYSTEM (CONTINUED) l Genersl Design Oriterion (GDC) 19 - Control Room, limits control room operator doses to 5 rem whole body, or its equivalent to any part of the body (30 rem I
thyroid).
When TVA postulated a LOCA, coupled with the unfiltered inleakage and the high winds, the resulting thyroid doses would be in excess of the GDC 19 limits unless componnatory actions were taken.
During the current unit 2 operating cycle (Cy:le 6), TVA temporarily modified the operability requirements for the Control Room Emergency Ventilation System (CREVS) in the Units 1, 2, and 3 Technical Specifications.
This change involved annotating the litaiting conditions for operation (LCCs) 3.7.E.1, 3.7.E.3, and 3.7.E.4 by an asterisk and defining the CREVS as being inoperable because it did not meet its design basis for essentially zera unfiltered inleakage. The Technical Specification Bases 3.7.E/4.7.E wc;e also revised to reflect this change. Power operations and fuel movement are acceptable until just prinr to startup for Unit 2 Cycle 7.
During Cycle 6, CREVS is being maintained functional by performing all applicable surveillanccs.
In the 1 vent that the applicable surveillances are not successfully performed, the actions required by the LCOs must be complied with.
Operation of Unit 2 during Cycle 6 was approved based upon the low probability of a postulated LOCA coupled with the high wind c?ndition and the compensatory actions instituted by BFN.
The compensatory actions included:
1)
The operation of all three trains of the Standby Gas Treatment System following an accident to maximite the negative pressure inside secondary containment, and 2)
The monitoring of plant radiological conditions to provide an early indication that the control room habitability zone may become degraded.
Upon determination that there was a possibility that the iodine uptake dose to the thyroid could exceed 10 rem, potassium iodide tablets would l
be Jistributed to control room and Technical Support Center personnel.
l DESCRIPTION OF ALTERNATIVES INVESTIGATED Studies have been performed to identify and evaluate potential alternatives.
l The alternativea considered were:
1 Replacing the existing duct with leak tight duct.
e Providing filtration of the supply air being introduced into the control Building.
l ENCLOSURE 1 BROWNS FERRY NUCLEAR PLANT CONTROL ROOM EMERGENCY VENTILATION SYSTEM (CONTINUED) seal and/or' repair the existing ductwork.
e Re-route the existing ductwork outside the control bay habitability zone.
Install a' kidney type filtre ion system.
1 Modify the control Bay ventilation tower intakes to offset the affects of a meteorclogical inversion by reducing the conce.tration of effluents I
being.5troduced into the control bay habitability zone.
Modify the Turbine Building and plant stack to reduce the concentration e-of effluents being introduced into the control room.
i Supplemerit the existing CREVS capacity.
i e
The following physical constrainto will significantly influence the selection of the-final corrective action plans e -.Haplacing or performing external modification to the ducto, which I
traverses the control bay habitability zone, would involve extensive l
work over the control room panels and operators.
construction noise and the potential for falling objects could challenge operations in the
~
Control roome some of the ductwork that traverses the control :ay habitability zone is e
insulated with asbestos.
Improper removal of the asbestos ~could pose a risk-to the health of the workers and the control room operators.
The current inicakage, from the control Building supply ductwork, assista=in pressurizing the control room habitability zone, Reducing l-this pressurized inleakage decreases the ability to obtain a positive pressure in the control room under isolation conditions, Any modifications / actions that would require additional diesel generator e
capaaity.
Tne available margin on the diesel generatora during certain accident scenarios is limited.
I
Page 4 of 7 ENCLOSURE 1 BROWNS FERRY NUCLEAR PLANT CONTROL ROOM EMERGENCY VENTILATION SYSTEM (CONTINUED) l DESCRIPTION OF CORRECTIVE ACTIONS AND CORRECTIVE ACTIONS PLANNED A list of corrective actions and an action plan for defining the remaining corrective actions, can be summarized as follows:
Modify the control bay ventilation towers to reduce the concentration of effluents being introduced into the control bay habitability zone.
This modification will involve extending the intakes and routing them to either side of the Turbine Building.
Increase the leak tightness of the control bay habitability zone.
This involves sealing penetrations, building txpansion joints, installation of redundant bubble tight isolation dampers, and sealing other sources of outleakage.-
Establish procedures and perform testing to periodically ensure the
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ability to maintain a positive preusure in the control bay habitability zone.
Remove tne changes implemented under temporary Technical Specification Amer.dment 265T. Temporary Technical Specification Amendment 265 expires gust prior to startup for Unit 2 Cycle 7.
Therefore, its removel is an administrative change.
Revise the applicable Technical Specification Bases section.
The current.section states:
"The control room emergency v ntilation system is designed to... maintain the control room pressure to the design positive pressure so that all leakage should be out leakage." The revised section will the control bay habitability zone be maintained at a positive pressure, Submit a Technical Specification amendment request to address the new e
-isolation dampers referenced from Surveillance Requirement 4.7.E.4.
The appropriate bases section will also be revised.
This cuandment request will require approval prior to Unit 2 restart from the Cycle 6 outage.
Perform analysis and/or tewting of the control bay habitability zone to determine if modifications to the existing CREVS units or if additional CREVS capacity will be required.
CREVS modifications or additional capacity will be installed, as required.
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Page 5 vt 7 ENCLOSURE 1 BROWNS FERRY NUCLEAR PLANT CONTROL ROOM EMERGENCY VENTILATION SYSTEM (CONTINUED)
Finalize calculations that document the control room operator thirty day e
integrated doses from a postulated LOCA are below the CDC 19 limit.
Submit a description of the results of the control bay habitability zone analysis / testing, a description of the corrective actions, and the results of the dose calculations by the end of July 1992.
. Update the Browns Ferry Updated Final Safety Analysis Report.
RESULTS OF PREUMINARY CONTROL ROOM OPERATOR DOSE CALCULATIONS After the CREVS corrective actions are implemented, the resultant control room operator thirty day integrated doses from a postulated LOCA will be below the
-GDC 19 limit.
IVA hns performed preliminary calculations that assume an unfiltered ~inleakage of 2750 cim,. completion _of the modifications to the control bay ventilation towers, and an additional 2500 cfm of CREVS capacity for-each redundant train. The resultant operator dose is 1.6 rem whole body gam.na,_0.65 rem beta, and 18 rem thyroid. The finalized calculation will determine the required CREVS capacity.
SUMMARY
OF DOSE CALCULATION METHODCLOGY The major attributes used in the preliminary dose calculations were:
1)
The extrema. wind conditions, which were originally assumed to produce ex-filtration from the Reactor Building, need not be postulated. A probabilistic risk assessmant was performed to determine the~11kelihood' of the exfiltration event occurring during the 30 day accident-recovery.
4 This probability of occurrence is below 10 and is no longer considered a credible event.
2)
The' primary containment leaks to the secondary containment (Reactor Building) at a rate of two percent per dt.y.
This is the. maximum allowable leakage rate specified by Technical Specification 3.7.A.2.b.
Pare 6 of 7 ENCLOSURE 1 BROWNS FERRY NUCLEAR PLANT CONTHOL ROOM EMERGENCY VENTILATION SYSTEM (COf4TINUED) 3)
In addition to the leakage from the primary containment to the secondary containment, the main steam isolation valves (HSIVs) were assumed to leak at a rate of 11.5 seth.
This is the maximum leakage rate allowed by Technical Specification 4.7.A.2.1.
The leakage was assumed through the HSIVs, to the low pressure turbines and condensors, out the low pressure turbine seals, and through the Turbine Buf1 ding roof vonts.
- 4) The lodine removal efficiency of the Standby Gas Treatment System is 90/70 percent and the iodine removal efficiency of the CREVS is 90/90 percent for inorganic and organic respectively.
COMPARISON OF CALCULATION METHODOLOGY WITH REGULATORY GUIDANCE While BFN was lironned prior to the issuance of the SRPs and the majority of the current Regulatory Guides, a comparison of the preliminary BFN dose calculation to selected regulatory guidance was performed to validate the major assumptions.
In general, BFN calculated ths doses to the control room operatcra in accordance with the guidelines provided by Regulatory Guide 1.3, Assumptions used for Evaluating the Potential Radiological Consequences of a Loss of Coolant Accident for Boiling Water Reactor, Rev. 2.
The recommended core inventory, effects of radiological decay during holdup, reductions in radioactive material due to e.ngineered safety features, Technical Specification cited containment Jakage rate, no suppression pool iodini retention, fumigation conditions for h hour, no ground effects plume depletion, breathing rates, elevated a*3 grouna level releases, semi-infin.tw cloud model for beta dosos, and site specific 2/g values were used.
BFH deviates from the Regulatcry Guide 1.3 recommendations by taking credit for half of the reactor zone volume for mixing, lodine dose conversion factors j
from the 10RP publication 30, 1979 instead of Publication 2, 1959, whole body L
gamma dose from a point kernel model instead of a semi-infinite cloud, and beta and gamma energies from TVA's Isotope Library Data File instead of the Table of Isotopes, 6' Edition.
t l
Although Regulatory Guide 1.3 does no. allow credit for iodine retention in the suppression pool, Standard Review Plan Section 6.5.5 subsequently allowed licensees to consider the pressure suppression pool 64 a fission product cleanup system. No credit was conservatively assumed for suppression pool cleanup.
s Page 7 of 7 ENCLOSURE 1 BROWNS FERRY NUCLEAR PLAN 7 CONTROL ROOM EMERGENCY VENTILATION SYSTEM (CONTINUED) l CONCLUSIOF' The corrective-actions taken to resolve previously identified CREVS concerns will ensure that. post-accident radiation doses to the control room operators
- are maintained below regulatory limita.
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ENCLOSURE 2 BROWNS FERRY NUCLEAR PLANT (DFN)
SUMMARY
OF COMMITMENTS The corrective actions, which will be implemented to vosolve the deficiencies identified with the current CREVS configuration, can be sun;marized as f ollows:
Modify the control bay ventilation towers to ruduce the concestration of e
effluents being introduced into the control bay habitability zone.
Increase tne leak tightness of the control bay habitability zona.
i Establish procedures and perform testing to periodically ensure the e
ability to maintain a positive pressure in the control bay habitability zone.
TVA will request the removal of the changes implemented under temporary e
Technical Specification Amendment 26bT and TVA will revise the applicable Technical Specification Bases section.
The revised section
-ill the control bay habitability zone be maintained at a positive pressure.
Submit a Technical Specification amendment request to address the new isolation dampers referenced from Surveillance Requirement 4.7.E.4.
The appropriate bases section will also be revised.
Perform analysis and/or testing of the control bay habitability zone to determine if modifications to the existing CREVS units or if additional CREVS capacity will be required.
CREVS modifications or additional capacity will be installed, as required.
Finalize calculations that document the control room operator thirty day integrated doses from a postulated LOCA are below the GDC 19 limit.
Submit a description of the resnits of the control bay habitability zone analysis / testing, a description of the corrective actions, and the results of the dose calculations by the end of July 1992.
Update the Browns Ferry Updated Final Safety Analysis Peport.
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