ML20099C848

Describes Corrective Actions Which Will Be Implemented to Resolve Previous Deficiencies Identified W/Crevs,For Compliance W/Gdc 19 Re Control Room.Requests NRC Review & Issuance of SE by 921030.Summary List of Commitments Encl

ML20099C848
Person / Time
Site:	Browns Ferry
Issue date:	07/31/1992
From:	Zeringue O TENNESSEE VALLEY AUTHORITY
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
NUDOCS 9208040331
Download: ML20099C848 (10)

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IDl#l imumwe V,eev Army Pcat Oke tu am om r u wt.ca awa 0 J %o*2ennguo vo N w.mtsww g m,o w s m JUL 31 IW2 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk

.Weshington, D.C. 20555 Gentlemen:

In the Matter of ) Docket Nos. 50-259 Tennessee Valley Authority ) 50-260 50-296

' BROWNS FERRY NUCLEAR PLANT (BFH) - RESOLUTION OF CONTROL ROOM EMERGENCY VENTILATION SYSTEM (CREVS) ISSUES This letter describes the corrective actions which will be implemented to resolve previous deficiencies identified with the CREVS. Upon implementation of these corrective actions, BFN will be in full compliance with General Design ',

Criterion (GDC) 19 - Control Room. Since these corrective '

actions represent a change in.BFNs previously reviewed approach to meeting-GDC 19, TVA requests a Safety Evaluation to document the acc.aptability of this method.

-In Reforcnces 1 and 0, TVA requested approval to allow '

operation of BFN Unit-2 during Cycle 6 with the CREVS inoperable only because it did not meet its design basis for essentially zero unfilterad inleakage. Compensatory actions were taken to ensure that this relaxation did not result in a significant reduction in the margin of safety. This request was approved by Reference 3. In Reference 4, DFN reviewed the alternatives investigated, discussed the physical constraints, and provided a list of corrective actions and an action plan for defining the remaining corrective actions.

Enclosure 1 to this letter summarizes the background of this ^

issue, provides a description of the modifications that will 9208040331-920731 DR p ADDCK 05000259 PDR lflf)/ _

l lll

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'2 U. S. Nuclear Regulatory Commission Jul. 311992 be implemented to resolve this problem, and reviews.the results and major assumptions of the operator dose calculations. Expeditious NRC review of these corrective actions and issuance of a Safety Evaluation Report is requested by october 30, 1992, in order to support the implementation of modifications that are required to be completed prior to the restart of Unit 2 from the next refueling outage.

A su m mary list of commitments contained in this letter is 1 provided as Enclosure 2. If you have any questions, please ,

contact R. R. Baron, Manager of site Licensing, at (205) 729-7566.

Sincerely, A

v i lu bb

us ,

.I '

O J. Zeringuo Enolosures cc (Enclosures):

-NRC Resident Inspector i Browns Ferry. Nuclear Plant j Route 12, Box 637 Athens, Alabama' 35611 Mr. Thierry H. Ross, Project Manager U.S. Nuclear Regulatory Commission one White Flint, North 11555 Rockville Pike '

Rockville, Maryland 20852 l- Mr. B. A. Wilson, Project Chief U.S. Nuclear Regulatory Commission Region II 101 Mariutta Street, NW, Suite 2900 Atlanta, Georgia 30323

-~ r

i U.S. Nuclear Regulatory Commission JUL 311992

References:

1) TVA lettor, dated February 14, 1989, Unit 1, 2, and 3 Technical Specification No. 265T

2) TVA letter, dated July 14, 1989, TVA Technical Specification (TS) No. 265T Control Room liabitability

3) HRC letter, dated September 18, 1989, Technical Specification Revisions Concerning operability of the Control Room Emergency Ventilation System (TAC 72198, 72199, 72200)

(TS 265T) browns Ferry Nuclear Plants, Units 1, 2, and 3

4) TVA letter, dated May 5, 1992, Control Room Emergency Ventilat.on System (CREVS) i

1. _ _ . _ _ _ _ _

EtJCLORURE 1 BROWi1S FERRY IJUCLEAR PLAtJT (BFtJ)

COtJTntOL ROOM EMERGEtJCY VElJTILATIOt1 SYSTEM DACKGROUND:

The Control Room Emergency Ventilation System (CREVS) is designed to protect the control room operators by a.utomatically starting on receipt of a control room isolation cignal and preocurizing the main control bay habitability zone with filtered outdoor air during accident conditions that could result in radioactive releases. This filtered air maintains the control room at a positive pressure so that all leakage should be outleakage. The CREVS uses charcoal adsorbers to assure the removal of radioactivo iodine from the air -

and high officiency particulato absolute (HEPA) filters for removing radioactive particulate mattor.

( The Control Bay ventilation towers, located on the north wall of the reactor building, provide the outside air for the Control Building oupply ductwork.

Vontilation fans, which are located in the ventilation towers, pressurize the cupply ductwork that traverses the main control bay habitability zone. These fans operate during the accident recovery period (30 days) to supply necessary cooling for essential equipment. The existing CREVS units take auction from theco pooltively prossurized ducts.

During the Unit 2 Cycle 5 outage, an employoo concern identified a specific condition that could impact the ability of the CREVS to provide an environment suitable for perconnel occupancy. The Control Bu!.lding air oupply ducto are not (Saigned or fabricated to be leak tight. Unfiltored outeldo air could leak from the seams / joints of the supply air ducts that traverse the contrcl bay habitability zone. This duct leakage could result in make-up air bypanning the CREVS and introducing potentially contami.iated and unfiltered outside air into the control bay habitability zone.

Duct leakage was not accounted for in the previous control room doce calculations. This was determined to be an unanalyzed condition and a condition advet ee to quality report was initiat ed. A survey of the ducts that paso through the habitability zone was completed and the ducts that contributed to the unfiltered inleakage were identified. A representativo section of duct was leak tooted and the results were uced to estimate the total lockago of the supply duct work. Duct leakage was catimated to be 2750 CFM.

Following a postulated lose of coolant aces . ic (LOCA), winds from the SSE, S or SSW sectors at speeds groater than thirtr Si miles per hour could offset the negative preocure maintained in the secondary cantainment by the standby gan treatment cyotem (SGTS) and produce ex-filtration from the reactor building. TVA evaluated the applicable design basis events and determined that a postulated LOCA is the controlling event in terms of radioactivity release and doao consequences to the control room operators.

Page 2 cf 6 ENCLOSURE 1 BROWNS FERRY NUCLEAR PLANT l CONTROL ROOfA EMERGENCY VENTILATION SYSTEM (CONTINUED)

General Design Criterion (GDC) 19 - Control Room, limits control room operator l doces to 5 rem whole body, or its equivalent to any part of the body (30 rem thyroid). When TVA postulated a LOCA, coupled with the u7 filtered inleakage, the high winds from a specific direction, and no compensator) actions, the resulting thyroid dosen would have been in excess of GDC 3~. limits.

I During the current Unit 2 operating cycle (Cycle 6), TVA temporarily modified )

the operability requirements for the Control Room Emergency Ventilation l i

System (CREVS) in the Units 1, 2, and 3 Technical Specifications. This change involved annotating the limiting conditions for operation (LCOs) 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 zero unfiltered inleakage. The Technical Specification Bases 3.7.E/4.7.E were also revised to reflect this change. Power operations and fuel movement are acceptable untti just prior to startup for Unit 2 Cycle 7. During cycle 6, CREVS is being maintained tunctional by performing all applicable surveillances. In the event 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 condition and the compensatory actions instituted by DPN. The compensatory actions included:

1) .The operation of all three trains of the Standby Gas Treatment System following an accident to maximize 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 lodide tablets would be distributed to control room and Technical Support Center personnel.

As diccuesed in DFN's May 5, 1992 submittal, studies were performed to identify and evaluate' potential corrective actions. The alternatives l

considered were:

• Replacing the existing duct with leak tight duct.

+ Providing filtration of the supply air being introduced into the control Duilding, a

Page 3 of 6 EtJCLOSURE 1 BROWilS FERRY T4UCLEAR PLAIJT COtJTROL ROOM EMERGEtJCY VEtJTILATIOt1 SYSTEM (COtJTitJUED) e Seal and/or repair the existing ductwork.

• Re-route the existing ductwork outside the control bay habitability zone.

• Install a kidney type filtration oystem, e Modify the Control Bay ventilation tower intakes to offset .he affects of a meteorological inversion by reducing the concentration of effluents being introduced into the control bay habitability zone.

e Modify the Turbine Building and plant stack to reduce the concentration of effluents being introduced into the control room, o Supplement the existing CREVS capacity.

The following physical constraints oignificantly influenced the selection of the final corrective action plani e Replacing, sealing, or performing external modification to the ducts, which traverseo the control bay habitability zone, would involve work over the control room panels and operators. Construction noise and the potential for falling objects could challenge operations in the cont -al room.

e Some of the ductwork that traverses the control bay habitability zone is "

insulated with asboatos. Improper removal of the asbestos could pose a risk to the health of the workers and the control room operators.

• The current inleakage, from the Control Building supply ductwork, assists in preocurizing the control room habitability zone. Reducing thin pressurized inleakage decreases the ability to obtain a positive presouro in the control room under isolation conditions, e Any modifications / actions that would require additional diesel generator capacity. The available margin on the diesel generators c'uring certain accident scenarios is limited.

I Page 4 of 6 EtJCLOSURE 1 BROWi1S FERRY tJUCLEAR PLAtJT COtJTROL ROOM EMERGEtJCY VENTILATIOt1 SYSTEM (CONTitJUED)

As committed in TVA's May 5, 1992 submittal, a Special Test was conducted on May 23, 1992 to determine if the existing CREVS was adequately sized to pressurize the CBHZ. This test was conducted by isolating the CBHZ, including the supply and exhaust fans and ductwork that provide ventilation air to and irom the outside, isolating the CREVS, and pressurizing the CBHZ by using a

+est fan.

From the results of this test, it was determined that the total unfiltered inleakage into the CBHZ was 3717 cim, instead of the 2750 cfm previously estimated.

DESCRIPTION OF CORRECTIVE ACTIONS The liot. of corrective actions from TVA's May 5, 1992 submittal included: 3 e Modify the control bay ventilation towers to reduce the concentration of offluents being introduced into the control bay habitability zone. This modification will involve extending the intakeo and routing them to either side of the Turbine Building. This ductwork will be seismically qualified and will include wind loadings.

However, tornado missile protection will not be provided since this ductwork in not required to mitigate the consequences of a tornado event. Administrative controls assure adequate ventilation is provided.

Outside air cupply to selected areas of the control Bay is isolated and cooling in entablished in the areas affected by the isolation of the outoide air. In addition, temperatures are monitored and additional _

cooling is provided, as necessary.

. Increase the leak tightneos of the control bay habitability zone. This involves sealing penetrations, building expansion joints, installation of redundant bubble tight ioolation dampers, and sealing other sources of outleakage, o Establish procedures and perform testing to periodically encure the ability to maintain a positive procuure in the control bay habitability zone, o Remove the changes implemented under terrporary Technical Specification Amendment 265T. Temporary Technical Specification Amendment 265 expires just prior to atartup for Unit 2 Cycle 7. Therefore, its removal is an administrativo change.

Page 5 of 6 ENCLOSURE 1 BROWNS FERRY NUCLEAR PLANT

$ COL (ROL ROOM EMERGENCY VENTILATION SYSTEM

g. . , .

(CONTINUED)

• zy

,El

^

e Revice the applicable TechnLol Specification Bases section. The current section states: "T > .ontrol room emergency ventilat.cn system is designef to ... maintai- 'le control room prer tre to the design

,ositive pressure so that all leakage should be t. leakage." The

.V* revised section will require the control bay habitability nane be

{p, maintained at a >oultive pressure, em ; " a Cubmit a Technical Specific. ion amendment request to ccdre s the new isolation dampers referenced from surveillance Requirement 4 . 7 . I; . 4 . The appropriate bases section will also be revised. This amenanient request will require apptova) prior to Unit 2 restart from the Cyc e 6 outage.

-] in addition, TVA will

.ce the two existing 500 cfm redundant CREVS trairs with 3000 cfm of city for each train. This additional CREVS capacity will increase Te iodine removal from the control room habitability zone after an accident and reduce the total integrated thirty day dose to the control

-com operators. The additional CREVS capacity will require swapping of 73 Wade from a more heavily loaded diesel generator to a diesel generator eith orcater margin. The Ts.:hnical Specificattora will ba amended to ref1r hi e ;hange.

d Jb w.e TVA hea vetermines that the 3717 cfm of unfiltered inleakage is .

C ac.;eptable in terms of m ating the requirements of GDC 19 and that further redue;tions t. - the CBH2 outleakage are also unnecessary, TVA has decided not to install redundant bubble tight isolation dampero in the CBHZ boundary and to

{

withdrew this commitment. <

RESULTS OF CONTROL ROOM OPERATOR DOSE CALCU ATIONS After the corrective actions are implemented, the resultant control room 1 operator thirty day integrated doses from a postulated LOCA will be below the CDC 19 limit. TVA has performed calculations for Unit 2 operatian that assume an unfiltered inleakage of 3717 cfm, MSIV 3 akage of 11.5 ccfh per alve, completion of the modifications to the control bay ventilation tow ra and 3000 c6n of CREVS captcity fer each redundant train. The resulting operator doses were well below the limits of GDC 19, i

,n &

e

'4 Page 6 af 6 ENCLOSURE 1 BROWNS FERRY NUCLEAR PLANT CONTROL ROOM EMERGENCY VENTILATION SYSTEM (CONTINUED)

SUMMARY

OF DOSE CALCULATION METHODOLOGY The dcse calculation methodology has not changed from the summary provided i.

TVA's May 5, 1992 Intter. As previously described, the major attributes used in the dose calculations were:

1) The e.'cremu wind conditions, which were originally assumed to produce ex-filtration from the Reactor Duilding, need not be postulated. A prceabilistic risk assessment was performed to determine the likelihood cr the exfiltration event occutring during#the 30 day accident recovery.

This probability of occurrence is below 10 and is no longer considered a credible event.

2) The primary containment leaks to the secondary containment (l? actor Building) at a rate of two percent per day. This is the maximum allowable leaknge rate specified by Technical Specification 3.7.A.2.b.

3) In addition to the leakage from the primary containment to the secondary contatnmont, the main steam isolation valves (MSIVs) were assumed to leak at a rate of 11.5 sefh. This is the maximum leakage rate allowed by Tecnoical Specification 4.7.A.2.1. The leakt.ge was ass'n d through the MSIVs, to the low pressure turbines and condeucars, out the low pressure tuchine senis, and through the Turbine Building roof vents.

4) The iodine removal efficiency of the-Standby Gas Treatment System and the CREVS is 90/90 percent for inorganic and organic respectively.

CONCLUSION The corrective actions described in this submittal will ensure that post-accident radiation doses to the control room operators are maintained below regula . ry [htits Expeditious NRC review of these corrective actions and issuance et a Safety Evaluation Report is requented in order to support the implementation of modifications that are required to be completed prior tc the restart of Unit 2 from the upcoming refueling outag+