ML19350A107

Interim Deficiency Rept Re Potential for Inadvertent Boron Dilution Event,Initially Reported on 801024.Westinghouse Is Investigating Event.Interim Recommendation Notes Acceptable Operation at Three Dilution Flow Rates

ML19350A107
Person / Time
Site:	Summer
Issue date:	11/20/1980
From:	Nichols T SOUTH CAROLINA ELECTRIC & GAS CO.
To:	James O'Reilly NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION II)
References
10CFR-050.55E, 10CFR-50.55E, NUDOCS 8012020493
Download: ML19350A107 (10)

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• SOUTH CAROLINA ELECTRIC a GAS COMPANY most ottict som te+ ,,

CotuMeiA, SouTM CAmo u NA 29218 1 T. C. NicMots, Jn. k '

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sevets.s Gnnere.s j November 20, 1980 f,

Mr. James B. O'Reilly 'i U. S. Nuclear Regulatory Commission '

Region II 101 Marietta Street, N. W.

Suite 3100 Atlanta, GA 30303

Subject:

Virgil C. Summer Nuclear Station, Unit 1 Reportable Item in Accordance with 10CFR21 Inadvertent Boron Dilution - Docket No. 50/395 i

On October 24, 1980, South Carolina Electric & Gas Company (SCE&G)

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' informed NRC of a substantial safety hazard concerning the potential for an inadvertent boron dilution event at cold or hot chutdown conditions while on the Residual Heat Removal System. A boror dilution results in a reactivity addition which could lead to a loss of shutdown margin and cause a substantial safety hazard as defined by 10CFR21. Under the SCE&G program, we are reporting this as a significant deficiency under 10CFR 50.55(e) . Additional information is presented in Attachments A and B.

Attachment A presents a discussion of the Westinghouse concerns and recommended actions. Westinghouse is continuing to investigate this potential event to obtain a long-term solution. If the Westinghouse -

evaluation is not completed 90 days prior to fuel loading, SCE&G will incorporate the Westinghouse recommendel interim corrective actions into the V. C. Summer Nuclear Station General Operating Procedures for mode 4 (Hot Shutdown) and mode 5 (Cold Shutdown).

! A final' report will be issued after the Westinghouse investigation i is completed and the long-term solution has been evaluated. If you have any questions concerning this repcrt, please call me.

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'Very truly yours, T. C. Nichols, Jr.

BSM:rm Enclosures 0 i

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4 Mr. James P. O'Reilly Page 2 CC: Messrs. V. C. Su==er G. H. Fischer E. H. Crews, Jr.

D. A. Nau=an O. S. Bradham O. W. Dixon, Jr.

R. B. Clary W. A. Williams, Jr.

J. B. Knotts, Jr.

J. Skolds B. Bursey NPCF/Whitaker File U. S. Nuclear Regulatory Connission Division of Inspection and Enforcement Washington, DC 20555 U. S. Nuclear Regulatory Co==ission Document Management Branch "#

Washington, DC 20555

U.g - C G'.!S-1055 3

ATTAC101ENT A W25t!Dgh00SB' V/Gi0f IICC010f :eur c=ecial Electric Corporation C#5" Di'lis!0ns Ecr355 N:sW7/tPav.s/vania15233 July 28, 1980 14r.-C. A. Price, Manager Nuclear Engineering South Carolina Electric & Gas Company QD P. O. Box 764 Columbia SC 29218 nQh U Q U \M

Dear Mr. Price:

SOUTH CAROLIHA ELECTRIC & GAS COMPANY VIRGIL C. SUMMER HUCLEAR STATION Inadvertent Baron Dilution On June 30, 1980, your Mr. R. Clary was notified of certain Westinghouse concerns and recom. ended actions regarding the potential for an inadvertent boron dilution event at cold or hot shutdown conditions while on the Residual Heat Removal Sys '

tem. This notification was in accord with Westinghouse determination that these concerns constitute an Unreviewed Safety Question under 10CFR Part 50.59. The NRC Office of Inspection and Enforcement was also notified on June 27,1980 that these concerns have generic applicability to Westinghouse-supplied nuclear power plants. Further clarification was made to the NRC Office of Inspection and Enforcement on June 30, 1980 that Westinghouse concerns are not applicable while the plant is greater than 5% shutdown.

This letter is intended to formally document these concerns and to provide ad-ditional relevant information. This letter also modifies the earlier recomend-ed actions by a more detailed specification of applicable plant operating conditions.

Inadvertent baron dilution at shutdown hap.been generally regarded as an event which can be identified and terminated by operator action prior to a return to critical. Automatic protection has ndt teen a standard feature for Westinghouse plants. !!cstinghouse has recently been conducting a general investigation of' this potential event relative to the licensing requirements imposed on newer plants not yet in operation. This investigation is not yet complete. However,

'it has been determined that under certain shutdown conditions and with certain assumed dilution rates, adequate time for operator action to prevent a return to critical may not be available.

The current-Westinghouse evaluations are based on plant conditions as noted below: .

The Reactor Coolant System effective volume is limited to the vessel and ~

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the active portions of the hot and cold legs when on RHR, i.e., steam gen-erator volumes are not included.

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D CCWS-1055 2

C. A. Price  %

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2. The plant is borated to a shutdown margin greater than or equal to 1%

t,k/ k. ,

3.

Uniform mixing of clean and borated RCS water is rot assumed, i.e., mixing of the clean, injected water and the affected loopTs assumed but instan-

, taneous, uniform mixing with the vessel, hot legs, Thusand a " cold leg front" dilution volumes upstream of the charging lines is not, assumed.

moves through the cold legs, downcomar, and lowcr plenum to the core vol-ume as a single voltae front. This results in subscquent decreases in shutdown cargin due to dilution fronts moving through the active core region with a tima constant equal to the loop transit time when on RHR (five to seven minutes).

If a return to critical occurs as a result of an inadvertent dilution, the fol-lowing potential concerns have been identified:

1. A re,nid, uncontrolled power excursion into the low and intemediate power ranges occurs, resulting in a power / flow mismatch due to the low flow (approximately 1 - 2% of nominal) provided by the RHR pumps.

Pressure

2. The potential exists for significant system overr essurization.

increases above the RHR cut off head (approximatCy 600 psig) further ac-centuate the effects of a power / flow mismatch when all RCS (RHR) flow is lo:t.

An investigation of the adequacy of existing cold overpressurization protection systems is necessary in order to assess the full impact of this potential problem.

This is not currently a

3. The potential exists for limited fuel damage. Preliminary evaluation indicates tha significant concern.for exceeding DIiB limits is low due,.to the cold initial operating co tions. Further investigation of this problem is underway.

I The recommanded interim actions to prevent or mitigate anIfinadvertent no cocked controlboron di-

' lution at shutdown conditions are detailed in Appendix A.

rods are required, as specified in Figure A-1, the plant operator has fifteen minutes from the initiation It isofthedilution event to Westinghouse teminate position that athefif teenevent min- before a return to critical occurs.

ute time interval from the initiation of the-dilution to the time shutdown mar-gin is lost is sufficient time for operator action. If cocked control rods are required, the source range reactor trip provides positive indication for imed-

,l iate operator', action to terminate dilution.

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. C. A. Price 3 CGWS-1055 9 ~

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._ 9 It is expected that the operator has available the follo::ing information for determination that a dilution event is in progress:

1. Source Range Neutron Flux with,

a. High Flux at Shutdown Alarm set at half a decade above background.

b. Use of the audible count rate indication to distinguish significant changes in flux, i.e. , a dcubling of the count rate.

c. Periodic, i.e. , frequent surveillance of the Source Range meters per-formed by the operator.

2. Status indication. of the Chemical and Volume Control System and Reactor Makeup Water System with, P. Indication of boric acid and blended (total) flow rate, or

, b.. Indication of boric acid and clean makeup flow rate,

c. CVCS valve positicn status lights, and

d. Reactor Makeup Water Pump " running" status light.

The operator action necessary upon determination that e dilution event is in pro-grass (bj High Flux at Shutdown Alarm, Source Range Peactor Trip, "P-6 Available" indication, high indicated or audible count rates, or make up flow deviation alarms) is:

1. Inmediately open the charging /SI pump suction valves from the RWST (that open on receipt of an "S" signal). (For 312 plants these are LCV-ll5-B, D.

For 412 plants these are LCV-112-D, E:)

2. Immediately close the charging /SI' pump suction valves from the VCT (that close on receipt of an "S" signal). (For 312 piants these are LCV-Il5-C, E.

Fcr 412 plants these are LCV-ll2-B, C.)

3. For two-loop plants, irmediately open ,the charging suction valves frcm the RWST. (For 212 plants these are LCV-ll3-B and LCV-ll2-C. ) Also irrediate-ly close the charging suction valves from the VCT. (For 212 plants these are LCV-ll3-A and LCV-ll2-B.)

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.- C. A. Price- -4 CGWS-1055 h

l Through the use of Appendix A and the above noted operator action requirements, 4 Westinghouse is attempting to minimize the operational burden placed on the plant to prevent or mitigate an inadvertent dilution event wnile maintaining adequate safety margin. -Our investigation of this event is continuing. A detailea analytical model of the system response to a dilution event at shut-l i down conditions is being developed and the potential for system overpressuriza-tion and fuel failure will. suosequently be assessed. The Westinghouse investi-gati,on is expected to be completed by September 15, 1980. We will keep you informed as to the results of our efforts. ,

Should you have any questions or comments', please advise.

Very truly yours, WESTillGHOUSE EL TRIC CORPORATION G. .'

R. A. Stough Project Engineer South . Carolina Electric & Gas Project RAS :gc'c Attachment cc: C. A. Price..ll'lA H. T. Babb, IL:lA

- H. E. Yocom, 4L 4A T. C. Nichols, Jr., IL 1 E. H. Crews, IL R. C. Holzwarth, IL 1A

- D.'A. Nauman, 1L.

H. Radin,IL- -

0. S. Bradham, IL. .A

Plant' Numerical Records System, IL 'l A Nuclear Project Central File.1C. 1A' W. A. Williams, Jr., il 1A i

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A C. A. price CGWS-1055 o .

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APPEf; DIX A Figure A-1, attached, provides the shutdown margin requirements as a function of Reactor Coolant System boron concentration and maximum possible dilution flow rate. Prior to use of this figure, the plant must determine the maximum dilution flow rate of all charging pumps not rendered inoperable once the plant is placed on R!!P.. To cover all codes, it should be assumed that the flow rate is based on pump runout unless there are flow limiting devices in the system (orifices, pip-ingresistances,etc.). The Reactor Makeup Uater pump capacity may be limiting in the deterr.ination of the maximum possible dilution flow rate.

Figure A-1 notes areas of acceptable operation of three different dilution flow rates as a function of RCS boron concentration and borated shutdown margin (Keff)*

For a given dilution flow rate, if the RCS boron concentration and shutdown margin result in a point placed to the left of the flow rate line, no control rod bank withdrawal is necessary. If the results place the plant to the right of tha line, then either the shutdown margin must be increased such that the plant is moded to the area of acceptcble operation, or 1% t.k/k in control rods must be withdrawn to provide additional shutdown margin. The tripping of the

('~' withdrawn rods provides positive operator indication that a dilution event is in progress and additional time for operator termination of the event. In all cases, 1

a shutdown margin of 5% ak/k (K 0.95) is considered sufficient for contin-ued operation without a require:gd) 't

. <for control rod bank withdrawal.

Figure A-1 is based on best estimate calculations for the "all rods in" configu-ration. It is recommended that the Westinghouse Nuclear Design Report for your' plant be used as a reference in determining the RCS boron concentration with the appropriate conservatism to be used in the figure. The Westinghouse Nuclear Fuel Division is available to provide assistance in meeting the constraints imposed by the Figure A-1 requirements.

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- Use of Figure A-1 is applicable any time there is boration/ dilution capability 4

from the normal boric acid blending sy' stem. The above procedure is not required if boration and/or makeup during cold and hot shutdown is perfomed utilizing water from the RUST. This requires that the normal dilution /boration path is isolated from the charging path. Two means of lockout to isolate the charging path are available: .

1. Lock out Reactor Makeup Water Supply.

This is acc'omplished by valve 8338 for 212 plants, valve 8457 for 312 plants, and valve 8455 for 412 plants.

OR:

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- 5T C. A. Price ' Appendix A'- Page 2/CG'45-1055 g-e i, e

2. l.ock out valves between the boric acid blender and the VCT.

These are FCV-1118, FCV-1108, 8339, '8355, and 8361 for 212 plants; FCV-ll4A,

'FCV-ll3B, 8454, 8441, and 8439 for 312 plants; FCV-111B, FCV-1108, 8453, (p. 8441', 8439 for 412 plants.

This recom.endation precludes the occurrence of'an inadvertent dilution while horating or making up water from the RWST under these conditions.

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• C, A. Pric.e CGUS-1055

. . APPENDIX A - Fa9e 3/ * .

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• ATTACHMENT B 10CFR21 - SUBSTANTIAI. SAFETY HAZARD

'l. Name and Address of Reporting Individual B. S. Mullinax P. O. Box 764 Columbia, SC 29218

2. Identification of Basic Component An inadvertent boron dilution event at cold or hot shutdown conditions while on the Residual Heat Removal System.

3. Identification of Firm Supplying Component Westinghouse Electric Corporation

4. Nature of Defect, Substantial Safety Mazard Created and Evaluation The inadvertent boron dilution tranrient is initiated by admission of unborated water into the Reactor Coolant System from the Reactor Makeup Water System. A boron dilut;on results in a reactivity addition which could lead to a loss of shutdown margin. Westinghouse is continuing to investigate the boron dilution event at shutdown conditions.

5. Date information of Defect Was Obtained July 31, 1980

6. Number and Location of Defect The inadvertent boron dilution transient is initiated by admission of unborated water into the Reactor Coolant System from the Reactor Makeup unrar System.

7. Corrective Action The inadvertent boron dilution transient is considered a 10CFR21 item and is being reported under the requirements of 10CFR 50.55(e).

If a long-term solution is not available 90 days prior to fuel loading, the Westinghouse recommended interim corrective actions will be incorporated into the Virgil C. Summer Nuclear Station General Operating Procedures for modes 4 and 5.

8. Advice to Purchasers or Licensees Westinghouse has reco= mended administrative changes to operating l procedures that prevent or mitigate the occurrence of an inadvertent i

boron dilution at shutdown conditions.

.