ML18228A440

04/10/1978 Letter Logic Inconsistency in Two of the Computer Codes Used in Westinghouse LOCA ECCS Evaluation Model

ML18228A440
Person / Time
Site:	Turkey Point
Issue date:	04/10/1978
From:	Robert E. Uhrig Florida Power & Light Co
To:	Stello V Office of Nuclear Reactor Regulation
References
Download: ML18228A440 (15)

Text

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REGULATORY INFORMATION DISTRIBUTION SYSTEM (RID DISTRIBUTION FOR INCOMING MATERIAL 50-250 25i REC: NRC ORG: UHRIG R E ATE: 04/i0/78 NRC =

FL PWR 8. LIGHT DATE RCVD: 04/i4/78 DOCTYPE: LETTER NOTARIZED: NO COPIES RECEIVED

SUBJECT:

LTR 3 ENCL 3 FURNISHlNG INFO CONCERNING INCONSISTENCY EXISTING IN TWO OF THE COMPUTER CODES < SATAN-VI< i) 8c LOCTA-IV 2) ) USED IN WESTINGHOUSE LOCA ECCS EVALUATION MODELS WHICH HAS AFFECTED APPLICANT"8 ANALYSIS'NVOLVING ZIRCONIUM-MATER REACTION HEAT GENERATION CALCU PLANT NAME:TURKEY PT 83 REVIEWER INITIAL:

TURKEY PT 83 DISTRIBUTER INITIAL:~

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FLORIDA POWER & LIGHT COMPANY

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r~yr~l Director of Nuclear Reactor Regulation r~ r Attention: Mr. Victor Stello, Director Division of Operating Reactors U. S. Nuclear Regulatory Commission tlat Washington, D. C. 20555 C7 rrt C7

Dear Mr. Stello:

Re: Turkey Point Units 3 and 4 Docket Numbers 50-250 and 50-251, ECCS Anal sis Florida Power 6 Light Company has been informed by Westinghouse Electric Corporation that a logic inconsistency exists in two of the computer codes used in, their LOCA ECCS Evaluation Model.

The SATAN-VIlll and LOCTA-IV[2'odes are the affected computer codes. All versions of the Westinghouse Appendix K evaluation model are affectedl3.4.5r6l. Therefore, our analysis on record is also affected.

This logic inconsistency involves the interface between the zirconium-water reaction heat generation calculation and the heat conduction equation. Both the zirconium-water reaction ectuation (Baker-Just) and the heat conduction equation are solved correctly. However, the heat conduction equation uses a volumetric heat flux from the zirconium-water reaction calcu-lation. The output of the zirconium-water reaction calculation is a surface heat flux. This surface heat flux is modified to obtain a volumetric heat generation rate by dividing by the thickness of the radial mesh size between the surface temperature node and the first node inside the clad. It is this calculation which was performed incorrectly. The inconsistency underestimates the volumetric heat generation rate due to the zirconium-water reaction by a factor of 2.

The presence of this logic inconsistency has been verified by visual inspection of the computer codes and by performing energy balances on some sample calculations. Correction of this error will result in higher calculated peak clad temperatures.

P EOP LE... SERVING P EOP LE

Mr. Victor Stello April 10, 1978 Page Two L-78-127 Westinghouse Electric Corporation has studied the effect of correcting this error on calculated peak clad temperature. In addition to correcting this error, some beneficial model changes were also studied. The result of their studies indicated a net increase in peaking factor from the proposed beneficial model improvements. Some details of these calculations follow.

Westinghouse Electric Corporation has proposed the use of the following improvements to the October 1975 version of their evaluation model:

1. Change the transition boiling correlation used during blowdown from the W Transition Boiling Correlation to the Dougall-Rohsenow correlation. Both correlations have been documented by Westinghousef3j and both termed "acceptable" in Appendix K of 10CFR50.46 and the NRC SER for the Westinghouse evaluation model.

2. Use of an em'.sqivity in the refill radiation heat. transfer model of 0.9<

3. Multiply the volumetric heat generation from the zirconium-water reaction calculation by a factor of 2 to correct the logic inconsistency.

4. Use of maxi-convqlqtion to improve the peaking factors being calculatedL8j .

5. Use of a new 15xl5 FLECHT correlation)91.

All of these modifications were discussed by Westinghouse with the staff on March 29, 1978. We understand that it will take the staff three months to review all of these model changes. We will work with Westinghouse and the NRC until then to arrive at a new approved LOCA ECCS Evaluation Model. At that time, we will submit to the Nuclear Regulatory Commission a schedule for reanalysis of the present limiting break size with the new model. A reanalysis will be performed for Turkey Point Units 3 and 4 as soon as possible following NRC approval of the Westinghouse model changes.

Until the Westinghouse model changes are approved and a new ECCS analysis can be performed for Turkey Point Units 3 and 4, Florida Power & Light Company will administratively limit F< for Turkey Point Unit 3 to 2.03 and to 1.91 for Turkey Point Unit 4.

Mr. Victor Stello April 10, 1978 Page Three L-78-127 These levels are conservative since Westinghouse estimates levels of Fq of 2.21 and 2.06 respectively, when credit is taken for the proposed model improvements with the October 1975 evaluation model.

The estimates have been confirmed by many actual calculations.

The derivation of the interim values of Fq of 2.03 and 1.91 are documented in the attached two tables. They are based on calcu-lations and sensitivity studies performed by Westinghouse and interim credits and penalties imposed by the NRC staff on the basis of these studies.

The revised Fq limits will require K(z) curves which are also attached.

The following clarification may be helpful in interpreting the attached tables. First of all, when talking about margin in the amount of steam generator tubes plugged, we are referring to the difference between the value assumed in the analysis of record and the actual percent of tubes curqengly plugged. ~;then the Westinghouse Perturbation technique!10~ is used to obtain a change in peaking factor related to this difference. The table lists the assumed percent steam generator tubes plugged versus the actual. You should also note that a small allowance has been made for the future possibility of having to plug additional tubes as a result of primary to secondary leakage.

Secondly, when correcting for as-built fuel temperatures, a sensitivity of ill'F initial pellet temperature per O.l change in peaking factor was used. This value is based on the 37'F pellet temperature at end of blowdown per 0.1 change in Fq (reported to the staff on March 29, 1978) and a bounding ratio of 3'F initial pellet temperature per 1'F pellet temperature at end of blowdown. Both sensitivities were conservative bounds of some actual calculations.

Based on Westinghouse core physics calculations, the maximum values of Fq that could occur for the remainder of the current cycles are as follows:

Turkey .Point, Unit 3, Cycle 5 Max. Fq = 2.02 Turkey Point, Unit 4, Cycle 4 Max. Fq = 1.97 For Turkey Point Unit 3, operation with a Fq less than or equal to 2.03 is 'ensured by the maximum predicted Fq being 2.02. For Turkey Point Unit 4, operation with a F< less than or equal to 1.91 is ensured by the implementation of operating procedures as discussed below.

l' Mr. Victor Stello Page Four l April 10, L-78-127 1978 The actual measured value of F< is a well behaved function of exposure for a particular base loaded plant, and for the Turkey Point Unit 4, cycle 4 it has been about 1.7 for equilibrium conditions. The Westinghouse design strategy, however, is based on defining a bounding envelope of Fc's resulting from the most adverse control rod placements ance xenon oscillations permitted by the plant's'echnical specifications. The maximum F< predicted by this most adverse envelope of values when increased by specified uncertainty factors is generally below the Fq limit established by the ECCS analysis, therefore requiring only monthly surveillance.

In the interim, while the maximum predicted F< (1.97) exceeds the allowable F (1.91), Turkey Point Unit, 4 will be operated as a base loadef plant. Procedures have been established and reviewed by the Plant Nuclear Safety Committee to provide for augmented surveillance when the reactor power exceeds the ECCS threshhold power, i.e., percent power greater than 1.91/1.97x 100%.

These procedures provide for Turkey Point Unit 4 to be at steady state equilibrium conditions prior to increasing power above the threshhold. Two incore detector thimbles will then be monitored to establish that sufficient Fq margin exists before increasing power above the ECCS threshhold.

Operation at power levels above the ECCS threshhold level will require that Bank D be inserted no more than 20 out of 228 steps.

In the event that, Bank D is inserted beyond this limit, and moved more than an accumulated + 5 steps, augmented surveillance will be instituted using two incore detector thimbles. Appropriate procedures have been established to reduce reactor power below the ECCS threshhold within 15 minutes limit If if the measured Fq exceeds Fq (1.91) by >48. the measured Fq exceeds the Fq limit by <4S, power would be reduced 1$ for each 1% deviation.

If the measured Fq is not within its limit within two hours, power would then be reduced below the ECCS threshhold level.

This matter has been reviewed by the Turkey Point Plant Nuclear Safety Committee and the Florida Power S Light Company Nuclear Review Board. They have determined that operation of Turkey Point Units 3 and 4 as described herein will ensure the continued safe operation of the plant and will not. endanger the health and safety of the public.

Very truly yours, Ro ert E.

President Uhrz.g'ice cc: J. P. O'Reilly, Region II Robert Lowenstein, Esquire

0 CURRENT ANALYSIS: = 1.90 PEAK CL EMPERATURE = 2019 F Tubes'Plugged = 15Ã . Turkey Point Unit 3 CHANGES TO CURRENT ANALYSES MODI FI CATION Fq CHANGE JUSTIFICATION/BASIS 1.= Zirc/Mater=Reaction Correction 0.00 Incorporated in base case

2. Current Analysis Margin to 2000'.

20 F/ X Fq NRC 2200'.00 Current Analyses Margin to +0.14 25 F/ A Fq - NRC

.4. Use of New 15xl5 I

Flecht -0.03 ncl uded in Ana1ys i s Performed by Westinghouse (-.03 is con-servatism imposed by NRC)

5. No use of Dougall-Rohsenow '.00 NRC Interim Position

6. No use of a=0.9 0. 00 NRC Interim Position

7. ESDR Power Used in Current 0.00 Margin already included in Analysis current analyses

8. Margin to Amount of Steam +0.02 -

(See note below)*

Generator Tubes Plugged

9. Use of As-Built Fuel Temperature 0.00

10. Margin In Containment Back 0.00 Pressure Net Change +0.13 Current Fq plus Net Change = New Fq 1.90 + +0.13 = 2.03

• Actual tubes plugged on Turkey Point Unit No. 3 is 11.7% as compared to 15/ used in analyses. Therefore, using 12.25 tubes plugged to allow some margin for additional plugging, a credit of .02 is applicable on'he basis of .007 units of Fq/per cent

,tubes plugged.

CURRENT ANALYSIS~ 2 ~ 05 PEAK CLAD TEMPERATURE = 2195 F Fq Tubes Plugged = 195 CHANGES TO CURRENT ANALYSES Turkey Point Unit 4 MODIFICATION Fq CHANGE JUSTI F I CATION/BASIS

1. Zirc/Water Reaction Correction -0.20 NRC

2. Current Analysis Margin to 2000'F 0.00 20'F/ 5 Fq

- NRC

3. Current Analyses Margin to 2200 F 0. 00 25 F/ '5 Fq NRC

4. Use of New 15x15 Flecht +0.05 (See note below)*

5. No Use of Dougall-Rohsenow 0.00 NRC Interim Position

6. No use of E=0.9 0.00 NRC Interim Position

7. ESDR Power Used In Current 0.00 Margin Already Included in Analysis Current Analysis

8. Margin to Amount of Steam +0.01 (See note below)*",

Generator Tubes Plugged

9. Use of As-Built Fuel Temperature 0. 00

10. Margin In Containment Back 0.00 Pressure Net Change -0.14 Current Fq plus Net Change = New Fq

2. 05 . + -0.14 = 1. 91

• Sensitivity study performed by Westinghouse indicated a 100'F benefit in PCT.

NRC converted this to +0.05 Fq benefit.

• • Actual tubes plugged on Turkey Point Unit No. 4 is 16.95 as compared to 19/, used in the analysis. Therefore, using 17.2/ tubes plugged to allow some margin for additional plugging, a credit of .01 is applicable on the basis of .007 units of Fq/per cent tubes plugged.

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REVISED K (Z) FUNCTION FOR TURKEY POINT UNIT 4 CYCLE 4 F = 1.91 1.0 k=

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'I References Bordelon, F. M., et al., "SATAN-VI Program: Comprehensive Space-Time Dependent Analysis of Loss-of-Coolant", WCAP-8306, June 1974.

t

2. Bordelon, F. M., et al ~, "LOCTA-IV Program" Loss of Coolant Transient Analysis", WCAP-8305, June 1974.

3. "Westinghouse ECCS Evaluation Model - Summary", WCAP-8339, Bordelon, F. M.,

Massie, H. W., and Zordan, T. A., July 1974.

4. Bordelon, F. M.. et al., "Westinghouse ECCS Evaluation Model Supplementary Information, WCAP-8471, April, 1975, (Proprietary) and WCAP-8472, April, 1975 (Non-Proprietary).

5. "Westinghouse ECCS Evaluation Model October 1975 Version", WCAP-8622, November 1975, (Proprietary), and WCAP-8623, November 1975, (Non-Proprietary).

6. Letter from C. Eicheldinger of Westinghouse Electric Corporation to D. B.

Vassallo of the Nuclear Regulatory Commission, Letter NS-CE-924, 1/23/76.

7. "High Temperature Properties of- Zircalloy - Oxygen Alloys", EPRI Report NP-524, March, 1977.

8. Little, C. C. et al., "Consideration of Uncertainties in the Specification of Core Hot Channel Factor Limits", WCAP-9180, September 1977.

9. "Westinghouse ECCS Evaluation Model, February, 1978 Version", WCAP-9220, February, 1978 (Proprietary), WCAP-9221, February, 1978 (Non-Proprietary).

10. Thompson, C. M. and Esposito, V. J., "Perturbation Technique for Calculating ECCA Cooling Performance", WCAP-8986, February 1977 (Non-Proprietary).

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