ML20012D478

Forwards First Annual ECCS Evaluation Model Revs Rept,Per 881017 Rev to 10CFR50.46.Rept Identifies Several Mods to Large & Small Break LOCA Evaluation Models Used at Facility & Provides Estimated Effects of Changes on ECCS Analyses

ML20012D478
Person / Time
Site:	Summer
Issue date:	03/16/1990
From:	Bradham O SOUTH CAROLINA ELECTRIC & GAS CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
NUDOCS 9003270390
Download: ML20012D478 (7)

Text

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Document Control Desk U. S. Nuclear Regulatory Commission Washington DC 20555

Subject:

Virgil C. Summer Nuclear Station Docket No. 50/395 Operating License No. NPF-12 ECCS Evaluation Model Revisions Report Gentlemen:

Attached is the first annual Emergency Cora Cooling System (ECCS) Evaluation Model Revisions Report for the Virgil C. Summer Nuclear Station (VCSNS).

This report is a requirement of the October 17, 1988 revision to 10CFR50.46 which requires licensees to notify the Nuclear Regulatory Commission on an annual basis of errors or changes in the ECCS Evaluation Models. If the changes or errors are determined to be significant, the licensee is to report the change within 30 days and include a proposed schedule for reanalysis and any other action required to confirm compliance with 10CFR50.46.

The attached report identifies several modifications to the large Break and Small Break LOCA ECCS Evaluation Models used at VCSNS. The report also provides the estimated effect of these changes on the limiting ECCS analyses.

SCE&G'has reviewed the model revisions and has determined that the char.ges are not significant per the definition of "significant changes" provided in 10CFR50.46(a)(3)(i).

I declare that the statements and matters set forth herein are true and correct to the best of my knowledge, information and belief.

Should you have any questions, please call at your convenience.

Very truly yours.

9003270390 900316 A > f f 4<ub nA DR ADOCK0500g5 EWR/OSB:1bs c: See page 2 Oh y

. Document Control Desk Match 16, 1990 Page 2 of 2 c: D. A. Nauman/0. W. Dixon, Jr./T. C. Nichols, Jr.

E. C. Roberts R. V. Tanner S. D. Ebneter-J. J. Hayes, Jr.

General Managers C. A. Price-R. B. Clary K. E. Nodland J. C. Snelson.

R. L. Prevatte H. G. Shealy J. B. Knotts, Jr.

E. W. Rumfelt W. T. Wood NSRC NPCF RTS (ANN 2300 File (813.12-4 (818.02-17)

.s Attachment I to Document Control Desk Letter

.- March 16, 1990

, Page 1 of 5 ATTACHMENT 1 EFFECT OF WESTINGHOUSE ECCS EVALUATION N00EL MODIFICATIONS ON THE LOCA ANALYSIS RESULTS FOUND IN CHAPTER 15 0F THE VIRGIL C. SUWER FINAL SAFETY ANALYSIS REPORT The October 17, 1988 revision to 10CFR50.46 requires licensees to inform the Nuclear Regulatory Commission of errors or changes in the ECCS Evaluation Models on an annual basis. If the errors or changes are determined to be significant, the licensee is required to provide the report within 30 days and include with the report a proposed schedule for taking action needed to prove compliance with 10CFR50.46. A significant change or error is defined in 10LFR50.46(a)(3)(1) as "one which results in a calculated peak fuel cladding temperature different by more than 50*F from the temperature calculated for the limiting transient using the last acceptable model, or is a cumulation of changes and errors such that the sum of the absolute magnitudes of the respective temperature changes is greater than 50*F."

Several modifications have been made to the Westinghouse ECCS Evaluation Models used to analyze loss-of-coolant accidents (LOCA's) at the Virgil C.

Summer Nuclear Station (VCSNS). These modifications have been evaluated to determine their impact on the resultant peak cladding temperature (PCT) in the small and large break LOCA analyses results found in Chapter 15 of the Virgil C. Summer Final Safety Analysis Report (FSAR). The following paragraphs provide a brief discussion of the assumptions made in the models, a summary of the nature of the modifications made to the models, and the estimated effect of each of these changes on the resultant PCT.

LARGE BREAK LOCA The large break LOCA analyses for VCSNS were examined to assess the effect of the applicable modifications to the Westinghouse large break LOCA ECCS Evaluation Model on PCT results reported in Chapter 15 of the FSAR. The

.large break LOCA analyses results were calculated using the 1981 version of the Westinghouse large break LOCA ECCS Evaluation Model incorporating BASH analysis technology. The analysis assumed the following information important to the large break LOCA analyses:

The licensing basis large break LOCA analysis for VCSNS was performed using the-1981 Evaluation Model with BASH. The analysis assumed a total core peaking factor (Fq) of 2.45 with uniform 15% steam generator tube plugging.

The analysis determined the limiting break size to be a double ended guillotine with a discharge coefficient equal to 0.4. The peak clad temperature for this case was 2141*F. However, due to the current cycle being a transition core with Vantage 5 fuel and Standard fuel existing in the

Attachment 1 to Document Control Desk letter

. March 16, 1990 Pase 2 of 5 core, a transition core penalty of 50*F was assessed. Thus, the effective peak clad temperature was determined to be 2191*F.

The.following modifications to the Westinghouse ECCS Evaluation Models affect

-the large break LOCA analysis results found in Chapter 15 of the Virgil C.

Summer Final Safety Analysis Report:

1981 ECCS EVALUATION MODEL INCORPORATING BASH ANALYSIS TECHNOLOGY Several improvements were made to the BASH computer code to treat special analysis cases which are related to the tracking of fluid interfaces:

1) A modification to prevent the code from aborting was made to the heat transfer model for the special situation when the quench front region moves to the bottom of the BASH core channel. The quench heat supplied to the fluid node below the bottom of the active fuel was set to zero.

2) A modification to prevent the code from aborting was made to allow negative initial movement of the liquid /two-phase and liquid-vapor interfaces. The coding in these areas was generalized to prevent mass imbalance in the special case where the liquid /two-phase interface reaches the bottom of the BASH core channel.

3) Modifications to prevent the code from aborting were made to increase the dimensions of certain arrays for special applications.

4) A modification was made to write additional variables to the tape of information to be provided to LOCBART.

5) Typographical errors in the coding of some convective heat transfer terms were corrected, but the corrections have no effect on the BASH analysis results since the related terms are always set equal to zero.

For the modifications specified in items 1 through 5 above, there is no adverse impact on the PCT calculation in the VCSNS large break LOCA analysis.

To summarize, modifications to the Westinghouse large break LOCA ECCS Evaluation Model combined with the transition core penalty alter the resultant PCT in the following manner:

A. Analysis calculated result 2141 'F B. Modifications to Westinghouse ECCS + 0 'F Evaluation Model C. Transition Core Penalty _+ 50 *F ECCS Evaluation Model Modifications 2191 'F Resultant PCT

Attachment 1 to Document Control D:sk Letter March 16. 1990 Page 3 of 5 i

SMALL BREAK LOCA The small break LOCA analyses for VCSNS were also examined to assess the effect of the applicable modifications to the Westinghouse ECCS Evaluation '

Models on peak cladding temperature results reported in Chapter 15 of the >

FSAR. The small break LOCA analyses results were calculated using the 1985 version of the Westinghouse small break LOCA ECCS Evaluation Model incorporating the NOTRUMP analysis technology. For VCSNS, the limiting size ,

small break resulted from a 3-inch equivalent diameter break in the cold leg. '

The calculated peak cladding temperature was 2095'F. The analysis assumed the following information important to the small break LOCA analyses:

The licensing basis small break LOCA analysis for VCSNS was performed using the NOTRUMP computer code. The analysis assumed a total core peaking factor (Fq) of 2.50 and a hot channel enthalpy rise peaking factor (F5) of 1.68.

The small break LOCA analysis assumed the plant was operating in mode 1 at 102% reactor power with 15% steam generator tube plugging. The analysis ,

considered break sizes of 2, 3 and 4-inch diameters and determined the '

limiting break size to be a 3-inch diameter break located in the cold leg.

The following modifications to the Westinghouse ECCS Evaluation Models would affect the small break LOCA analysis results found in Chapter 15 of the Virgil C. Summer Final Safety Analysis Report: ,

NOTRUMP ECCS EVALUATION MODEL The Westinghouse small break LOCA ECCS Evaluation Model analyses for VCSNS were performed with a version of the NOTRUMP computer code which has since been revised to incorporate the following potentially significant ,

modifications:

1) The modification to preclude changing the region designation (upper, lower) for a nova in a stack which does not contain the mixture-vapor interface was not incorporated in the small break LOCA analyses for VCSNS. The purpose of the modification was to enhance tracking of the ,

mixture-vapor interface in a stacked series of fluid nodes and to preclude a node in a stack, which does not contain the mixture-vapor interface, from changing the region designation. The update does not affect the fluid conditions in the nodes representing the reactor coolant system, only the designation of the region of the node. The region designation does not typically affect the calculations, except for the nodes representing the core fluid volume (core nodes). In core nodes which are designated as containing vapor regions, the use of the steam cooling heat transfer correlation is forced on the calculation in compliance with the requirements of Appendix K to 10CFR50, even if the node conditions would indicate otherwise. This modification could affect the heat transfer calculation if the region designation was improperly reflected, but is expected to result in a small decrease in the PCT if the correction were taken into account.

4 Attachment I to Document C ntrol Desk letter

.. March 16, 1990 t Page 4 of 5

2) The modification to correct typographical errors in the equations which calculate the heat transfer rate derivatives for subcooled, saturated, and superheated natural convection conditions for the upper region of interior fluid nodes was not included in the small break LOCA analyses for VCSNS. The heat transfer rate derivatives for subcooled, saturated, and superheated natural convection conditions for the upper .

region of interior fluid nodes used the lower region heat transfer area i instead of the upper region heat transfer area which could, in rare '

instances, affect the amount of heat that could be transferred to the fluid. Incorporating the modification into the small break LOCA analyses could result in an increase in the PCT of 36.8'F.

3) The modification to correct typographical errors in equations which calculate the derivatives of the natural convection mode of heat transfer in the subroutine HEAT was not included in the small break LOCA analyses for VCSNS. However, incorporating the correction into  !

the analyses would have no effect on the analysis results.

4)Themodificationtocorrectatypographicalerrorinanequationwhich calculates the internal energy for nodes associated with the reactor coolant pump model when the associated reactor coolant pump flow links are found to be in critical flow was not included in the small break LOCA analyses for VCSNS. Since the small break LOCA Evaluation Model calculations did not encounter critical flow in the reactor coolant pump flow links, including this modification would have no effect on the analysis results.

Modifications were made to the small break LOCTA-IV computer code used in the small break LOCA ECCS Evaluation Model. Since the small break LOCTA-IV code modifications could, at most, result in a very small benefit, the effects of ,

the modifications to the small break LOCTA-IV code do not need to be assessed or tracked.

To summarize, the effect of the ECCS Evaluation Model modifications on the small break LOCA analyses for VCSNS could result in a penalty in the peak cladding temperature calculation. To be conservative in determining the -

available margin to the limits of 10CFR50.46, a peak cladding temperature penalty of approximately 37'F should be added to the analysis calculations as

! a result of ECCS Evaluation Model changes.

l- -

As discussed above, modifications to the Westinghouse small break LOCA ECCS Evaluation Model could affect the small break LOCA analysis results by altering the PCT in the following manner:

A. Analysis calculated result 2095 *F B. Modifications to Westinghouse ECCS + 37 *F Evaluation Model ECCS Evaluation Model Modifications 2132 *F Resultant PCT'

oI Attachment 1 to Document Control Desk letter

. - March 16, 1990 Page 5 of 5 CONCLUSIONS An evaluation of the effect of modifications to the Westinghouse ECCS Evaluation Model was performed for both the large break LOCA and smell break LOCA analysis results found in Chapter 15 of the Virgil C. Summer final Safety Analysis Report. When the effects of the ECCS mode 1' changes were combined with the current plant analysis results, it was determined that VCSNS remains in compliance with the requirements of 10CFR50.46.

Additionally, because the modifications to the codes do not effect resultant PCT's different by more than 50*F from the previously calculated results, South Carolina Electric & Gas Company has determined that the modifications to the ECCS Evaluation Models are not significant.