RC-94-0313, Application for Amend to License NPF-12 to TS Re Changes to Current SBLOCA Evaluation Model Applicable to Plant

Application for Amend to License NPF-12 to TS Re Changes to Current SBLOCA Evaluation Model Applicable to Plant

ML20077E914
Person / Time
Site:	Summer
Issue date:	12/07/1994
From:	Skolds J SOUTH CAROLINA ELECTRIC & GAS CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
RC-94-0313, RC-94-313, NUDOCS 9412130180
Download: ML20077E914 (8)

Text

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South Carolina Electric & Gas Cornpeny 10CFR50.46 John L. Stolds

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P.o. Box 88 Son;or Vice President Jenkinsvdle. SC 29005 Nuclear operations (803)345-4001 .

SCE&G l December 7, 1994 Refer to: RC-94-0313 Document Control Desk U. S. Nuclear Regulatory Commission Washington, DC 20555 Gentlemen:

Subject:

VIRGIL C. SUMMER NUCLEAR STATION i DOCKET NO. 50/395 OPERATING LICENSE NO. NPF-12 l

10CFR 50.46 30 DAY REPORT (ANN 2300)

Westinghouse has recently provided South Carolina Electric & Gas Company (SCE&G) with a Nuclear Safety Advisory Letter documenting significant changes to the current Small Break Loss of Coolant Accident (SBLOCA) Evaluation Model which are applicable to the Virgil C. Summer Nuclear Station (VCSNS).

The effects of the individual changes to the ECCS Evaluation Model has been determined to exceed 50 F and are thus significant in accordance with 10 CFR 50.46 (a)(3)(i). Consequently, this letter constitutes a 30 day report pursuant to 10 CFR 50.46 (a)(3)(ii) requirements.

This report is based on VCSNS's new analysis of record, provided in order to support the SG replacement Tech Spec changes (see Reference (1)in Attachment IV Table B).

The reference PCTis 1839 F which includes + 32 F penalty to account for fuel rod burst / blockage effects and a -16 F benefit for previous SBLOCA model assessments.

Prior to NRC approval of the SGR Tech 5 3ec changes,'Nestinghouse identified changes / corrections to the SBLOCA analysis which modified the PCT to 1860 F.

This submittal provides PCT changes of 161 F to the reference 1839 F.

The first change is due to Boiling Heat Transfer Correlation Errors in which drift and slip errors for mixture velocity were not properly accounted for in NOTRUMP. The estimated effect of this change is -6* F on the Peak Cladding Temperature (PCT).

The second change is due to Steam Line isolation Logic Errors which is the result of incorrect logic that is inconsistent with the standard logic used in the analysis. The estimated effect of this change is + 18 F.

The third change is due to Axial Nodalization, Rod Internal Pressure Model Revision and SBLOCTA Error corrections. The first two effects are due to non-conservative and synergistic behavior with respect to PCT. The final effect is due to effects in the SBLOCTA code based on the previous two effects. The estimated effect of these changes are +96 F.

Attachments I,II, and lll provide a more detailed technical description of these issues.

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'lh addition, the Burst and Blockage / Time in Life Penaltywas increased by +41*F to a i value of +73*F. This is included into the absolute value of the total PCT change.

Attachment IV summarizes the collective impact of the ECCS Evaluation Model changes. These changes affect the SBLOCA Model only and result in a net increase in PCT of 150' F. Based on these impacts,it is concluded that VCSNS' current Analysis of Record (2/94) remains conservative and continues to demonstrate compliance with 10 CFR 50.46 requirements.

The undersigned affirms that the statements and matters set forth herein are true

- and correct to the best of his knowledge,information, and belief.

Should you have any questions, please call Mr. Michael J.Zaccone at (803) 345-4328 at your convenierice.

Very truly yours, John L. Skolds MJZ/JLS/nkk Attachment c: O. W. Dixon J. B. Kn otts, J r.

R. R. Mahan (w/o att.) L. R. Cartin R. J. White NSRC S. D. Ebneter RTS (ANN 2300)

G. F. Wunder Central File System General Managers File (813.12-4, 818.02-17)

NRC Resident Inspector NUCLEAR EXCELLENCE - A SUMMER TRADITION I

Attachment i

. l ANN 2300 Page 1 of 1 l

BOILING HEAT TRANSFER CORRELATION ERRORS Backaround

' This closely related set of errors deals with how the mixture velocity is defined for use in various bo~iling heat transfer regime correlations. The previous definition for

mixture velocit,y did not properly account for drif t and slip effects calculated in -

NOTRUMP. This error particularly affected NOTRUMP calculations of heat transfer coefficient when using the Westinghouse Transition Boiling Correlation and the Dougall-Rohsenow Saturated Film Boiling Correlation.

In addition, a minor typographical error was also corrected in the Westinghouse  !

Transition Boiling Correlation.

This was determined to be a Non-Discretionary Change as described in Section 4.1.2 of WCAP-13451 and was corrected in accordance with Section 4.1.3 of WCAP-13451.

Affected Evaluation Model 1985 Small Break LOCA Evaluation Model Estimated Effect Representative plant calculations for this issue resulted in the estimated PCT effect -

documented in the attached Margin Utilization Sheet.

A a

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- Attachment il ANN 2300 Page 1 of 1 STEAM LINE ISOLATION LOGIC ERRORS Backaround This error consists of two portions: a plant specific effect which applies to analyses which assumed Main Feedwater Isolation (FWI) to occur on S-signal, and a generic l effect applying to all previous analyses.

The plant specific effect was the result of incorrect loc it which caused the main steam line isolation to occur on the same signal as FW . Therefore, when the S-signal i was chosen through user input to be the ap the steam line isofation to occur on S-signal.propriate signal This is inconsistent forstandard with the FWl, it also caused I conservative assumption of steam line isolation on Loss of Offsite Power coincident l with the earlier Reactor Trip signal.

The generic effect was the result of incorrect logic which always led to the steam line isolation function occurring at a slightly later time than when the appropriate signal was generated.

This was determined to be a Non-Discretionary Change as described in Section 4.1.2 of WCAP-13451 and was corrected in accordance witfi Section 4.1.3 of WCAP-13451.

Affected Evaluation Model 1985 Small Break LOCA Evaluation Model l Estimated Effect Representative plant calculations for this issue resulted in the estimated PCT effect documented in the attached Margin Utilization Sheet.

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' Attachment 111

' ANN 2300 Page l of 2 l

AXIAL NODAllZATION, RIP MODEL REVISION, l AND SBLOCTA ERROR CORRECTIONS ANALYSIS l

Background j 10CFR50.46, Appendix K arescribes the acceptable features and required documentation for ECCS Evaluation Models. More specifically, Section 11.3 requires ,

that documentation be in place to verify that sensitivity studies have demonstrated I the adequacy of nodalization schemes used in the analysis models. A study was i recently undertaken with the Westinghouse small break LOCA Evaluation Model to examine the sensitivity of predicted results to the nodalization used for the hot rod model. The results of that study raised concerns regarding the adequacy of the '

standaro axial nodalizationprescribed for use in the SBLOCTA code for licensing  !

basis analyses. As a result of this concern, Westinghouse investigated this as a Potential Issue per 10 CFR 21, which was determined not to be a substantial safety hazard or failure to comply pursuant to 10 CFR 21.21(a).

Issue Description The standard rod model (developed in the 1970's) used in performing SBLOCTA calculations has 19 axial nodes with a finer distribution in the top elevations.

However, sensitivity studies tojustify the number and distribution of these nodes can not be documented. A senes of calculations were performed using increasingly finer axial nodalizations than prescribed for the 19 node model and indicated that the standard SBLOCTA 19 node model was not conservative. Nearly all cases demonstrated a significantly non-conservative behavior with respect to PCT. The penalty is attributed to a net increase in single-phase steam enthalpy rise as these nodes uncover sooner and heat up more than coarser nodes partially covered by the mixture level. Thus,it was concluded that a revised model that included a much finer axial nodalization could potentially lead to less favorable results than those predicted in the current analyses possibly challenging the 10 CFR 50.46 acceptance cntena.

As a result of further investigation into the SBLOCTA code, several additional related issues associated with nodalization and the overall solution of the fluid conservation equations were subsequently identified and corrected. As a separate, but re!ated issue, Westinghouse has implernented a revised model for calculating transient fuel rod internal pressure in the SBLOCTA code. Fuel rod pressure is a governing factor in defining the clad creep, burst and blockage behavior for small break LOCA l transients. The NRC was informed of this modeling change per Westinghouse letter NTD-NRC-94-4253, " Revision to the Rod Internal Pressure Model in the Westinghouse SBLOCTA Code (Proprietary)." The letter also informed the NRC that Westinghouse has validated and instituted the model as a methodology improvement to the small break LOCA model for standard implementation on a forward-fit basis in accordance with WCAP-13451, Westinghouse Methodology for implementation of 10 CRF 50.46 Reporting, October,1992.

Technical Evaluation At this time Westinghouse has completed the generic technical evaluation of the ,

fuel rod axial nodarization methodology. A revised standard for rod nodalization has been established with insures an adequate solution to the hot channel calculation by specifying a fine nodalization of 0.25 ft nodes for all elevations that i are predicted to uncover during the transient.  ;

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• Attachment lll ANN 2300 Page 2 of 2 Since the improved axial nodalization methodology and revised fuel rod internal pressure model can have significant synergistic effects on the predicted peak clad temperature, the SBLOCTA calculation from the limiting,small break LOCA transient has been rerun with the revised code and methodology m order to obtain an accurate estimation of the net effect of these changes on the analysis of record.

Several recent code revisions and error corrections of lesser magnitude have also been incorporated in the code version used to conduct this calculation. Normally these items would have been reported in the 10CFR50.46 year-end reporting summary along with estimates of effects. As a consequence of using the revised code to obtain results for this evaluation, these items have been implicitly addressed in the results provided. Since this portion of the ECCS Small Break Evaluation Model has already been reanalyzed, Westinghouse believes that no additional reanalysis is necessary to satisfy 10 CFR 50.46 even for those plants that have a significant PCT change as a result of this issue.

Since all of the issues relate to aortions of the SBLOCTA code and/or its associated input methodology, they may ae reported as a single closely-related group of changes. Attached to this letter is a revised Small Break LOCA Marg.in Utilization Summary table which contains a compilation of the net effect of this evaluation, as item " Axial Nodalization, RIP Model Revision and SBLOCTA Error Corrections."

Where necessary, Westinghouse has provided notes as an attachment to explain which items have been affected.

Estimated Effects Representative plant calculations for this issue resulted in the estimated PCT effect documented in the attached Margin Utilization Sheet.

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• ANN 2300 Page 1 of 2

- Small Break Peak Clad Temperature Marain Utilization

. Revision Date: 10/12/94 Plant Name: Virgil C. Summer Eval. Model: NOTRUMP Fuel: Vantage +

Utility Name: South Carolina Electric & Gas FO = 2.45 FAH = 1.62 SGTP = 10%

Reference * - Clad Temperature Notes A. ANALYSIS OF RECORD (2/94) PCT = 1823'F 1 B. PRIOR PERM ANENT ECCS MODEL ASSESSMENTS APCT= -16'F C. 10 CFR 50.59 SAFETY EVALUATIONS Table A APCT= 1*F D. 199410 CFR 50.46 MODEL ASSESSMENTS (Permanent Assessment of PCT Margin)

1. Boiling Heat Transfer Correlation Error 2 APCT= -6'F

2. Steam Line Isolation Logic Error 2 APCT= 18'F

3. Axial Nodalization, RIP Model Revision, APCT= 96'F -

and SBLOCTA Error Corrections Analysis E. OTHER MARGIN ALLOCATIONS

1. Burst and Blockage / Time in Life APCT= 73*F 2 LICENSING BASIS PCT + MARGIN ALLOCATIONS PCT = 1989'F References for the Peak Clad Temperature Margin Utilization summarv can be found in Table B.

Notes:

E 1. AOR performed for core power = 2900 MWt and A75 steam generators.

2. This assessment is a function of base PCT plus permanent margin allocation and as such will increase / decrease with margin allocation changes.

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,* Attachment iV ANN 2300 Page 2 of 2 TABLE A - 10 CFR 50.59 Safety Evaluations

- Revision Date: 10/12/94 l Plant Name: Virgil C. Summer Utility Name: South Carolina Electric & Gas Reference Clad Temperature . Notes

' l. SMALL BREAK ECCS SAFETY EVALUATIONS' A. Fuel Reconstitution APCT= 1*F 1 TOTAL 10 CFR 50.59 SMALL BREAK ASSESSMENTS PCT = 1'F Notes:

1. This penalty is due to a fuel assembly reconstitution in assemblies K21 and K46 for Cycle 9. The penalty will be removed when assemblies K21 and K46 are removed from the core.

TABLE B - References 1, Steam Generator Replacement Technical Specification Change Request (TSP 930019) from J. L.

Skolds to Document Control Desk, dated March 11,1994.

2. 10CFR50.36 30 Day Report (ANN 2300) from J. L Skolds to Document Control Desk, dated September 22,1994..

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