LIC-93-0249, Informs of Changes & Errors in Loca/Eccs Codes & Methods Utilized by Westinghouse for Evaluation of FCS Unit 1 Per 10CFR50.46(a)(3)(ii)

Informs of Changes & Errors in Loca/Eccs Codes & Methods Utilized by Westinghouse for Evaluation of FCS Unit 1 Per 10CFR50.46(a)(3)(ii)

ML20058N605
Person / Time
Site:	Fort Calhoun
Issue date:	10/13/1993
From:	Gates W OMAHA PUBLIC POWER DISTRICT
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LIC-93-0249, LIC-93-249, NUDOCS 9310200047
Download: ML20058N605 (3)

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- 3@E Omaha Public Power District P.O. Box 399 Hwy.75- North of Ft. Calhoun FortCalhoun NE680234399 402/636-2000 October 13, 1993 LIC-93-0249 U. S. Nucler.r Regulatory Commission Attn: Document Control Desk Mail Station P1-137 Washington, DC 20555

References:

1. Docket No. 50-285

2. WCAP-13451, " Westinghouse Methodology for Implementation of 10 CFR 50.46 Reporting," dated October, 1992

3. WCAP-13027-P, " Westinghouse ECCS Evaluation Model for Analysis of CE-NSSS," dated July 1991

4. WCAP-13194-P, "NRC Presentation " Westinghouse ECCS Evaluation Model for Analysis of CE-NSSS and Results of Large and Small Break Loss of Coolant Accident Analysis for Fort Calhoun Station Unit 1," dated January 1992

5. Letter from Westinghouse (N. J. Liparulo) to NRC (Document Control Desk) dated October 30, 1992 (ET-NRC-92-3755)

Gentlemen:

SUBJECT:

Report of Changes or Errors in Loss of Coolant Accident (LOCA) /

Emergency Core Cooling System (ECCS) Models

, Omaha Public Power District (OPPD) is As required reporting by 10 CFR changes 50.46(a)(3)(ii)LOCA/ECCS and errors in the codes and methods utilized by Westinghouse Electric Corporation (Westinghouse) for the evaluation of Fort Calhoun Station (FCS) Unit No.1. During the 1992 reporting period three errors and two changes were identified by Westinghouse. With one exception, these errors and changes are not considered significant.

When Cycle 14 operation was initiated, the previous LOCA/ECCS evaluation by ABB-Combustion Engineering was replaced by the Westinghouse evaluation described in References 3 and 4. The LOCA/ECCS evaluation was reviewed and approved by the NRC as part of the FCS Cycle 14 core reload design.

During 1992, Cycle 23 of the NOTRUMP code was found to contain an error in SUBROUTINE BESSJO which led to calculation of incorrect values for the zeroth order Bessel function of the first kind. This calculation is used in the algorithm designed to limit heat transfer out of a quenching fuel rod to the theoretical conduction limit. The error impacted the small break LOCA evaluation model and was determined to produce a 108* F reduction in the Peak Clad Temperature (PCT). The 108'F reduction, although conservative, constitutes a significant change in PCT. Westinghouse failed to notify OPPD in a timely manner of the change. Reference 5 transmitted to the NRC documentation of actions that have been taken to ensure that future significant changes in PCT will be identified in a timely manner.

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e l U. S. Nuclear Regulatory Commission LIC-93-0249 4 Page 2 The second error was found in the steam generator auxiliary feedwater (AFW) flow rate timing 1). A minor logic error associated with this variable ledvariable TIMESG(in to a step change the AFW flowrate once the transient time passed the value of TIMESGC7 . Typically, this value .is set equal to 11000 seconds so this error would only) affect very long transient calculations. The error would allow the AFW flow rate to immediately revert to the full flow value of main feedwater flow. This large step change has led to code aborts in the cases where

it has occurred. This error has no effect on the small. break LOCA PCT.

The third error was discovered during investig,ations into anomalies of wetting '

-and dryout behavior demonstrated by the BART grid model. The error resulted from a programming logic error in the grid heat transfer model. The error caused the *

solution to be performed twice for each timestep. Calculations performed with the revised code have consistently demonstrated significantly better grid wetting

< and lower clad temperatures. A conservative benefit of zero degrees PCT has been assigned _to this change.

'I Two changes to the Westinghouse LOCA Evaluation Model codes have been incorporated. The first change was in- the WREFLOOD-INTERIM code used to calculate heat transfer from structural metal in the vessel during the reflood phase. It was ' noted that the material properties available in the code corresponded to those of stainless steel. While this is correct for internal

' structures, it is inappropriate for the vessel wall which consists of carbon steel. with a thin stainless steel cladding on the internal surface. The -

estimated effect of this change is a 25'F PCT benefit for. the large break LOCA evaluation.

The other change,- constituting a set of changes which make steam generator secondary side modeling more convenient for the user, was implemented into i NOTRUMP, The model improvement involved several facets of feedwater flow modeling.- Because the enhancements only allow greater ease in modeling plant specific steam generator secondary side behavior, it is estimated that no effect will be seen in the PCT result.

The attached Tames 1 and 2 summarize the small break and large break LOCA/ECCS temperature margin utilizations.

'If you should have any questions, please contact me.

Sincerely, AW h. h W. G. Gates Vice President WGG/pjc Attachment c: LeBoeuf Lam S.D._ Bloom,b,Leiby&MacRae NRC Project Manager J. L. Milhoan, NRC Regional Administrator, Region IV R. P. Mullikin, NRC Senior Resident Inspector

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TABLE l' J 1992 SMALL BREAK PEAK' CLAD TEMPERATURE MARGIN UTILIZATION FORT CALHOUN STATION UNIT NO. 1

• L CLAD TEMPERATURE p :A. Analysis of Recordf(8/31/91) PCT - 1444* F

h. B.. Prior Permanent ECCS Model Assessments a PCT = 0*F C. 10 CFR 50.59 Safety Evaluations a PCT - 0*F

' D. . 199210 CFR 50.46 Model Assessments NOTRUMP Bessel Function Error a PCT = -108'F E. Other Margin' Allocations:

- None A PCT - 0*F .

LICENSING BASIS PCT + MARGIN ALLOCATIONS PCT = 1336*F

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l TABLE 2 g -1992 LARGE BREAK PEAK CLAD TEMPERATURE MARGIN UTILIZATION

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FORT CALHOUN STATION UNIT NO. 1 l

CLAD TEMPERATURE 1

A. Analysis of Record (8/31/91). PCT 2066*F B. Prior Permanent ECCS Model Assessments a PCT = 0*F C. .10 CFR 50.59 Safety Evaluations A PCT - 0* F D. 199210 CFR 50.46 Model Assessments:

Structural Heating Model' 6 PCT = -25*F E. 0ther Margin Allocations:

Spacer Grid Heat Transfer Error in BART a PCT - 0* F LICENSING. BASIS PCT + MARGIN ALLOCATIONS PCT = 2041*F L

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