ML17229A442

Special Rept:On 970731,determined That Use of Conservative Min Value for Pin/Box Factor in Analysis of Record,Compared to Actual Value of Cycle 10 Min Pin/Box Factor,Introduced Conservation of .0073 in Energy Redistribution Factors

ML17229A442
Person / Time
Site:	Saint Lucie
Issue date:	08/08/1997
From:	Stall J FLORIDA POWER & LIGHT CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
L-97-205, NUDOCS 9708140132
Download: ML17229A442 (4)

Text

CA.'l'P'6 UK X J REGULAT INFORMATION DISTRISUTIONOZSTEN (RIDE) r ACCESSION NBR:9708140132 DOC.DATE: 97/08/08 NOTARIZED: NO DOCKET ¹ FACIL:50-389 St. Lucie Plant, Unit 2, Florida Power & Light Co. 05000389 AUTH. NAME AUTHOR AFFILIATION STALLFJ.A. Florida Power a Light Co.

RECIP.NAME RECIPIENT AFFILIATION Document .Control Branch (Document Control Desk)

SUBJECT:

Special rept:on 970731,determined that use of conservative min value for pin/box factor in analysis of record, compared to actual value of Cycle 10 min pin/box factor, introduced conservation of .0073 in energy redistribution factors.

DISTRIBUTION CODE: IE22D COPIES RECEIVED:LTR ENCL SIZE:

TITLE: 50.73/50.9 Licensee Event Report (LER), Incident Rpt, etc.

NOTES:

RECIPIENT COPIES RECIPIENT COPIES ID CODE/NAME LTTR ENCL ID CODE/NAME LTTR ENCL PD2-3 PD WIENS,L.

INTERNAL: ACRS AB 2 AEOD/SPD/RRAB FILE C ER 1 NRR/DE/ECGB 1 NRR/DE/EMEB NRR/DRCH/HHFB 1 NRR/DRCH/HICB NRR/DRCH/HOLB 1 NRR/DRCH/HQMB NRR/DRPM/PECB 1 NRR/DSSA/SPLB NRR/DSSA/SRXB 1 RES/DET/EIB RGN2 FILE 01 1 EXTERNAL: L ST LOBBY WARD LITCO BRYCEFJ H 1 1 NOAC POORE,W. NOAC QUEENER,DS 1 1 NRC PDR NUDOCS FULL TXT 1 1 K I NOTE TO ALL "RIDS" RECIPIENTS:

PLEASE HELP US TO REDUCE WASTEI CONTACT THE DOCUMENT CONTROL DESK, ROOM'WFN. SD-5(EXT. 415-2083) TO ELIMINATE YOUR NAME FROM LISTS. FOR DOCUMENTS YOU DON"Z'NEEDl 'ISTRIBUTION TOTAL NUMBER OF COPIES REQUIRED: LTTR 25 ENCL

0 Florida Power 8 Light Company, 6501 South Ocean Drive, Jensen Beach, FL 34957 August 8, 1997 L-97-205 10 CFR 50.46 U. S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, D. C. 20555 Re: St. Lucie Unit 2 Docket No. 50-389 Error in the use of LBLOCAEvaluation Model; 30Da 10CFR50.46Re ort Ref: CENPD-132P, "Calculative Methods for the CE Large Break LOCA Evaluation Model," August, 1974, and approved supplements thereto.

This report is submitted pursuant to 10 CFR 50.46(a)(3)(ii) to provide notification of an error discovered in the use of an acceptable large break loss of coolant accident (LBLOCA) evaluation model (Reference) which is employed to calculate cooling performance of the St. Lucie Unit 2 emergency core cooling'ystem (ECCS). The error led to lesser values of peak cladding temperature (PCT) than should have been calculated in the limiting ECCS analyses of record. Corrected PCT values for the present and previous operating cycles conform to the required acceptance criterion and demonstrate continued compliance with =

10 CFR 50.46.

On July 11, 1997., Florida Power and Light Company (FPL) was informed by ABB Combustion Engineering (CE), the fuel vendor for St. Lucie Unit 2, that CE had initiated an evaluation of Energy Redistribution Factors (ERF) used in the large and small break LOCA/ECCS evaluation models, and approved supplements thereto. ERFs represent the fraction of the total energy generated by the limiting fuel rod which is actually deposited in that rod, and the evaluation was initiated due to concerns involving proper consideration ofthe effect on ERF from moderator voiding that occurs during a LOCA. On July 16, 1997, CE informed FPL that the ERF cited in the topical reports for the large break evaluation model did consider the effects from moderator voiding, but the ERF calculated in 1975 for 16 x 16 lattice fuel assemblies did not consider the effects of moderator voids. St. Lucie Unit 2 uses the 16 x 16 lattice fuel design. /gz.z-h It was determined that the ERF deficiency applied to the limiting analysis of record (LBLOCA), but did "riot impact analyses performed. for the sm'all break LOCA: As a result of the deficiency, the energy.

deposition in the hypothetical hot fuel pin is underpredicted by 0.5 to 1.5%, depending upon the. location

• of the hot pin within the lattice, arid a generic assessment indicated that ERFs should have been 0.015 greater than the values employed. On July 31 1997, CE provided FPL with a unit-specific estimate of the impact on PCT from the modeling error. In contrast to the ERF deficiency, it was determined that use of a conservative minimum valu'e for the pin/box factor in the analysis of record, compared to the actual value of the. Cycle 10 (present operating cy'cle) minimum pin/box.factor, had introduced a 9708140i32 970808 PDR ADQCK 05000389 .

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L-97-205 Page 2 of 2 conservatism of.0073 in the ERF. Combining this conservatism with the increase in ERFs due to voiding results in a net non-conservative ERF value of.0077. A unit-specific sensitivity of PCT to increases in the ERF was determined, and it was concluded that an increase in the ERF of.0077 will increase the calculated PCT by less than 30 F.

The PCT calculated in the limiting ECCS analysis of record, updated through the present operating Cycle 10 prior to discovery of the ERF deficiency, was 2143 'F. The revised PCT is 2173 F. This peak cladding temperature, and the PCTs for previous operating cycles after considering the ERF deficiency, demonstrate continued conformance to the criterion for PCT set forth in 10 CFR 50.46(b).

Without taking credit for the contrasting conservatism associated with the minimum pin/box factor assumed in the analyses of record, it was determined that the impact of the modeling error (an underestimate of ERF by 1.5%) could increase the calculated PCT by as much as 60 'F. The impact of this error on PCT is greater than 50 'F and, therefore, is reportable as significant. The impact on the limiting PCT calculated using the last acceptable model, based on the sum of the absolute magnitudes of respective temperature changes resulting from a cumulation of all modeling changes and/or errors to date, is approximately 61 F.

Interim compensatory actions taken by FPL to ensure compliance with 10 CFR 50.46 during the time that evaluations were in progress at ABB-CE to confirm or revise, as necessary, the ERF values employed for LOCA analyses included: (a) Initially, a penalty was applied to the measured values of the Total Planar Radial Peaking Factors- F ~ and (b) subsequent to this initial action, the F penalty was replaced with a penalty applied to the Local Power Density (peak linear heat rate) alarm setpoint.

Please contact us ifthere are any questions.

Very truly yours, J. A. Stall Vice President St..Lucie Plant

'AS/RLD cc: Regional Administrator, Region II, USNRC.

'enior Resident Inspector, USNRC, St. Lucie Plant.