ML17241A339

Forwards Notification of Change to Small Break LOCA ECCS Evaluation Model Used for St Lucie Unit 1.Anomaly Was Discovered & Corrected That Resulted in Reducing Calculated PCT for Limiting SBLOCA by More than 50 F

ML17241A339
Person / Time
Site:	Saint Lucie
Issue date:	05/20/1999
From:	Stall J FLORIDA POWER & LIGHT CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
L-99-112, NUDOCS 9905270115
Download: ML17241A339 (4)

Text

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ACCESSION NBR:9905270115 DOC.DATE: 99/05/20 NOTARIZED: NO DOCKET FACIL:50-335 gt. Lucie Plant, Unit 1, Florida Power &. Light Co. 05000335 AUT K.NAPE .AUTHOR AFFILIATION STALL,u.A. Florida Power 8 Light Co.

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

SUBJECT:

Forwards notification of change to small break LOCA ECCS evaluation model used for ST Lucie Unit 1.Anomaly was discovered E corrected. Limiting PCT is well within 10CFR50.46 acceptance criteria.

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Fiorida Power & Light Company, 6351 S. Ocean Drive, Jensen Beach, FL 34957 May 20, 1999 L-99-112 10 CFR 50A6 U. S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555 Re: St. Lucie Unit1 Docket 50-335 SBLOCA Evaluation Model 30-Da 10 CFR 50A6 Re ort The attached report is submitted pursuant to 10 CFR 50A6(a)(3)(ii) to provide notification of a change to the small break loss of coolant accident (SBLOCA) emergency core cooling system evaluation model used for St. Lucie Unit 1. An anomaly was discovered and corrected that resulted in reducing the calculated peak cladding temperature (PCT) for the limiting SBLOCA by more than 50'F, and meets the criteria for reporting within 30 days.

The limiting PCT is well within the 10 CFR 50A6 acceptance criteria.

Please contact us if you have any questions about this matter.

Very truly yours, J. A. Stall Vice President St. Lucie Plant JAS/RLD Attachment CC:

Regional Administrator, Region II, USNRC Senior Resident Inspector, USNRC, St. Lucie Plant 99052701i5 990520 05000335i PDR ADQCK P PDR an FPL Group company

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St. Lucie Unit 1 L-99-112 Docket No. 50-335 Attachment SBLOCA Evaluation Model 30-Da 10CFR50.46 Re ort Siemens Power Corporation (SPC) analyzes the St. Lucie Unit 1 emergency core cooling system (ECCS) performance in postulated loss of coolant accidents pursuant to the requirements of 10 CFR 50.46. SPC identified and resolved a deviation from the 10 CFR 50, Appendix K, requirements in that the specifications for nodes in the core model for the small break loss of coolant accident (SBLOCA) evaluation did not provide a converged solution. Correction of the anomaly resulted in a significant decrease in the calculated peak cladding temperature (PCT) for the limiting small break. It should be noted that the large break LOCA continues to be the limiting loss of coolant accident analyzed for St. Lucie Unit 1.

Nature of Deviation:

Specifications for nodes in the SBLOCA core model did not provide a converged solution for the hot assembly PCT.

Descri tion of Error and Corrective Action:

A concern involving proper demonstration of solution convergence for core hot assembly calculations was identified in SPC Condition Report (CR) 6272. The concern led to the determination that the nodal representation used in the SBLOCA methodology did not result in a converged solution with respect to core node length.

Two models were identified as principle contributors to the anomaly: the two-velocity cross-flow model and the level model. The two-velocity cross-flow model lacks sophisticated treatment for the two-phase momentum equations that are used, and causes severe oscillations during two-phase flow conditions. In addition, the level model, which sharpens the liquid-vapor interface, can have spurious behavior under some conditions. When these two models are used in conjunction, solution convergence is not demonstrated for the hot assembly PCT.

The anomaly was corrected by specifying homogeneous flow (single-velocity) at the cross-flow junctions and by using small equal-sized nodes in the core model. These revisions to the application of the SBLOCA model result in consistent and converged core solutions in both steady state and transient calculations. Calculations with the corrected model included a break-size spectrum analysis, which determined that the limiting small break changed from .05 ft to 0.1 ft. The final result is a significant reduction in the calculated peak cladding temperature.

The corrective action eliminates the oscillatory behavior and resolves the issue of solution convergence. SPC reviewed the benchmark calculations for the current SBLOCA model and concluded that the benchmarks were not sensitive to the model changes.

Im act of the Error/Deviation:

For the St. Lucie Unit 1 SBLOCA analysis, the impact of the model change on the calculated PCT is -1 88'F. The revised PCT for the limiting SBLOCA is 1765'F.