ML20043G462

Advises That Util Will Control Operations to Abide by More Restrictive Required Shutdown Margin Curve While Awaiting NRC Approval to Place Revised Curve in Tech Specs

ML20043G462
Person / Time
Site:	Summer
Issue date:	05/03/1990
From:	Bradham O SOUTH CAROLINA ELECTRIC & GAS CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
NUDOCS 9006200328
Download: ML20043G462 (5)

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th C r lina Electric & Gas C:mpany le Eradharn ainxin voie, sc 29065 Nuetsar Op; rations (803) S45 4040 SCE&G May 3, 1990 Document Control Desk i

U..S. Nuclear Regulatory Commission Washington, DC 20555

Subject:

Virgil C. Summer Nuclear Station

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Docket No. 50/395 l

Operating License No. NPF-12 i

Required Shutdown Margin Curve Gentlemen:

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1 While performing the final calculations in the Reload Safety Evaluation for

,the Virgil C.-Summer Nuclear Station (VCSNS) Cycle 6 core reload, South h

Carolina Electric & Gas Company (SCE&G) discovered that the calculated boron worths for Modes 3 and 4 in the new cycle are more negative than the worths assumed in the existing Boron Dilution Accident Analysis. As a result, the

-Required Shutdown Margin curve (Figure 3.1-3 in VCSNS Technical Specifications) will not ensure that the Operator has sufficient. time (previously accepted as 13.4 minutes) from receipt of an alarm to terminate a boron dilution event. prior to a loss of shutdown margin. Because the calculated boron worth for Mode 5 has not changed from the value used in analyzing the Boron Dilution Accident, the discrepancy exists only'for Modes

-3 and 4.

l To resolve this discrepancy, SCE&G will administratively control operations

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to abide by a more restrictive Required Shutdown Margin curve (provided in l

l ) while awaiting NRC approval to place the revised curve in I

Technical Specifications.- The proposed curve was generated using analytical methods previously reviewed and approved by the NRC in granting Amendment Number 75 to the VCSNS Operating License (TAC N0. 68644 - VANTAGE,5 IMPROVED FUELDESIGN).

For reference, the Required Shutdown Margin curve currently in i

Technical Specifications is provided as Attachment 2.

4 The lines. defining the regions of acceptable operation for Modes 3 and 4 in the revised figure exhibit definite curvature, whereas the current Required i

-Shutdown Margin curve consists of straight lines. The method for calpulating L

'the data points is identical for both curves. The data for the curve currently in Technical Specifications was fitted to produce the straight lines. To linearly fit the data in the proposed curve would, however, result in requiring prohibitively high shutdown margins for operations at increased boron concentrations.

Because of this, the lines defining the regions of acceptable operation for Modes 3 and 4 in the proposed figure are not linear.

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Document Control Desk May 3, 1990 Page 2 of 2 The curves for Modes 3 and 4 have been shifted upward in the revised figure to account for the increased shutdown margin required due to the more-negative boron worths in the Cycle 6 core. The relative magnitude of the proposed change is best illustrated by the figure in Attachment 3, which provides the adjusted curves for Modes 3 and 4 superimposed on the current figure 3.1-3 in VCSNS Technical Specifications. As mentioned previously, the Mode 5 boron worth remains unchanged. As such, the Mode 5 line in the revised curve is identical to the corresponding line currently in the Technical Specifications.

As shown in Attachment 3, the revised Required Shutdown Margin curve requires greater shutdown margins for boron concentrations between 750 and 2500 ppm than are required by the existing Figure 3.1-3.

For all other concentrations of boron, the requirements are the same. The curve to which SCE&G will b,e adhering to administratively is, therefore, more restrictive than the current Technical Specifications requirements.

SCE&G has verified that the boron worth discrepancy exists only for Cycle 6.

The Required Shutdown Margin curve was correct for all previous cycles utilizing the curve. Additionally, SCE&G has determined that the adjusted boron worth does not impact on any other analyses or documents at VCSNS.

SCE&G will expeditiously process and submit a Technical Specifications Change Request to replace Figure 3.1-3 with the more restrictive Required Shutdown Margin curve.

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

Very truly yours,

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0. S., Bradham EWR/0SB:lbs.

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0. W. Dixon, Jr./T. C. Nichols, Jr.

E. C. Roberts R. V. Tanner R. L. Prevatte S. D. Ebneter J. B. Knotts, Jr.

J. J. Hayes, Jr.

H. G. Shealy General Managers E. W. Rumfelt C. A. Price NSRC R. B. Clary NPCF K. E. Nodland RTS (TSP 90003)

J. C. Snelson File (813.20)

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SHUTDOWN MARGIN VS. TECHNICAL SPECIFICATION REQUIREMENTS 7

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