ML20214Q459

Discusses Results of Analysis to Support Deletion of wet-bulb Temp Operating Restriction on Essential Svc Water Cooling Towers.Design Basis Need Not Consider Coincident LOCA & Tornado Generated Missile Due to Unlikelihood

ML20214Q459
Person / Time
Site:	Byron
Issue date:	05/29/1987
From:	Ainger K COMMONWEALTH EDISON CO.
To:	NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM)
References
TAC-63350, NUDOCS 8706050043
Download: ML20214Q459 (2)

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? / ') Commonwealth One Fast Nabonal Plaza, Edloon Chicano, lilinois

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\ ' ) Address Reply to: Post Mce bot 76T~

Ad Chicago, Illinois 60690 0767 May 29, 1987 U.S. Nuclear R39ulatory Commission 1 Attn: Document Control Desk ,

Washington, DC 20555 l f

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Subject:

Byron Station Units 1 and 2 Essential Service Water Cooling Towers TAC No. 63350 NRC Docket Nos. 50-454 and 50-455 Reference (a): May 26, 1987 letter from K. A. Ainger to U.S. Nuclear Regulatory Commission Gentlemen:

Reference (a) presented the results of an analysis to support deletion of a wet-bulb temperature operating restriction on Byron Station.

That operating restriction was imposed because preoperational test data on the essential service water (ESW) cooling towers indicated a heat removal capacity that was lower than the initial design value. Until further t wling tower testing could be completed, a commitment was made to place the

. 2's in a shutdown condition upon occurence of a high ambient wet-bulb temperature.

The analysis discussed in reference (a) replaced the wet-bulb temperature restriction with a temporary operating limit of 90*F on the ESW pump discharge temperature. This analysis considered a time-dependent LOCA heat load, the thermal capacity of the EsW, an initial ESW pump discharge ,

temperature of 90*F and only three of eight cooling towers cells available l l

for heat removal. This analysis and the approach to the problem was i I

! reviewed by our technical staff and the conclusions were consistent with those of the NRC Staff.

l Subsequent discussions among our technical staff about the

' analytical approach to the problem led to a more detailed and more ,

conservative time-dependent evaluation method. The new evaluation method I supports the 90'F ESW pump discharge temperature limit assuming four cooling l tower cells are available. However, with only three cooling tower cells available, the 90*F lirait results in a peak ESW pump discharge temperature ,

of approxi'aately 100.5'F. This slightly exceeds the technical specification  !

limit of 98'F. This letter is to inform you of the change in initial conditions that must be assumed to demonstrate the 98'F technical i specification limit will not be exceeded. That is, four cooling tower cells must be assumed to be available to accomodate the design basis accident at the design basis wet-bulb temperature.

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8706050043 870529 4 DR ADOCK 050 od

7 The time-dependent analysis discussed above has been forwarded under separate cover to the NRC Staff for their review today.

We believe it is appropriate to assume that four cooling tower cells will be available during the design basis conditions discussed above.

There are eight cooling tower cells amongst the two cooling towers at flyron Station. Six of the eight cooling tower cells are required to be or:rnble Per technical specifications during two unit operation. A single active failure of an emergency diesel generator will cause the loss of two cooling tower fans. Therefore, four cooling tower cells will be available to remove  :

the design basis heat load from one unit undergoing a LOCA and the othee a safe shutdown. '

Less than four cells can be postulated only if a LOCA is considered l to occur coincident with a tornado that generates vertical missiles which ,

destroy one of the operating cooling tower fans. Our position is that the i design basis need not consider a coincident LOCA and tornado generated l vertical missile due to the extrenely low likelihood of such an occurence. '

Please direct any questions regarding this matter to this office.

Very truly yours

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K. A. Ainger Nuclear Licensing Administrator cc: Byron Resident Inspector NRC Region III Office 1

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