ML20199K868

Notifies Staff of Intention to Maintain Present Setpoint for ATWS Mitigating Sys Actuation Circuitry (AMSAC) C-20 Permissive,In Response to Concerns Raised by Westinghouse PWR Staff Re Acceptability of Subj Setpoint

ML20199K868
Person / Time
Site:	Farley
Issue date:	02/02/1998
From:	Dennis Morey SOUTHERN NUCLEAR OPERATING CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
NUDOCS 9802090042
Download: ML20199K868 (2)

Text

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Dave Mor:y Southern Nucleat

%ce heutnt Operstitig Company Iarley hoject P0. 00: 12% -

Birmingham. Alat>ama 35201 kl2% 932 5131 h

SOUTHERN February 2,1998 COMN Energy to terve li,ur %Id'*

Docket Nos.

50 348 10 CFR SOE 4

50-364 U. S. Nuclear Regulatory Commission NITN.: Document Control Desk Washington, DC 20555 Joseph M. Farley Nuclear Plant ATWS Mitigatine System Actuation Circuitrv (AMSAC) C 20 Permisig Ladies and Gentlemen:

Recently, some Westinghouse PWR staffs raised concerns regarding the acceptability of the ATWS Mitigating System Actuation Circuitry (AMSAC) C-20 permissive setpoint. He C-20 permissive is derived from turbine impulse chamber pressure. It automatically arms the AMSAC logic during power ascension and blocks automatic actuation during ramp down. He concern is i

related to: 1) part-power operating conditions, where indicated / measured differences between thermal power and turbine power can result in thermal power being higher than turbine power; and

2) the NRC AMSAC safety evaluation reports which refer to reactor power (rather than turbine power) in regard to the C 20 setpoint. At Farley Nuclear Plant (FNP), the C 20 input signal is derived from turbine impulse chambe pressure, and the current setpoint is 40% nominal turbine power. He purpose of this letter is notify the Staff of our imentions to maintain the present setpoint.

He basis for the AMSAC logic and the C-20 setpoint is described in WCAP 10858 P A, Resision 1,"AMSAC Generic Design Package." FNP compliance with 10 CFR 50.62 was dwumented by APCo letters to the NRC dated Febmary 17,1987, December 9,1987. April 28,1988 and August 12,1988. NRC letter dated October 31,1988, which includes the Staffs Safety Evaluation (SE),

documents the acceptance of the FNP AMSAC design and compliance with 10 CFR 50.62. In the Westinchuse generic report, AMSAC is required to actuate above 70% nominal power based on the supporting A*1%S analyses. Dese analyses demomtrated RCS peak pressure will not exceed the ASME Senice Level C stress limit for pcwer levels less than 70% without AMSAC armed. In response to Staffs questions, the C 20 permissive setpoint was conservat vely reduced from 70%

i to 40% turbine power to limit the amount of RCS voiding which may occur during an ATWS event

(

Reference:

Addendum I to WCAP 10858 P A).

He SE for FNP sta,, in part, that above 40% reactor power, the C-20 permissive will automatically arm the AMSAC logics. This statement is inconsistent with WCAP-10858 P-A, Revision 1, which states that AMSAC arms at 40% nominal turbine power, and APCo letter dated December 9,1987, which states that the C 20 signal is based on turbine load. This SE also states that the C-20 permissive signal will originate from two diverse turbine first stage impulse chamber

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f l]. S. Nuclear Regulatory Commission' Page 2 pressure transmitters, his is consistent with the FNP as-built condition, previous APCo letters, and FSAR section 7.8. -

To address the concerns which have been raised ryarding potuntial non conservative diferences between thermal power and turbine power at part power operating conditions, the following points can be made. WCAP 108$8 P A, Revision I, defmes the accuracy requirements of the C 20 permissive signal; the FNP AMSAC instmmentation satisfies these requirements, Engineering i

evaluations based on FNP performance data determined that the maximum discrence between turbine power and thermal power at approximately 40% RTP is less than 3R Westinghouse performed an additional ATWS analysis at *0% nominal power (

Reference:

Westinghouse Technical Bulletin ESBU TB 97-08 RO, dated November 26,1997). De analysis supports

- discrences beth, turbine power and thermal power of up to 10% at part power conditions by i

demonstrating that:

De amount of RCS voiding that would occur without an AMSAC actuation at 50% reactor power remains !ces thn the amount previously predicted to occur (at full power with

AMSAC);and A substantial amount of margin to core uncovery exists at the time when operator action (i.e.,

10 minutes) can be taken to further mitigate the consequences of the ATWS event, i

- Therefore, arming AMSAC at a C 20 pennissive based on turbine impulse pressure corresponding to 40% nominal turbine power remains appropriately conservative, even considering the diWerences r

between turbine power and reactor power when at part power condhions, As such, the basis for i

NRC approval of the FNP AMSAC implementation is preserved, and FNP will maintain the C 20 pennissive setpoint at 40% turbine power as specified by the current FNP design documentation.

If you have any questions, please adsise, Respectfully submitted,

(

_ SOUTHERN NUCLEAR OPERATING COMPANY

$h 771(1f' Dave Morey L

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- MOE/maf:setpt32. doc.

'cc:

Mr. L. A. Reyes, Region 11 Administrator Mr. J, I.' Zimmerman, NRR Project Manager Mr. T. M. Ross, Plant Sr. Resident inspector l

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