ML20203C973
| ML20203C973 | |
| Person / Time | |
|---|---|
| Site: | Vogtle  |
| Issue date: | 02/19/1998 |
| From: | Mccoy C SOUTHERN NUCLEAR OPERATING CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| LCV-1165, NUDOCS 9802250314 | |
| Download: ML20203C973 (2) | |
Text
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C. K. McCoy Southern Nucl:ar
%ce Prevdont Operding Cornpa:y,IIc.
Vogde Project 40 invemess Center Parkway P0. Box 1295 0,rmingham Alabama 35201 4
Tel 205 992.7122 f ax 205932 0403 k
SOUTHERN h COMPANY Energy to Serr e nur%rl?
February 19, 1993 Docket Nos.
50-424 50-425 U. S Nuclear Regulatory Commission 1
ATTN: Document Control Desk Washington, D. C. 20555 Ladies and Gentlemen:
VOGTLE ELECTRIC GENERATING PLANT AMSAC C-20 INTERLOCK PERMISSIVE Recently, some Westinghouse PWRs raised concerns regarding the acceptability of the ATWS mitigating system actuation circuitry (AMSAC) C-20 interlock permissive setpoint.
The C-20 interlock pennissive is derived from turbine impulse chamber pressure. It automatically arms the AMSAC logic during power ascension and blocks automatic actuation during ramp down. The concern is 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 the Vogtle Electric Gerarating Plant (VEGP), the C-20 input signal is derived from turbine impulse chamber pressure, and the current setpoint is 40% nominal turbine power. The purpose of this letter is to notify the Staff of our intentions to maintain the present setpoint.
The basis for the AMSAC logic and the C-20 setpoint is described in WCAP 10858-P-A, Revision I,"AMSAC Generic Design Package." VEGP compliance with 10 CFR 50.62 was documented to the NRC by letters dated July 30 and November 18,1987 and April 11,1988.
NRC acceptance of the VEGP design and confirmation of VEGP compliance with 10 CFR 50.62 was documented by NRC letter dated October 3,1988. In the Westinghouse generic report, AMSAC is required to actuate above 70% nominal power based on the supporting ATWS analyses, which demonstrated RCS peak pressure will not exceed ASME Service Level C stress limits for power levels less than 70% without AMSAC armed, in response to i
the NRC Stairs questions, the C-20 permissive setpoint was conservatively reduced from I
70% to 40% turbine power to limit the amount of RCS voiding which may occur during an p
h.,
ATWS event (
Reference:
Addendum I to WCAP-10858-P-A).
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9802250314 980219 ll l
PDR ADOCK 05000524 P
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U. S. Nuclear Regulatory Commission LCV.1165 Page 2 The NRC Jet *er dated October 3,1988, states, in part, that above 40% reactor power, the C-20 interlock will automatically arm the AMSAC logics. This statement is inconsistent with WCAP-10858-P-A, Revision I, which states that AMSAC arms at 40% nominal turbine power. The NRC letter does state that the C 20 permissive signal will originate from existing Grst-stage turbine impulse chamber pressure sensors. This is consistent with the as-built condition and the description in VEGP FSAR section 7.7.1.11.
In response to the conceras that have been raised regarding potential non-conservative differences between thermal power and turbine power, Westinghouse performed an l
additional ATWS analysis at 50% nominal power (
Reference:
Westinghouse Technical i
Bulletin ESBU-TB-97 08-RO, dated November 26,1997). The analysis supports differences between turbine power and thermal power of up to 10% at part-power conditions by demonstrating that:
The amount of RCS voiding that would occur without an AMSAC actuation at 50%
reactor power remains less than the amount previously predicted to occur (at full power with AMSAC);and A substantial margin to core uncovery exists at the time when opecitor action (i.e.,
e 10 minutes) can be taki.n to further mitigate the consequences of the ATWS event.
SNC has veriGed that the difference between turbine power and thermal power at approximately 40% reactor power remains less than 10% for VEGP Unit 1. It is our plan to conGrm that this difference is less than 10% for Unit 2 during the upcoming refueling outage that is scheduled to begin on or about March 8,1998. Therefore, arming AMSAC at a C-20 permissive based on turbine impulse pressure corresponding to 40% nominal turbine power remains appropriately conservative, even considering the differences between turbine power and reactor power when at part-power conditions. As such, the basis for NRC approval of the VEGP AMSAC implementation is preserved, and it is our intention to maintain the C-20 permissive setpoint at 40% power based on turbine impulse chamber pressure.
Sincerely, C. K. McCoy CKM/NJS KdWINWORD\\LCVI165. DOC a
U. S. Nuclear Regulatory Commission l.CV-1165 Page 3 SculltcIn Nuclear Onerating Comnany xc:
hir. J. fl. !!easley, Jr.
Mr. M. Sheibani NORMS U. S. Nuclear Regulatory Commission Mr. [.. A. Reyes, Regional Administrator Mr. D.11. JafTe, Senior Project Manager, NRR Mr. John Zeiler, Senior Resident inspector, VEGP l
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