ML20044A885

Forwards Response to NRC Question on Steam Generator Level Instrumentation Setpoints,Per Revised Instrument Line Tap Locations.Tap Location Will Be Changed from Above Transition Cone to Below Transition Cone

ML20044A885
Person / Time
Site:	Vogtle
Issue date:	07/06/1990
From:	Hairston W GEORGIA POWER CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
ELV-01828, ELV-1828, NUDOCS 9007160259
Download: ML20044A885 (4)

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Ad&ess-40 invemess Center Parkway Post Offce Box 1295 . t 0,rmw/wn, Alabama 35201 .r Telephone 20$ 868 5581 July-6, 1990 ve soven e*cuc srstem W. O. Heirston, m sena vce eresusent s- ELV-01828 Nudear Operahons 0461 j ' Docket Nos. 50-424 <

425 4 U. S. Nuclear Regulatory Commission ,

ATIN: Document Control Desk Washington,-0. C. 20555 Gentlemen:

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V0GTLE ELECTRIC GENERATING PLANT TECHNICAL SPECIFICATI0i1 CHANGE STEAM GENERATOR LEVEL INSTRUMENTATION ADDIT 10NAL INFORMAT10N ,

in letter ELV-01536 dated May 29,1990, Georgia Power Company (GPC) requested a-Technical Specification change to revise the steam generator water level y

., instrumentation,setpoints in accordance with revised instrument line tap

1. locations.

This change 'will provide additional margin between the trip setpoint and:the -

nominal: operating steam generator water level. The tap location will.be changed ,

from above the' transition cone to below.the-transition-cone. .This change will-

. mitigate sh rin k;and swell effects as well as provide additional operating' margin.

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cSteam generator level behavior at. low power levels is difficult to control- for the Model F steam generators which are used at the Vogtle Electric Generating Plant (VEGP). For' example, if the water level drops below the nominal level setpoint and:the operator-increases feedwater flow, the indicated steam

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generator-level may either show minimal response or shrink even further while >

the long term effect wi.ll be to increase level, in control terminology, this t characteristic is referred to as non-minimum phase response, and it-represents a-significant- control system challenge while in automatic control or a significantt l challenge'to operator ~ training.and experience for manual operation.

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Relocation of the lower water level taps to the downcomer, region below the

. transition cone, of the steam generator introduces a compensatory effect (due to v'elocity head changes when flow rate changes) which significantly mitigates the shrink and swell effects, thereby resulting in smoother operation. Relocation

ofnthe level; tap will also result in- an increased band of indication,-which will

. allow' adjustment of the low-low steam generator level reactor trip setpoint and

.( _ additional operating margin. This additional margin to' the trip setpoint should af , -enable Vogtle to withstand a feed pump trip from 100% power without having an

o junnecessary reactor trip.

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The benefits associated with this modification include 1) the effects of level I shrink and swell at low power levels will be reduced thus. reducing the potential for reactor trip 2) the time necessary to recover indicated level .following a e Lreactor trip will be reduced, thus reducing the potential for an overcooling event due to excessive auxiliary feedwater 3) the margin to the low level trip

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'setpoint' will be increased thus reducing the potential for a reactor trip due to

- a feedwater malfunction at 100% power, The results are that unnecessary plant trips will be prevented while retaining adequate safet'y margins. During 1989 the -Vogtle Electric Generating Plant uperienced three unplanned reactor: trips involving feedwater pump trips and a fourth ~u nplanned trip occurred as a. result of a failure of the turbine intercept valves to open. These trips could-probabaly have been avoided with this modification. ,

Table 1 of- Enclosure 2 to GPC letter ELV-01536 prosideo a comparison of the previous analysis limits and the limits associated with the modified tap-location. Enclosure 1 of this letter is a table that describes the relationship between the proposed Technical Specification limits and the analysis' limits. In addition, the NRC staff requested the references for the WFLASil progran used for.

the small break LOCA analysis.and referenced in Section P.2 of Enclosure 2 of letter ELV-01536. These references are the same as discussed in the VEGP Final Safety Analysis Report Section 15.6.5.3.1.2, and are reproduced as Enclosure 2 of this letter.

Sincerely, 2 /

% W. G. Hairston, 111 i

WGil, Ill/lidli/gm Enclosures

.xc: Georaia Power Company Mr. C. K. McLoy Mr. G. Bockhold, Jr.

Mr. R. M. Odom

-Mr. P. G. Rushton NORMS ,

Southern Company Services Mr. L. B. Long.

V. S. Nuclear Reaulatory Commissim Mr. S. D. Ebneter, Regional Administrator

'lir. T. A. Reed, Licensing Froject Manager, NRR Mr. B. R. Bonser, Senior Resident Inspector, Vontle State of Georata ,

Mr. J. L. Ledbetter, Commissioner, Department of Natural Resources V

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ENCLOSURE 1 'E

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Response to NRC Question on Steam Generator i t 1

Level Instrumentation t j -l 1:

In ' response to the request for c'larification withLrespect to the relationship 'r between the safety analysis 'limitsiand the technical specification trip setpoints resulting from the steam generator levelttap relocation program, the' F

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following information is provided:

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flon-L'OCASafety '

Tech. Spec ' '

'!F Analysis Limits (l) Irin Va hg(l) Total A119wanca ,

to 4- SG-level lit-Hi 100% NRS. 86% NR$ 14% -

f l(566 inches) (533 inches) (33 inches)

SG level Lou-Low 16% NRS 37.8% NR3 21.8%  !

(Feedline brcak) (370 inches) (42) inches) ' (51 inches)

.y SG level Low-Low 30% NRS 37.8% NRS 7.8%

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(Loss of norm FW) (403 inches) (421 inches) (18 inches) a

., l NOTE: The total allowance consists of a statistical comtiination of ' V

, uncertainties associated with the sensor, rack and' process measurement ast' y well as available niargin. Specific components of the_statistica'l , '

i;~n combination'of uncertainties include such items as sensor. calibration, sensor drift, sensor temperature and pressure-effects rack calibratVoa, '\o rack temperature ef fects, proces5 measurement, envirornental alloirdnces, e

, and velocity head ef fects when appropriate, Remaining margin is i!' it

, typically a result of an operational limitation with t.he equipmentt when?

(compared to the safety analysis limit, as well as for conservatisni. .This - i nothodology.is consistent with that used in-WCAP-ll269 Rev. 1 (proprietary) (WCAP-ll270 Rev.1, non-proprietary) for deterriination of i j reactor protection system setpoints. ,

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(1) inches as measured from top of the tubesheet.

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.ih q"c E" REFERENCES FOR WFLASH

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Esposito; V. 'J.LKesavan, K., and Haul, B. A., "WFLASH A FORTRAN IV W p 1.

b 4 "4 Computer Progra:n for Simulation of Transients in a Multi-Loop PWR,"

M& E&P3?&7 Pavision 2 (proprietary) and WCAP-8261, Revision 1 l (nonpropriotary), July 1974, yh)g'f M J D .p + ,

2. .Skwarek, R,, Johnson, W., and Meyer, P.,;" Westinghouse Emergency Core Ql,+ Cooling System Small Break October 1975 Model, "WCAP-@]0-P-A (proprietary)'

and NfAPd2Zh6.(nonproprietary), April 1977. -

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