ML20055A913

Responds to NRC Re Errors in BWR Water Indication.Level Measurement Sys Includes Degree of Enhancement to Allow Plant to Withstand Failures Beyond Design Basis.Supplemental Procedures Unnecessary

ML20055A913
Person / Time
Site:	Fermi
Issue date:	07/15/1982
From:	Tauber H DETROIT EDISON CO.
To:	Youngblood B Office of Nuclear Reactor Regulation
References
EF2-57-987, NUDOCS 8207200123
Download: ML20055A913 (3)

Text

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Harry Tcuber Erg nd Construct!on Detroit Edson Edisin>>-

July 15, 1982 EF2 - 57,987 Mr. B. J. Youngblood U. S. Nuclear Regulatory Commission Licensing Branch No. 1 Division of Licensing Washington, D. C. 20555

Dear Mr. Youngblood:

Re ferences : (1) Enrico Fermi Atomic Power Plant, Unit 2 NRC Docket No. 50-341 (2) Letter of May 5, 1982, B. J. Youngblood to H. Tauber

Subject:

Errors in BWR Water Level Indication This is in response to your letter of May 5, 1982 regarding errors in BWR-water level indication.

Reactor vessel water level indication is accomplished on BWR designs by employing the hydraulic differential pressure measurement method. The particular design implemented on Fermi 2 is the cold reference leg-design. Condensing chambers connected to the steam space in the vessel are used to generate a fixed reference level for the cold reference. leg. Pressure taps at different elevations in the water space of the

!, vessel are employed as the variable leg of each dif-ferential pair. No hydraulic connection or path exists normally between the two differential legs of the measurement at the instrument end. Each leg includes a restriction orifice near the primary coolant pressure boundary and an excess flow check valve at the primary containment wall. The differen-tial pressure instruments are mounted on racks which are located in the reactor building near the drywell wall.

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Mr. B. J. Youngblood July 15, 1982 f. [ /4 * *a i EF2 - 57,987 i rb g , ,

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i it The Fermi 2_ design utilizes two types'of level trainsmit- .J ters, the Rosemount 1151 series andtthe Gener59 E16ctr/c i GE-MAC 555. The Rosemount is a .capacitivv sens'er .

• design with a 4 to'20. milliampere output, and the General Electric is a bellows / force balance primary senk Q J ,' /

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sor design with a- 10 to 50 milliampere output span. >

The level instrument designations *and arrangements are '

/4 shown on Figurc 7 3-9 of the FSAR., All of the. Fermi N level indicators are referenced to ~ the . top o'f active fuel as discussed in Section H. ILK.3 27 of the FSAR.- 3 h Reactor vessel. taps, lines, racks and' instrument's are ( ,

divided into two divisional groups to maintain spatisi d

diversity. The ability of the ECCSisystems to'N'i,thstan'd a single failure is dependent upon the physical.sepaga- -

tion inherent in the level sensing logic desigd.i 'I In order to minimize the effects of drywell;t[emperature - 'I changes, each pair of sensing linea is run withiapproxi- r/

mately equal vertical drops within the drywell. This j J

approach essentially eliminates the effectfof drywell ,

temperature changes on level by producing-a self- i compensating effect. A further measure to preclu.de -

potential level measurement errors is limiting the ver- i tical drop of both sensing lines within the drywell..-

Only the amount of slope necessary to assure self-venting g/

of the lines is maintained within the drywell... As a result, the potential error due to a loss of liquid in7' the sensing lines is minimized. The nominal lev'el sen- 'h..

sing line vertical drop in the drywell is.; sixteen 1

y inches. This distance is measured from vessel tap ele- Ii vation to the drywell penetration. ,

h As a result of the preceding design features;'drywell (

temperature effects on level instruments are judged to be very minor. Potential for' instrument line vaporiza- f.

tion (flashing) is very nearly zero due cto the' fact that cold reference leg designs have a saturation pressure of ,

118 psia when the drywell is at the maximum predicted temperature of 3400F. /

An analysis of the potential failures which result in high drywell air temperatures has,beenfperformed by '/4 General Electric Company and the'results published in /

NEDE-24801, " Review of BWH Reactor Vessel Water Level ,4 Measurements". This report conc'lu' des! that the cold , r, reference leg level measurement design wit)f parallel I.3 ~$

lines in the drywell and restricted serftical drop will n /,p

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.r perform adequately under any postulated degradation without further modification. In addition, the Fermi 2 level measurement system includes a degree of enhan-cement to allcw the plant to withstand failures beyond the design basis as discussed in Section 2.4 of the e NEDE report.

Edison has concluded that supplemental procedures in addition to those presently developed in response to the emergency procedure guidelines are not necessary.

Should you have any additional questions, please con- /

tact L.' E. Schuerman, (313) 649-7652.

Sincerely,

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'( " I L cc: L. L. Kintner

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