ML19253C592

Forwards Responses to NRC 791004 Questions on Water Level Measurement Sys Inside Containment.Responses Based on Mods to Steam Generator Ref Leg Design Which Will Be Completed Before Initial Criticality

ML19253C592
Person / Time
Site:	Sequoyah
Issue date:	12/03/1979
From:	Mills L TENNESSEE VALLEY AUTHORITY
To:	Rubenstein L Office of Nuclear Reactor Regulation
References
NUDOCS 7912060525
Download: ML19253C592 (8)

Text

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f, TENNESSEE VALLEY AUTHORITY CH ATTANOOGA TENNESSEE 374o1 400 Chestnut Street Tower II December 3, 1979 Director of Nuclear Reactor Regulation Attention: Mr. L. S. Rubenstein, Acting Chief Light Water Rea-tors Branch No. 4 Division of Project Management U.S. Nuclear Regulatory Commission Washington, DC 20555

Dear Mr. Rubenstein:

In the Matter of the Application of ) Docket Nos. 50-327 Tennessee Valley Authority ) 50-328 Enclosed are TVA's responses to the questions on the water level measure-ment systems inside containment transmitted by your letter to H. G. Parris dated October 5, 1979. The Sequoyah Nuclear Plant Final Safety Analysis Report will be revised to be consistent with the enclosed responses.

The enclosed responses are based on modifications to the steam generator reference leg design (i.e., insulation) to be completed before initial criticality.

Very truly yours, TENNESSEE VALLEY AUTHORITY I

\ \% \n:

L. M. Mills, Manager Nuclear Regulation and Safety Enclosure 1506 278 Bool sE 7912060 SRS gj An E;ual Opportunity Er clever

i ENCLOSURE RESPONSE TO L. S. RUBENSTEIN'S LETTER TO H. G. PARRIS DATED OCTOBER 5, 1979 WATER LEVEL MEASURDiFliT SYSTEMS INSIDE CONTAINMENT

1. Describe the liquid level measuring systems within containment that are used to initiat; safety actions or are used to provide post-accident monitoring information. Provide a description of the type of reference leg used, i.e., open column or seal refer-ence leg.

Response

Two types of level messurement systems used inside containment are described below along with the particular application:

A. An open column reference leg is used for steam generator (SG) level measurement. The instrument is connected to the SG liquid by a condensate chamber at the upper tap. The liquid in the reference leg will be at essentially ambient temperature.

Steam Generator Narrow Range Water Level Safety Functions Turbine trip and feedwater isolation on high-high steam generator water level Reactor trip on low steam generator water level in coinci-dence with steam flow - feed flow mismatch Reactor trip on low-low steam generator water level Au::iliary feedwater pump initiation on low-low steam genera-tor water level Post-accident monitoring function Steam Generator Wide Range Water Level Safety Function Post-accident monitoring function B. A sealed reference leg is used for pressurizer level measurement.

The instrument uses a seal liquid which is not part of the pressurizer liquid and has a physical barrier (a diaphragm) that transmits the hydraulic pressure frcm the liquid (steam) to the seal liquid. The diaphragm is located a sufficient distance from the condensate chamber to be at ambient temperature.

Pressurizer Water Level Safety Function Reactor trip on high water level 1f>06 279 Post-accident monitoring function

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2. Provide an evaluation of the effect of post-accident ambient temperatures on the indicated water level to determine the change in indicated level relative to actual water la. vel. This evaluation must inc;ude other sources of error including the effects of varying fluid pressure and flashing of reference leg to steam on the water level measurements.

Response

A. Re ference Leg Heatup High energy line breaks inside containment can result in heatup of level measurement re ference legs. Increased re ference leg water column temperature will result in a decrease of the water column density with a consequent apparent increase in the indi-cated steam generator water level (i.e., apparent level exceed- -

ing actual level).

The following formula can be used to calculate the magnitude o f this bias: -

H L

1. L, cal - p

. E = 77- ( 7,c a l - og , c a l )

where:

E = level error due to reference leg heatup, as a fraction of level span, H = level span = vertical distance between narrow range taps on steam generator, HL

= height o f reference leg,

- maximum vertical distance from lower tap to water level in condensing pot on upper tap.

This must be determined for the simiting instru-ment connec.tions, pt, cal = water density at containment temperature and steam generator or pressurizer pressure for which the level indication system was calib-r a ted. If this in formation is not available, an upper-bound density (lower-bound temperature) must be assumed.

pl = water density in reference leg at the time of interest (p f, cal - og, cal) = di fference betwcen saturated water density and dry saturated steam censity at the steam generator' or pressurizer pressure for

'which the level indication system was calib-rated. An upper-bound pressure must be assumed.

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A This procedure is based on the assumption that the tubing from the upper and lower taps, below the elevation of the lower tap, have the same temperature at all times.

B. Re ference Leg Boiling In addition to.the above reference leg density change under subcooled conditions, boiling could conceivably occur in the reference leg following dc tssurization of any steam generator with high containment temperature. This combination of condi-tions could only occur following a steamline or.feedline rupture inside containment. If such boiling were to occur, it could cause a major bias in the indicated level for a short time period, in the extreme case indicating 100 percent level when the vessel is actually empty.

. C. Coolan't Density Changes A bias in indicated water level may also be introduced by -

changes in pressurizer or steam generator prissure, due to changes in the density o f the saturated water and steam within those vessels. While prediction of the e f fects o f rapid depres-surization requires complex calculations for each speci fic case, the bias which would exist at low power under quiescent condi-tions can be calculated directly, using the following formula: .

og-og pL, cal - ol -8 9, cal + o g H pf, cal - o g, cal H p g, cal - pg, cal wherc-E = level error due to density changes in both the ves-sel and the re ference leg, as a fraction of level span, L = true water level in the vessel, above the lower level tap, pp = saturated water density at the pressure of interest, og = dry saturated steam density at the pressure o f interest, and other symbols han the same meanings as in Section A above.

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3. Provide an analysis of the impact t hat the level measurement errors in control and protection systems (question 2) have on the as,sumptions used in the plant transient and accident analysis. This should inc3 ude a review of all safety and contrn) setpoints derived from level tignals to verify that the setpoihts will initiate the action required by the plant safety analyses throughout the range of ambient temperatures encountered by the instrumentation, including accident temperatures.

If this analysis demonstrates that level measurement errors are greater .

than assumed in the safety analysis, address the corrective action to be taken. The corrective actions considered should include design changes that cculd be made to ensure that containment temper 1ture effects are automatically accounted for. These measures may include setpoint changes as an acceptable corrective action for the short term. However, some form of temperature compensation or modification to elimin. . or reduce temperature errors should be investigated as a long term solution.

Response

A. Steam Generator flarrow Rance Water Level Tri(Jetpoints The only high-energy line rupture within the containment for which the steam generator water level provides the primary trip function is a fbedline rupture. For such a case the loa or low-low water level trip must be actuated when the pressure di fference between the narrow range level taps corresponds to a zero-level value. Thus the trip setpoints must ba at or above the value that would be indicated at zero true level. Because large steam generator pressure changes are not expected before trip, only the re farence leg heatup e ffects need be considered, and not the effects of system pressure changes.

The determination of the steam generator low-low level trip for Sequoyah is as follows:

Bottom of span (percent) 0 Normal channel accuracy (percent) +5 Post accident transmitter error (percent) -+To Insulated re ference leg effects (post accident heatup) (percent) +3, -0 TOTAL ERROR, percent o f span *

+18, -15 Trip setpoint

'18 percent o f span Allowable setpoint 17 percent o f span

. The value of +3 percent, -0 percent used for re ference leg effects was obtained from the formula in the response to question 2 part A, assuming that the reference leg temperature does not exceed 340 F before reaching the High containment pressure setpoint.

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d As the steam generator narrow range reference legs will be insulated and bounding temperatures are available, the formula in the response to question 2 part A has been used for each section of vertical length to which a discreet temperature can be assigned.

The above setpoint revisions will be made on those trip set-points that provide primary protection for accidents that results in an adverse environment inside containment.

The recommended setpoints derived above result in operating restrictions. Westinghouse is there fore evaluating two alter-nate long term solutions which will permit the lowering of the steam generator water trip setpoints. The two systems under a consideration are as fcilows:

- Mechanical compensation of sealed reference legs

- Temperature compensation of transmitter output B. Pressurizer Water Level Trip Setpoint i;o credit is taken for this reactor trip function following a

- high energy line rupture inside containment. Thus the trip setpoint need not be revised to include environmantal errors.

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4. Review and indicate the required revisions, as necessary, of emergency procedures to include specific information obtained from the review and evaluation of items 1, 2, and 3 to ensure that the operators are instructed on the potential for and magnitude cf erroreous level signals. Provide a copy of tables, curves, or correction factors that would be applied to

______ __ non-accident monitoring systems that will be used by plant operators.

Response

As listed in the response to question 1, the steam generator narrow range water level and pressurizer water level instruments are used for post-accident monitoring. Because of reference leg heatup and process variable changes, tri indicated para-meters may provide erroneous information to the operator fol-lowing a high energy line rupture. The limits of allowable indicated water level are provided based on conservative upper bound error calcuations from the response to question 2.

Indicated steam generator water level can be maintained within a range that will assure that adequate heat removal capacity exists.

For Sequoyah this range of indicated water level is determined as follows :

Maximum Minimum water level water level Transmitter error, total, adverse environment +25 percent -25 percsat Level re ference leg, 90* F to 340*F +15 percent 0 _

Process pressure error, 800-1100 psia 0 bottom o f span +1 percent Otop of span ,

-4 percent Total bottom o f span +40 percent Total top of span -29 percent There fore, to assure the steam generator tubes are cuvered, t.h e indicated water level must be greater than 40 percent. To assure the steam generator is not filled, the indicated water level must be less than 71 percent.

I ,

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Indicated pressurizer water level can be maintained within a range that will ensure that a water level exists in the pressurizer. For Sequoyah this range is determined as follows (the reference leg error given below is a boundary analysis and may be reduced by further analysis):

Minimum Maximum water level water level Transmitter error total +25 perce..' -25 percent Level reference leg, 90*F to 340*F 22 percent 0 Process pressure error 200 - 2350 psia 0 bottom of span +3 percent Otop o f span ,,

. -5 percent Total bottom of span +50 percent Total top o f span -30 percent Therefore to assure a water level exists (i.e. not full or empty) the indicated water level must be greater than 50 percent and less than 70 percent.

Furthermore, a remote possibility exists that the fluid in the 'open reference legs may flash to steam in the depressurized steam genera-tors following a secondlary high energy line rupture. There fore to alert the operator to the possibility of erroneous indications, the following caution will be inserted in all plant emergency instructions for indicated steam generator water level.

CAUTION: The operator should use several plant indicated variables to veri fy the existence of water level in one or more steam generators due to the possibility of erroneous level indication due to measurement system errors. The backup variables that should be used include auxiliary feedwater flow, steamline pressure and primary wide range Th at and Tcold-In particular, the operator should not rely upon steam generator water level indications in any depressurized steam generators following a high energy line rupture inside containme.nt. This is due to the possibility o f re ference leg boiling.

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