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WESTINGHOUSE EVALUATION OF |RVLIS PERFORMANCE AT TE SEMISCALE i

TEST FACILITY FOR TEST S-IB-1 |

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May 1982 ,

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i Westinghouse Electric Corporation

: Nuclear Energy Systems,

' P.O. Box 355,

! Pittsburgh, PA.!

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The purpos2 of this report is to pres:rt tha Westinghouse evaluation ofthe Reactor Yessel Level Indication System (RVLIS) performance duringtest S-IB-1 at the Semiscale Test Facility in Idaho Falls, Idaho. The

objectives of this test, concerning RVLIS, were to:

1) verify that short tena conditions, such as over-ranging duringthe large pressure transient will not adversely affect the longterm performance of the RVLIS and

2) verify that the RVLIS will provide a useful indication of the

level in the vessel during the reflooding portion of the tran-sient.

Test S-IB-1 was intended to simulate a 100 percent communicative, coldleg break loss of coolant accident. The experiment incorporated earlyprimary coolant pump trip and normal emergency core cooling injectioninto the intact loop cold leg. The Semiscale mod-2A system configura-tion, with the upper internals modified to more accurately nodel a PWR,was used for this test.

The results indicate that the RVLIS performs as expected during therefilling portion of the transient; that is, the indication follows the

trend in level and provides a conservative underprediction of the level

in the vessel.

II. Applications of Level Measurements

The Westinghouse Reactor Vessel Level Indication System (RVLIS) was

developed to provide:

- a supplemental indication of the approach to inadequate corecooling (ICC)

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- a supplemental indication of the effectiveness of operator actionto prevent ICC

an aid to the use of the reactor head vent systeu.-

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The RVLIS, togeth:r with core exit th2rmocouples, can be used to indicatethe approach to ICC. ICC is defined as a high temperature condition inthe core such that operator action is required to cool the core beforesignificant core damage occurs. When the reactor coolant pumps are notoperating, the RVLIS narrow-range indication is used. This indicationrepresents the collapsed liquid level in the vessel, which is a conserva-tive indication of the mixture level. A prerequisite of ICC is a mixture

level below the top of the core.

The RVLIS indication will be most useful during transients characterizedby slowly decaying pressure where appropriate operator action can aiti-gate the consequences of the event. The RVLIS will not be used by theoperator during a large LOCA event to A:termine the approach to inade-quate core cooling. The transient progressas so quickly that theoperator would not have time to take any action based on the RVLIS indi-cation. In addition, no operator action is required during the event,since the safeguards systens would actuate autouatically to bring thereactor coolant system to a safe condition. The operator is instructed

to verify that the autouatic functions performed as expected regardlessof the RVLIS reading. If any necessary safeguards equipment is notoperating, the operator is instructed to take steps to establish opera-tion, regardless of the RVLIS indication. The primary objective ofoperator action in the Inadequate Core Cooling Procedure Guidelines is todepressurize the reactor coolant system to maximize the ECCS flow deliv-ery and to allow the SI accumulators and low head safety injection toinject. For a large break LOCA, the RCS inherently reaches a pressurethat maximizes ECCS flow delivery and allows full SI accumulator dis-charge and low head safety injection without operator action.

The RVLIS narrow range indication will be useful following a large breaktransient only as a confiruatory indication of the effectiveness of the

dutomatiC safegudrds system The operator can use the RVLIS indication todetermine if the vessel is refilling and to determine when the core has

been recovered.

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Th2 RVLIS upper ranga indication, measuring tha kvel frou the hot Icg,

elevation to the top of the vessel, will be used during long-tena recov-ery operations to aid in the ube' of the reactor vessel head vent. The t ,'/ ,

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benergency procedure for ope 7dtwg the head vent requires that-the system i

be returned to a subcooled condition, so, any v,0fd taeasured by the upper '' '' '

range would indicate non-condensible g'a'ses. AltNugh ific indication

provides infomation on voids, the indication ,is not used for euergency/

operations until subcooled congic.irps are re, ached in the reac' tor coolaritsystem as indicated by the core pxit thenaoccupies or hot Veg RTD's.' <

/Since this ueasureraent extends dod orily to the hot legs,.the indication

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is not used in the ICC procedure. , F r/d large break transient, a stable '

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condition is reached when the level in the ccie is at the hot anicoldzleg elevations. The upper range will, ther4 fore, never te ssed for this

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III. Test S-IB-1~

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A. Test Description..

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Af ter the break initiation, the Seuiscale systwa pressure decreased froiathe steady state operating pressure of apprcximately 15.5 MPa to a pres-sure of less than 0.5 MPa. This blowdown occurred in approxiiaately 50

seconds. During this tirae the systera inventory was drasticallydepleted. The level of liquid in the vessel dropped below the bottora ofthe active fuel. 3

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At the end of the blowdown, tiie system pressure stabilized. The system_

was slowly refilled until the test was teminated at approximately 8ninutes.

The standard RVLIS configuration'was used for this test; the narrow and

wide range DP's were measured from the, top'of the vessel to 'the bottora ofthe vessel and the upper range AP was'ueusured fros.: tiie top of thevessel to the hot leg. In PAR applicat.ioris the RV:.lS output will se

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displayed in tenas of percent ofispan,l In this report ,the RVLIS outputis presented in units of centiueter[ dove or below the cold leg pipeelevation. ,

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Tha RVLIS acasures the diffsrential pressura across a giv;n span. Thi s

differential pressure is compensated for coolant density to determine theequivalent collapsed liquid level. A more detailed description of the;

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j RVLIS is available in Reference 2.'

The Seuiscale Facility instruuentation neasures level or fluid inventoryi

with differential pressure instruuents and gauma densitoueters. The' loctions of the RVLIS pressure taps, the Semiscale pressure taps, and the

densitometers are shown in Figure 1.

B. Couparison of RVLIS and Semiscale Measurements

A comparison of the RVLIS narrow range indication and the level deter-mined from the Semiscale differential pressure ueasured from just belowthe cold leg elevation to the bottom of the vessel is shown in Figure 2.There are dif ferences between these two ueasurements early in the tran-sient due to the rapid pressure fluctuations in the vessel, in additionto the difference in the measurement spans. This behavior was expected

during the blowdown and is not important in evaluating the RVLIS sincethe RVLIS will not be used during the early stages of a large break tran-sient. Af ter approximately 100 seconds the agreement between the twoindications is good. This indicates that conditions in the upper portionof the vessel (flows and water level) do not significantly affect theRVLIS indication during the refill transient.

A comparison of the RVLIS narrow range indication and the gauma-densito-aeter recovery times is shown in Figure 3. This comparison shows that,

except for fluctuations in the indicated two-phase level early in thetrainsient, the RVLIS gives a good indication of the vessel mixturelevel. The RVLIS collapsed liquid level indication accurately followsthe trend in the vessel two-phase mixture level and ccy ' 'n tly

under-predicts the actual level.'

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These comparisons indicate that the RVLIS uill provide useful inforuationduring the refilling portion of a large break transient.

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R3fer:nces

1. Stephens, A. G. , " Experiment Operating Specification for SemiscaleM00-2A Experiuents S-I-B-1 and S-I-B-2." EG&G, EGG-SEMI-5722.

2. Westinghouse Electric Corporation, " Westinghouse Reactor Vessel Level

Indicating Systei.: for Monitoring Inadequate Core Cooling." December1980.

3. Westinghouse Electric Corporation, " Westinghouse Evaluation of RVLISPerformance at the Semiscale Test Facility." December,1981.

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Figure 1. Locations of RVLIS and Semiscale differential pressure|

measurements and Semiscale densitometer measurements.|

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