ML20202B253

Emergency Operating Procedure 74-1122058-00, Revs to Abnormal Transient Operating Guidelines

ML20202B253
Person / Time
Site:	Arkansas Nuclear
Issue date:	04/22/1986
From:	BABCOCK & WILCOX CO.
To:
Shared Package
ML20202B246	List: 1CAN078606, Forwards Emergency Operating Procedure 74-1122058-00, Revs to Abnormal Transient Operating Guidelines. Rev Incorporates Inadequate Core Cooling Monitoring Sys ML20202B253
References
RTR-NUREG-0737, RTR-NUREG-737 74-1122058, 74-1122058-00, NUDOCS 8607100252
Download: ML20202B253 (6)

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4. During a small break, tripping the RC pumps at a time period when the RC void fraction is about 70% or greater.

The intent of the Inadequate Core Cooling (ICC) guidelines is:

1. To allow the operator to identify when core cooling is inadequa te.

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2. To provide the operator with a. way to estimate the severity of the accident.

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3. To identify those systems which are vital so that the operator's attention will be~ focused on these items in his attempts to re-establish core cooling.

4. To identify some known alternative actioris to try to correct or minimize' the consequences of the accident untii normal cooling can be re-esta511shed. These actions ' are based on the severity

. .i of the accident. . .,

Monitoring Approach to ICC The method for monitoring the approach to ICC is discussed. below.

Although the info,rmation provided by this method is not a basis for operato r actions, it is available to . provide the operator wi th advance warning that ICC may be approaching.

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• . *>g DATE: 4-21-86 PAGE 113 J

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BASCOCK *& WILCOX NUMSER NUCLEAR POWER DIVI $10N TECHNICAL DOCUMENT 74_11220ss-co Inadequate core cooling (ICC) conditions exist when the core e,xi t.

temperature . (CET) becanes superheated. This will not occur until the core starts to uncover due to RC inventory loss. Consequently, trendi ng toward ICC can be monitored by observing changes in the reactor coolant parameters as the RC gues from a subcooled liquid to uncovering the core.

The progression to ICC occurs in two phases. Fi rs t, the amount of subcooling at' the core exit decreases until the core exit RC becomes saturated and second, the saturated liquid inventory decreases until the core starts to uncover leading to ICC.

The amount of core exi t subcooling is detennined by measuring RC pressure ( to determine the saturation temperature) and core _ exit temperature. The core exit temperature (CET)- can be directly measured by the CET thermocouples or indirectly measured by the RC hot leg tem-perature indicators if RC 1oop flow exits. q As the amount of RC subcooling decreases, the loss of subcooling' margi,n i

limit will be reached indicating the RC could be saturated and ,

requiri ng trippi ng of the RC pumps. The l oss-of-subcooling margin -

limit presents pressure and temperature combinations whjch are more subcooled than saturation conditions. The intent of this limit 1.s to assure that the RC is subcooled if the limit is not exceeded. The DATE: PAGE 113-a 4-22-86 l 1

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BABCOCK '& WILCOX NUCLEAR POWER OlVl!!ON NUMBER TECHNICAL DOCUMENT 74-1122058-00 extira subcooling is chosen based on the abili ty to measure the RC l

pressure and temperature (instrument errors) and for temperature and pressure variations from the point of measurement. Because of the extra subcooling, the RC may not actually become saturated until the j indicated RC conditions approach closer to saturation conditions. The

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reasons for tripping the RC pumps are provided in the "Best Methods for Equipment Operation Chapter."

After the RC becomes saturated, changes in the RC inventory are measured by changes in the water level in the RC hot leg pipe and in the reactor vessel. This is possible because the RC pumps were tripped when the loss-of-subcooling margin was reached. This allows the steam and water to separate creating a ~ water level which can be measured.

The water level in the ho't leg pipes is detennined by measuring a di f ferential pressure be, tween the head of water in the hot leg pipe and in a ~ reference leg.

.The indicated RC hot leg ~ level can be less than the actual water level if onon-condensible ~ gases accumulate in the RC hot leg pipe at RC temperatures above 550F. Above 550F, steam becomes significa'n tly denser than noncondensible gases. Because the densi ty of the non-condensible gas is less than the densi ty of ' steam, accumulating non-condens,1bles Wil'1 reduce ~ the sensed pressure in the 1C hot leg pipe.

DATE: 4-22-86 PAGE 113-b

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SABCCCK' & WitCOX NUCLEAR power DIVISION NU E TECHNICAL DOCUMENT The hot leg l evel instrumentation is designed to measure static pressure di f ferences between the hot leg pipe and a respective reference leg. Consequently, the hot leg level instrumentation will provide an erroneous indication if pressure changes are introduced by RC pump operation.

The reactor vessel water level is measured using a sensor containing di fferential thermocouples located at fixed elevations. When a thennocouple elevation becomes surrounded by steam, the heat transfer at the sensor surface will decrease causing a change in the differential thennocouple output. Thus, discrete changes in water -

level are indicated. -

Because of the errors involved with trying to measure the RC hot leg and reactog vessel l evel s, the measured values should ~ be considered.

for trending information rather than for absolute measurements; e.g.,

the water l evel is increasing or decreasing. Also, the RC level measurements could indicate the core is covered when it is not.

Therefore, once the incore thermocouples indicate superheat, ICC actions should be inplemented . These actions should be taken regardless of what the RC hot leg and reactor vessel level instrumentations may indicate.

DATE: 4-22-86 PAGE~ 113-c

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74_11220sa-oo REACTOR HOT LEG AND REACTOR VESSEL LEVEL MEASUREMENT RULE:

a. Do not use the-RC level indications' when the core exit thermocouples indicate superheat' conditions.

b. Do not use hot leg level indication or reactor vessel plenum level indication when a Reactor Coelant pump is operating.

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In summary, the CET thermocoupl es, in conjunction wi th RC pressure l detectors, provide the only valid indic,ations of ICC. The RC sub-cooling measurement, RC hot leg level measurement and reactor vessel measurement can be used to alert the operatorjthat the RCS may be ap-proaching ICC. However, just because the RCS is approaching ICC does not mean it will occur. The RC conditions 'may approach ICC conditions, then stop and even reverse wi thout reaching ICC condi tions. This behavior can be detected with the methods ;Qsed to detect approach to ICC. The most important objective to p'revent ICC is to keep the core covered. The core will be adaquately cool e'd as long as the core remains covered with water. When the core is covered with water, the core outlet temperature an not be superheated. -

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Mitigating ICC

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ICC is indicated when the ' reactor coolant pressure and temperature (incore thermocouples) enter the superheat region. This condition can occur wi th or wi thout fo rced ci rcul ation.

If the RC pumps are operati ng, superheated condi tions imply that the reactor coolant is nearly all steam (see Figure 24a - Time IV). That is, the liquid in DATE: 4-22-86 PAGE 113-d