ML19331D629

Forwards Evaluation of Instrumentation for Detection of Inadequate Core Cooling to Support 800711 Ltr Stating That No Addl Instrumentation Is Needed to Meet NUREG-0578 Requirements

ML19331D629
Person / Time
Site:	Arkansas Nuclear
Issue date:	08/26/1980
From:	Trimble D ARKANSAS POWER & LIGHT CO.
To:	Reid R Office of Nuclear Reactor Regulation
References
1-080-18, 1-80-18, NUDOCS 8009030393
Download: ML19331D629 (22)

Text

ARKANSAS POWER & LIGHT COMPANY POST OFFICE BOX 551 LITTLE ROCK. ARKANSAS 72203 (501)371-4000 August 26, 1980 1-080-18 Director of Nuclear Reactor Regulation ATTN: Mr. R. W. Reid, Chief Operating Reactor Branch #4 U. S. Nuclear Regulatory Corm 11ssion Washington, D.C. 20555 SUBJ ECT: Arkansas Nuclear One - Unit 1 Docket No. 50-313 License No. DPR-51 Instrumentation for Detection of Inadequate Core Cooling (File: 1510.3)

Gentlemen:

Our letter to you dated July 11, 1980, stated that our evaluation determined that the existing instrumentation could detect inadequate core cooling and that nc additional instrumentation was needed.

Attached is a more thorough discussion as to why no additional instrumentation is needed to detect inadequate core cooling.

As stated in our July 11, 1980, letter, we feel we are in compliance with the requirements of NUREG-0578 concerning inadequate core cooling, and do not plan to install any additional instrumentation.

Very truly yours ,

b &f. V David C. Trimble, Manager, Licensing DCT/fg/lp Attachment 8 009030 N MEMPER MiCOLE SOUTH UTauTIES SVSTEM

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EVALUATION OF INSTRUMENTATION TO DETECT INADEQUATE CORE COOLING

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! '- TABLE OF CONTENTS PAGE

1.0 BACKGROUND

1

2. 0 DEFINITION OF INADEQUATE CORE C90 LING 2 3.0 OPERATOR GUIDELINES FOR INADEQUATE CORE COOLING 3 ji;

4. 0 DISCUSSION OF METHODS TO DETECT INADEQUATE CORE COOLING 5 g

4.1 Core Outlet Thermocouples 5

.EE 4.2 Axial Incore Thermocouples 6 4.3 Ultrasonic Techniques 6 j 4.4 Neutron and Gamma Beams 7

..

4.5 Differential Pressure Transmitters 8

5.0 CONCLUSION

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n APPENDIX A - NUREG-0578 POSITION ON INSTRUMENTATION A l FOR DETECTION OF INADEQUATE CORE COOLING A-2 AND-CLARIFICATION FROM H. R. DENTON'S '

LETTER OF OCTOBER 30, 1979 TABLE 4.0 PROPOSED INADEQUATE CORE COOLING INDICATIONS COMPARED TO ESTABLISHED CRITERA FIGURE 3.1 CORE-EXIT THERM 0 COUPLE TEMPERATURE FOR INADEQUATE CORE COOLING

! FIGURE 4.1 LAYOUT OF CORE THERM 0 COUPLE FIGURE 4.1 LOCATION OF THERMOCOUPLE J

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,. ' 1.0_ BACKGROUND m

-The major concerns raised in the aftermath of the TMI-2 accident were identified in the "TMI-2 LESSONS LEARNED TASK FORCE STATUS REPORT, NUREG-

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F 0578". Section 2.1.3.b of that report addressed additional

'= instrmentation which could assist in the detection of inadequate core cooling. The NRC position on additional instr m entation was that

=. " licensees shall provide a description of any additional

" instr mentation or controls (primary or backup) proposed for 4

the plant .... giving an unambiguous, easy-to-interpret

'E indication of inadequate core cooling."....

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Subsequently, the NRC's position was clarified and amplified in Enclosure 1 to H. R. Denton's letter of October 30, 1979 to all operating nuclear

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9 power plants entitled " Discussion of Lessons Learned Short Term Requirements". This letter addressed the following requirements for any additional instr mentation proposed. (The complete clarification is reproduced in Appendix A.)

a. Design of new instrdmentation should provide an unambiguous

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indication of inadequate core cooling.

b. The indication should have the following properties:

!!= (1) -It must indicate the existence of inadequate core cooling caused by various phenomena.

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(2) It must not erroneously indicate core cooling because of l- the presence of an unrelated phenomena.

c. The indication must give advanced warning of the approach of inadequate core cooling.

d. The indication must cover the full range from normal operation ij to complete core uncovering.

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H. R. Denton's letter of October 30, 19 79, clarified the requireme tn s that i

any investigation of additional instrumentation include an evaluation of reactor water level indication.

In r-onse to NUREG-0578 B&W and AP&L have developed operator guidelines for action to recover from a condition of inadequateusing core cooling existing instrumentation (References 1-3).

The evaluation provided in the following sections reviews the adequacy of existing and proposed instrumentation to indicate inadequate core coolingTo(ICC) . perform this review, it is important to understand when ICC ccurs actually o

, what operator actions occur prior to ICC, and the guidelines followed once ICC has occured.

The next two sections describe ICC and the actions tTken before and after ICC is indicated.

These sections are then followed by a comparison of existing and proposed equipment for indicating ICC which concludes with a section describing why the existing installed instrumentation provides the best indication.

2.0 DEFINITION OF INADEQUATE CORE C00LIfG In a depressurization event, the recctor coolant systemrs(RCS) t must fi reach saturation conditions before them is any danger of inadequate re co cooling.

Subsequently if the RCS inventory is reduced and uncovery of the core begins, temperatures in the uncovered region will increase causing s upe rheati ng.

It is important to note in this discussion that inadequate core cooling cannot begin befom reactor vessel (RV) water inventory falls below the top

.' the core thus resulting in an increasing fuel clad temperature.

y l_ 3.0 OPERATING PHILOSOPHY AND GUIDELINES FOR INADEQUATE CORE COOLING 7

The. goals of the operator prior to ICC are different than those once ICC has occured. Prior to an indication that ICC has occured, the operator is taking actions which will stabilize pressure and refill the RCS. The goal

s is to re-establish the subcooling margin at the high pressure condition or cooldown and depressurize to low pressure injection plant conditions.

Indication that ICC has occured changes the operator's guidance because i the goal of refilling at the high pressure cannot be attained. The

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operator at this point is instructed to partially depressurize using the PORV to increase RCS inventory addition rate. Note: If this f ails the operator is instructed to further depressurize and establish low pressure injection (LPI). These last two steps are based on conscious decisions i that recovery at the higher pressure is not possible and that depressurization will cause more immediate core voiding, but in the longer term will result in improved core cooling by increased RCS inventory.

Based on this logic it is important that the indication not be ambiguous and not occur prematurely. It is important to provide as much time as possible for recovery at the higher pressure which leads to the preferred mode of operation.

-. Symptoms of an overcooling transient are similar to the small break loss of coolant transient up to the point of inadequate core cooling. At this point, if the operator has taken actions for inadequate core cooling when

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in f act overcooling exists, an unnecessary serious transient would result.

4 Thus, the operator must not proceed with the inadequate core cooling b

actions until inadequate core cooling is confirmed.

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The following sections describe the actual operator actions taken prior to ICC and those once ICC is indicated.

3.1 Operator Actions During Approach to ICC ,

Operator actions during the approach to an inadequate core cooling condition are summarized as follows:

1. Initiate HPI

2. Maintain OTSG level'

! 3. Trip RC pumps if ESFAS initiated by low RC pressure t

4 4. Monitor incore thermocouple temperatures to determine if 1

inadequate core cooling exists.

f These actions are verified when saturation conditions exist. No further actions are taken until thermocouple temperatures reach a predetermined temperature from Small Break Operating Guidelines (see Figure 3.1-1, Curve 3

1). This indicates that superheating is occurring,- that fuel clad temperature has increased above saturation and that inadequate core cooling exists.

3.2 Operator Actions Once ICC is Indicated Once inadequate core cooiicq is indicated the operator is instructed to take the following actions:

1

! 1. Start one RCP per loop

2. Depressurize operative 0TSG(s) to 400 psig as rapidly as possible f 3. Open the PORV to maintain RCS pressure within 50 psi of OTSG pressure

4. Continue cooldown by maintaining 100 F/hr decrease in secondary saturation temperature to achieve 150 psig RCS pressure.

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CORE EXIT THERl.:0 COUPLE TE:.1PERATURE FOR INADEQUATE CCP.E COOLING q: .

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T CLAD LESS THAN lE00 F 1000' -

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T CLA0 LESS THAN 1400 F l

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!E 200 600 1000 1400 1800 2200 j; Pressure, psia n .

These actions are taken to reduce RC pressure thus increasing HPI flow and RCS inventory addition rate. If thermocouple temperature continues to

> rise above a higher predetennined temperature which indicates a significant increase in fuel clad temperature (see Figure 3.1-1, Curve 2) the operator should:

1. Start all RCPs

-2. Depressurize OTSG(s) to atmospheric pressure

3. Open the PORV to depressurize the RCS and allow LPI to restore core cooling 4.0 DISCUSSION OF METHODS TO DETECT INADEQUATE CORE COOLING The following methods of indicating core cooling were examined in this evaluati on:

1. Existing core thermocouples

2. Additional axial core thennocouples

3. Ultrasonic RV level indication

4. Neutron or gamma beam RV level indication

5. Differential pressure (dp) transmitters for RV level indication The capabilities and evaluations associated with each type of indication are discussed below. Table 4.0-1 provides a summary of the methos and their capabilities.

4.1 Core Outlet Thermocouples The existing core thennocouple instruments indicate inadequate core cool _ing when interpreted using the operator guidelines of Reference 1.

The location of these thermocouples provides indication of sharply

-increased temperatures at the top of the core when the top of the core reaches conditions of inadequate cooling. The locations of the thermocouples in the core and fuel assembly are shown on figures 4.1-1 and 4.1. 2 . _ _ _ , _ _ _ _ _

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Figure 4.1-1 Layout of Core Thermocouples y INLET .

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i 4.2 Axial Incore Thermocouples

.f;jj Additional thermocouples installed axially in the incore instrument guide tube will provide an indication of the extent of inadequate core cooling;

} but, an indication that the middle of the core is inadequately cooled will not elicit any further operator action over and above the actions taken when the top of the core indicates inadequate core cooling. There would be no change in operator guidance even if this thermocouple information i"s were available.

3 4.3 Ultrasonic Techniques Several methods of ultrasonic techniques were considered. These included f using existing internal structures as wave guides, installing an y

externally excited ultrasonic vibrating rod and installing a head mounted i;;

j:i transducer. In simple applications, all of these methods have been

. proven. However, in the reactor vessel the core provides a heat source

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E which changes the density of the fluid. 1he fluid changes state from a

single phase liquid to a two phase fluid, and finally to a single phase I vapor. Ultrasonic level measurement techniques are frequently used where there is a sharp density change at the fluid interface. The level created in a reactor vessel as a result of a LOCA will be a frothy, two-phase mixture height rather than a fixed phase interface. The variable density change will not provide an easy-to-interpret indication, and could provide an ambiguous output signal.

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The ambiguous signal could lead the operator to believe that the core was inadequately cooled when in fact sufficient heat transfer was causing the frothy condition and adequate cooling was in progress. As a consequence of the incorrect b'elief, the operator would take the incorrect actions of depressurizing the RCS.

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- 4.4 Nautron and Gamma Beams Neutren and gama beams have been used successfully to determine the level of fluid in a vessel. The application of this method to a RV level would be the use of the core as a source and use the existing out of core detectors to monitor the water level through changes in count rate.

Normally, the detector count rate decreases at rates characteristic cf the various mechanisms of neutron production that exist following a reactor trip.

95 It One concept of water level measurement uses the installed source range is g detectors which respond to a decrease in water density. As water level i

. decreases, the detector output increases. However, if the water level M decreases to below the top of the core, the detector output decreases.

g The intensity of the neutron beam and thus detector output would be very w..

dependent on previous power history, thus requiring calibration prior to each use of the instruncnt. Tnis is not reasonable during accident conditions. For this reason, further investigation of this method was if terminated. A more detailed discussion of the application of this nuclear radiation method is included in Reference 6.

Another concept of RV water level casurement system has been tested at three reactor sites. The system employs BF3 neutron detectors above and below the reactor vessel. Data was collected and extrapolated to

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determine neutron count rate between one and six days af ter shutdown as a function of water level above the core. The data showed a relatively slow

! increase in count rate as the water level decreased from a full condition, with a marked increase in count rate when the water level reached five feet above the top of the core. At this level water was still above the l hot leg nozzles. This indication system is capable of providing a

, discrete data point -indicating that reactor vessel level is five feet i=

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above the core. Evaluation of the remaining data requires interpretation by the operator to determine the correct reactor vessel water level. The capability of this instrtsnent must be evaluated imediately af ter a g

shutdown to show its effectiveness in a high background level which would be the case following a LOCA.

4.5 Differential Pressure Transmitters The use of differential pressure transmitters to measure reactor vessel level was considered. Three level measurement ranges, one across the W reactor vessel, a second across the hot leg, and one combining these ranges, were evaluated.

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S The first, a reactor vessel differential pressure (dp) measurement, would fJ g require new penetrations in an incore nozzle at the bottom of the reactor vessel and at tha top in a control rod drive mechanism (CRDM) closure. An

! instrunent could be installed to provide a differential pressure between j the bottom of the core and the top of the reactor vessel, but the J. .

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differential oressure (dp) would be affected by not only the water level head, but aiso by shock loss, friction loss, and flow acceleration loss.

During forced flow cond tions, the shock loss, friction loss, and flow acceleration loss terms dominate the signal.

Additionally, the magnitude of these terms varies depending on the density, and thus flowrate, of the pumped fluid. Due to the changing magnitude of these terms, it is not possible to compensate the dp signal p to achieve a water level from head only. During stagnant boiloff, the decay heat in the core will cause the level of Coolant in the core region

jj to swell to a level greater than that in the downcomer region of the reactor vessel. A dp level measurement would measure the collapsed level h in the downcomer region. A swelled level of 12 feet might be indicated by

= a collapsed level of between 7.4 and 8.625 feet, depending on system

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pressure. The unpredictable peak power distribution and decay heat level preclude compensating the dp signal for this error. Although the parameter of interest in this case is the mixture height, the dp cell f would measure a collapsed level which means that under some conditions this signal would be ambiguous, and could lead to premature fhf depressurization of the plant by the operator's misinterpretation of the indication.

D 15 The second method, a hot leg differential pressure measurement would b- require new penetrations at the bottom of the hot leg and the vent line at

the top of the hot leg. This instrunent would provide a dp signal and not d an actual water level. In this instance, measuring any water level would p be a valid indication that the core was covered. During flow conditions, E the output signal would be affected by the same effects as the reactor pi vessel dp signal discussed above. However, the hot leg dp signal could be E temperature compensated. The fact that the hot leg contains coolant would indicate that the core was covered and thus no new operator actions for inadequate core cooling would be required. However, if the operator takes Q actions for inadequate core cooling based on only a level in the hot leg k

then he would be taking incorrect actions for some casualties which could also be indicated by a level in the hot leg; i.e., overcooling, partial steam voiding in the hot leg caused by transients.

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The third method, a differential pressure measurement from the bottom of p

the reactor vessel to the top of the hot leg, would require new penetrations in an incore nonle at the bottom of the reactor vessel and E

p.7 at the vent line at the top of the hot leg. This range is a combination of the two previous instrument ranges. It provides an advantage ove the hot leg level measurement in that it can measure the entire RV level span, but it would still exhibit the same ambiguity as the reactor vessel dp l

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described earlier. In addition due to the greatly expanded rangeo the e

inaccuracy of the instrunent would be greater, perhaps as large as + 4.0 ff ,

feet. This measurement would be inaccurate in the hot leg range and would be ambiguous in the reactor vessel range as discussed above.

E All three methods of dp level measurement require additional structural g penetrations or modifications. Additionally, the operator would not be s

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directed to take action until he confirmed the existence of inadequate core cooling with the core exit thermocouple, thus these additions would

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not change any operator guidance.

EF g 5.G CONCLUSIONS As has been discussed, no proposed method of indication of inadequate core cooling would meet all the established criteria. The introduction of a ad iguous information provided by some proposed systems of inadequate core p"

cooling indication would cause operator confusion. This confusion could l lead to-incorrect and unsafe actions in some sit.uations; i.e., premature b

depressurization during LOCAs, or incorrect actions during overcooling events.

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Reliance on existing core exit thermocouples and previously published e.. operator guidelines for interpreting the available information is the 5 best and most direct method of determining that the inadequate core cooling condition hus ocurred. The existing instrunentation in the B&W designed nuclear steam supply system is able to detect inadequate core cooling. The incore thermocouples provide an unambiguous indication of the existence of inadequate core cooling, and will not erroneously indicate inadequate core cooling. The thermocouples provide the most discriminating capability of defining the existence of inadequate core

'f=E cooling.

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S The basis for this ' conclusion is further supported by the following:

The recently installed T sat meter provides a long term indication of the approach to inadequate core cooling since saturation conditions must be achieved prior to the onset of

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inadequate core cooling. Saturation conditions would be reached a significant time before inadequate ccre cooling, thus the operator

f. would be alerted to the condition.

The existing core thermocouples will indicate the intnediate G approach, the existence of and termination of the inadequate core g cooling condition.

The instrunents will ensure direct, appropriate interpretation of s plant conditions by the operator when used in conjunction with E

previously published operator guidelines.

Each proposed reactor vessel level measurement system concept fails to provide any additional aid to the operator for detection of L inadequate core cooling. Core cooling is'directly indicated by

temperature measurement, not level measurement. Secondly, each of b the level measurement concepts fails to meet all of the established E criteria as outlined in Table 4.0-1.

The potentially ambiguous information provided by the proposed RV level indication instrunent systems could lead to unsafe and incorrect actions if the operator actad on the level indication.

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No new or additional detectors are required to cover the full range

, of plant conditions. Adequate co.e cooling is determined by core 1 heat removal capabilities. It is directly indicated by the reactor coolant .,ystem temperature / pressure relationship The approach to inadequate core cooling is indicated in sufficient time by the T meter to allow the operatcr to take mitigating action. If a

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his actions are unsuccessful and inadequate core heat removal conditions exists, sufficient indication for the operator is available by means of the core thermocouples. As su,arheated conditions are reached the thermocouple temperature will increase.

If additional operator actions of partial depressurization of the RCS are successful and he can regain control of the core heat removal, the thermocouple indication will provide the necessary feedback to tell him that his actions were effective.

It is B&W's and AP&L's technical judgement that the existing plant sensors pro-vide a reliable and accurate method of detecting the approach to and existence of inadequate core cooling for all modes of plant operation.

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TABLE 4.0-1 LEVEL MEASUPEMENT METil0D W!!ICil MEET EXISTING CRITERIA LEVEL MEASUREMENT METiiODS CRITERIA GTsting AdditlonT '

Ranked in Order of B&W Neutron or j Sohcooling Incore incore Ultra. Gamna Assigned Priority  ; ion t tor T/C 1/C Sonics Bean

• lbt leg RV SPND Level AP

1. Must be direct indica- X X tion of ICC

2. Unanbiguous - not X X erroneously indicate ICC

3. Cover full range fran X X normal operation to X core uncovery

- - _ - p L ..

4 Provide advanced warn- X X

ing of ICC X X S. tlanhlguous - indicate

! C during pinnped high X X voni fraction and stag unt bollof f

6. No major structural X X X X changes to plant

7. Unambiguous - meeets safety grade criterla**

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• Develop work is still required to prove capability of this method inanediately af ter shutdown.

• • State-of-the-art hardware to meet safety grade criteria is not available to comply with the schedule installation date.

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l APPENDlX A lc NUREG-0578 POSITION ON INSTRUMENTATION FOR DETECTION OF INADEQUATE CORE COOLING AND CLARIFICATION FROM i! H. R. DENTON'S LETTER OF OCTOBER 30, 1979 k

P POSITION l

Licensees shall provide a description of any additionai instrmentation or

• 1 l controls (primary or backup) proposed for the plant to supplement those devices cited in the preceding section giving an unambiguous, easy-to-interpret

, indication of inadequate core cooling. A description of the functional design requirements for the system shall also be included. A description of the procedures to be used with the proposed equipment, the analysis used in developing these procedures, and a schedule for installing the equipment shall be provided.

CLARIFICATION I

1. Design of new instrmientation should provide an unambiguous indication of inadequate core cooling. This may require new measurements to or a synthesis of existing measurements which meet safety-grade criteria.

2. The evaluation is to include reactor water level indication.

3. A comitment to provide the necessary analysis and to study advantages of

= various instruments to raonitor water level and core coolng is required in the response to the September 13, 1979 letter.

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4. The indication of inadequate core cooling must be unambiguous, in that, it

! should have the following properties:

t a) it must indicate the existence of inadequate core cooling caused by l

l various phenomena (i.e. , high void fraction pumped flow as well as l

l stagnant boil off).

b) It must not erroneously indicate inadequate core cooling because of the presence of an unrelatec phenomenon.

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APPENDIX A (Cont'd)

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5. The indication must give advanced warning of the approach of inadequate

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core cooling.

6. The' indication must cover the full range .from normal operation to 3 complete core uncovering. For example, if water level is chosen as the unambiguous indication, then the range of the instrument (or instruments) must cover the full range from normal water level to the '

[j' bottom of the core.

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' REFERENCES l, ;

1. Small Break Operating Guidelines, B&W Document 69-1106002-00, November 1979

2. Inadequate Core Cooling Decay Heat Removal System Mode of Operation, B&W Document 69-1106921-00, December 1979

3. Inadequate Core Cooling - DNB at Power, Site Instruction 3/4/9/187,5/355, 7/364, 8/172, 11/191. 14/402 dated December 21, 1979

4. Analysis Summary in Support of Inadequate Core Cooling Guidelines, B&W .

Document 86-1105508-01, December 5,1979 I