ML20062E541

Discusses WCAP-11916, Loss of RHR Sys Cooling While RCS Is Partially Filled. Rept Is Timely & Appropriate Response to Concerns Raised in Generic Ltrs

ML20062E541
Person / Time
Issue date:	12/11/1989
From:	Thadani A Office of Nuclear Reactor Regulation
To:	Newton R WESTINGHOUSE OPERATING PLANTS OWNERS GROUP
Shared Package
ML20062E533	List: ML20062E530 ML20062E535 ML20062E541
References
CON-IIT05-181-90, CON-IIT5-181-90 NUDOCS 9011210074
Download: ML20062E541 (7)

Text

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REFERENCE 3 i

DEC 11 1989 l'

Roger Newton, Chairman l

Westinghouse Owners Group Box 2046 i i Milwaukee, Wisconsin 53201 1

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SUBJECT:

" LOSS OF RHR$ COOLING WHILE THE RCS IS PARTIALLY FILLED," WCAP-11916, j

JULY 1988, AND OTHER RELATED WESTINGHOUSE OWNERS GROUP ACTIVITIES J

Dear Mr. Newton:

You provided us with copies of the subject report in November, 1988 While no review was requested, we reviewed the report because of its impact upon activi.

ties discussed in Generic Letters 87 12 and 88 17. Our evaluation is provided as enclosure 1.

Overall we find WCAP.11916 to be a timely and appropriate response to the concerns, raised-in the generic letters.

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We contracted with the Idaho National Engineering Laboratory to assist in the review.

Their review is provided in enclosure 2.. Idaho stated, and we agree.

l that "Overall, the broad' scope of WCAP-11916 indicates that Westinghouse and the Westinghouse Owners Group recognize that the potential for loss of RHRS capability exists, that losses of RHRS capability can lead to serious complica.

tions in coolin t

timely manner."g the reactor core, and that this issue must be addressed in a "The methodologies for applying the results of the thermal.

hydraulic analyf es to individual plants are clearly written and well. conceived."

i WCAP.11916 results, when considered with our minor Enclosure 1 coments,. are appropriate for use in responding to the GL 8817 recomendations.

We understand you have issued procedures guidance for reduced inventory opera. -

tion and that you are preparing background documentation for that guidance.

We would appreciate receiving a copy of any of this material that may be readily available.

We commend you for these forward-looking efforts and we look forward to a continuation of this approach to nuclear safety.

N Ashok C. Thadani, Director Division of Systems Technology Office of Nuclear Reactor Regulation

Enclosures:

DISTRIBUTION Evaluation of WCAP.11916 central Files WLyon R/F SRXB R/F cc w/ enclosures:

AThadani Lawrence Walsh, Seabrook station CBerlinger(11A.1)

Alan Ladieu, Yankee Atomic Electric CTramm611-(13E.16) 9011210074 90110' JConran(MNBB.3701)

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"Overall, the broad scope of WCAP-Il916 indicates that Westinghouse and the Westinghouse Owners Group' recognize'that the potential for loss of RPRS[residualheatrenovalsystem)capabilityexists,thatlossesofRHPS capability can lead to serious complications in cooling the reactor core, and that this issue trust be addressed in a timely manner."

• The methodologies for applying the results of the thermal-hydraulic analyses to individual plants are clearly written and well-conceived."

• Overall, WCAP-11916 is very thorough and well-written, and provides the licensees with valuable infomation:regarding prevention of and recovery.

from losses of RHRS capability."

i' Plant and scale model data are reported that cover flow phenomena in the RCS, I

and air ingestion and_ vortex femation at the RHp inlet nozzle.

These data =are used to correlate RCS level and RHR flow rate relative.to air ingestion into RHP suction pipes, to establish leve1Tvariations within'the'RCS

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3 to-provide-insight into operations to restore RHp-operation if RHR 'should be lost, and to provide bases for plant coerating procedures for reduced RCS inventory opera-tion.

Analyses are proiiced that address. time to boiling, RCS pressurization, core uncovery, and related topics. Our review of WCAP-11916.is provided below.

B.

AIR INGESTION CORRELATION AND RHR LEVEL - The WCAP-11916 air ingestion correlation is stated to be both reasonable and conservative for limiting air entrainment to maintain acceptable RHR pump operation.

Usage is qualified in-that monded operating limits are not' intended to: replace operating expc cerating at low RHp system' intake flowrates during mid-loop opt

stated to greatly reduce the risk of_ entraining sir.

We agree taat reduced RhR flow rate is effective. in reducing the risk of air -

ingestion and we support this change.for reduced RCS inventory operation.

Industry experience leads us to believe that RHR systems have been. operated with-unrecognized air ingestion and we consequently recomend that licensees carefully review any operations outside the safe region as-determined by the ur--

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WCAP correlation. We further believe the air irgestion correlation to be of a best estimate nature, and sugoest a small safety factor be applied in determin.

ing a lower bound for RCS level.

Finally, we observe that RCS wster level-1 should be maintained as-high as is consistent with other needs so as to mini.

mize the chance of losing RHR dce to air ingestion or inadvertent' loss of RHA

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l inventory. The limiting upper water level may be to maintain a gas communica -

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tion space via the pressurizer surge line when the head is on the reactor

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i vessel.

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C.

AIR INGESTION SYMPTOMS - The plant test data suggest that the first symptom i

of air entrainment is noise at the RHR pump, followed by a drop.in suction f

pressure, and finally by oscillations in suction pressure, flow rate, or motor 4

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current, The authors recomend that RHR noise be monitored when entering into i

a level / flow combination where experience is limited.

We concur.- We also suggest monitoring pump ncise when reducing inventory while in a mid-loop-configuration to guard against inaccurate level indication.

D.

RCS RESPONSE TO LOSS OF RHR - Analyses are reported covering time' to boiling following loss of RHR, RCS pressurization rate, and time to core uncovery for various RCS configurations.

These show that boiling can inttfate j

in less than 10 minutes, although time for more Ifkely conditions. is 20 to 30 minutes.

A large cold leg opening with hot legs isolated can lead to core uncovery several minutes after boiling starts. More likely is core uncovery later than 30 minutes following loss of RHR for large cold side openings and in excess of one hour for hot side openings, s

This knowledge is valuable for procedures preparation and for training.

Its application should include the influence of the RCS pressure boundary configu -

ration and usability of SGs, and it should be'used in conjunction with indicat-ed temperature and level. Care should be 'used with indicated level since RCS behavior can introduce errors that depend on the instrument and its connection to the RCS, Vessel. temperature indication should i,e accurate under all condi-tions in which an :dvarse containment environment-does'not occer.

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We expect procedures writers will consider the effect of the containment t

environment upon actions which must take place inside containment.

Although GL 8817 recomended containment closure prior to core 'uncovery, closure l

actions may be limited after initiation of boiling'due to steam in the contain-i ment.

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KCAP-11916 shows that RCS pressure can reach 400 psia about an hour after loss of RHR if the RCS is closed and the steam generators ($Gs) are empty, and that providing water to $Gs can significantly reduce pressurtration rate.

We agree, although we believe there may be film coefficient and gas transport modeling

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difficulties that cause pressure to be over-predicted when there is water'in the SGs.

The WCAP results apraar to be inconsistent with the Diablo Canyon event pressurization, where the SGs apparently limite'd RCS pressure to a few '

osi (Ref. 8).

As identified in' the WCAP, pressurization-behavior strongly I

influences gravity makeup from water storage tanks, and an over-prediction, if t

real, could erroneously eliminate gravity feed as a inventory addition option.

i E.

OETERMINATION OF VENT ADEQUACY WCAP 11916 showed that a pressurization of several psi can eject much of th) RCS inventory when there is a cold leg I

opening in the RCS pressure boundary, and that such a pressurization can prevent cold leg injection water from reaching the core.

Large, low resistance hot leg vents are necessary to compensate for the low steam density and lar i

steam flow rates that occur during boiling if pressurization is to be avoided.

j Consequently. licensees should fully consider vent paths to accurately determine backpressure.

Two examples will illustrate-typical-considerations.

Consider an open pressurizer manway. An unrestricted opening of this size-might be adequate, but the surge line diameter is smaller and there is heater hardware in ~the lower pressurizer.

These must.be considered.. Changing conditions may also change pressurization behavior.

Suppose RCS boiling L

initiates with hot leg water covering the surge line connection'at the hot leg.,

Water will be forced into the pressurizer, causing a pressure elevation in the_

upper reactor-vessel.

If there is a cold leg opening, water-may remain in the pressurizer long enough to cause early. core uncovery.

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5 Of ten, licensees provide a path to the pressurizer relief 'ank (PRT) and remove the rupture disk te provide an opening to containment.

There are a number of potential restrictions via this path in addition to the disk opening.

For I

example, water in the PRT can cause several feet of water backpressure since, if the sps'*0trs are covered, there is ilttle coteunication between the relief tank inlet ard the disk opening.

F.

ALTERNATE REANS OF DECAY NfAT REMOVAL - One or more methods are identi'te in WCAP 11916 for increasing RCS inventory following loss Cf RhR, with some variation depending upon the RCS configuration.

If RHR cannot be easily reestablished, at least one alternate mode of decay heat removal is identified.

l Two are necessary to comply with OL 8817 reconnendations.

The potential for cold leg injection failure due to steam flow from the vessel clearly requires thst hot leg injection be provided as an option in procedures.

G.

LEVEL VARIATION WITHIN THE RCS - The magnitude of level dif forences within the RCS is shown to be roughly 1 or 2 inches, which is stated to be signiff.

This should be considered in applying the air ingestion correlation to cant.

account for levci variation between instrument locations and level at the RRR suction pipe entrance, 111. _ REFERENCE $_

1.

T. S. Andraychek, et al., ' Loss of RHR$ Cooling While the RCS is Partially Filled," Westinghouse Electric Corporation WCR 11916 kw 0, July 1982.

2.

F. J. Miraglia. " Loss of Residual Heat Removal (RHR) While the Reactor Coolant System (RC5) is Partially Filled (Generic letter 87-12)," letter from NRC to all holders of operating licenses or construction permits for pressurized water reactors, July 9, 1987.

3.

D. M. Crutchfield, "Less of Decay Heat Removal (Generic Letter No. 88-17),

10 CrR 50.54(f)." letter from NRC to all holders of operating licenses or ennstruction oemits for pressurited water reaeteri, October 17, 1988.

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M. E. Waterran, " Technical Evaluation Report, WCAP 11916, 'Less of RHRs Cooling While the RCs is Partially Filled,'" EGAG Idaho, Inc., EGG.

l EAST 8681, September 1989.

5.

P. A. Newton, " Westinghouse Owners Group, farly Notification of Mid Leep Operation Concerns, letter to Westinghouse Owners Group Frimary Represen-tatives(IL,IA),0G8821,May27,1988.

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F.,

C. E. Rossi, "Possible $udden Loss of RC5 Inventory During Low Coolant

,I Level Operation," NRC Infomation Notice 88 36, June 8,1988.

>I I I 7.

R. A. Newton, " Westinghouse Owners Group Reactor Cociant System Mid Loop Operation, Sumary of Jure ?3,1988 Meeting with NRC," letter to trecutive Advisory Comittee, Westinghouse Owners Group Primary Representatives, I

OG 88 30, July 14, 1988.

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8.

" Loss of Posidual Heat Removal System, Diablo Canyon, Unit 2, April 10, 1987."

U.S. Nuclear Regulatory Comission, NUREG-1269 June 1987.

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i ENCLOSURE 1 l

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Npt STAFF EVALUATION OF WCAP.21916 REY. O "LO$$ OF RHRS COOLING WHILE THE RCS IS PAPTIALLY FILLE 0" i

ii EVALL'ATION COMPLETED NOVEMPER, 1989 i

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SLWARY WCAP 11916 (Ref.1) was writuen under the direction of the Westinghouse Owners Group (K00) Analysis Subcomittee with guidance from the WOG Operations Subcom-l mittet.

It's stated purposet are to provide "... information applicable to the fluid systems perfomanco when the RCS (reactor coolant system) is partial-l ly filled and themal hydraulic analysis of the RCS following the loss of RHR (residual heat removal) during operattent with the RCS loops partially filled."

I "It is intended that the WOG plants participating in this program will be able to utilize the results of this report based on the categortretton applicable to their particular plant." The work was initiated to provide infomation re.

ouested in Generic Letter (GL) 87-12 (Ref. 2) and is applicable to the recomen-dationsofGL8817(Ref3).

We find WCAP-11916 to be thorough and well-written.

It provides the licensees with valuable infomation regarding prevention of and recovery from loss of PHR.

WCAP-11916 results, when applied with consideration of the comments provided in this enclosure, are appropriate for use in responding to the GL 8817 recomendations.

The results may be used based upon the plant catego-rizations described in WCAP-11916.

II. LVALUA710N OF _WCAP-11916 A.

OVERVIEW - The Idaho National Engineering Laboratory (INEL) reviewed WCAP-11916.

INEL states (Ref. O. and we agree. that:

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