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ACCESSION NBR:9303030121 DOC.DATE: 93/02/26 NOTARIZED: NO DOCKETFACIL:50-315 Donald C. Cook Nuclear Power Plant, Unit 1, Indiana M 05000315

50-316 Donald C. Cook Nuclear Power Plant, Unit 2, Indiana M 05000316AUTH.NAME AUTHOR AFFILIATION

FITZPATRICK,E. Indiana Michigan Power Co. (formerly Indiana a Michigan EleRECIP.NAME RECIPIENT AFFILIATION R

Document Control Branch (Document Control Desk)~I

SUBJECT: Provides summary of analyses rept for analytical effort.Analyses demonstrate that containment structural integritywill not be threateded by hydrogen generation & combunstion.

Nim tinDZSTRZBUTZON CODE: ROZZD COPZES RECEZVED:LTR I ENCL ~ SZSE: ~f 'K S

TITLE: Generic Ltr 88-20 re Individual Plant Evaluations

NOTES:

RECIPIENTID CODE/NAME

PD3-1 PD

INTERNAL: ACRS HOUSTONPMNRR AN,R

G L 01RES MITCHELLPJRGN 1RGN -3

COPIESLTTR ENCL

1 1

1 11 11 11 11 11 1

RECIPIENTID CODE/NAME

DEAN,W

AEOD/DSP/TPABNRR/OGCBRES FLACK,JRES/DSIR/SAIB/BRGN 2RGN 4

COPIESLTTR ENCL

1 1

1 11 13 37, 71 11 1

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EXTERNAL'RC PDR NSIC 1 1

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NOTE TO ALL"RIDS" RECIPIENTS:

PLEASE HELP US TO REDUCE WASTE! CONTACT THE DOCUMENT CONTROL DESK,

ROOM Pl-37 (EXT. 504-2065) TO ELIMINATEYOUR NAME FROM DISTRIBUTIONLISTS FOR DOCUMENTS YOU DON'T NEED!

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1

Indiana MichiganPower CompanyP.O. Box 16631Columbus, OH 43216

R

AEP:NRC:0500Y

Donald C. Cook Nuclear Plant Units 1 and 2Docket Nos. 50-315 and 50-316License Nos. DPR-58 and DPR-74HYDROGEN CONTROL PROGRAM (10CFR50.44(c))SUBMITTAL OF ANALYSES

U. S. Nuclear Re'gulatory CommissionDocument Control DeskWashington, D. C. 20555

Attn: T. E. Murley

February 26, 1993

Dear Dr. Murley:

In an NRC letter dated April 10, 1989, it was indicated that the10CFR50.44 hydrogen control review effort for the Donald C. CookNuclear Plant, which had been terminated by the NRC in 1986, wouldbe subsumed into the Individual Plant Examination (IPE) effort.As a result of this direction, the IPE and 10CFR50 ~ 44 analysisefforts for Cook Nuclear Plant were performed as par'allel butseparate interfacing programs. Indiana Michigan Power Company(I&M) submitted the Individual Plant Examination results in letterAEP:NRC:1082E dated May 1, 1992. In letter AEP:NRC:500X datedAugust 5, 1992, I&M committed to complete by February 28, 1993, theanalyses pursuant to 10CFR50.44 to demonstrate the ability of thehydrogen control system to mitigate the consequences of the releaseof hydrogen into Cook Nuclear Plant containment during postulateddegraded core accidents. The purpose of this letter is to providethe report of this analytical effort.The attachment to this letter provides a summary of the analysesrequired by 10CFR50.44 paragraph (c)(3)(vi)(A). The sequencesselected for analysis were based on significant sequences found inthe IPE. These analyses demonstrate that the containment structural

9303030121'30226.PDR 'DOCK 05000315

PDR

Dr. T. E. Murley 2 AEP:NRC'0500Y

integrity will not be threatened by hydrogen generation andcombustion, and that necessary equipment will survive theconditions created by the burning of hydrogen.

Sincerely,

PL~p~gVice President

rbb

Attachment

cc: A. A. Blind - BridgmanJ. R. PadgettG. CharnoffNFEM Section ChiefA. B. Davis - Region IIINRC Resident Inspector - Bridgman

Dr. T. E. Murley 3 AEP:NRC:0500Y

bc: S. J. BrewerW. M. Dean, NRC - Washington, D.C.D. H. Malin/K. J. TothM. L. Horvath - BridgmanJ. B. Kingseed/R. B. BennettJ. B. ShinnockW. G. Smith, Jr.AEP:NRC:0500YDC-N-6015.1

ATTACHMENT TO AEP:NRC:0500Y

HYDROGEN CONTROL PROGRAM (10CFR50.44(c))

FOR THE

DONALD C. COOK NUCLEAR PLANT .

SUBMITTAL OF ANALYSES

ATTACHMENT TO AEP'NRC:0500Y

1.0 INTRODUCTION

PAGE 1

The Nuclear Regulatory Commission requires in 10 CFR 50.44 thatreactor containments be able to accommodate, without loss ofcontainment integrity or degradation of vital equipment, thehydrogen that may be generated during degraded core accidents. Asa result, the ice condenser containment design at the Cook NuclearPlant was required to include hydrogen control systems capable ofaccommodating an amount of hydrogen equivalent to that generatedfrom the reaction of 75X of the cladding in the active fuel regionwith'team. The hydrogen generated by such a reaction has beenjudged to bound the amounts of hydrogen likely to be generated indegraded core accidents in which core degradation is arrested priorto core meltdown.

In response to the above requirement, the Distributed IgnitionSystem (DIS) was installed at the Cook Nuclear Plant. This systemis currently operable and it has been reviewed and approved by theNRC staff in a Safety Evaluation dated December 17, 1981 [1].Considerable analyses and experiments have been performed todemonstrate conformance with the above requirements, as requiredby 10 CFR 50.44 paragraph (c)(3)(vi)(A). In an NRC letter datedApril 10, 1989, it was indicated that the final confirmatoryanalysis effort, which was terminated in 1986, would be subsumedinto the Individual Plant Examination (IPE) effort. The base IPEanalysis was submitted to the NRC on May 1, 1992 in letterAEP:NRC:1082E [2]. This current report describes the use of the IPEanalysis to address compliance with the requirements of 10 CFR

50.44. As described in this report, the IPE analysis was used toestablish the more likely degraded core sequences. The containmentresponse analysis performed for the IPE was modified to include therequirements of 10 CFR 50.44, and analysis of appropriate sequenceswas then performed.

The computer analysis of the containment response to hydrogengenerating sequences provides an understanding of the timing andrelated hydrogen combustion rates in the various regions of thecontainment. This information is useful in determining heat loadson equipment required to survive the hydrogen combustionenvironment. Due to the approximations necessary to perform anintegrated analysis of both the reactor coolant system andcontainment, the detailed combustion characteristics predicted bythe computer code only approximate the conditions expected in acontainment region, particularly for the pressure response tohydrogen combustion. In containment regions wher'e pressureresponse is of particular interest, hydrogen flow conditionspredicted by the sequence analysis are used to re-evaluate the codepredictions based on the response expected from a review of

ATTACHMENT TO AEP:NRC:0500Y PAGE 2

applicable experiments. Conclusions on the adequacy of the DIS forhydrogen control are then drawn on the combination of experimentand analysis, as required by 10 CFR 50.44 paragraph (c)(3)(iv)(A).

Two other hydrogen control systems are installed in the CookNuclear Plant, the hydrogen skimmer system and the hydrogenrecombiners. The hydrogen generation criteria of 10 CFR 50.44(c)(3)(vi) are well beyond the capability of either of thesesystems. The hydrogen skimmer system was installed to draw airfrom confined areas in the containment to prevent a combustiblemixture of hydrogen from building up in these areas in the eventof a design basis accident. In a design basis accident, less than1X of the zirconium is expected to react with water. The DIS hasigniters in these same confined areas, which can take over for thehydrogen skimmer system in controlling hydrogen in those areas inthe event for the more severe accident conditions. Likewise, thehydrogen recombiners were not intended to control hydrogen in theconcentrations and at the generation rates required under thissection of the regulation. The use of hydrogen recombiners willbe reserved for long term hydrogen control after the accidentconditions have stabilized. This report only addresses theeffectiveness of the DIS in controlling hydrogen. The existingUFSAR addresses both the hydrogen skimmer syst: em and the hydrogenrecombiners.

Section 2 provides a summary description of the methodology usedto choose accident sequences for analysis, the computer code andmodels used to analyze those sequences, and the results of thesequence analysis. A complete report on the sequence analysis isprovided in the Appendix. The containment structural adequacy isreviewed in Section 3 of this summary, and equipment survivabilityanalysis is reviewed in Section 4. Section 5 discusses the use ofthe head vents on the hydrogen control analysis. The conclusionsof the hydrogen control analysis are summarized in Section 6.

ATTACHMENT TO AEP:NRC'0500Y

2.0 ACCIDENT SEQUENCE ANALYSIS

PAGE 3

In an accident sequence that leads to a degraded core, waterinventory is lost from the reactor coolant system until the coreis uncovered. With the core uncovered, there may be insufficientsteam to carry the decay heat away, resulting in a heatup of thefuel rods. When the fuel rod zirconium cladding reachesapproximately 1800'F, the zirconium begins reacting with steam toform zirconium dioxide and hydrogen in an exothermic reaction.Within minutes, the core will reach the uranium dioxide andzirconium melting point, and the core will begin to slump. Duringthe time of core uncovery, hydrogen generation is usually limitedby a lack of steam to a relatively low rate. To stop theprogression of the accident, the core must be recovered with water.This recovery will produce a large amount of steam as the core isquenched. This steam, with the already overheated cladding, willallow for a large but brief peak in the hydrogen generation rate.The thermal shock may also shatter the intact fuel rods at thattime.

The hydrogen generated by the zirconium water reaction is releasedto the reactor coolant system and then out the coolant systembreach into the containment. This breach could be a break in thereactor coolant system or a release path through the reactorcoolant pump seals or through the safety or power operated reliefvalves if the accident was initiated by a transient. The hydrogenmay reach a burnable concentration in containment, threateningequipment by the high heat flux. If the hydrogen builds up to asufficiently high concentration before the burn begins, theresulting pressure pulse may even threaten the containmentintegrity.

This section overviews the methodology and results of the sequenceanalysis. The complete report on the hydrogen control sequenceanalysis is provided in the Appendix to this report. Theacceptance criteria for this hydrogen control analysis arecontainment structural integrity and survivability of vitalequipment. The various sequences and analytic assumptions used inthe analysis provide a basis for showing compliance with theacceptance criteria, which appears in subsequent sections of thisreport.

Accident Sequence Selection

Sequences were selected for analysis based on several criteria.First, the most probable sequences were chosen based on theIndividual Plant Examination. These were then binned into similar

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sets of sequences based on the hydrogen generation rates, and arepresentative case for each set was chosen for analysis. Note thatthese are not necessarily the most probable sequences. The corepressure is important to the rate of core reflood, since it limitsthe ECCS flow rate available to reflood the core. Thus, forhydrogen generation, the core pressure at the time of core recoveryis the primary variable in dictating the hydrogen generation rate.Therefore, a large LOCA sequence, a small LOCA sequence, and asequence initiated by loss of component cooling water was analyzedto cover the spectrum of low, intermediate, and high systempressures at core recovery. These sequences cover the range ofnear atmospheric pressure, steam generator pressure (about 1100psi) and full reactor coolant system pressure (above 2250 psi).In addition, several sensitivity analyses were performed.Detailed justification for the selection of these sequences isprovided in the Appendix.

Accident Sequence Modelling

To analyze the sequences, the MAAP3.0B [3] code was used. Thiscode combines a nuclear steam supply model with an ice condensercontainment model, and was used in the IPE analysis. The originalMAAP hydrogen burn models were modified to reflect the hydrogenburn completeness correlation provided in an earlier submittal [4].This correlation is the same as is used in the HECTR code [5,6],and was supported most recently by the Nevada Test Site (NTS) [7]large scale hydrogen burn experiments for premixed conditions.

For any scenario in which the core geometry is sufficiently intactto be recoverable, less than 75X of the cladding would havereacted. To have a reasonable expectation of a recoverable core,recovery is initiated so that approximately 50X of the core nodeshave reached the melting point. The core geometry is keptartificially intact to maximize the surface area for steam waterreaction on recovery. To achieve the 75X zirconium claddingreaction required by 10 CFR 50.44 (c)(3)(vi), an artificialhydrogen tail was added to the analysis. This was similar to thestrategy used by the Boiling Mater Reactors with Mark IIIcontainments [8].

A maximum ignition criteria of 6X hydrogen concentration fortypical containment regions was used in this analysis, consistentwith the most recent hydrogen analysis effort for the Cook NuclearPlant [4]. This ignition criteria bounds the 5.3X hydrogenconcentration for a pre-mixed volume which is supported by the NTS

experiments [7] . The MAAP hydrogen burn models were developed basedon the assumption of discrete burns in pre-mixed volumes. Forcontinuous injection of hydrogen into a well mixed volume, as would

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be the case for most containment regions of the Cook Nuclear Plantin a degraded core accident, ignition would be at a sufficientlylow average hydrogen concentration that no significant pressurepulse would typically be expected [7,8]. Since the assumption ofignition at a maximum of 6X hydrogen concentration does produce asubstantial pressure peak, this assumption was used to provide aconservative bound on the pressure response to a hydrogen burn incontainment.

The hydrogen ignition criterion of 6X was used in both the upperand lower regions of containment as an initial ignition criterion.The first ignition would promote very good local mixing. After thefirst ignition, the ignition criterion is reset to the lowerflammability limit of hydrogen. The lower flammability limit isthe lowest concentration of hydrogen that will propagate a flame,and in the MAAP code is calculated by including the affects oftemperature and steam concentrations. For containment conditionstypically calculated in the MAAP code, the lower flammability limitis about 4.9X hydrogen. The code results will then more closelyapproximate a continuous burn as seen in continuous injectionexperiments. This lowering of the ignition criterion in the lowercontainment is assumed to affect both the lower compartment andannular regions of the lower containment, since they are adjacentwith a large interconnection area.

In the presence of a water fog at the outlet of the ice condenser,the energy required to vaporize some or all of the water in the fogwould suppress the flame propagation. Therefore, an ignition andflame propagation penalty would be expected in this region.Earlier analysis [4] expected a 2X penalty on the ignition ofhydrogen in this region, for an ignition criterion of 8X hydrogen.More recent experiments have measured the production of fog at theoutlet of the ice condenser [9] and a new analysis has addressedthe impact of fog on the propagation of hydrogen flames [10]. Basedon the newer work, ignition and relatively continuous burning wouldbe expected at the outlet of the ice condenser for most sequences.This work found a best estimate ignition criterion of slightly over7X when using the worst fog concentration measured in theexperiments. The ice condenser inlet steam conditions for typicalaccident sequences are less severe than the inlet steam conditionsof the experiments. Therefore, even lower ice condenser outlet fogconcentrations would be expected for typical accident sequences,resulting in an ice condenser outlet hydrogen ignition criterionsignificantly lower than 7X. However, adequate proof of this wouldrequire 'extensive work. Therefore, a fogging ignition criterionof 8X at the outlet of the ice condenser was used in this analysis.The upper plenum ignition criterion was not set lower after a burnsince the fog actually inerts the incoming flow stream.

ATTACHMENT TO AEP:NRC:0500Y

Hydrogen Production Results

PAGE 6

The three base case sequences provide a spectrum of hydrogengeneration rates and mechanistic zirconium oxidation fractions.For each case, a non-mechanistic hydrogen production "tail" of O.llb/sec. was added to obtain an effective hydrogen productionequivalent to 758 zirconium oxidation. The low pressure sequenceproduced the least mechanistically produced hydrogen and thehighest peak hydrogen generation rate at reflood. The highpressure sequence showed the opposite pattern, with the lowest peakrate on reflood. The intermediate pressure sequence producedintermediate results on both of these parameters. These aresummarized below.

Mechanistic Hydrogen Generation

Large LOCA Small LOCA CCW

Generation Rate (lb/sec)- prior to reflood- during reflood

(peak)

0.13

8.1

.012

5.0

.018

3.0

Mechanistic oxidation 19.3a

Hydrogen Combustion Characteristics

35.6s 51.0a

In the three base cases, the first burn does not occur until thehigh production rate associated with core reflood has begun. Thisfirst burn is in the lower containment for all cases. For the CCW

case, the first lower compartment burn is nearly simultaneous withthe first burn in the upper plenum of the ice condenser. In allcases, significant burning is predicted in the lower compartment.The lower containment sprays keep the steam concentration in lowercontainment sufficiently low that the burning is nearly continuous.Only brief periods of inertion by steam were observed.

Burning is predicted at the outlet of the upper plenum of the icecondenser in only the loss of CCW sequence, although the ignitionpoint is nearly reached during reflood in the large break LOCA

sequence. Although the peak hydrogen production rate is higher inthe large LOCA case, the total amount of hydrogen generated duringthe large LOCA reflood is not very large, resulting in a somewhatsmaller hydrogen concentration passing through the ice condenser.

The loss of CCW sequence is also the only case that shows hydrogenconcentrations reaching the ignition point in upper containment.

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The relatively high hydrogen production rate for the long refloodperiod allows sufficient concentration to build up in this regionto reach the ignition point.

The hydrogen laden air is drawn from the upper containment to theannulus, which is a region of lower containment. With the hydrogenburning in the adjacent lower compartment region, the ignitioncriterion has been reset to the lower flammability limit. For thesmall and large break LOCA cases, the initial ignition criterionfor the upper containment was not met, but the mixture exceeded theconcentration of the lower flammability limit for a long period oftime. It should be noted that the lower flammability limit wasexceeded for these two cases only during the non-mechanistic periodof the hydrogen release. This leads to significant burning in theannulus. In the loss of CCW sequence, the initial burn in the uppercontainment resets the ignition criteria to the lower flammabilitylimit, shifting the majority of the burning from the annulus to theupper containment for this case, If the upper containment initialignition criterion had been set closer to the lower flammabilitylimit in the small or large break LOCA cases, a similar patternwould have been seen. In any event, it can be concluded that theair returning to the lower compartment willhave no larger hydrogenconcentration than allowed by the lower flammability limit.

The following table summarizes the amount of hydrogen burned ineach region for the 'base sequences.

Hydrogen Burnt by Region (lbs)

Large LOCA Small LOCA CCW

Lower ContainmentUpper PlenumUpper ContainmentAnnulus

16000

900

34000

605

231198583

46

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3.0 CONTAINMENT STRUCTURAL ADEQUACY

PAGE 8

The containment compartment pressure and temperature results of thesequence analyses were used to determine if the acceptance criteriawere met. The maximum pressure peak is used to determine if thecontainment maintains structural integrity. The acceptance criteriaof 36 psig was used for the ultimate containment structuralcapacity, as approved in a Safety Evaluation from the NRC datedFebruary 21, 1985 [11].

Only relatively low pressure pulses are shown in the sequenceanalyses which were summarized in Section 2 and the Appendix. Themaximum pressure pulse of 28 psia, or 13.3 psig, was seen in theloss of CCW sequence. This pressure pulse was caused by theinitial burn in the upper containment. This pressure is well belowthe acceptance criteria of 36 psig.

In addition, the conclusion of the series of experiments ending inthe NTS experiments [7] was that large deflagrations are notexpected in well. mixed containment atmospheres with continuoushydrogen injection in the presence of active igniters. Thissupports the conclusion of the sequence analyses.

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4.0 EQUIPMENT SURVIVABILITY

PAGE 9

The equipment needed to achieve and maintain shutdown conditionsfor a degraded core accident were provided in the Safety EvaluationReport for the DIS [1] and are reproduced below. The originalanalysis of the survivability of instrumentation and equipmentutilized heat transfer calculations of the critical equipmentsupplemented by evaluations of equipment subject to hydrogen burnenvironment [12]. This methodology took credit for the submergenceof equipment when possible. Subsequent to that date, more extensiveexperiments have been performed at the Nevada Test Site facility,which support both the methodology and conclusions of the earlieranalysis [13]. For this study, survival of the critical equipmenthas been verified for the conditions calculated in the MAAP

analyses.

Essential Equipmenta) Narrow-range steam generator level monitorsb) Pressurizer level monitorsc) Pressurizer pressure monitorsd) RCS wide range pressure monitorse) Core exit thermocouplesf) RCS loops RTDs

g) Air recirculation/ hydrogen skimmer fansh) Distributed ignition system componentsi) Containment water level monitoring system

also to be considered

a) Containment isolation valvesb) Gaskets and seals for flanges, electrical boxes, air locks, andthe equipment hatch

~ c) Electrical penetrations

Consistent with the Safety Evaluation [1], the pressuretransmitters located in the lower containment are taken asrepresentative of the critical equipment, and specifically analyzedfor thermal response. The adequacy of this approach is supportedby the extensive NTS equipment experiments. In these experiments,a broad range of equipment was exposed to hydrogen burn conditions,including cables, pressure transmitters, solenoid valves, a motoroperated valve, limit switches, a fan motor, resistance temperaturedetectors, hydrogen igniters, and containment penetrations. Allbut a few equipment specimens operated normally during and afterall tests; and all cables passed pose-test electrical checks.

ATTACHMENT TO AEP'NRC:0500Y PAGE 10

Hydrogen Burn Thermal Effect on Equipment

In the sequence analyses, the high initial ignition criteria in thevarious regions allows the maximum amount of hydrogen to return tothe lower containment volumes. This will maximize the hydrogenburning in the lower volumes, which will maximize the thermal loadon equipment in those volumes. It was shown above that thehydrogen to be returned to the lower compartment is limited to thelower flammability limit in the upper containment. However, in allbut the loss of CCW case, the high upper containment ignitioncriterion forces the burning to be in the annular regionsurrounding the lower compartment, which maximizes the heat flowinto the lower compartment. In the loss of CCW case, the bulk ofthis burning occurs in the upper containment.

A simple, thin steel (1/4") heat structure was added to the MAAP

model to represent a piece of equipment. The peak temperature ofthis heat structure was used as an indicator of the most severeconditions in the lower compartment for further analysis. Thelarge break LOCA base case was found to be limiting for thermalresponse, since the hydrogen burns in that region occurred shortlyafter blowdown when the equipment is still quite hot. This is trueeven though the small break LOCA case predicted over twice theamount of hydrogen to burn in the lower compartment.

To ensure the limiting loads on the equipment were obtained, twovariations of the large break LOCA case were analyzed. First, twoinstead of one containment air recirculation fans were assumed tobe operating. This is shown to not significantly affect thehydrogen burn characteristics and locations; the base case largeLOCA was found to remain limiting.

The second variation of the large break LOCA case was to use coreparameters representative of Unit 2 instead of Unit 1. Unit 1 was

originally chosen since it contains more zirconium than Unit 2.Unit 2, however,. operates at a 5X higher power. In addition, thefuel rod array in Unit, 2 is 17x17 instead of 15xl5, with a greatersurface area for zirconium oxidation. The Unit 2 sensitivity caseresults in a slightly greater peak hydrogen generation rate atreflood, and a slightly greater peak equipment temperature in thelower compartment.

A third sensitivity with a potential interest to equipment thermalresponse is the CEQ fan failure case. Although fan failure is notexpected as discussed below, it was the intent of this sensitivityto determine whether survival of the containment air recirculation(CEQ) fan was necessary to obtain, acceptable equipment

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survivability results. The loss of CCW sequence provides thegreatest challenge to the fans, so that base case was modified toassume CEQ fan failure at the time of the first burn in the uppercontainment. The lower circulation flow has the effect ofincreasing the hydrogen concentration exiting the ice condenser,where most of the hydrogen burning now occurs. The impact of theCEQ fan failure was found to have no effect of the peak equipmenttemperature in the lower compartment.

A thermal model of a Foxboro pressure transmitter was developed toobtain the temperature response of the transmitter internals. Theexternal conditions for this heat transfer calculation werecalculated by the MAAP code for the limiting sequence for thermalresponse, the Unit 2 large break LOCA. The analysis determined thetemperature of the pressure transmitter cover, the air inside thecover, and the surface of the electronics inside the cover. Theresults indicate that the electronics temperature is not expectedto exceed 200'F. The NTS experimental measurements of theelectronics of a Foxboro pressure transmitter reached 246'F, andcontinued to function. Therefore, essential equipment is concludedto survive the thermal environment caused by hydrogen burning underthe rules of 10 CFR 50.44 (c) (3)(iv)(A).

Containment Air Recirculation Fan

The assumption of the high ignition criterion of 8X hydrogenconcentration at the ice condenser outlet due to fog'ging has asignificant impact on the evaluation of the survivability ofContainment Air Recirculation fans. Given a more moderate foggingignition penalty at the ice condenser outlet, .significant burningwould be expected there. As described above, the ignition criterionof 8X includes significant conservatism in the estimation of thefog concentrations at the ice condenser outlet. As a result ofhydrogen burning in the upper plenum, insufficient hydrogen wouldbe expected to build up in the upper containment to support aglobal burn on a best estimate basis. A global burn is a burn thatpropagates through a large fraction of a volume, consuming asignificant fraction of the hydrogen. A global burn initiated ata relatively high hydrogen concentration in this region has beenshown before to cause the collapse of the Containment AirRecirculation fan housing [14]. However, based on the evaluationof continuous injection experiments provided below, a hydrogen burninduced pressure excursion sufficient to destroy the fans is notexpected.

In the MAAP analyses presented in the Appendix, the initialpressure rise of 10 psid which accompanied hydrogen burning in theupper compartment is due to the conservatism employed in the

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analysis, i.e., ignition criterion of 6 vol.X Hz and the use of theHECTR code burn completeness correlation. The HECTR code burncompleteness correlation represents an upper bound of scatteredburn completeness test data of premixed combustion in a closedchamber without continuous injection. In the NTS tests withcontinuous injection, the observed pressure rise was modestcompared to that without injection. The upper compartment is anopen through-flow volume with continuous injection of hydrogen fromthe upper plenum. The pressure rise associated with burning isexpected to be small. Therefore, the hydrogen modelling combinationis overly conservative in the prediction of pressure pulses for theCook Nuclear Plant upper containment region. Insights gathered fromthe NTS tests support this observation [7, 15]. Although pressurepulses due to hydrogen burning of up to 6 psid were observed in NTS

continuous injection tests at the time of first ignition, thesepressure pulses were only seen for special conditions. In caseswhere a significant pressure pulse was observed, a wide boundarylayer of a combustible mixture of hydrogen grew until it came incontact with an igniter. The pressure pulse was due to the rapidpropagation of the hydrogen burn through this boundary layer. Theboundary layer was fed by the injection source of hydrogen andsteam.

The configuration and hydrogen injection source in the Cook NuclearPlant upper containment differs from this scenario. The hydrogeninjection source in the upper containment is diffuse with a maximumhydrogen concentration of 8X, inerted by fog. The rate of increasein hydrogen concentration in the upper compartment is very slow(40,000 cfm fan flow at a maximum 8X concentration into the 746,000cu. ft. volume upper containment). Containment spray induced uppercompartment flows are expected to be high (in the range of severalmeters/second [16]), promoting good mixing. No boundary layerbetween ignitable and non- ignitable regions should exist, andcombustion should initiate at the lower flammability limit.Therefore, a significant pressure producing deflagration is notexpected in the Cook Nuclear Plant upper containment. For thisregion, the rate of hydrogen burning will rise to equal thehydrogen flow rate into upper containment. Therefore, the CEQ fansare concluded to survive the conditions caused by hydrogen burningunder the rules of 10 CFR 50.44 (c) (3)(iv)(A).

Even though the fans are not expected to fail due to a burn in theupper containment, as a sensitivity analysis these'ans wereassumed to fail at the first burn in upper containment. Since thissensitivity analysis does not produce more severe results than thebase case analyses, it can be concluded that the design functionof the CEQ fans would have been met, and that failure of the CEQ

fans would have been acceptable.

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There are two design objectives for these fans. First, the fansare intended to provide good circulation of the steam released inthe lower containment through the ice condenser to reach uppercontainment. This ensures that the ice condenser and uppercontainment sprays remove steam effectively, keeping thecontainment pressure to within its design basis value of 12 psig.However, due to the fact that the subject degraded core accidentconditions are well beyond the design basis, the acceptancecriterion for containment pressure for this analysis is thecontainment ultimate strength of 36 psig. The sequence analyzedshows that, even with a postulated fan failure, the peakcontainment pressure is 13.3 psig, (28 psia) which is well belowthe containment ultimate strength of 36 psig.

The second design basis for the Containment Air Recirculationsystem is support for the Hydrogen Skimmer System (HYS) flows.The HYS draws gases from the steam generator and pressurizerdoghouses and the containment dome region to prevent combustibleconcentrations of hydrogen from accumulating in a design basisaccident, in which a small percentage of the cladding is assumedto react with steam. For the degraded core accident sequences, theHYS would be greatly overloaded and this criteria cannot be met.For degraded core accidents, the DIS takes over, eliminating theneed for HYS operation.

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5.0 REACTOR HEAD VENTS

PAGE 14

The reactor head vents were installed in the Cook Nuclear Plant inresponse to 10CFR50.44 (c)(3)(iii) to vent a hydrogen bubble in thereactor head in the event of a degraded core accident withsignificant hydrogen generation. This head vent is also used in theEmergency Operating Procedures to aid in vessel depressurizationafter all other mechanisms have failed, including both the primaryand secondary power operated relief valves. The reactor head ventvents to the lower level of upper containment, and as such is apotential hydrogen source that can bypass the igniters in the lowercontainment and ice condenser upper plenum.

The primary purpose for the head vent is to relieve a "hard bubble"after hydrogen generation has occurred. At this time, it is assumedthat the core is covered, and that hydrogen has accumulated in thereactor head. Since the core is covered, no time criteria existsfor the speed of venting this bubble. The Emergency OperationProcedures direct the operator to read the hydrogen concentrationin the upper containment before venting the hydrogen, and limit thetime of venting so that a combustible mixture of hydrogen could notaccumulate in the upper containment. The hydrogen could then beremoved by the hydrogen recombiners. Since the venting procedureexplicitly prevents a combustible mixture from accumulating, thisuse of the reactor head vent is supported.

The second use of the head vent is to relieve primary systempressure when all other depressurization system fail. Given thenumber of system failures needed to reach this point, this use isconsidered extremely unlikely. The head vent geometry and flowcharacteristics are very similar to the continuous injectionexperiments performed at NTS [7,15). If the head vent is used toaid in depressurizing the core, a mixture of steam and hydrogenwill be released at a high flow rate and momentum to the uppercontainment. For this configuration in the NTS experiments, a slowapproach to ignition was found, with no significant pressure pulsewhen ignition occurred. In fact, the only cases where significantpressure pulses were seen in the NTS continuous injectionexperiments were those cases with only bottom igniters. The CookNuclear Plant upper containment has igniters both in the upper domeof the upper containment and on the steam generator and pressurizerdoghouse walls. As seen in the NTS tests with continuous injectionof a jet of hydrogen and steam, ignition would either be at a pointwhere the j et passed a relatively low igniter, or as the hydrogenreached the top of the containment. In either event, ignitionwould occur while the hydrogen occupies a relatively small portionof the containment volume, and a pressure pulse sufficient to

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damage or destroy the CEQ fans would not occur. Therefore, the useof head vents to depressurize the primary system in the event ofa degraded core accident is acceptable.

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6.0 SUMMARY AND CONCLUSIONS

PAGE 16

To meet the hydrogen control requirements of 10 CFR 50.44, theDistributed Ignition Syst: em was installed at the Cook NuclearPlant. As required by Section (c)(3)(vi) of that regulation,analyses have been completed to support the design of the hydrogencontrol system.

The analyses were based on the accident sequences identified in theCook Nuclear Plant Individual Plant Examination. These analysesassumed that the core was recovered before the core became severelydegraded, and that 75X of the zirconium in the core reacted withsteam to generate hydrogen. The containment conditions thatresulted from this hydrogen burning in the presence of theDistributed Ignition System was then calculated.

The regulation requires that the containment maintains itsstructural integrity, and that the equipment necessary to maintainsafe shutdown and containment integrity be capable of performingtheir function during and after exposure to the environment createdby the hydrogen burning. The analyses show that the maximumpressure predicted for hydrogen burn conditions is far less thanthe ultimate structural capability. In addition, the maximumtemperature calculated in important equipment is shown to be lessthan the temperature required to threaten the capability of thatequipment. Therefore, the analyses show that the design of theDistributed Ignition System is sufficient to control hydrogen underthe requirements of 10 CFR 50.44.

APPENDIX TO

ATTACHMENT TO AEP:NRC:0500Y

HYDROGEN CONTROL PROGRAM (10CFR50.44(c))

FOR THE

DONALD C. COOK NUCLEAR PLANT

SUBMITTAL OF ANALYSES

' A