ML19350B588

Rough Draft, Analysis of Hydrogen Mitigation for Degraded Core Accidents in Sequoyah Nuclear Power Plant, Prepared for Ofc of Nuclear Regulatory Research

ML19350B588
Person / Time
Site:	Sequoyah, McGuire
Issue date:	12/01/1980
From:	Berman M, Jamarl Cummings, Sherman M SANDIA NATIONAL LABORATORIES
To:	NRC OFFICE OF NUCLEAR REGULATORY RESEARCH (RES)
Shared Package
ML19247D089	List: ML19350B588 ML19350B592
References
NUDOCS 8103230073
Download: ML19350B588 (130)

Text

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1. ANALYSIS OF HYOROGEN MITIGATION FOR DEGRADED CORE ACCIDENTS IN THE SEQUOYAH NUCLEAR POWER PLANT December 1, 1980 M. Berman M. P. Sherman J. C. Cummings l

M. R. Baer S. K. Griffiths i

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i Prepared for Divisica of Reactor Safety Research Of fice of Nuclear Regulatory Research U. S. Nuclear Regulatory Commission WashingOn, DC 20555 I

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ACKNOWLEDGEMENT Contemporary research has changed significantly from the days of Newton, Einstein and Edison. Present studies tend to be very broad and complex, frequently involving many different disciplines and spe-cialties. Research performed by one or two investigators has become rare. The coordinated efforts of multi-faceted teams are row commonly required for the solution of complex problems. Space and p, cpriety frequently limit the number of contributors whose names can appear as authors. The ef forts of many other individuals frequently pass unno-ticed and unrecognized, even though their contributions to the success of the integrated effort were quite significant.

We would like to acknowledge those efforts here, and inform the ,

readers of their important contributions. Blaine Burnham performed the MARCH calculations for this report, becoming a user and code =cdi-fier within a space of 8 weeks. Rupert Byers performed the CSQ deto-

' Pat Rosario and Coris Jackson typed the two ver-nation calculations.

sions of this manuscript in record time and with a minimum of errors.

It is generally easier for managers to achieve notoriety than fame.

When things go wrong,.their names begin to appear; but when things go well - anonymity. Glen Otay and Bill Snyder contributed significantly to this work and to many other projects in reactor safety. They pro-vided outstanding moral and physical support, which is hereby grate-fully acknowledged. We also acknowledge the encouragement, guidance, and material support provided to us by our NRC program manager, Don Heatson.

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, -i FOREWO RD Late in August 1980, the U.S. Nuclear Regulatory Commission asked Sandia to perform a short term (2-3 months) analytic study to investi-gate the behavior and utility of three hydrogen control mitigation j schemes: deliberate ignition, water fogging and Halon injection. At-tachment 1 describes the scope for this short term effort. The urgency of the request was based on the following circu= stances: (1) Licensing decisions needed to be made on Sequoyah and other PWRs equipped with ice condensers and containments s= aller and weaker than standard large, dry FMR containments; and (2) these containments are more vulnerable to hydrogen combustion damage, for quantities of hydrogen comparaole to that produced during the TMI-2 accident.

Sand'a staff began to investigate these schemes during the first l

week in September. This report summarizes the results of twelve weeks i

of intensive study. Although we believe that significant progress has  !

I been made to date, the reader must keep in mind the extremely short j duration of the. study. We have attempted to ensure that no major er-i rors were made, that no significant omissions occurred, and that the i

report is an accurate evaluation of the three mitigation schemes in terms of present knowledge and experimental information.

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SUMMARY

This report is intended to provide a preliminary assessment of i three mitigation schemes proposed for hydrogen control in nuclear pow-

, er plants: deliberate ignition, water fogging and Halon inerting. We have been requested to respond to 21 questions addressing various as-pects of these mitigation schemes including efficacy, practicality, operational strategies, post-LCCA behavior, design concepts and costs, and most importantly, whether overal! plant safety will be improved or i degraded by the deployment of such mitigation systems.

As a result of this study, we have concluded that no one of these mitigation schemes is clearly superior to the others under all accident conditions. Advantages and disadvantages have been identified for all systems. In addition, there are accidents for which all schemes would provide improved safety. However, accident scenarios can also be iden-i - i" ,tified'for which deliberate ignition or Halon inerting could be detri-l .e

, \ " {l g -m, ental to safety.

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'g [ . t. yitem at moderate rates, and where mixing occurs rapidly, deliberate ig-

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.d g nition of lean mixtures should be beneficial. Preliminary calculations +

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<pa significant-T[. ,y' indicate that deliberate ignition in the lower compartment

,P ly reduces the pressure rise in coatainment under the following condi-tions: hydrogen release rates up r.o at least 35 lbs/ minute (the maxi-mum rates observed for MARCH calculations of small and large break LCCAs during the degraded core pection of the accident); and sufficient-ly rapid mixing to prevent high concentration pockets from forming.

Under certain accident conditions, the lower compartment could be in-erted either by high concentrations of steam, or by low concentrations of oxygen. If this should occur, the interim deliberate ignition sys-tem (IDIS) as presently planned for Sequoyah has a serious shortcoming.

The inerted gas mixture entering the bottom of the ice condensers will

8 emerge as an extremely rich mixture at the top. Concentrations could i

. approach or exceed the detonability limits in a toroidal region around the periphery of the containment at the top of the ice condensers.

Four igniters are presently planned for this region. We strongly rec-ommend that those igniters be removed. Instead, we suggest that upper compartment deliberate ignition strategy should attempt to burn lean mixtures high in the upper compartment. The upper compartment is much

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larger than the lower; furthermore, it does not have either the heat-removal capacity of the ice condensers or thr: expansion into the lower u

volume available to it. Consequently, burns in the upper compartment can yield pressure rises approacPing those computed for adiabatic, con-stant volume deflagrations. Igniters placed high in the upper compar:-

ment will tend to burn more dilute mixtures than igniters immediately above the ice condensers. If the burns occur below the downward-propagation limit of 9%, only part of the hydrogen will be burned. We therefore recommend placement of tre-igniters high in the upper compart-ment. In the lower compartment, the present number and arrangement of r igniters appears adequate for an interim system.

The Sequcyah IDIS presently involves the automatic initiation of all igniters simultaneously. While we believe that this system will improve safety, we also believe that an operator-controlled, computer-and detector-assisted system would be superior. Such an advanced sys-tem would permit each igniter to be separately controlled, but would require a much more extensive monitoring system than presently extses.

Important questions which remain to be quantitatively answered in determining the relative merits of the deliberate ignition systems in-clude: more accurate determinations of the rates, quantitics and loca-tions of hydrogen generation for various accident scenarios; the prob-ability of producing locally high concentrations of hydrogen; the im-pact of the ice condensers and fans on hydrogen transport and combus-tion in tha upper and lower compartments; the effects of combustion on

safety equipment; and the fraction of hydrogen consu=ed in each ec==us- .

tica event. Experiments to date indicate tha: 00==us:10n 00=pleteness is a sensitive function of gec=e:ry, source strength, nu=ber and loca-tion of igniters and hydrogen concentration.

0:nceptually, water fogging appears to be a very attractive .1:1-gation sche =e. We have perf0r=ed kinetic vaporization calcula:::ns which indicate that the droplets will be vaporized rapidly encugh to validate the ther=0 dyna =ic calculations; i.e., significant reducti:ns in pressure and te=perature resulting fr0= the addi:icn of 3.05 vol. %

liquid water will occur for droplets of all pra :ical sizes (10-100 =

radii) during deflagrations in contain=ent. These sa=e drople: sizes will not be vapori:ed in the fla=e front, but ra her a short distance behind. This implies that the deflagration will not be quenched, per-

=itting the c0=bining of deliberate ignition with water fogging into an integrated syste=. Cur. analysis, however, has.un:cvered a p0:en-tially severe limita icn on the practicacility of water fogs; 1.e.,

the requirements for its =aintenance during the accident period. In Sequoyah, only about 4500 gallens of water are required to reach 0.05 volu=e 4. To maintain this level, however, will require the spray sys-te= to provide water Oc make up for losses by se:: ling, agglameration and collisions with surfaces. Under the assu=ption of gravitational settling and instant coagulati:n upon contact, cal:ulations have indi-cated that losses =ay be severe. Additives whi:n increase surface ten-sion =i-htv reduce agg10:eration losses, but this is presentiv. unpr,ven.

1: may also be possible to. int:0 duce the required volume of wa:er by means of a foa=. This is also being investigated. Fr0= our studies, then, we conclude that fcgging could be very beneficial if procle=s associated with its =aintenance in the con: sin =ent at:0 sphere can be solved.

Cur investigations have uncovered a signift:an body of infor=a-t ion concerning the use of Halen as a hydr 0 gen :0=bustion preven:ive L

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measure. Halon has been previously employed for such uses on a large l scale (106 g 3). At known concentrations, Halon will inert hydrogen:

air: steam mixtures. For Sequoyah, an adequate system would cost approx-imately three million dollars and could probably be installed within a few months. There are, however, so=e potentially detrimental aspects to the.use of Halon as a hydrogen control measure. The addition of the Halon itself (264,000 lbs) will raise the containment pressure about 9 psi. For accident scenarios where steam overpressuri:ation poses a greater threat than hydroge6 combustion, this 9 psi Halon incre-mental pressure could be very detrimental. If the Halon concentration f alls below the inerting level, the Halon itself could contribute to the explosive yield, and the high deflagration temperatures would decompose the Halon to produce highly corrosive halogens and halogen acid compounds.

Even if the Halon accomplishes its inerting objectives, some means would be required to remove the . hydrogen from the post-accident ' environ-ment without decomposing the Halon. Present recombiners would require modification to accomplish this. Halon will also radiolytically decom-pose, and prolonged exposure could lead to significant decomposition.

It is important to note that, unlike water fogs, Halon and deliberate ignition are incomoatible mitigation schemes.

_4_

I INTRODUCTION The three mitigation schemes were investigated separately. The three major sections of this report reflect that separation. Each sec-tion is divided into two parts. The first part is an introduction cen-cernir.g the general nature of the specific mitigation system. In :ne cases of deliberate ignition and water fcgging, a nore de siled discus-sien is presented concerning the important phenomena and conclusions.

Following the introduction and discussion, eacn of the questiens asked in the work secpe 'see A::achmen: 1) is specifically answered, based en our best engineering judgment at the present time.

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. 1.0 DELIBERATE IGNITION 1.1 Introduction and Discussion

" Failure" pressures for the Sequoyah containment have been esti-mated by several groups with the results shown in Table I. (1,2,15)

Table

" Failure" Pressures for the Secuevah Containment Pressure, pcia Group Yield Ultimate TVA ( Re f . 1) 33 43.5 TVA (Ref. 15) 22.7 43.2 NASA-Ames 35.6 66.7 R&D Assoc. 27 --

NRR 34 --

'Jhe highest value in the table, 66.7 psig, is admitted to be not very accurate. The NRC recommended the value of 31 psig as a conserva-tive " failure" pressure in Ref. 1. In subsequent conversations, how-ever, we learned that 45 psig is now accepted as a conservative maxi-mum.(16) For the purposes of this report, we have performed calcula-tions for a range of failure pressures which encompass both the 31 and 45 psig values. It should be noted, structural engineers at Sandia indicate that failure often begins at local stress concentrations and weaknesses (flaws), and propagates throughout the structure. Idealized analyses, such as those shown in Table I, may be too optimistic.

We can conservatively predict the pressure rise due to the defla-gration of homogeneous hydrogen: air or hydrogen: air: steam mixtures in containment by considering the-combustion as a complete, adiabatic,

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. . constant-volume process. In reality, anticipated deflagrations during

. possible accidents in Sequoyah will not be adiabatic, nor will they oc-cur at constant volume. In Sequoyah, the upper and lower compartments are quite distinct and separated by paths running through the ice con-densers and recirculation fans, Figure 1. Burns which take place in the lower compartment can expand into the upper compartment and, in the process, transfer large quantities of heat by cooling and condensation in the ice condensers. These mechanisms can greatly reduce the pressure and temperature rises compared to adiacatic, isochoric deflagrations.

Burns in the larger upper compartment, however, can approach the adia-batic, isochoric results. Pressure or temperature reductions result only from heat transfer to the walls and expansion through the return-air fans; these mechanisms are much less effective than those available for lower compartment burns.

Results for adiabatic, isochoric, complete combustion are shown in Figures 2 and 3. Thirty-one psig overpressure corresponds ,to a .ccmbus-tion of about 6% hydrogen by volume; 45 psig corresponds to about 91 hydrogen. Table II summarizes some pressures which would result from deflagrations of various concentrations of hydrogen for certain initial l

conditions. The percentage of metal-water reaction (MWR) is based on a I total 100% yield of 1950 lb.(15) (Note that other references consider 2250 lbs to represent 100% yield; see discussion at end of Section 1.)

i CCMBCSTION OF LEAN HYDROGEN: AIR MIXTURES Factors whien may reduce the pressure and temperature rises illus-trated in Figures 2 and 3 involve the assumptions of no heat transfer, constant volume, and combustion completeness. As Table II shows, com-bustion may be incomplete for lean mixtures of hydrogen in air. Belcw about 9% hydrogen (the downward propagation limit) the combustion of hy-drogen by weak igniters, such as spark plugs and glow plugs, is found to be incomplete in large chambers. Results f or the ignition of

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. hydrogen: air mixtures by a spark plug at the center of a 12-foot sphere .

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by Furno, Cook, Kuchta and Burgess,(3) and for similar experiments in a 6-foot-diameter sphere by Slifer and Feterson(4) are shown in Figure 4.

These results show very incomplete consumption of hydrogen up to 8i in-y itial hydrogen volume fraction, rapidly approaching complete consumption

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in Figure 5. These experiments indicate that the strength of the igni-tion source influences combustion completeness for hydrogen concentra-tions below about 9 volume % hydrogen.

Table II Hydrogen-Air Deflagrations Assumptions Homogeneous, Constant-Volume, Adiabatic 1 Atmosphere, 298 K, .100 % Relative Humidity Overpressure, psig Hydrogen Concentration Complete Incomplete in Air MWR Equivalent Combustion Combustion

6% 20% 31 1 20 8% 27% 40 4 33

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Clegg(5) on the spark ignition of lean mixtures in a' vertical tube of 10.16 cm diameter and 26.53 cm height, shown in Figure 6, is in agree-l

l. ment with the much older work of Coward and 3rinsley(6) on the l

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combustion of hydrogen: air mixtures in a 4.5 cm diameter tube. The re-sults of Harrison, et al., Figure 7, show a much higher fraction of hy-

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drogen consumed, for mixtures with initial hydrogen concentrations below 8%, than the results of similar experiments in large spheres.(3,4)

If we assume the data for spark ignition in large spheres are typi-cal of the fraction of hydrogen that would be consumed for deliberate ignition in reactor containments, then a 31 psig pressure rise corre-sponds to just over 3% initial hydrogen concentration. Because of the rapid rise in pressure expected as the initial hydrogen concentration goes from 8 to 9%, the uncertainty in the failure pressure of the con-tainment, shown in Table I, has little ef fect on the maximum hydrogen concentration that can be burned without containment failure. A 9%

hydrogen concentration in containment corresponds to about 45 psig and a 30% oxidation of the zirconium cladding in the reactor core.

Without the addition of a second mitigation scheme, such as fcg-ging, the combustion of hydrogen mixtures of over 8% could result in -

containment overpressurizing. If the generation of hydrogen is slow enough, the hydrogen could be slowly consumed in multiple burns with interim cooling of the atmosphere by heat transfer and water sprays, preventing a continuous buildup of pressure. Hence, deliberate igni-tion is a .ni:igation scheme which is limited in the amount of hydrogen it can centrol for a single ignition, and limited in the rate of hydro-gen it can control for multiple ignitions with interim cooling.

In sammary, the ec=pleteness of-combustion of lean hydrogen mix-tures, below 9% hydrogen (the downward propagation limit) but above 4%

hydrogen (the upward propagation linit), for a given ho=cgeneous mixture can depend on the following:

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We will now attempt to give a"po'ssible explanation for the disparate ex-

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perimental results and present our thinking on the application to hydro gen co=bustion in containment.

The work reported by Slifer and Peterson(4) indicates that the frac-tion of hydrogen consumed using strong ignition sources, such as pyro-fuses, is much higher than the f raction consumed using spark igniters, as shown in Figure 5. This is not surprising since the pyrofuse is a longer lasting source of larger size, igniting =cre hydrogen. The dif-ferences between the results of tube experiments, Refs. 5 and 6, and those concucted in large spheres can be explained as follows.

The upwardly propagating flames in lean hydrogen-air mixtures ap-pear in the form of burning globules. Below 6% hydrogen concentration, the flame temperature is of the order of 500-700 K, below the spontane-ous ignition temperature of approximately 800 K. It has been observed that hydrogen rapidly dif fuses in*" the flame zone giving locally rich-er and hence, hotter flames and idepleting .the region .between the glob-

- ules.(7}- Part of the reason for the incompleteness of combustion of .

hydrogen, then, is the failure to bcrn the residual hydrogen in the re-gion between the globules. Presumably, in tube experiments the glob-The results shown in Figure 7 ules fill the cross section of the tube.

may be typical of tne fraction of hydrogen consumed in the region tra-versed by the globules.

Upwardly propagating flames are expected-to propagate in a roughly conical volume, surrounded by a region of :ero co=oustion, when the ir-itial atmosphere is quiescent. This result was found by Sapko, Furno and Kuchta(8) for methane: air: nitrogen near-flammability-limi: mixtures, Figure S. The idea of some conical-type flame propagation upward was mentioned by Carlson, Knight and Henrie(9) for lean hydrogen flames.

One can. visualize the combustion of lean hydrogen: air: steam mix-tures as shown schenatically in Figure 9 for a single weak igniter and a quiescent' atmosphere. If this model of lean hydrogen combustion is a TIME, msec 9C0 720 p- -- q l i I \ 03 l \ 0.hf'-

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valid approximation for ignition of a homogeneous mixture in containment, then the pressure rise can be computed given two pieces of information:

the completeness of combustion of hydrogen as a function of initial hy-drogen concentration in the zone of partial combustion, and the cone angle or other quantity specifying the increase in the partial combus-tion zone area versus height from the igniter. The completeness of com-bustion in the partial combustion zone may be approximated by the data taken in tubes, Figure 7. The main uncertainty then would be in the rate of growth of the partial co=bustion zone with height. The upper compartment in Sequoyah is relatively uncluttered, and the model in Fig-ure 9 may be appropriate. The lower compartment, however, is extremely cluttered and crowded. The complex geometry and large number of obsta-cles might tend to increase the degree of combustion completeness.

If the combustion takes place in the upper compartment of the Se-quoyah containment, the steam present in the atmosphere of the lower com-partment would presumably have been condensed in passing through the ice condenser.- However,-it is possible that hydrogen may continue to evolve after the ice has melted. Combustion in the lower chamber may take place in the presence of a large fraction of steam. The effect of steam is to act as a diluent in hydrogen combustion, more effective than nitrogen or l

! excess oxygen in lowering flame temperature because of its higher specific heat. The effect of steam on the flammability limits is shown in Figure 10.(10) At low steam concentrations, there is little effect on the lower j flammability limit of hydrogen. It takes almost 60% steam to inert hy-drogen: air: steam mixtures. The presence of up to 15% steam seems to have l

little ef fect on the Otmpleteness of hydrogen consumption for lean mix-tures.(5)

! IGNITION l

Experiments have been performed to determine the minimum ignition energy required to ignite hydrogen

air mixtures. (ll,12) The results I

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Stean E x:ures are st Ant in Figure 11. For near stoichiometri; mixtures at atmospheric pressure, energies of less than one-tenth of a millijoule are required, but the energy required increases rapidly as ne flammability limits are approached. At approxima:ely 7% hydrogen, an energy of 0.6 millijoule is required. Drell and Belles (ll) caution that these energies are mini-mums attained in laboratory settings and tha t gas motion, turbulence and spark duration may increase the required energy. There is a lack of ex-perimental information on the minimum spark energy required for ignition very near the flammability limit. The gap be: ween the electrodes of a spark plug used for lean hydrogen combustion must be larger than that used in automotive spark plugs. The gap must be larger than the quench distance, the distance to cool down an incipient flame. The quench dis -

tance for lean hydrogen mixtures increases rapidly as the flammacility limit is approached in a fashion similar to the minimum required spark energy. For about a 7% hydrogen mixture the quench distance is nearly 0.3 cm. The failure to obtain ignition- in mixtures usually considered .

flammable using automotive spark plugs by Carlson, Knight 'and Henrie(9) -'

may be due to quenching by the electrodes.

Glow plugs were selected for ignition in the Sequoyah containment because of concera about possible radio frequency interference caused

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. SEQUOYAH INTERIM DELIBERATE IGNITION SYSTEM (IDIS)

. TVA has proposed a deliberate ignition system (IDIS) for hydrogen i

control in Sequoyah. Tnirty-two glowplugs have been installed in light-ing fixtures throughout the containment. TVA plans to have the operator activate all the igniters simultaneously, based on some acciden signal (e.g., ECCS activation). We will refer to this IDIS system as " automat-ic" deliberate ignition 'DE), in contrast to an operator-controlled de-liberate ignition scheme which would permit each igniter to be activated separately by operator action. These two philosophies of deliberate ig-nition are discussed in more detail in the question-answer section fol-lowing this section. We will assume for this discussion, however, that TVA is constrained to use the " automatic" DE system in the near term, -

and the following remarks concern that system.

Basic questions dealing with DE include the efficacy and reliabil-ity of the igniters under accident conditions. These issues are pres-

- ently being addressed experimentally .in.a program at Lawrence Livermore National Laboratory. We assume that the igniters will work; i.e., be capable of igniting mixtures of hydrogen in the range of 5-10% in the presence of large volumetric fractions of steam, and not be adversely affected by sprays or other environmental factors.

l Deliberate ignition will be beneficial if the hydrogen can be con-sumed slowly enough so that neither containment integrity nor equipment survival is jeopardized.* This can be accomplished in several ways:

burn lean mixtures; burn in the lower compartment; burn slowly enough to prevent pressure and temperature build-up. Lean mixture combustion has i

already been discussed extensively. Bur'iing in tPe lower compartment

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l will generate pressures much lower than would occur for an adiaratic, i

l

• We have tacitly assumed that DE would not lead to the combustion of other materi als (e.g., flammable insula tion) . TVA must insure that such combustib. materials do not become secondary sources of deflagra-

, tions.

1 i -

isochoric cc=bustion. Pressaro. reduction results frc= the_ expansion of

- gases through the ice condenser into the larger upper compartnent, and .

frc= the condensation of steam and cooling of the other gases as they pass through the ice. Deliberate ignition in the icwer cc=part=ent could; on1 be harmful if it led ec de:cnations, pseudo-de:cnations, cr, pcssi-bly, if all the ice has been =elted. Ex:re=ely high concen: rations are unliiel' to develop in the lower cc=part=ent for =cs: accident scenarics, except perhaps TMLB (loss of main feedwater and all pcwer) .

TVA has presently allocated 25 gicwplug igniters to the lower cc=-

partment. 'We have encountered conflicting state =ents concerning :ne number and location of the igniters; we believe this is due to revisions reflecting TVA's continuing evaluation. However, it is possible that the-following data no lenger reflect the present situation.) The number and location of these igniters are shown in Table III.

,ab_,e t..

Glcwplue Locations in Secuevah Lcwer Compartmen _ . . . _ .

t

Entrance of Ice Condensers 5 Outer Equip =ent' Annular Space 9

" Raceway" Annular Space 4 Main Subec=partment (including primary pe=ps 7 and stea= generators)

The locations are indicated in Figures 13 and 14. This dis:ribution cov-ers all subcc=partments except the s=all cen:ral ec=partmen above ne reacter vessel and below the control red drive asse=bly missile snield.

Because of the possibility of a local decenation in this cc=partment, we are, at present, unable to determine whether or not an igniter should ce

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installed in this subec=partment. We intend c study this question fur-ther. Otherwise, for the interim system. we believe tha: the presen sche =e provides adequate ccverage in the lower cc=partment. The

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l question of ignitor redundanoy can be.more profitaoly addressed af ter

.. the results of the LLNL tests become availaole. .

Burns in the upper compartment (UC) can occur for two reasons: the entire containment has been overwhelmed by an extremely rapid and large release of hydrogen f cm the primary system (a very unlikely scenario);

or the lower compart=ent (LC) has been inerted by excess steam and/or deficient oxygen. Since DE will result in repetitive deflagrations in the lower compartment, fresh oxygen needs to be supplied from the upper compar tme n t . This oxygen supply can be impeded if the LC burns occur more frequently than the time required to resupply the oxygen (order of minutes) or if the fans are partially or completely inoperative. Steam inerting can occur whenever the stea= fraction in the LC exceeds about 56%. This can occur either because of large steam releases accompanying the hydrogen, increased steam concentrations from previous burns, or failure of the air-return fans.

Conditions for deliberate ignition in the upper cc partmen are very different than~in the lower co=partment. The pressure rise due to com-bustion can approach that for an adiabatic, constant-volume combustion.

As a r e s ul t , mole fractions above 9% hydrogen should no be burned, based on a " failure" pressure estimate of 45 psig.

The volume of the icwer compartment is only about 43% of the upper i

compartment. It is doubtful if significant flow can occur from the upper l *o the lower compartment in time to diminish the comoustion pressure rise l

l l

The doors to the ice condenser normallv. permit flow onl* one wav., from l lower to upper compartments. There are two air-return fans of about 40,000 CFM capacity. In an upper compartment deflagration, the higher pressure in the uc.e.er compartment would presumably cause an increase in I

this flow rate. Nevertheless, the flow rate through the fans is too low to significantly reduce upper compartment pressure rise.

There are seven glow plug igniters in the upper cc=partment: four a:

i

he ext: of the ice condenser and three high up in the deme near the wa:e 1

l spray no::les. The action of the water sprays in reducing the upper t

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compartment gas temperature miy permit the use of repeated burns in the upper compartment provided each burn consumes less than a given amount of-hydrogen (less than 9% hydrogen), the exact amount depending on the ini-

'tial pressure, and provided there is sufficient time between burns for the water sprays to cool the gases (typically several minutes).

The concentration of hydrogen in the upper compartment will be a maximum at the exit of the ice condensers. It is possible for a steam-inerted mixture entering the ice condenser to become detonable at the ex-it. The mole fraction of hydrogen at the exit, XH2 cut' mole fraction in, Xg , and the mole fraction of the steam condensed in the ice condenser, X H ,0, by s

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For example, consider a modest hydrogen mole fraction cf 9%..inerted by a 60% ' steam fraction in the lower compartment.- -If all the steam is con-densed in passing through the ice condensers, as expected, the hydrogen l concentration at the ice condenser exit will be 22.5%. This concentra-tion is extremely detonable, and would form a 300* partial torus adja-cent to the containment wall. Under such conditions, the rationale for locating four igniters in this region is very questionable. We would l

l strongly recommend that'TVA remove these igniters for the IDIS.

In our early analysis of deliberate ignition, we were inclined to place ignitors near the top of a volume to benefit from the difference l

in upward and downward flammability limits. This strategy also appears desirable for Sequoyah upper compartment DE. Igniters placed near the top would tend to generate lean mixture deflagrations which would not l- propagate downward, and which would only involve a fraction of the hy-drogen contained in the UC. Such deflagrations could gradually remove i

I i l

the hydrogen, while still allowing the normal heat removal mechanisms (sprays) to limit pressure and temperature rises.

f LOCAL DETONATIONS With 32 igniters distributed throughout containment, the possibility of developing unifor= hydrogen concentrations in excess of 13% (the de:-

onability limit) seems very remote, if not i=possible. It is also of no interest in terms of reactor safety f. o r Sec.uev. ah , since an 13% deflagra-tion, by itself, has a very high probability of failing containment.

Local hydrogen accu =ulations, however, could conceivably reach detonable concentrations. The previous section indicated that the plenum at the top of the ice condensers is such a region. Another suscectible re~sion might be the subcompartment above the reactor vessel head, Figure 1.

A local detonation will produce a shock wave propagating outward from the detonating region and decaying with distance. A rough es ti=a te can be =ade of the distance within which the shock wave can be damaging by using the data for high explosives.(13,14) For a given energy re-lease of explosive, E, the ratio of overpressure to a=oient pressure, (p - po)/po, is usually given as a decreasing function of the dimension-less distance from the center of the explosion, R/Ro, where R is the distance from the center of the explosion and Ro is a scale distance given by l

Ro = (E/po)1/3 i

The energy released in~a hydrogen detonation can be considered as the product of the molar energy of reaction and the number of =cles of hy-I drogen consumed.

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where r is the radius of the detonable cloud, XH is the mole fraction -

2 of hydrogen in the cloud,)E is the univs:: sal gas constant, To the ini-tial gas temperature and Q the energy release per mole of ydrogen con-s umed , 2.4 x 105 kj/kmole. For an initial temperature of 300 K and a hydrogen mole fraction of 0.29, we have Ro = 4.3 r The blast overpressure is a weak function of explosive size, speed of energy release and other parameters. However, we can take the fol-lowing values for rough approximation.(13)

R/Ro (p - po)/po 0.5 2.0 1.0 0.5 2.0 0.2 If'we consider a peak pressure of 30 psig (2.0 po) as nondamaging, then the shock wave pressure will decay to acceptable values in approximately 2.5 cloud radii. A more complete analysis would have to consider shock wave propagation into deflagrable mixtures, reflections, etc. Nev-ertheless, the above analysis indicates that small detonable clouds may not damage cuntainment even if they detonated. .The effects of larger detonating clouds would have to be examined more carefully.

A det onation could also threaten containment by the generation of missiles from explosively-produced debris. Referring to Figure 1, the control rod drive missile shield (or pieces thereof) might become such a missile if a detonation occurred in the subcompartment below it. We performed a 2-dimensional axially symmetric calculation of a detonation within this compartment using the CSQ hydrodynamic computer code.Il7)

The calculational geometry is shown in Figure 15, with the reactor head below and the missile shield above. Other initial conditions included a hydrogen mol'e fraction of 20%, and initial temperature and pressure

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Figure 15 also illustrates the de cnation waves (pressure contours) proceeding inward and downward from the ignition source region at 1.4 ms.

The pressure history at top center for the full 50 ms calculation is shown in Figure 16. The highest pressure of about 49 bars occurred about 2.5 ms after ignition. Pressures continue to oscillate due to multiple internal reflections of the shock waves.

A very simple analysis of the missile shield was performed. The complex pressure behavior was approximated by a constant 10 bar pres-sure pulse lasting 50 ms. The impulse delivered to an intact missile shield was insufficient to make it strike the containment dome. A more complex structural analysis, which would also treat the pcssibility of spallation and fragmentation, is beyond t.*,e scope of this shcr: term effort.

ACCIDENT ANALYSES One method of assessing the performance of the Secuoyah interim de-l liberate ignition system would be to perform accident calculations with and without the IDIS, and compare the threats to containment survival.

fwo ec=puter codes have been used to perform such calculaticas: the Westinghouse CLASIX code, and MARCH from Sattelle Columbus. Both codes are unverified, and are essentially still under development. Results of CLASIX calculations were reported in Ref. 1. For a small break LOCA, S 2D, they= showed that multiple burns would occur in the lower compar:-

ment resulting in pressure rises of only a few psi per burn. The burns were separated generally by_two or more minutes. Containment failure S.9.0As . . . . . . . . .

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was calculated to occur only if heat removal in the ice condenser was ar-tificially reduced, or if both fans were inoperative. The results were also relatively insensitive to the ignition criterion employed.

MARCH S 2 D calculations were qualitatively similar to the CLASIX re-suits.(lilS) Peak pressures were generally below anticipated failure conditions prior to the core slumping into the lower plenum. Calcula-tions carried beyond core slump always predicted containment failure.

Sandia has obtained a uopy of the MARCH code and the S2 D input deck from Battelle (BCL). With their assistance, we have set up and run many MARCH calculations investigating both small and large break LOCAs. We will briefly review some of our results for this report. More extensive documentation will be provided in subsequent Sandia Quarterly and topical-reports.

Figures 17-21 illustrate the threat to Sequoyah from hydrogen gen-eration during a small break LOCA (S2D). Hydregen and .nteam are released from.the primary system into containment. The. hydrogen . is ass umed to ac-cumulate without burning. Hydrogen production begins approximately 61 minutes after the break (Figure 17).. About 800 lbs of hydrogen are gen-erated prior to core slumping, which occurs at 93 minutes. Hydrogen is generated extremely rapidly following core slump, but for this interim study, significant core melt will not be considered.* The average rate of hydrogen production during the " degraded core" fraction of the acci-dent is about 25 lbs/ min. Figures 13 and 19 show the mole fractions of hydrogen in the lower and upper compartments, respectively. We see that detonable concentrations are reached in the lower compartment, and con-centrations of about 9% in the upper compartment, prior to core slump.

Figure 20 illustrates the very large steam fractions reached in the LC;

• We have reason to believe that_ MARCH may be overly conservative in the post-slump period, both for numerical reasons and because of physi-cal assumptions.

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Figure 21 shows that the ice condensers have been very effective in re- -

ducing steam concentrations in the UC.

Figures 22-26 illustrate a similar no-burn calculation for a large break LOCA (AB). The hydrogen release begins much earlier in the acci-dent, 2.5 min (Figure 0;, Hydrogen is generated for about 20 min until core slump. Seven huncred pounds of hydrogen are released during this period, with an average generation rate of 35 lbs/ min. As with the small break, detonicle concentrations of hydrogen develop in the LC for the degraded core portion of the accident. Hydrogen concentrations i.- the UC, however, are lower than for S 2D (Figures 23 and 24). The efficacy of the ice condensers in reducing steam fractions is again evident in Figures 25 and 26.

Many calculations were performed where the hydrogen was burned when its concentration exceeded a given number, for example 8 or 10%. Fig-ures 27-32 illustrate such a calculation for an igni: ion concentration of 101. Multiple burns occurred in the LC, spaced approximately 5 min-utes or less apart, Figure 27. Pressure rises were very small, Figure

31. No burn occurred in the UC, since the concentration there never reached the 10% ignition point (prior to core slump, of course). Fig-ure 29 shows that the steam concentrations frequently reached concentra-tions high enough to inert (approximately 40 to 531, depending on hydro-gen concentration, initial temperature and pressure; see Figure 10) .

The highest steam concentrations, however, tended to occur both before and after the major hydrogen production period for degraded cores. The capability to recogni:e steam inerting and prevent burning under those conditions was programmed by Sandia and incorporated into the MARCH log-ic. For these calculations, 56% was selected (input number) as the steam inerting concentration. Ref erring to Figure 10, and the discusaion on IDIS, we can conclude that this selection was not necessarily conserva-tive. The S 2 D calculation with 10% burn indicated that steam inerting did not occur. This conclusion needs to_be regarded with skepticism,

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since it requires a significant precision in the MARCH calculations of

," hydrogen and steam inventories as functions of time - a precision which .

MARCH may not possess. Furthermore, the calculation did not employ the lower steam-inerting concentrations that would prevail at high ambient-temperature and low hydrogen concentrations.

Figures 33-33 illustrate the AB accident behavior under the assump-tion of burning at 10%. Again, no significant pressure rises occur due to combustion in the LC prior to core slump.

One final calculation will be reported here; i.e., a small break LOCA with both fans inoperative, and 10% burn criterion. Figure 39 shows the hydrogen concentration in the lower compartment. Note that it has reached nearly 30% prior to core. slump; i.e., although burns were permitted at 10%, they did not occur. The reason for this is evi-dent in Figure 41 - for this calculation, the LC was stean inerted (con-centration in excess of 56%). This LC inerting permitted the concentra-tion in the UC to steadily increase, as shown in Figure 40. Because cf the ice condensers, the UC still had a relatively low steam fraction (Figure 42).

In summary, our MARCH calculations qualitatively support the pre-vious work performed by BCL and Westinghouse-TVA. The important con-clusion is that deliberate ignition in the lower compartment appears to be teneficial for large and small break LOCAs for degraded core acci-f.ats.

We should note, at this point, that the agreement is qualitative.

We believe that all accident calculations to date, including ours, have been performed under stressful conditions with short deadlines. Such

. conditions are not conducive to producing reliable and error-free re-suits. We believe that-the continuing hydrogen _research program at Sandia (supported by the NRC) will provide a higher level of quality assurance for these results. We would also like to note some important discrepancies which <e uncovered during this work. From various scurces,

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we have seen zirconium inventories quoted for Sequoyah of 41,993, 43,000 ,

and 50,913 lbs. Such dif ferences lead to large dif ferences in antici-pated hydrogen generation. These differences may be due to heated and unheated sections, but no reference has ever stated this. They may also be due to 17x17 bundles replacing 15x15. We are also not convinced that the Sequoyah FSAR provides the most reliable information, since recent TVA reports sometimes disagree with the FSA.7 Containment volumes also vary significantly depending on source. PSA2s, FSA Rs , I&E documents and various references have provided us with the folicwing discrepant information.concerning volumes:

4 upper compartment including ice condenser: 397900, 355000, 303500 cu. ft.

ice condenser volume: 131380, 157500, 125000 cu. ft.

lower compartment: 387700, 383000, 2S9000, 237000 cu. ft.

total containment: 1.236, 1.284, 1.192 million cu. ft.

1- Part of the discrepancy may be due. to the ambiguous definition' ofE " dead . .

ended" volumes.- This could not explain the differences in ice conden-ser or total volumes. We believe that it is essential for TVA to pro-

-duce a document providing ' the latest information on Sequoyah including

. volumes, geometries, =irconium masses, final number and location of

glowplugs and additional pertinent. data. If the Sequoyah FSA2 is in-deed the most reliable source of data, then all researchers should ce persuaded to use the same set of initial conditions.

L

- e

1.2 Answers to Questions

1) What ignition strategy should be followed: (on continu-ously?, turn on for accident?, or turn on for specific times?)

4 An extrapelation of a simple core boildown calculation indicated that it would be very difficult to generate hydrogen earlier than 10 minutes after a break. However, a large break LCCA calculation with MARCH indicated that hydrogen would begin to appear in less than 3 min-utes. Therefore, for an all-or-nothing automatic deliberate ignition system, the igniters should be activated very early in an accident -

perhaps at trip time or ECCS activation. There is no reason to keep the glowplug igniters on continuously.

It is our understanding that the Sequoyah IDIS system is an interim mitigation scheme. At this stage of our research, however, we feel that an operator-controlled. system would be preferable in the long ' term. The

- system should pe rmit. the '. selective . activation of . some. igniters and the deactivation of others. If the operator can monitor, in real time, the hydrogen concentration as a function of position and time, selective 10-cal ignition of low concentration pockets could greatly improve the safe-I ty and efficacy of deliberate ignition and decrease the possibility of i

l accidental overpressurization. It is essential that the reactor opera-1 i

tor understand the principles of - deliberate ignition, the hydrogen con-l-

l

(- centration limits for which upward and downward propagation are possible, and the' desirability of lean mixture combustion over an extended period of time.

2) For degraded. core accident scenarios (short of core melt),

will ignition. avoid containment threat?

For almost all degraded core accident scenarios in'the large, dry .

PWR containments, we believe' that del'iberate ignition of hydrogen would 1 . . . ..

be beneficial, even for 100% metal-water reaction. For Sequoyah, how- .

ever, the smaller free volume and the weaker containment combine to make deliberate ignition an imperfect mitigation scheme (assuming it would f l be possible to minimize or eliminate accidental ignition sources) . As-

} suming a failure pressure of 45 psig, thy containment could nor, survive i

the adiabatic, isochoric, complete combustion of 9% hydrogen by volume in air (about 30% zirconium oxidation) . For deliberate ignition to be successful, uniform concentrations less than 9% need to be burned,

and at a rate and frequency sufficient to permit the normal heat removal mechanisms to operate (to prevent pressure buildup). Lower compartment burns enhance the efficacy of deliberate ignition because of pressure rise' reductions from heat removal in the ice condensers and expansion into the upper compartment. Upper compartment burns, however, can ap-preach the maximum. adiabatic, isochoric pressure rises. A real-time hydrogen concentration r cnitoring scheme coupled with the intelligent application of-deliberate. ignition. by well-informed operators

can re :

i

' duce the probability of accidental, unwanted ignition. The system

could also be augmented with an intentional flaring system near high point vents on the primary system.

l Although we believe that the IDIS is not the complete. solution to the hydrogen problem, we do expect it to be beneficial for most types I

i of degraded core accidents.

i

3) Are there negative aspects to deliberate ignition versus I

j existing potential ignition sources?

L To th'e best of our knowledge, no. engineering study has been per-t iformed -to evaluate the possibility of accidental ignition, nor to con-i sider means-to reduce' that possibility. .The absence of such informa-

-tion has not prevented strong opinions from1 eing b made both for and against the-likelihood of unintentional-(non-deliberate) ignition. In I

L the short. period of this - study, Jwe. have not investigated ~ this problem,.

~

but strongly suggest ~ that this matter be pursued. The que5 tion we can address is: Are there situations under which deliberate ignition could be harmful? The answer is yes.

If the lower compartment is either steam- or oxygen-inerted, then hydrogen concentrations could build in the upper compartment, where com-bustion could be more dangerous. If the hydrogen is released from the primary system at an extremely rapid rate, the igniters in the lower compartment could be overwhelmed and large concentrations could develop.

It is also possible that pockets of high concentration could develop, which might detonate. One especially dangerous scenario could be a re-covered loss-of-power accident. That is, for a period of time, the de-liberate igniters would be deactivated, while very high concentrations -

of hydrogen develop and uniformly fill the containment. When power is restored, all the igniters could come on simultaneously, thereby guar-anteeing a large deflagration or detonation. (Note that an operator-controlled system would not_be forced.into this situation.) -

If accidental ignition is both inevitable and unavoidable, the IDIS system is clearly desirable. However, a future study may show that there are only a small number of potential ignition sources in containment, and that these sources can be eliminated (perhaps by spark arrestors or simi-lar devices or enclosures). The new question, then, is whether deliber-ate ignition is preferable to no ignition. We cannot answer this ques-tion now.

4) _ Calculate pressure rise for partial combustion in H2/

air / steam mixtures-and compare this to literature data to estimate completeness of combustion as a function of H2 concentration.

At present, there is no way to compute the pressure rise due to par-tial combustion because we cannot compute the f raction of hydrogen that

_will be consumed. Given the fraction of hydrogen consumed, computation

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-~

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REFERENCESFOR m

" Hydro r.=~,T,e- r -:

LR.C., gen Control DELIBERATE . m:. '.

Aug. IGNITION --?.D.g _ :

13, 1980for Sequoyah Nucl .q;..g -W . -~

. E. Blejw as, .;.y :C,}.&

ear Plant,

.,3T ~ . . . -

cGuire Con tainmen Ul t V" Yield andUnits 1 and 2 " 54.ii>y;:/p: ,,

india Laboratorie essels,"timate 10, LetterPressures L. Furno s, July to W.

E 1980 te rva t i ons, on. 3. . C 16 A. Von Riesemannof "..Sequoyah

.. r .- + 6 Q :., D a Inst., Near ook,

• . w  ::r

9. .:

.e n.e Limit J.F M. Kuchtaand D S93-599, -

O. Slifer and 1971) . lanes, " 13 Svm. . S. Burgess, I i

,, D ...j.1-- '

Characteristicn SWR ral T. G.

i Pe te rson, on Comb. "Some }- ..,.% E:~.

~'

(Pittsburg, I Electric (April Cont " Hydrogen Fl -

. Harrison, M. 19 73) . ainmen ts, " NEDO-10812ammability and

>gon smant,"Combustion StudiTamm, R. MacFarl , 73NED49,Burn-1980 tute, 1980 ~

Western States Ses Relatedane, L. S.

to Nuclear RClegg,

~

Cow  :

of ard and F. ection Meeting,eactor Safety" Canadian -

imits. Gaseous MixturesBrinsley, Combustion Part II. Part I."The Dilution mits of Li bon Monoxide The in r,"

Low Ai er Li J. Chem.mits Soc. for HydrogenThe n

of Dilu- Determina tio .-

ton, A. S. Go d (105: , Mett.an ion of ga -"' #[u..

~',~

r on Hydrogen and 1859-1885 ) , e ,

~

and Comb. & Flame 33 Methane Flam- F. A. Williams, -

2ko, A. :33-45 (1978). es in Oxygen "Nea r--L , 3- ...c..-:- .

L. F imit Downward g

g ,

w-l*<*"-YE+-M . --" T*^

nt of Large urno, and Ni tr0 gen Mix-

ng 1, 1976 Requirements," 4BScale CH -Air-NJ. "Fla M. Kuchta, i

ureau 2 Expl  :

of Mines,osions,meand Pressure t g >'-a " . ,- c #' k Report ofBuoyancy Inv estiga-Effects son, R.

scion and M. Knight ~ .

Propagat, and J.

.thout 973). Water Spray, ion in Various "Fla HO. Henrie,

" AI-73-29, Atomydrogen and me , .

To Air Detona-and T. R Int. Div. Mixtures, to PWR ' , . . s :- ~'.

me , Rockwell .

er 195Loss. Moffette, Hvdrocen Fl . ,/ '.

7) . -of-Coolant .. 1 tnd F. Accicent, ammability D g", .

WAPD-SC-54$ a and

, Bettis ta . .-

R 1383, NationalE.evSelles, of Surv  !

,.7-Advisory CommittHvdrocen Combusti

~

. v I.

on Proo- .c, (New York:on Elbe, Comoustio ee for Aeronautics ,

l ixolosiv Academic Pressn,and ,

Flames . Mg*gk.,g , x ../I -&:. ~ ~-%'c. -

" ~ . h. 4 1961).Exolosions of

• A. Cox,e Shocks in Air k_ ,' ,P,,, gp,,,-- ~ ,. g .p. Lv- "

P. S.

m Exolo ien MazardsWestine, s

J. Ku J., Macmillan, 1953 ., . w gg@

._ - s, 1"

Evalua tion,(Southw lesz, R. A. Strehl ov, w

~~ est Research /

• * . . . . .. r--

- ' 5+- .

' k'a-- -.*******w==~-~~.;,a"~

-o.-

.= V f.,L k_f** ef ..

,w.-~* '

- a . _.

REFERENCES (cont. ) -

15. Sequoyah Nuclear Plant Hydrogen Study, Vol. I, April 15, 1980, Re-port Issued Septencer 2, 1980, Tennessee Valley Authority.

16. Statement by ACRS Chairr.an during ACRS meeting with NRC Chairman i

and Commissioners on September 5, 1980.

17. S. L. Thompson, CSQ -- A Two Dimensional Hydrodynamic Program with j Energy Flow and Material Strength, San.ila National Lacoratories, SAND 74-0122, (August 1975).

f

(

b

.' 2.0 WATER FOGGING 2.1 Introduction and Discussion Fogging is the suspension of a large mass of liquid water in the form of small droplets in the containment atmosphere. The fog acts as a large thermal capacitance, greatly reducing the temperature rise which would otherwise occur as a result of hydrogen combustion or steam re-lease from the reactor vessel. Figures 1 and 2 show the theoretical temperature and pressure expected for the complete, adiabatic, constant-volume, combustion of hydrogen: air mixtures including the evaporation of added droplets. (The calculations neglect combustion-product dissocia-tion, and hence somewhat overest'imate the temperature and pressure at high concentrations, a region of no interest in dequoyah safety studies since the high pressures there will surely overpressurize containment.)

Table I illustrates the large pressure reductions which would occur.if 0.05% water droplets (fog). were suspended in the hydrogen: air mixture prior to combustion.

Taole I Hydrogen Concentration Required to Attain Given Pressures Hydrogen Volume Percent 30 Fog Pressure Complete Incomplete 0.05% Water (psig) Combustion Comoustion Croplets 31 6 8-9 15 45 9 9 19 The complete, adiacatic, isochoric comoustion of 9 volume s of hydrogen homogeneously' distributed throughout containment would raise the pres-sure from 0 to 45 psig, as shown in the table. Lean mixture incomplete cembustion would not change the results appreciably, since combustion is'quite efficient at this concentration. However, the addition of a g _

INITIAL CONDIT!CN$:

T = 298 K = 25 C

. P = 100 kPa a 1 ATM. p STOCH!CM E*RIC 100% RELATIVE HUMICITY RATIO 7- l uk 1

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g 0 4 8 12 16 20 24 2S INITIAL HYORCGEN CONCEN*RATICN. Vlc

' Figure 1. Rati: Of final pressure to initial pressure as ; functica of initial hyd: qen ==ncentratien and velur.e fracti:n of water drops.

" - * - -- - --m _ _ _ _ _ - _ _ ___ _-_-a

e i

INITIAL CONDITIONS:

T = 298 K = 25 C 2500 -

P = 100 kPa a 1 ATM.  :

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1000 -

oh

• o?

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- INITIAL TEMPERATURE I

0 O 4 8 12 16 20 24 28 INITIAL HYDROGEN CONCENTRATION, Vlo i

Figure 2. Final te::1perature se : function of initial hydrcgen concentratica and vol=:te fraction j of water drops

- a . . . .

fog (0.05% by volume) prevents that large a pressure rise (45 psig) until, the hydrogen concentration reaches as high as 19%, corresponding to more than 70% metal-water reaction. The benefits of water fogging are both obvious and significant. For fogs with concentrations lower than 0.05%

by volume, the benefits would, of course, be proportionally lower, as shown in Figure 1.

An experimental study of the ef fects of 0.054 water drops on hydro-gen: air combustion showed the expected reductions in pressure rise due to deflagrations, and the suppression of detonations up to the highest

! hydrogen concentration tested, i.e., stoichiometric.(1) The suppression of detonations could be very important for large dry PWRs, where the strength of the containment is much higher. For the ice condenser con-tainments, only a local detonation in a hydrogen rich pocket would be of interest. All other detonations will lead to containment failure.

The fog would be generated by spraying droplets into the contain-ment atmosphere sufficient to suspend a large mass of water in a finely -

divideo state. Methods are available for doing this. We have also just begun to address the use af foams. Foams may have advantages over drop-lets in terms of engineering and design requirements for replenishment.

KINETICS OF DROPLET 9APORIZATION To achieve the benefits of pressure and temperature reduction from fogging ( Figures 1 and 2, , the droplets must be small enough to vapori:e not f ar behind the flame front. If the droplets are too large, their vaporization would-be delayed. This could result in a large region of higher temperature burned gas corresponding to the curves in Figures 1 and 2 for no droplets. For these overs :e droplets, pressure rise dur-ing combustion would initially overshoot -the equilibrium value, and even-tually achieve' equilibrium as the droplets vaporized. The overshoot in pressure could last long enough to generate higher static loads on the containment structure. On the other hand, however, if the droplets are j . .

- a .

too small, they could vaporize inside the thin (~1 mm)( 2) flame front ,

and inhibit or even quench the flame. For these reasons, the optimum droplet size is bounded both above and below.

The time required to burn the atmosphere in containment will be of the order of the characteristic length of the flame path divided by the effective flame speed. Typical flame paths could vary from 10 to 40 me-l ters. A discussion of flame speeds to be expected in hydrogen: air com-t bustion can be found in Refs. 3 and 4. The effective mean flame speed is expected to be higher than the laminar flame speed which is below 1 m/s for lean hydrogen mixtures. Assuming a flame speed of about 2 m/s ,

.the time for combustion would range from about 5 to 20 seconds. For this analysis, we will assume that the fog would be effective in re-ducing pressure if.it vaporized in less than 1 second.

The rate of vaporization of the fog is.primarily dependent on the following quantities:

i

1) ~ Mean droplet diameter;and size distribution,

2) Ambient temperature, 3). Relative speed of ambient gas to droplets,

4) Composition of ambient gas. .

We examined a theoretical model of the rate of vaporization of water

. droplets caused by a flame propagating past the droplets. A pictorial representation of the model is shown in Figure 3. Vaporization begins when the flame impinges on the droplet, and a steam film composed of

-water vapor, air, and combustion products forms around the droplet, dif-fusing outward. Because.cf the thermal expansion of the burned gases, a relative velocity will develop between the droplets and the surround-ing-burned gases. The relative motion of the gases will distort the surrounding steam film.and-increase the rate of heat transfer to the

-. droplets'and hence. increase the' rate of vaporization.

The steam film surroundingLthe droplet grows with-time and finally mixes with the

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combustion products as the drepret is ultimately consumed.--The vaporiza-

- tion time for the droplet is defined as the time between flame impinge- -

ment and complete vaporization of the droplet.

Details of the mathematics of this vaporization mocel can be found in Ref. 5. Shown in Figure 4 are the vaporization times of droplets with initial radii between 1 and 10 4 m for several values of initial hydrogen mole fraction. Note that droplets of initial radius less than 200 m will vaporize in less than 1 second. A 200 sm radius droplet is a fairly coarse droplet and is easily obtained by many different types of spray no::les.

We will not examine the possibility of the fog vaporizing in the thin flame front. The thickness of a hydrogen laminar flame front is of the order of 1 mm.(2) For a flame speed of 2 m/s, the residence time is of the order of 0.5 x 10-3 seconds. From Figure 4 we see that droplets of initial radius less than about 4 m will entirely vaporize in the flame zone.

Another critical aspect of flame propagation.within a, cloud of water droplets is the possibility cf quenching; i.e., droplets acting as heat sinks and extracting the thermal energy generated by the propa-gating flame. As the droplets become closely packed, the area available for energy loss increases. A critical spacing of the droplet field ex-l i

ists such that a large fraction of the heat released is absorbed, thus preventing flame propagation. This spacing is called the " quenching l distance". This parameter is experimentally determined by propagating flames in tubes, and for.these geometries the quenching distance is de-l termined by:

i dq = [4Ve/S T l eritical where 7 = combustion gas volume and S; = heat transfer surface. area.

For a cloud of spherical water droplets of a given water content

by 'solume) , this volume-to-surf ace ratio is

h:

~[ 11 - Air mixtures .05% H 0,.2q f

.. 1  % relative humidity . tx g =0.17 y S:-

(xH) -initial 112fraction (O.14 e 2 / (O.108) 3 g hg (0.075) 5 flame zone vap. /

5 / '

., ~. basedon (0. 0 57) .

Y fE 0 . Sg"100. cmIsec /

/A

. Vaporization Time (SEC) l l

(

t l

s'i g u re 4. Vaporization 'I'ime vs. Droplet Ita<lius at Various Comlauntion Stoictiiomet ries

4Ve 4 pri _ n )

S. .

a n where R = mean droplet radius a..d n = volume fraction of water. When this ratio becomes of the same order as d q, the heat transfer effects due to droplet spacing approach a critical limit. A critical droplet radius is reached when:

3 nde Ec

• I ( 1 - 7 )

Droplet arrays which are composed of droplets with radii greater than Re will allow free propagation of the hydrogen flame. Figure 5 is a plot of the critical droplet radius for various water contents of the fcq and two values of combustion stoichiometry. (Data for quenching distance 7

were taken from Lewis and von Elbe.(6))

If the fcq is made such that the great majority of drople:s are larger than 5 m in radius, then we do not, expect -the droplet vapori:a-

tion and droplet spacing to significantly affect the flame structure and

[h hence influence the fla=mability limits. Fogging could enerefore =e used in conjunction with deliberate ignition.

The predictions of the vapori:ation model have been checked against available experimental data (7) for isolated water droplets evaporating in heated air. ' Figure 6 shows this ca=parisen and our predictions of vaporization time agree to within a few percent. (The long vaporization times seen in this figure are due to the large droplet size and low am -

bient gas' temperature used.in the experiments.)

The model predictions'also agree with en experimental work which examined hydrogen flame propagation in a vessel with water spray addi-tion.(1) Using a mean droplet radius of 250 um (as was determined in the experimental work) cur model would predict a vaporization time of 0.5 sec, well within the time at which the pressure rise was observed (this corresponds'to the time required-for the flame to propagate across the 1

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l Figure 5. The Effect of Drople: Spacing on Flame Cuenching 1

I l

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heated iir

• REF:'Evaacrc:n...cf Sir: qts Ore;tet at Elevated Pressures and Temperatures.Hircyasu and Kadota. Tran., Jap. So.M.E.

, - 40.(1974) t-200.. 300. 400. 500.

Ambient tem;erature 'C Figure 6. Comparison Between Theoretical Vaporization Time and Experimental Cara on Isolated Droplets R . . . . . .

16 % H 2

40.

2

n ,

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e.

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y... .y ..__....-r e . . s = is a i. is to time (SEC

- - Vaporization t!=e fer 500gm dia, water dt:;feu REF: '7tarne and Detnation initiation and Pr:pagatien in Various Hyfragen-air mixtures with and eithout nater scrrf'. L W.Cartsen, R. Knight,and J. Henrie.Roctaell Intern repcrt Al-7}-29.1971 Figure 7. Prediction of 500 gm ciameter Droplet in Hydrogen-Air Combustion Environment

- a . _ . .

. vessel). Figure 7 shows the pressure vs. time Of a tes; at 15% 1n.:131 -

hydrogen concentration and the =cdel prediction of vapori:ation time is the barred interval on the time axis.

PROCCCTION OF WATER FCGS Based on the ther:0 dynamic calculations, a water volume fraction approaching 0.05% mav. be needed to sufficien:lv reduce the c.ressure and temperature following cc bustion. For a 3.4 x 104 =3 containmen: vol-ume, a water centent of 0.05% correso.cnds to 4500 c.als of water .claced in suspension. Since a lar e e n uantity of dreple:s with radius less than 200 p= =ay be required, a high output zerosol generation device must be used.

Fine mists can be generated in lar;e quantities using no::les er mechanical a cmi:ation devices. Tve.ical cutputs frc these svstems

. are usuallv. less than 10 c.als/ min (6) and thus T.an'I no::les will be needed.

Conventional no::le systems produce c.olv. dispersed scrav.s (varv.ine. dro -

let sizes) with a leg-ncr=al distribution of d: ple: si:es centered about a geccetric mean radius of 50 gm. Sprays with smaller droplets (10 g = ean radius) can be produced using speciali:ed no::les with swirl and air blasting.(3) Fogs with very small droplets, less than 1 um radius, are difficul: to generate in large gusntites.

Since no::les have small discharge areas, large pressure frops across the no::le crifices can be anticipated. For sufficient a:0=i:a-tion, typical pressure dr:ps through pressure no::les are 100 psi.(1)

The jressure drop across a "N' orifices is calculated using the fol-lowing relationship:(2)

-2 I M

- ~

E g

- Cn as w

e a where bb-

a . _..

? = pressure d op across the nc:zle syste:

O = liquid water density Q. = total outptt of water (volume flow rate)

A = no::le discharge area of a single orifice C3 = discharge coefficient = 0.3(3)

Consider a total water flew of 104 gal / min input through 5000 orifices of 0.1-inch disaeter; the calculated pressure drop is 70 psi. (If 1000 orifices were used in this calculation, the pressure dr:p would be

~2000 psi.) Therefore, we conclude that, for pressure ateni:ing no::les, a large number (>5C00) of orifices will be required.

Probably a superior a:c=i:ation scheme is the use of air blast ne:-

les. Water can exit through reasonably large no::les (1/4 inch diam-eter or larger) with very 'cw pressure drop and with high resistance to clogging. .The atomization is acccmplished by an impinging high-speed compressed-air jet.

FCG MAINTENANCE AND STA3::.!TY Although high volume production of small water drep t is possible, stable, high-density fogs may be dif ficult to =aintain if :here are very large settling losses. . Estimates made under the ass.=ption that d ps do not interact and ccalesce, yield loss rates far belaw these given by a more ec=plete formulation of the problem. The range of these losses, when drop interactions are and are not considered, extends from =cderate values (<100 gp=) to values so large (~100,000 gp=) that fogs of accept-able density cannot be attained given reascnacle injectica rates. Thus, the mechanisms affecting losses from containment must.be examined =cre

. closely to determine the feasibility of maintaining a high denst y fog.

Available nc::les do nor creduce menodispersed (single si:e) sprays; rather, they produce polydispersed sprays covering seme range of drop si:es.- As the constituents of a polydispersed fog settle, larger drops,

~

with higher terminal velocities, will evertake, collide and coalesce-with smaller ones, having relatively lower terminal velocities. This is -

callr J gravitational coagulation (12,13) . The result is to increase the mean drop size and therefore to increase the rate of water loss. Because the effect is so significant, coagulation plays a critical role in fog maintenance and stability.

$ost calculations of aerosol coagulacion are based on scme form of the " geometric sweep" concept (14) of collisions; i.e., if any portion of a drop lies within the volume swept out by another drop as it falls some distance, then the two will collide. This method gives too high a rate of collision and so the swept volume is modified by a collision effi-ciency, Ec, to account for close-proximity hydrodynamic interactions of the two drops. A commonly used collision efficiency is that given by Lindauer.(13).

An efficiency of zero (Ee = 0) yields no collisions and hence no coagulation, while a value of unity (Ee = 1) returns the geometric sweep collision rate. The actual value must lie between these extremes.

Figure 3 shows losses from containment for two values of the colli-sion efficiency: ero and that given by Lindauer. Calculations are basec i

i on a widely accepted formulation of gravitational coagulation (16,17,13)

(

and drop settling.(19) The containment volume, V e , is initially empty and the uniform injection rate is constant at 310 kg/s (5000 gpm).

! Note that the effects of coag. atio. .re very pronounced. At less i

than 60 seconds, the loss rates when coagulation is considered (E =

f

'Lindauer')'are roughly'two orders of magnitude greater than when they are not (Ec = 0). The dramatic consequence of this difference is shcwn in the instantaneous fog density, Figure-9.

When coagulation of drops is considerec, the water content peaks at slightly over 2.0 ac 10-4 g/cm3 (0.021 by volume) in less than 30 seconds; this is the highest attainable density for the stated conditions and is c ly marginal fer a successf ul mitigation' design. Af ter this time

.the density oscillates regularly with seme' decay in. amplitude.

The long-n -

10' Injection Rate -

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4 7 S 10'=:

• o

= 'Lindauer' M

W M

M O

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E =0-

". c 10' O.0 1d.0 24.0 36.0 4$5.0 60.0 TIME (S)

I l

Mean Drop Radius = 10 pm Log-Standard Deviation = 1.2 m

InjectionofRate Volume = 310 kg/=s3.4x104m Containment (5000 g5 )(1.2x10 6 gg3)

Height of Containment = 40 m (130 ft) l l

l Figure 8. Settling Losses for Steady Fog Generation - Comparative

! Effect of Collision Efficiencies l

l

-a6-

i I

O e

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, A l M l 2 4.0 -

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0.0 12.0 24.0 36.0 48.0 60.0 TIME (S) l l

Mean Drop Radius = 10 um '

Log-Standard Deviation = l.2 Injection Rate = 310 kg/s (5000 gem) 6 volume of Containment = 3.4x104 m3 (1.2x10 ft3)

Height of Contain=ent = 40 m (130 ft) l t

l Figure 9. Water Content for Steady Fog Generation - Concarative l Effect of Collision Efficiencies t

I i

i t

time behavior is shown in Figure 10. In contrast, when coagulation is not considered the density continues to grow almost linearly for several hours. The asymptotic water content in this case is about 2.5 x 10-2 gf em3 ( 2. 5 % by volume) attained .. around 5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br />. Note that this value is over 100 .imes that of the maximum mentioned before.

As a means of decreasing losses, we have examined the ef fect of the mean drop size being injected. With no coagulation, halving the mean would result in approximately a four-fold decrease in the settling losses and a corresponding four-fold increase in the steady-state fog density.

As shewn in Figure 11, however, when coagulation is considered, the mean drop size being ic]ected has almost no effect on the water content; be-yond an initial transient, the quasi-steady density is nearly the sa=e for mean drop sizes in the range 10 to 50 gm - again, coagulation is clearly dominating the dynamic behavior. Similar comments apply to the effect of spread or log-standard deviation of the drop si:es being in3ec-

~

+

ted. Figure 12 shows:that a wide range of log-standard deviation, 1.2'to

, 1.4, does not significantly alter the quasi-steady fog density. These somewhat surprising results indicate that generating very fine drops of j nearly uniform size will neither significantly reduce losses from con-i tainment nor increase the fog density.

I Finally, the role of the injection rate has been examined for rates r

in the range 62 to 620 kg/s (1000 to 10,000 gpm); some results are given in Figure 13. Considering the value of the water content at the first maximum to be a characteristic measure of the fog density, the density i

I appears to grow as the injection rate to.the one-half power. This again may be contrasted with the case when coagulation'is not considered, in which the water content would grow linearly with the injection rate.

i.

Thus, merely increasing the injection rate is not.an effective way to achieve significant increases in the fog density.

In summary, we find that the maintenance of high fog densities re-q uired to substantially mitigate the effects of hyd:cgen combustion may I

O

- - 5.0 e

m M

y 4.0 -

0 N

O v

g 3.0 -

Z w

Z O 2.0 -

O Qll w

< 1.0 -

0.0

'0.0. 3b.0 6b.0 9b.0 12'0.0 15'0.0 TIME (S)

I

(

Mean C cp Radius = 10 ;.:n Log-Standard Deviation = 1.2 (500 Injection Rate = 310 kg/s3.4x10ggm)5 m (1.2x106 f:3)

Volume of Centain=ent =

Height of Containment = 40= m (130 f t)

Collision If ficiency -(I c) 'M.Mauer' Figure 10. Long Tine ~4a er Content for Steady Feg Generatica 1

7 o

- - 5.0 e

m 2 4.0 -

O N Mean Drop e

v Radius (um) 3.0 -

Z w

b-2 10 0 2.0 -

U 20 w

< t.0 -

/ - 7 3 30 50 0.0 , , . . .

0.0 12.0 24.0 36.0 48.0 .60.0 TIME (S)

Leg-Standard Deviation = 1.2 Injection Rate = 310 kg/s (5000 gem) 6 volume of Centain=ent = 3.4x104 =3 (1.2x10 ft3)

Height of Contain=ent = 40 m (130 ft)

Collision Efficiency (E c) = Lindaue:'

Figure 11. Water Content for Steady Fog Generation triation with Mean Radius of Injected Drops l

\

O

- - 5.0 e

A m

2 4.0 -

U N

8 3,0 -

Leg-Standard Deviation Z

w F-Z O 2.0 - 1.2 U

l.3 Ct:

w 1.4 P-

< 1.0 -

0.0 , , , ,

0.0 12.0 24.0 36.0 43.0 60.0 TIME (S)

Mean Drop Radius = 10 pm Injection Rate = 310 kg/s (5000 70m) 6 volume of Containment = 3.4x104 =3 (1. 2x10 ft3)

Height of Containment = 40 m (130 ft)

Collision Efficiency (E e) = Madauef Figure 12. Water Content for Steady Fog Gener icn - Variation with Log-Standard Deviation of Injec:ed Crops a . .

l i

l C

g - 5.0 m

M 2 4.0 - Injection Rate U [kg/s (gpm)]

\

O V 620 (10,000) g 3.0 -

Z w

Z O 2.0 -

O g 310 (5000) w 3 62 (1000) 3 0.0 , , , , ,

_ 0.0 12.0 24.0 36.0 48.0 60.0 TIME (S)

Mean Drop Radius = 10 pm Log-Standard Deviation = 1.2 4 3 0 Volume of Containment = 3.4x10 m (1.2x10 ft3)

Height of Contain=ent e 40 m (130 ft)

Collision Efficiency (Ee ) = M ndauer' rigure 13. Water Content for Steady .ro g Generation - Variation with Injection Rate

.w ,

be difficult to achieve. The mechanics of drop collisions, and e v iticu .

-larly the coalescence of droplets in high density fogs, are crit::,ai .c; determining the losses from the fog. The mechanics of droplet collisions will require further investigation, both theoretical and experimental.

We will also be considering the addition of surfactants to increase sur-face tension and possibly reduce the losses from the fog.

THE EFFECTS OF FOGGING ON DETONATIONS AND SHCCK WAVES Carlson, Knight and Henrie(7) found that the use of a coarse (250 m radius) liquid droplet spray of volume fraction 0.05% suppressed detona-tions for hydrogen: air mixtures up to stoichicoetric in a 16-inch-diam-eter horitental tube. The droplets act to dissipate energy from detona-tion and shock waves both by mechanical action of the drop drag,(22) and by the thermal action of droplet evaporation in cooling the gas.

The possible beneficial effect of fogs in preventing detonations may be of;1imited interest in the Sequoyah safety study. If a hc=ogene -

+ ous detonable mixture burns without detonation, the containment will not-withstand the resultant static overpressure, even with fogging at 0.05 vol.%. For the stronger large, dry PWR containments, the suppression of detonation is more relevant. However, it may be possible to withstand the dynamic effects of a detonation of a locally rich pocket of gas in Sequoyah. When the detonation wave leaves the detonable region, it be-comes a shock wave of ever decreasing strength. The rest of this sec-tion of the report will address the following three questions:

1) What is the effect of fog on detonation waves?

2) What is the ef fect of .og on transition to detonation?

3) What is the effect of fog on shock waves from local detonations?

THE EFFECTS OF FCGS ON DETONATION WAVES Pierce (23) in his model of snock wave propagation through sprays, considers that the droplets evaporate behind the shock front in a

-dE-

distance negligible compared to the shock wave radius of curvature. If

- this is the case for either shock waves or detonations, we can model the wave as a discontinuity with the jump conditions across the wave given by the usual Chapman-Jouquet theory. We have done this for hydrogen: air detonation waves. The reduction in post-detonation temperature is sub-stantial, as seen in Figure 14. The reduction in pressure, however, is much smaller, Figure 15. Consequently, the use of fogging, if it does not prevent a detonation, will not greatly reduce the dynamic loads pro- i duced by a detonation.

THE EFFECT OF'FCGS ON THE GENERATION AND SUPPRESSION OF DETCNATION WAVES In the study of Carlson, Knight and Henrie(l) the water spray was able to suppress a detonation wave propagating into the mixture from an adjacent tube which had a stoichiometric hydrogen: air mixture without the spray. This is a more severe requirement than that needed to prevent the transition of a deflagration into a detonation.- Evidently, the dissipa-tion of energy in the detonation wave by the evaporating drops as enough to weaken and dissipate the detonation.

The transition from deflagration to detonation is believed to usu-ally occur as a result of the formation of shock waves ahead of the de-flagration and the resultant shock pre-heating of the gas. The effect of drop evaporation cooling the burned gas will reduce the expansion of the burned gas and hence weaken subsequent shock waves. On the other hand, the droplets also introduce turbulence in the flame tone ard increase the flame speed. The increase in flame speed would tend to promote detena-tion.

Obviously there any many unknowns concerning suppression of detona-tions by fogs. Further analytic and experimental work will be required to clarify the possible benefits of fogs-in detonaole mixtures.

e .

e. .

16.0

~

1417146 CONC 17!aNS

• f a 299 R 15.0 - *= 1 ATN. -

1002 R E L. . +0 "

~ O MUMIDITY 14.0 -

g e#*<,*c -

w .

a e

S 13.0 m

- '#e , -

w - ,

2 Q- 12.0 - .#,d -

J .

1 o#* -

. 11.0 -

z y 10.0 - 4 -

i m

e, *,+

m 9.00 -

w z .

Q.

a 8.00 -

z l w 7.00 '

10. 20. 30. 40. 50. 60.

( INITIAL HYOROGEN CONCENTRATION. VOL. PCT.

i l

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Figure 14. Etfect of Water Drops on resonation Pressures for Hydrogen:

Air Mixtures (VL is the vol.1 of droplets) l .

93-

a - _.

P a 3.0

• • +o x 2.8 -

O*o#

4 ei.# -

2.6 -

2.4 - "4 .

o 2.2 -

'o# # -

2.0 -

1.8 -

1.G -

8.oy, -

• 1.4 -

0

• y ff cc 1.2 -

o

.m. O g, y 1.0 -

w I

w

.80 -

>= ' ' ' ' ' ' ' ' '

.50

10. 20. 30. 40. 50. 60.

INITIAL HYOROGEN CONCENTRATION. VOL. PCT.

Figure 15. Theoretical Chapman-Jouguet Detonation Temperature for a mixture of Hydrogen: Air: Liquid-Water-Drops (VL is the vol.% of droplets)

, . l

~

l

' l

- s . -

THE EFFECT OF FCGS ON SHOCK WAVES l

. -1 If a cloud of gases detonates, the detonation wave after leaving l the detonable region will decay into a shock wave of decreasing strength.

(See the discussion in the " Local Cetonations" section. ) Pierce (20)

, considered a simplified theoretical model of the propagation of a shock I

wave from a point explosion into an inert gas with liquid droplets. For the same shock wave speed (Mach number) the effect of the vaporization of l the droplets is to decrease the temperature rise behind the shock wave, but to increase the pressure rise. The vapori:ation of the droplets ,

1 causes the Mach number of the expanding shock wave to decrease f aster l

than it would without the droplets. The result of these two ef f ects of i vaporization on the expanding shock wave is that the pressure behind the I

wave is larger near the explosion center, but soon becomes lower as the wava propagates further from the explosion center.

We are performing calculations to determine pressure and impulse from point explosions in a fog, and.will extend..the work, eliminating

> 1 some of the assumptions used which limited Pierce's =cdel to lower drop- '

let volume fractions. We will also extend the work to consider the ef-feet of a spherical volumetric detonation rather than a point explosion.

The Pierce model cannot be valid at lower Mach numbers because the vapor pressure of saturated steam is below the partial pressure of water vapor I if there were complete vaporization. At low Mach numbers the vapori:atior i will be incomplete or even nil.  ;

F i 1

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- s . -

2. 2 Answers to Questions -

1) What would the design concept of a water fog system be?

The practicability of the water fog concept we have been consider-ing depends on the ability to suspend large masses of water in the con-tainment atmosphere. If 0.05% liquid water by volume could be main-tained, we expect that the concept would work well. Aerosol codes which assume all drop collisions result in drop agglomeration, predict very large losses for droplets in containment-sized volumes. This would re-quire very large flow rates for the renewal of the fog. We believe that these loss rates are overly pessimistic; droplet collisions will not always re sult in agglomeration because of surface tension effects. The addition of substances to increase surface tension cenld further reduce drop agglomeration. However, until the loss rates can be more accurately estinated or measured, it is premature to design a fogging system.

Alternatives to fogging may be possible which also could suspend large masses of water in the atmosphere. We have begun to look at the use of foams. Foams have been used in airplane hangers for fire suppression.

It would not be necessary for the foam to be able to stay indefinitely in place, since it could be renewed.

2) How effective would water fog be in preventing a hydro-gen combustion or detonation threat to containment?

We know that droplets of appropriate size will evaporate soon after the flame front passes. Hence, the adiabatic constant-volume combustion calculations assuming complete droplet evaporation should be reasonably accurate in predicting final pressures. For 0.05% droplet volume frac-tion, Sequoyah could survive a 16% hydrogen deflagration - i.e., not ex-ceed the conservatively estimated 31 psig failure pressure. For lower volume fractions of droplets, the maximum hydrogen initial concentration e

would be lower. For 100% =irconium oxidation, deliberate ignition Oc=-

bined with fogging would be able to handle :ne hydrogen only if i: came out slowly enough for it to be burned over an extended period of time with cooling of the atmosphere taking place. If the burning was rapid enough to be nearly adiabatic, the containment would be overpressuri:ed, even with fogging.

There is some experimental evidence that fogging can prevent the propagation of detonation waves. This has to be examined further. Sim-ple . thermodynamic calculations of the jump cond : ions across a de:ona-tion wave, assuming complete droplet evaporation, indicate tha: :ne em-perature rise is considerably reduced, but that the pressure rise is re-duced much less. If the fogging does not prevent a detonation, it will not reduce the dynamic loads produced by the detonation sufficiently to prevent damage.

3) What water fog density and particle size are required to suppress containment threat as a funcAion of He/ .air / steam ratios likely to result from dominant accident scenarios?

From our studies it is clear that a very wide range of drople: sizes will be satisfactory for evaporation behind flames. The drople: si:e will be governed by considerations of loss by settling. Drople:

l diameters on the order of 100 gm appear to be satisf actory for pressure s uc.

. ere s s ion f but may be too large for limiting loss rates. We have been using 0.05) droplet volume fraction as a' goal since this was the value used in a previous study. Cur calculations, however, also indicate tha:

0.05)'is ressenable to achieve _significan: pressure reductions.

4) Assess the problem of maintaining a water fog in the pos:-

accident atmosphere.

The suspension of a large mass of water droplets in.the containment atmosphere for an extended' ime may be the most difficul: technical problem with fogging. The t. . .f computer codes (which assume all drop collisions produce agglomeration) indicate very large drople: loss-es, requiring very large wa er ficw rates to renew the fog. We believe that these loss rates are too high and that surface tension effects may limit agglomeration. This issue must be settled before fogging systems can be designed.

5) Discuss the pros and cons of a water fog system.

Pros

1. It will reduce the pressure and temperature rise caused by hydrogen combustions.

2. It will act to condense steam, reducing containment steam pressure during a LCCA.

3. It may prevent detonations. This issue is not yet re-solved.

4. It requires no new penetrations of containment. It may be simple and inexpensive to install compared to other mitiga: ion schemes.

5. In' combination with deliberate ignition, it will perma-nently remove hydrogen from the post-LCCA environment, while mini-mizing the overpressurization threat to containment.

Cons

1. It is an active system requiring pcwer to pump water to maintain the fog.

2. If the loss rate of drops is as high as predicted by aero-sol codes, then the water flow rate required to renew the fog may be impractically high.

3. In condensing the staan, it may cause a hydrogen-rich mix-ture to go from nonflammable to highly flammable.

-100-

- ~ . .

6) Will water fog reduce steam concentrations and yield a

. more co'mbustible or detonable mixture?

Yes. The fog will condense steam. If a high concentration of hy-drogen exists which has been inerted by steam, the condensation of that steam will produce a combustible mixture.

7) What steps are to be taken after the water fog is in place in a containment containing H /2air / steam / fission products? What are the final steps to recovery after a water fog has initially prevented or reduced the effects of combustion?

We do not expect the fog to significantly alter the flammacility limits. Therefore, fogs can be deployed in conjunction with deliberate ignition without interference. Deliberate ignition would be carried out until the atmosphere was no longer flammable. The. fog is used to reduce the danger of overpressurization to the containment by combustion and/or steam. When the hydrogen has been removed, the fog can be stopped.

8) How long will it take to install one?

It is premature to answer this question since the basic practicality of the idea is in question. If the loss rates are shown to be reasonable it should not take too long. It may be possible to modify the existing spray nottles and have them produce the' fogs.

- The following two ques.: ..ns were not part of the original NRC work scope. They were raised at later' discussion sessions -

9)- What ef fect will the existing water sprays have on. fog stability?

If water fogging is adopted, it should serve as a complete replace-ment for the spray system; i.e., the fogs will provide all the protection

-101-

- a - -

that the sprays did, in addition to greatly enhancing the hyd cgen com- .

bustion mitigation features. If, for some presently undetermined reason, it was desired to maintain both capabilities, and that simultaneous oper-ation could occur, the integrated effects can be analy:ed.

We have performed a computer calculation using a bi-modal injection to simulate the concurrent operation of a fog and a spray generation sys-tem. The =ean droplet si:es of the fcg and spray were taken to be 10 and 500 u=, respectively. The injection rate of the fog was 310 kg/s (5000 gpm), while that of the spray was 62 kg/s (1000 gpm); a frecuency dis-tribution by mass of the injected drops is shewn in Figure 16. Figur: 17 gives the fog density or water content, as a f unction of time. The audi-tion of a water spray does not produce any. si3nificant chanse in the fo~3 density - either increase or decrease. The =cs: notable result is an apparent stabilicine effect of the spray - the limits of oscillatica of the overall fog density are significantly reduced.

10) What effect will ice condensers have on :he fcg?

Si=ple calculations shcw that the nu=ber of drops suspended in the turbulent flew through a duct decays exponentially with distance X frc:

the inlet:

N=N o exp(-(2"3X)/( UTR) ]

where "3 is a characteristic transverse velocity, U is the axial velec-ity, and R is the duct radius. The solutien presupposes that all d: cps which contact the duct wall are lost from the system and cannot be re-entrained. For turbulent ficw, the transverse velocity may be taken as the RMS eddy velocity, U, which is in_ general sone fraction of the axial velocity - typically on the order of 3%.

Us = U = 0.03 U .

This assumption gives

-167- .

10' ;_

3

^2 10 -:

U  :

Ne V -

>= 2 a 10 3:

z W -

"3 O

w -

% 1 6 10 3 .

s  ! I 10 , . .. .., . . ., . ., ..,

10-4 10-> 10- 10-i 10 DROP RADIUS (CM)

FCG: .ean M Orop Radius = 10 ;.: 2 Lcg-Standard Deviation = 1.2 Injection Rate = 310 kg/s (5000 g;=)

SPRAY: .Mean Drop Radius = 500 p I,og-Standard Devia:10n =.1.3

)

Injection Rate = 52 kg/s (1000 gg=3 Vol=.e of Containment = 3.4x104= (1.2x10 ft3) 5 Height of Contain=ent = 40 = (130 ft)

Figure 15. Frequency Distribution by Mass Of Generated Fog and Spray

-103-

O e

- 5.0 m

m 2 4.0 -

0 N

O v

g 3.0 -

Z w

b- Withont Z Sprav O 2.0 -

O W

W 1.0 -

\

L With Spray 0.0 , , , , ,

0.0 12.0 24.0 36.0 48.0 60.0 TIME (S)

FCG: Mo' . rop Radius = 10 pm

_;-Standard Deviation = 1.2 Injection Rate = 310 kg/s (5000 gp=)

SP F.AY : Mean Drop Radius = 500 pm Log-Standard Deviation = 1.3 Injection Rata = 62 kg/s volume of Containment = 3.4x10 (10004=

gpg) (1.2x10 6 ft3)

Height of Contain=ent = 40 = (130 ft)

Collision Efficiency (E c) = ' indauer' Figure 17. Water Content for Binodal Fog Generation - The Effect of Sprays on Fog Stasility

-104-m

- 9- . -

- N = No e xp { -0. 0 2X/R] .

Appropriate lengths for an ice condenser are X = 15 m (48 ft) R = 0.10 m (.33 ft) giving N = 0.05 No .

That is, virtually all fog / drops entering the ice condensers will bt 10s:

from the system.

l l

L I

J

-105-

REFERENCES FOR WATER FCGGING

1. L. W. Carlson, R. M. Knight, J. O. Henrie, " Flame and Detonation Initiation and Propagation in Various Hycrogen-Air Mixtures, With and Without Water Spray," AI-73-29, 1973.

2. I. Glassman, Combustion, Academic Press, 1977, p. 58.

3. J. Warnat=, " Calculation of the Structure of Laminar Flat Flames II: Flame Velocity and Structure of Fully Propagating Hydrogen-Oxygen and Hydrogen-Air Flames," Ber Bunsanges Phy Chem, 82, pp.

643-649, 1978.

4. M. P. Sherman, et al., "The Behavior of Hydrogen During Accidents in Light Water Reactors," SAND 80-1495 ( Albuquerque: Sandia Labora-tories, August 1980).

5. F. Williams, Combustion Theory, Addison-Wesley Publishing Co., 1965, pp. 47-56.

6. 3. Lewis and G. Von Elbe, Combustion, Flames and Explosions of Gasses, 2nd Ed., Academic Press (1961).

7. H. Hiroyasu, et al. , " Evaporation of a Single Drople at Elevated Pressures and Temperatures (Experimental Study), "Trans. Japan Soc.

Mech. Engrs., 40, (1974), 3147.

8. .R. Dennis,'ed.,-Handbook on Aerosols, Tech. Inf. Center, USE RDA, 1976, Chapter 2.

9. N. P. Fenger, " Dimensional Analysis of Several Atomiters," Engi..-

eering, Dec. 1976.

10. Design of Liquid Propellant Rocket Engines, NASA SP-125, p. 128.

11. Liquid Propellant Rocket Combusuien Instability, " Injection and Atom 1:a:Lon,* NASA SP-194, p. 4 '. .

12. N. A. Fuchs, The Mechanics of Aerosols, MacMillan Co., N.Y., 1964, Chapter 7.

13. G. C. Lindauer, A. W. Castlemr.n, " Behavior of Aerosols Undergoing 3rownian Coagulation and Gravitational Settling in Closed Systems,"

Aerosol Science, 1971, _2 c o. . 85-91.

14. . G. D. Kinzer and W. E. Cobb, " Laboratory Measurements and Analysis of the Growth and Collection Efficiency of Cloud Droplets," J. of Meteorology, 15, 1958, pp. 138-148.

15. G. C. Lindauer and A. W. Castleman, Jr. , "The Importance of Gravi-tational Coagulation on the-Settling of Hign-Mass Density Aerosols,"

Nuclear Science and Engineering, 42, 1970, pp. 58-63.

16. V. M. Voloshchuk and T. S. Sedunov, Eds. , Hydrodynamics and The rme-dynamics or Aerosols, John Wiley & Sons, N.Y., 1973.

-106-l_ - - -- -

t REFERENCES (cont.) -

17.

A Kovet: and 3. 01und, "The Effect of Coalescence and Condensatica on . Rain Formation in a Cloud of Finite Vertical Exteht," J. of the Atmospheric Sciences, 26, 1969, pp. 1060-1065.

18. F. Tolfo, "A Si=plified Model of Aerosol Coagulation J. of Aerosol Science, _B , 1977, .o o . 9-19. .

i

19. R. Gunn and G. D. Kin =a, 2

Droplets in Stagnant Air," J. of Metecrolocy, "The Terminal Velocity of Fall for Water 6, 1949, p. 243.

20. V.

Shafrir and M. Neiberger, " Collision Efficiency of Two Spheres Falling 4141-4147. in a Viscous Medium," J. Geophysical Res. , 63, 1963, pc. -~ '

21. A. A. Borisov, 3. E. Gel'fand, S. A. Gubin and S. M. Kogardo, "Ef-feet of inert Solid Particles on Oetonation of a Combustible Mix-ture," Combustion, Exelosion and Shock waces 11, 909-914 (Nov-Dec. 1975) in Russlan}Translatec in Englisn, Oec. 1976.

l 22. T. H. Pierce, " Blast-Wave Propagation in a Spray," J. Fluid Mech.

88, 641-657 (1978).

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W

-107-

. s . -..

3.0 HALON (CHEMICAL) INERTING 3.1 Introduction and Discussion This section discusses the feasibility of installing a Halon-addi-tion systen as a hydrogen control measure in a nuclear plant containment building. The concept is to chemically inert the containment air, i.e.,

prevent a hydrogen burn, at some time after a serious accident begins but before significant quantities of hydrogen are generated. Fortu-nately, most of the issues concerning a Halon 1301 (CF 33r) system in a nuclear containment have already been examined experimentally in a pro-gram (1,2) conducted for the U.S. Maritime Administration. Answers to specific NRC questions will be addressed in the sections which follow this introductiu.t.

It is well known that CF 3 3r extinguishes combustion by means of chemical, rather than physical (heat removal), processes.- However, de-tailed interpretations.of the exact chemical mechanisms remain somewhat controversial.(3,4) Bromine is generally thought to play the key role in combustion inhibition by CF 33r. For H2:02 flames, three reactions will directly inhibit combustion:

H + H3r A

-H7 Br O + HBr -OH + Br OH + H3r - H2 O + 3r This direct inhibition r'ssults f rom the replacement of highly reactive radicals (H, O, OH) by less reactive bromine atoms. Direct inhibition will not completely explain-the magnitude of the phenomenon, however, and it is necessary to consider additional reactions that maintain the

" radical pool" at more or less its original overall si:e.(3) For H2:02 flames, the reactions that contribute to this " regenerative inhibi-tion"(4) are:

-108-

3r + HO2 - H3r + O2 --

. Br + H.302  :

HBr + HO2 B r + 3 r + M --- B r 2 + M Br + H + M H3r + M where M represents any gas-phase species.

One final point should be mentioned before we begin to address spe-cific questions. At Halon concen: ations below that required := inert a combustible gas mixture, the presence of Halon may not be beneficial at all and could ce detrimental. Macek(5) studied the effect of additives on detonctions of H2:02 mixtures. The presence of 2-4 vol. i Halon 1301 actually decreased the minimum ignition energy in such mixtures. Johnson, Furno, and Kuchta(6) observed the ccmbustion-inhibiting effect of Halon '

1301 en CH4: air mixtures. While 3.6 vol. % of the 1301 rendered the mix-tures nonfla=mable, 3.4 vol. % of 1301 had no beneficial effect in terms of decreasing the post-combustien pressure. McHaleCl) ocserved a simi-lar behavier'with H2:02:N2:1301 mixtures when-he reduced the 1301 con-centration below that required to inert. Furthermore, if combustion ec-curs in the presence of non-inarting quantities of Halon, the Halen will thermally decompose into extremely corrosive halogens or halogen acids, which could adversely affect the plant and safety systems. The conclu-sion to be drawn from these studies is tha: the Halen concentration must be maintained above that required to iner at all times and in all loca-tions.

-109-

3.2 Answ7rs to Qumstions * - -

1) How much Halon is required to prevent deflagration of -

H2: air: steam mixtures?

Several researchers have answered this question experimentally for Halon 1301. The concentration of Halon 1301 required to inert H2: air ,

mixtures varies with the H2 concentration. Maximum inerting concentra-tions occur for a H2 concentration of about 14-17 vol. % in a mixture of H2: air:1301. Bajpai and Wagner I7) used a vertical Pyrex glass tube of 5.8-cm diameter and 127-cm length to measure H2: air:1301 flammability limits at 25'C and 760 mmHg. Their ignition source was an electrical spark (energy unspecified) between needle-point electrodes at the bottom of the tube. The criterion for inerting limits was based on flame propa-gation (determined by thermocouple response) less than 25% of the tube height. Their result for peak Halon inerting concentrations was 29 vol.

% Halon 1301 and 32.5 vol. 1 Halon 1211 (CF2ClBr).

McHale(l) examined Halon 1301-inerting limits in a similar labora-tory-scale device and then confirmed his results with a few tests in in-

- termediate-scale (5.6 f 3 volume) and large-scale (1200 f 3 volume) ves-sels. The laboratory-scale device was a vertical Pyrex tube (5.08-cm diameter _and 122-cm length) with ignition at the lower end by either an electric are or an electric squib.(energy not specified for either source). McHale's criterion for the inerting limit was for flame propa-gation _ (apparently determined by thermocouple response) less than 50% of the tube height. His result for peak Halon 1301 concentration required to inert H2: air mixtures at 70*F and 1 atmosphere was 23 vol. % with spark ignition and 24 vol. % with squib ignition (see Figure 1).

McHale also examined the ef fects of H2O addition, increased pres-sure, and increased temperature (see Table 1). His results for peak Halon concentration required to inert mixtures of H2:1301:N2:02 (Wita N2 = O2) were: .a) the addition of steam to the mixture lowers the

-110-

. .m. _ . . _ .., , __ - . - .

13C1 hn

s. . . . . . . , = - \.Ch.\. . ..

O E=.iciien Pre..;. e:

[

e..,...,........., ..

, SCWiU li n'iICM

'9:' \ hg\f/

\/\/\,/Ny'\/

\/ \/\/\/M/

\/\/\/\/N/hi/\

l x,'/\/\/\/\M N h/\

H 2

\/ N/\/\N!!M/NY N/\

^

W Tigure 1. Exp 10 S iOP. Limit 3r 32: Air:13Cl (T:00 Refere.ce 1) l 1

l l

-111-

- s - - . .

Table 1 Summary of Experimental Studies of Halen 1301 Inerting Peak Halon Concentration Gas Ignition to Inert Mixture Conditions Source (Vol. 4) Reference H2: air No steam Spark 28 7 25'C, 1 ats.

H2: air No steam Spark 23 1 21

• C , 1 a t= .

H2: air No steam Squib 24 1 21'O, 1 atm.

H2:02:N2: steam Saturated steam Squib 51 1 N2"02 49'C, 1 ats.

H2:02:N2 No steam Spark 54 1 N2=O2 21*C, 1 a tm .

H2:C2:N2 No steam Squib 58 1 N2=02 21*C, 1 atm.

H2:02:N2 No steam Squib 58 1 N2*O2 49'C, 1 atm.

H2:02:N2 No steam Squib 63 1 N2=O2 49'C, 3 atm.

-112-

required 1301 concentration; c) 4hanges in temperature (70-120'F) havo no measurable effect; and c) increased pressure (1-3 atm) raises ne re .

quired 1301 concentration. Extrapolation of these data to mix:ures of H2:1301: air leads one to the conclusion that the peak Halon concentra-tion required to inert would: be unaffected by temperature over the range 70-120*F; decrease to about 23 vol. t with low temperature satura-ted steam present; increase to about 25 vol. % for increased initia; pressure up to 3 atmospheres.

The National Fire Protection Association requires 31.4 vol. % Malon 1301 to inert H2: air mixtures (NFPA-12A). This value includes a safety margin of approximately 10-20%. Based upon the experimental data dis-cussed above, the 31.4 vol. % requirement should prevent deflagratien of any H2 : air: steam mixture over the lower range of temperatures, pres-sures, and ignition sources to be expected in a containment building following an accident.

The actual quantity of Halon 1301 required to inert the Sequoyan containment,can be calculated as follows. The number of moles of air inside Sequoyah at 120*F (49'C) and 14.7 psia is 1.3 x 106 If the peak 1301 concentration (31.4 vol. %) is required when the H2 concentration Ls 17 vol . % , then the initial quantity of air represents 51.6 vol. % in the final mixture of H2: air:1301. This requirement means that the final mixture has 8 x 105 moles (264,000 lbs) of Halon 1301 and 4 x 105 moles (1900 lbs) of hydrogen. This quantity of 1301 corresponds to a partial pressure of about 9 psia at 120*F.

In une course of his investiga:ica into the suitability of using Halon 1301 in a nuclear plant, McHale discovered two mechanisms whereby "The experimental da:a do not cover the full range of temperatures and steam concentra:icns that might be expected in a serious accident.

Neither parameter would be expected to significantly increase the re-quired Halon inarting~ concentration, but this should be confirmed ex-

.perimentally.

-113-

1301 could bo removed from :30 containmont gas.(1,2) Tho first procoas i

. is solution of the 1301 in the water in containment, and the second is -

radiolytic decomposition of the dissolved 1301. In order to calculate the quantity of 1301 lost by these mechanisms, we need to know the quan-tity of water inside containment. The maximum amount of water available to dissolve 1301 is 3.3 x 106 kg (8.4 x 106 lbs) at Sequoyah.(8)

The solubility of 1301 in water at 120*F (49'C) is 8 x 10-6 kg 1301/

kg H 20/ psi 1301 gas. For 9 psi of 1301 in Sequoyah, this is a quantity equal to 270 kg (600 lbs). McHale(2) determined that radiolytic decom-position of the 1301

water solution could attain a maximum level of 5.2 x 10-3 moles 1301/11:er water. Radiolytic decomposition would re=ove less than 3000 kg (6500 lbs) of 1301 from the containment gas. Adding the maximum losses of 1301 to the desired quantity in the gas phase, we determine that 1.2 x 105 kg (271,000 lbs) of Halon 1301 should iner: the Sequoyah containment with about a 10% safety margin for low temperatures and steam fractions.

2) How much Halon is required to prevent detonation of H2:

l air: steam mixtures?

The detonation limits of H2 in air are 13 (lower) and 59 (upper) vol. 4. The lower limit of 13 vol. 4 H2 requires a Halon inerting con-i centration that is slightly-less (~1 vol. 4) than the peak Halon concen-trations discussed in.the answer to Question 1. In other words, 23-27 vol. % 1301 should prevent a deflagration in a detonable H2: air =ixture.

It follows that a detonation.must also be_ prevented by a Halon concen-

.tration that is less than or equal to 27 vol.  %. Including a 10% safety.

margin, we conclude that 30 vol. % Halon 1301 should prevent the detona-tion of any.H2: air:staam mixture in a post-accident containment environ- 1 l

ment. The actual quantity of Halon 1301 needed to prevent ' detonation in -

the Sequoyah containment is slightly less than that calculated in answer-ing Question 1.. Based' en the discussion. in the introduction, it would  ;

, -114-

..... ~~m. m. _- _. , - - ~

not be productive to determine the minimum amount of Halon required to ,

prevent detonation. Since that minimum would not prevent deflagrations, all the detrimental aspects of Halon in non-inerting concentrations would ensue.

3) Will thermal recombiners operating on a H2: air steam:

Halon mixture produce halogens or halogen acids in quantities likely to adversely affect stainless steel?

If state-of-the-art hydrogen recombiners could burn the mixture de-scribed above, then they would produce halogens and halogen acids in suf-ficient quantities to adversely affect stainless steel and virtaa11y all 4

other materials inside containment (see Cuestion 5). However, if the Halon concentration in the mixture remains abcve the inerting level, none of the mixture will burn. Current state-of-the-art hydrogen re-combiners are designed to operate on containment gas with a content of f 4 vol. % H.3 and b 5 vol. t 02.(9) ' Containment gas outside these'de-sign concentrations can be treated by recirculating the burned effluent or by adding 02 - but this could lead to the adverse situatica described in the first sentence above. Cbviously, Halon-inerted containment gas must-be treated in a special way and this will be discussed in the an-swer to Question 4.

4) How should a containment filled with H;: air: steam:

Halon: fission-product mixture be handled after an accident?

As incicated in the response to Question-3, state-of-the-art hydro-gen recombiners cannot handle the mixture described above without caus-ing significant problems. There may' be a number of methods to saf ely de-fuse Halon-inerted containment gas. One of the most obvious is to modify existing recombiners such'that they remove the Halon before the contain-

- ment gas mixture enters the burn region. Cryogenic traps or chemical.

-115-

.. -. - = _ - -. -

. a . _.

.- getters could be installed at the recombiner inlet. In this way, the hydrogen could be safety burned with oxygen - without the fear of ther-mally or chemically decomposing the Halen and producing copious amounts of halogens and halogen acids.

We contacted technical represente ives of three firms that manufac-ture hydrogen reco=oiners.(10) These people were briefed as to the prob-lems associated with recombiner perfor=ance in a containment filled with aH2: air: steam:Halon: fission-product mixture. Then we discussed two spe-cific questions: 1) Can recombiners be modified to remove Halons before the mixture enters the recombination region?, and 2) Can recombiners be modified to treat mixtures containing as much as 20-30 vol. %H7 2 All of the people contacted indicated a definite "yes" in reply to Question 2, but none of them had enough knowledge of Halons to answer Question i de-finitively. Clearly, some engineering development work will be required in order to modify existing recombiners so that they can safely treau a Halon-inerted containment atmosphere.- -- - -

5) Are chemical rections between Halon and the post-accident atmosphere likely to produce halogens or halogen acids in quantities likely to adversely af-feet stainless steel?

Chemical reactions between Halon and the post-accident containment

. gas would not be expected to produce significant quantities of halogen products except at elevated temperatures. Tests of the high tempera-ture stability of Halen 1301 have been conducted using sealed : ces con-taining liquid 1301 and either stainless steel 316 or mild steel.(ll)

When these sealed tubes were maintained at 600*F (316*C) for 25 hours2.893519e-4 days <br />0.00694 hours <br />4.133598e-5 weeks <br />9.5125e-6 months <br />, the metal corrosion penetration rates were 7.6 x 10-3 and 14.4 x 10-3 in-ches/ year for stainless and mild steels, respectively. Similar tests (ll) were conducted with a variety of common =etals, at two moisture levels (2 and 72 ppm), for a duration of 44 months, and at temperatures between

-116-i . _ . . ._ __. __

. room temperature and 250*F (121*C). These tests revealed very low cor-

~

. rosion rates. Common elastomers and plastics, sealed sith liquid 1301 at room temperature for a period of two weeks, appear to be compatible 1 with Halon 1301, except for silicone rubber, ethyl cellulose, and possi-bly cellulose acetate /butyrate.(ll)

At temperatures above 950*F (510*C) Halon 1301 will thermally se- '

compese. In the presence of wster, Her and HF acids will be formed as well as small s uantities of 3r2 and carbonv.l halides (CCF,. and 003r.,).

If the concentration of Halon 1301 were to be below the inerting concen-tration and a burn were to start in a H2: air:.1alon mixture, tne Halen would carticipate in exothermic reactions with He. and C.,

. to eroduce halo-gens and halogen acids.

As indicated in the answer to Question 1, Halon 1301 will radiolyti-cally decompose when dissolved in water. The dece position p;0 ducts of concern are HF and H3r acids. McHale(2) found that a solution that was buffered to a pH ut 10.3 would. attain a saturatica pH of about 3 and an unbuffered solution would attain a saturation pH of about 2.3 (5.2 x 10-3 olar H3r, with HF ioni:ation suppressed). These acidic solutions would cause significant corrosion of steel and other =aterials(12,13) in ==n_

tainment if the solution were act neutrali:ed in a timely manner. The actual decrease in solution pH follows an exponential function of the product P R, where P is the Halon 1301 gas pressure in psi and R is tne integrated radiation dose absorted by the solution in Mrad. F0r the Sequoyah containment P is about 9 psi. The solution would be capable Of significant corrosion (pH 4 4) after a radiation dose of approximately 10 Mrad. 'In order to neutrali:e the pH of the solution it would require the addition of a strong base to the containment water. The maximu:

quantity of base would be equal to the maximum H3: concentration, 5.2 x 10-3 ol/ liter.- Using ::aCH in the Sequoyah water, the neutrali:stion

- would require about 800 kg (210 ppm by weight).

_117.-

. a . _..

This problem of Halon decomposition is probably the most difficult .

technical issue faced by Halon inerting systems for nuclear plant con-tainments.

4

6) Are there adverse effects which result from the energy absorption on the addition, expansion, or evaporation of liquid Halon? ,

The heat of vaporisation of Halon 1301 is roughly 3000 cal /mol (35.5 Btu /lb) at 70'F (21*C). We calculated earlier that 3 x 10 5 mol< s of 1301 would be required to inert the Sequoyah plant. This means that 2.4 x 109 cal of heat must be supplied to evaporate all of the 1301.

The air in containment (~ 1. 3 x 106 moles) cannot supply this quantity of heat by itself, but the solid surfaces exposed to the gas can. A simple analysis will be developed below and the data recorded by McHale(l) will be discussed.

The surf ace areas of steel and concrete inside the Sequoyah con-tainment have been estima cd(3) as 3 x 105 gt2 and 1 x 105 gg2, respec-tively. Using these surface areas and average values of neat capacity

and density (see Table 2), we compute a thermal capacity of 2.5 x 108 cal /K-cm and 0.6 x 108 cal /K-cm for the steel and concrete surfaces, respectively. This means that if all steel and concrete surfaces are cooled from 120'F-(49'C) to 70*F (21*C) to a depth of 0.3 cm, the heat so removed will be sufficient to evaporate all of the 1301. If the 1301 is injected into the. containment over a sufficiently long time (t.g., 1000 seconds), the thermal diffusion rates into steel and con-crete are sufficient to supply the quantity of heat needed (see Table 2).

The thermal capacity of the contai 1 ment air is only 7 x 106 cal /K or 0.2 x 109 cal for the change in temperature' described above.

The net result of these considerations is the conclusion that the cooling: capacity of the liquid Halen is very large but that the walls and surfaces inside_ containment can easily supply the necessary heat

- -118-

-.-. -- _ - . . - . . .. - _ _ - __. - . .. .. - . _ - _ _ = - .

. . i 1

r

- s . -..

(

"l

. I Table 2 l Physical Precerties of Steel and Concrete l Inside ne Secuoyan Containment  :

Parameter Value for Steel Value for Concrete Surface area 3.3 x 105g2 0.9 x 105g2 3.1 x 103 cm,- 8.4 x lo,e em-,

i Density 487 lb/ft3 157.5 lb/ft3 7.30 gm/cm3 2.52 gm/cm3 Heat capacity 0.113 Stu/lb *F 0.233 Stu/lb *F 0.113 cal /gm-K 0.238 cal /gm-K I

25 Stu/hr-ft *F 0.8 Stu/hr-ft *F '

Heat conductivity

, 0.103 cal /s-cm-K 3.3 x 10-3 cal /s-cm-K Thermal diffusion 0.117 cm2/s 0.0055 cm2/3 coefficient Diffusion depth 1.1 cm 0.2 cm

in 10 s 4

t Diffusion depth 3.4 cm. 0.7 cm in 102s Diffusion deoth "

10.3 =m 2.3 cm in 103s r

k b

L I

o 196 '

.c3 46eu uuc cocaA ovoporation.

, If the Halon is injected over a time 4

scale of minutes, then the containment pressure will increase during -

the injection.

For the example considered above, if the initial pres-sure was 14.7 psia at 120*F, then the final pressure would be about 21.9 psia at 70*F.

McHale(l) conducted tests of Halon 1301 injection in a 1200 f 3 tank.

The tank was a cylindrical vessel with a volume-to-surface ratio of about 2 ft. McHale injected 1200 moles (400 lbs) of 1301 in about 20 minutes; the tank contained 220 moles of air initially. The tempera-ture of the gas decreased by only 15-20*C due to the Halon evaporation .

The Sequoyah containment has a volume-to-surface ratio of about 3 f t and consequently McHale's results are roughly applicable to Sequoyah if 1301 injection times are comparable. His data support the contention dis- .

cussed above that injection of Halon will not cause adverse effects (in the form of reducing containment pressure below its initial value).

7)

What would be the~ design concept for a Halon :ddition sys tem? -

The standard design concept of a Halon addition system is quite simple.(14elS)

Tanks of pressurized liquid Halon are stored outside of, but close to, the hazard area.

When detectors indicate a fire dan-ger or fire in progress, the Halon is discharged by manual or auto.tatic controls.

High-flow-rate nozzles discharge the Halon as a liquid spray that rapidly mixes and vaporizes.

One possible design concept of a Halon addition system for a nu-clear containment will be described now. The.Halon would be stored in bulk tanks such as an ISO tank or tank trailer, either of which will hold 40,000 lbs of Halon.

A minimum of seven of these would be located outside the containment building.

Each tank would feed a numoer of noz-

les (say 3 or 4) located inside the containment. Commercially-available no::les have discharge rates up to 300 lbs Malon/sec(16) so sizes would I

-120-

. be selected based upon the time required to inert the containment. The discharge system would be actuated either manually or automatically.

Accident scenarios capable of producing significan: quantities of hydro-gen would be factored into the automatic actuation system. Since the Halen' system uses high-pressure gas to inject the 1301, a passive dis-charge option ..equiring no electrical power: us ing manually-c,:erated i

svalves) could be designed into the system. Detectors capable of sensing both H 2 and CF 3 3r would be loca:ed throughout the containmen; building.

Post-accident treatment of the containment gas would require special attention (see Questions 3 and 4) in order to safely remove both H2 2nd CF 33r-

8) What are the pros and cons to its (Halon) use?

The pros and cons associated with using Halon to inert a nuclear

. plant containment building are summarized-belcw.

Pros

1. There is extensive experience with Halon systems includ-

. ing large-volume (106 5:3) applications.

2. The ability of Halen to inert H;: sir: steam mixtures is i proven.

3. Most questions for nuclear reactor application have been addressed previouriv in McHale's(Ir2) study.

4. Without decomposition,_ aH 2:cir:Halon: steam mixture

should be stable for a long time.

5. The Halen discharge system can be designed to'be passive (requiring no electrical power).

I

-121-

~ * - _.

Cona

1. Addition of the Halon will increase the gas pressure in containment (about 9 psi for Sequoyah) .

2. Corrosion by the decomposition products of Halons can be significant (especially at elevated temperatures and for long peri-ods of time).

3. Treatment of the post-accident containment gas will prob-ably require a new or modified hydrogen recombiner.

4. The Halon concentration must remain above the required inert level at all times or the presence of Halon in the mixture can actually be detrimental.

9) How long would it take to install one (a Halon-addition system)?

In order to get estimates of cost and time to install, we contac-ted two major manuf acturers of Halon - fire-extinguisher sys tems: cthe-Ansul Company (16) and' Walter Kidde and Company,"Inc.(17) ~3oth of'these firms have experience with Halon protection systems for very large

(>105 g 3) volumes. In particular, the Ansul Company is presently in-stalling Halon systems in oil tanker engine rooms (0.75 x 106 g:3),

Costs for a complete Halon system, including engineering, installation, and'.all hardware (not including H2 detectors or reccmbiners), range frcm S.50 - S3.00/ft3 Halon 1301 can be purchased under a GSA contract for

$2.50/lb. The total cost of a Halon system for Sequoyah would probably be 1.5-4.5 x 106 dollars. Both firms believe that if they used contain-ment drawings to prefabricate modular piping components, an all-welded pipe system could be installed inside a containment in 1-2 weeks (pos-sibly using several daily shifts of men). Installation of all compon-ents outside containment would probably require 1-2 months.

.-122-

. s . - .

n e. .r

.~ e. R.r..e*L.r.d .r ^ pv. . u.n.

. . .w^ h+

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- ~

.v.CM.. a. ' a. , " 9 V. d. . .^ ", *. 7 s uy *e ". a. s s .' O r. s * "..d v. a *. d. *. a. s . 4 . .. . ~. ~v#. M. . a .' .- .

.0.1- . r.".ases 7 a...d .2

?'," a' . .' a . . 4 .- .s a. s a. a . . .".v^ .^ . ,. o . a . .' * .*. o.sa. r - . . "..

ARC 47-5647 (Oec. 1976); Mari:ime Adn.nistration, ".S. Cepar men:

Os. Com.e. .. e, c.,......

...... R._.,o.00.

s. e.

a s . .v.e.q a .i , a u..m z a.... - , e . . 3 u.--

~

re . a. s s .4..- . s~.udv. a n d *. a. a- . 4. . . ,-

^#

.. . =..=.'^...

1301: Phase !!!," Atlantic Research Corporation Reper: No. ARC 47-5702 (Mar. 1973); Maritime Administration, " S. Cepar :en: cf .

Commerce, Contract -38169.

3. G. Dixon-Lewis, Ccmbust. Flame 36, 1 (1979). --

4. L. A. Lovachev and L. N. Levachev, C0mbust. Sci. ~ecn. '9, 195 (1979).

~

5. A. Macek, AIAA Jcurnal 1, 1915-1913 (1963).

6. A. L. Johnson, A. L. Furno, and J. M. Kuchta, " Infrared Spectral Radiances and Explosion Properties of Inhibited Methane- Air Flames," Su Mines RI B246 (1977).

7. S. N. Bajpai and J. P. Wagner, Ind. Eng. Chem., Prod. Res. Cev. --14, s ,.-s9 ( ., 9., s. ) .

S. 3. W. Burnham (input data for the March computer code), Sandia hsa . 4vn..a ., r .aa c.a.-.4es,

. .... f ri.,73.. .. .. nn..~u...va.4-n.- ...

9. n . .w. e ar e w.. .s n. 4-.- . ..4- -

a . .- ... a..... n- .e4.....a

.. o. . ....a..,

. . . .. 1.1., .s u..v.... .

4

. .7 .? .i, .". . Wa.a.- .,e a c .0. s ,

. " s a . .d.

. i a .N a . .i ..a .' ." a ".c. .= .0 .4 =.a- R . . a.

r - . . . .N e .

5ANc80-1495 (Aug. 1950).

10. L. R. Secne (Atomics International, Canoga Park, CA); J. C' Hare

. 4 .g .u.nL. a. e , o....

( .n.e s ... . . a. , .oA), .,,. . a. R. 3 0.... . , ., . ( n...4 . v....-d....s 4 . s u, ..v. ... a..a.

Chemicals, Inc., Allen Own, PA), private cc munication, (Nov. 1980).

l.3 .. ..

.a.a 2cn a... ee -L,.s

- an.a v0.,

- an,. 2. c n . u.a .i . n ., .,0 .i r e. . .. .

.. . . . r. x ...4 . . _

c.uishant," Report No. 3-290 (1977).

.i .n . r o.

u. Sh..4 .. . (e4.), Co...e s 4 ... yee.' . .e 1 v. e a . / ..r .u. 4... n.. . . . o . ., - . . . . . s ,

Newnes-3utterworths, Loncon (1976J.

.s .

a.. . .u. . r.tw ... 13 4 - (ra.),

.- '. .". e C .+ . . .a s .4 a.

. u. . a . ..d. " c c k , ?..". . . W .' .' a. v .= . . d .~ ~ .a. . a ,

Inc., N.Y. (1943).

14. J. . .. .". ". a. . . a w- ". . , " u.. a .i ^..~. e. x . 4 . . " 4 -m "..' . . ~., .c ,v- a .a. n. s. , e s .:

. . . 4 - .. u . '. . a. . 4 a , "

S e v a. n .v. .7 4 .# . .". N. . .o 'n n' . . . . ". a .' v.a. a. . 4 . . 3 , < a n . . .=.. c .isc^, c'en ( .v. y .= " .,. '.',

~

19 a,. t ) .

15. C. L. Ford, Actual S.=ecifv.inc. Engineer, 74-33 (Jan. 1972).

3e. D. . e. .i ...

, , , .< e... . , n. . .e . . -.. en.,

. . . . . . , .a.

.v. a. .. 4. .... . . , -.4va..

y.. .. c m.aun4.r. 4

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1990).

17. 3. L. Warner, Kidde 3elieville, selt.v4'.'., e. . , e 4. ,, a . . . . ..... . .r . .. -- .

. .. 4. . 1_

tion (Sept.-Cet., 1930).

-123-

. ATTACHMENT 1: NRC WCRK SCCPE, August 25, 1980 .

SCCPE FOR 2-3 MCNTH EFFORT CN SECCCYAH Investigate using existing .31:erature (i .e . , no exper =e nta.3 work is required) the ef fectiveness and practicality of three nydrogen centrol measures:

1. Deliberate ignition

2. Halen addition after accident initiation

3. Water fcq Cbiective - Provide an early assess =en { prior :0 December 1, 1930) of the ef ficacy and .cracticality of these three =itiga:icn schemes for de-graded (still ecolable - no: =cl:en) cores. Assess whether enese =itiga-tion sche =es will, in the near ter=, =itiga:e the ef fects of a signifi-cant fraction of the' accident scenarios tha t lead to the degraded =cde.

Determine--if installation of the =itigation syste= will degrade er i=-

c.reve safe v..

Cn Celiberate Icnition (based on TVA c.lans for a c.lew c. luc. system . in Sequcyah):

- What ignition strategy should be felicwed: On continuously; turn on for accident; or turn on at specific ti=es?

- Ter degraded core accident scenarios (shcr cf core cel:),

will ignition avoid containment threat?

- Are there negative aspects to deliberate ignition vs. exist-ing potential igni:ica sources?

- Calculate pressure rise for partial c0=bustion in H;/ air /

stea =ixtures and c0= pare this with literature data to

-124-

w.g 1 by.,sp. ta heme _ ' - - -

t

.* estimate completehess of Ecabustion as a function of H2 ^

. concentration (small effort anticipated). -

On Halon If Halon is added to containment early in an accident sequence:

- How much Halon is required to prevent deflagration of H 2/

air / steam mixtures?

- How much for detonation?

- Will thermal recombiners operating on a H / air 2 / steam /Halon mixture produce halogens or halogen acids in quantities likely to adversely affect stainless steels?

How should a containment filled with a H / air 2 / steam /Halon/

fission product. mixture be handled af ter an accident?

- Are chemical reactions between Halon and post-accident a:-

mosphere.likely.to produce halogens or. halogen. acids-in 1

quantities likely to adversely af fect stainless steels?

- Are there adverse effects which result from the energy ab-sorption on the addition, expansion, or evaporation of liquid Halon?

- What would be the desie~n concept for a Halon addition sys-l i

i tem?

- What .are the p:os and cons to its use?

- How long would it take to install one?

On Water Foc

! - What would tne design concept of a water fog syste.T be?

.-125-

• y _ -.

p- ,+ .,.7 a w -

p . . ,, , ~ . , -.-r- - , + w

- ~ - . _ .

- How effective would water fog be in preventing a hydrogen combustion or detonation threat to containment?

- What water fog density and particle size are required to suppress containment threat as a function of H / 2air / steam ratios likely to result from dominant accident scenarios?

- Assess the problem of maintaining a water fog in post-accident atmosphere.

- Discuss pros and cons of a water fog system.

- Will water fog reduce steam concentrations and yield a more combustible or detonable mixture?

- What steps are to be taken after the water fog is in place in a contaiment containing H2/ air / steam / fission products?

-- What are the final-steps to recovery af ter . water fog has initially prevented or reduced the effects of.comoustion?

- How long would it take to install one?

l l

l l

l l

e i

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i l - -126-

-. - -