Regulatory Guide 5.8

Design Considerations for Minimizing Residual Holdup of Special Nuclear Material in Drying and Fluidized Bed Operations

ML13038A435
Person / Time
Issue date:	06/30/1973
From:	Office of Nuclear Regulatory Research
To:
References
RG-5.008
Download: ML13038A435 (5)

June 1973U.S. ATOMIC ENERGY COMMISSIONR EGU LATO.RY GU I D EIDIRECTORATE i:.OF REGULATORY STANDARDSRmIl -NIPt4 GI1015.8!"DESIGN CONSIDERATIONS FOR MINIMIZING RESIDUAL HOLDUP-CIALNUCLEARMATERIAL IN DRYING AND FLUIDIZED BED OPERATIONS.iNTROUCTION~"'=505 ldi, or hidden inventory. A characteristic amount of'Wmateal that is difficult to locate, sample, identify,o.22 -"Cbiiitets 6r'applications,"'of'Io0, !quantify, and analyze is held up in equipment for a given:.I:SpeCial Nuclear Material,";requires," indg; design, mode of operation, and type of process material.:applicationfor alicenseto possess certainri , :I' This holdup may be decreased as the status of thespecial nuclear material (SNM) contain a changes progressively through the four stagesi of the applicant's procedures for control: of operation, shutdown, draindown, andinting;.for.: SNMWTwhichý,will.be, in ..hi! !,Siý,:cleanout.. Simultaneously, the number of sources ofider ;license,':ý!.including'-`procedureso uncertainty and the levels of uncertainty in the physical4M .during its '.processing.,,or, use- in -the:ii inventory may be reduced.-. ..procedures' by which' proces'losses are -.......'.quihermore; Section','nth'.51fo"rm ofnventory ui e' " relation of SNM in process equipment inquirements,".requires;in pa rt; that certain the form of residual holdup following shutdown,iduct"<' at ;specified intervals physical draindown, and cleanout could have adverse effects on a.the NM i their possession under license. 4t'ý materials control program. Minimizing that quantity ofif.and .accounting for SNM can be'made'.ý- -ýmaterial retained in process equipment enhances the'tby minimizing the residual holdup after of a materials protection program in theer.; draindowni and after cleanout of the -following wy"A'.'4t0oprocess SNM,'thereby reducing the' followin'gway: ,,.. ..Uncertai...'..ontributed. residual,,,,,,. aý.,. Te quality of a physical inventory is improved4it ne y ' Vy'reducing uncertainty due to holdup. For example,issociated: with determination :of residual the contribution of unmeasured holdup material to the:.. ...... .rý',.. ..,'j: .'d. .. t heA n r b t o f u m a u r d h l u a ei l t hSe:re gulatry i;ide',describes".,ý'acceptable6-,; icategory of material unaccounted for (MUF) may besand characteristics 'for. minimizing thel -lessened,:' and, the influence of uncertainty associatedIpof-.'SNM 'in'dryiri"id -fluidized bed gwith measured holdup on the limit of error on materialer shutdown;'.draindown,' or cleanout unaccounted for (LEMUF) in a material balance may beitate', material 'control and accountability~fll lessened. Furthermore,. the extent to which thehese 'features and characteristics are eruncertainty ,;in.. holdup contributes to a physicalterfere with process operations.-K i 'ý,inventory depends on the magnitude of the amount, -,, : present and how precisely and accurately that amount is'DISCUSSION being measured. Therefore, reducing theq..uantityof holdup directly decreases the uncertainty..contributed .. to a physical inventory.amt~ laton ,,,o ,h b.'A reduction in the quantity of holdup materials'i N which are referred to as residual' ;.ts o SNM~ tmustbe:recovered. following the three stages of.......... ..-. ..shutdown, ...drain down.-. and cleanout of processumtlty exceeedla-u an yor tltni, m s o 'equipment decreases the quantity of SNM susceptible tourn' ..plutoniumtor, any..the diversion during sampling, identification, and subsequentuclei"eaciorlicensed pursuaatitoi 1 CtFeR Partan separation .'.or: recycle of this material as may be!necessary to complete a physical inventory. A reduction*j~j;,This Is a qwi6ontained Vurani'combination'" thoperation of 'n50 brfasiealed 3t)'USAEC EGULATOCopUies of published gulide may be obtained by request Indicating the divisions~ ,~ *~,&"y~desired to the US. Atomic Energy Commission. Washington. D.C. 20545,Reuaoy'udsae'sud tdecbendrk ailbtthpulc Attention: Director of Regulatory Standardls. Comments en suggestions formethds ace 4able oteACRgltrrtf fIpeetn pcfcprso K im'provemnentt In these guides are encouraged and should be gent to the Secretarythe Commission's regulations, to delineate techniques used by the staff In' kOf the Commissuion, U.S. Atomic Energy Commisusion. Washington, D.C. 20645.evaluating tpeciflc problems or postulated accidents, or to provide guidance to Attention: Chief. Public Proceedings Staff.applicants. Regulatory Guides era not substitutes for regulations and compliaAce Z .... -with -them is not required. Methods and solutions diffternt from those set out in The guides ae Issued In the following ton brood visions:the guides will be accoptable if they provide a basis for the findings requisite tothe issuance or continuance of a permit or iicensa by the Commission. -'1. Power Reacto ' ' '. Producu"" '2. Research and Test Reactors 7. Traneportation7 ..' 3. Fuels and Materials Facilities I. Occupational HeatthPublishe gudes will be revse perlodcally,* as appropriate, to aoenmmodate ,; 4. Environmental and Siting 9. Antitf it Reviewcomments andto reflect new Inforrmation or eXperience. -.. .t :ta n lP.0 en in holdup also can enhance process operations bydecreasing the extent and time of., unit, processinnterruptions for material accountabllity. For example.jfor a process amenable to dynamic inventory techniques,icredibility'.In" such .a '. technique may be. increased by;:ducingthe;detrimental effects of residual holdup onc an invent o, effort requnIredo"stablish the presence ofan :io .remove residual material for a physical inventor" "Ws:ireduced "Consequently, '.the amount of time and theiThme: of people who need access to SNM are reduced.'The,'opp ortunity..r for unauthorized .individuals to gain:Sccess to SNM during this stage of a physical Inventory,l"iso ninv he retplme J.A -ski3. h1, ri~ For certain processes, however, batch drying isadvantageous. With individual batches it is possible toimaintain batch identity. However, batch driers require,,more physical handling of material than continuousdriers, may be difficult to automate, and may necessitateiadditional equipment cleanout between batches.SFor' directly heated fluid-bed ders and spray driers,'t re is a tendency for dry product to adhere to the,walls and bottom of the drier, particularly if the productis very fine. Internal mechanical scrapers constitute an4additional Impediment to cleaning the drying chamber., 6T ,. ndirectly heated driers inwhich 'the materialbeing dried contacts a heated surface, sweep gas may be,.used to carry away the vapor from the drying solid.ýýUnless the exit .velocity, of this vapor is low, some4:material will be entrained in the gas and may be retainedJin: ducts. :'Also,: for indirectly heated continuous screw'conveyor driers, as the feed drys to an adhesive paste It!has a tendency to cake and choke off the flow or to spilljout. of the drier. Furthermore,-. the screw, conveyor isidifficult to clean and tends to be a significant source ofýholdup.-`'f`:';(.7,'Rotary driers have been used to combine a drying;.peration with a calcining operation in separately heated;zones. An intermediate paste having a high bulk densityand viscosity has a tendency to adhere to most materials.which it contacts such as flight carriage surfaces. Such'surfaces contribute to holdup by impeding material flow",during emptying. Therefore, special attention, may befor cleanout or draindown. Directly heatedr'.7otary driers also may be accompanied by carry over of,`,dust or ultrafine powder which becomes a form ofmetal belt driers have a high surface area andhigh porosity. Consequently, they may be a significant:~,source of material holdup. .;,.,-, Batch pan driers are completely enclosed andrlsuaUy are equipped with agitators which constitute asource'of material holdup upon emptying. Agitators alsoScauseg~more,,,difficulty,". for a complete.,, cleanout.-Continuous. driers'.with open :static beds such as pans,.boats, ,or trays. do not contribute significantly to holdup.inless"'they are accidentally tipped -or the contents are'.,otherwise Spilled.' Trays` pans, and boats also may be.,j..easy to empty and clean if necessary.:!3;,Holdup n'iFluidized Bed Operations! Because. of the numerous interrelated componentsofa fluidized bed operation that contact the processrmaterial, there 'exists a large number of locations whereholdup may occur and numerous sources initiating its,.,occurrence.".'.';,However,' holdup in fluidized beds`commonly '.depends upon .the 'size and growth of'ý;%ýparticles throughout the system.I Beginning with the....:..,.£1:inif a' bnt'ainei .';.the ýgas l'steam and must be,collected 'ri:filtersand recovered from the process. The,entrainment hcontrbutes'to holdup' of, fines in ductwork"and Wfiters.'!Oný itihie ',bthfer':,,!hand;:,dirctlyv1heated dr..ieriýlap ea ps % entrainýmifent the':niis'irnimal.'Useý':of agitators in' diictll3heated driers 1rmay'.:..be:,'.desirable',,,from ".",an "',ioperatingstandpoint However,'.;'driers with agitators are difficult toempty and clean '.". ',, "" ' i' ' ' ...-high.thioughput ope6ration. '~;arises.:incrieased :;continuity iof 'operation :iandin creased'unimiforrnity! of fmat erial handling.' C ontinuousoperatioi~.have;the:' advant ager s of ,firinimaphysica1r potential. processautomation -'.

input, screw conveyors, pneumatic carriers, dip pipes, connecting lines is sensitive to process variables such asseal legs, injectors, or other means are used to introduce temperature. Severe .operating conditions such assolids into, the reactor proper. Difficulties with holdup temperature excursions are particularly conducive toarise and :speclal '.techniques become "necessary when , severe segregation and packing of SNM against the base1 sol:dsare not free flowing, such as filtered precipitates' .ý'.and interior walls. The existence of holdup mayand other moist solids. Since it is difficult by means of ag,-gi. compound an excursion by its sudden release to the* screw conveyor to feed wet material to a reactor, it may ¢fluidized -bed for reaction, which induces furtherbe necessary to assist handling by wetting the material t excursion and additional resulting holdup by thei~iattainý.a>,slurry,! consistency be fore feeding.-J If $1urrr'5,0,Itmechanisms of packing and fusion..V' 2.conststency.,dts `iundesirable for the'material,, the solieh of packing and fusion.Tproduct may.be recycled to mix with the feed to dry txl AVeral i fsesand~herby. acheve~~eterjhanlinRL-Exiting materil consists of product and offgassiand*.-therebyR'(. a chi eve'ýIcbe tter .*'ihan dlin gqualities.-I --Prouct may be batch removed by freely drainingNonhomogeneous :--feed ý,containing lumps of through a bottom outlet valve or by a screw conveyor,i smidry',solids, i~tend -to .,ýcompound the problem; by`?& ;Howeve, depending upon the characteristics of the' aglomeratingand fusing together instead of breaking 4i ' particulate matter, solids may adhere to the walls or the* apartiy" Agglomerate's 'which are 'much larger. than'. the a s distribution plate. If excessive caking occurs, larger%:Monriginal particle size subsequintly. tend to segregate outý, :agglomerates may form that do not drain throughf:the'bed *rAfter a period of time,'the fluidized bed may* '- niopeng in the exitvalve -D'becomrei sstatic 1&.thereby Wnecessitating,-, shutdown-., and ", .s: ' "nou s er "ypes of outlets for continuous product':. " iremoval are: (1) a simple weir pernmtting overflow, (2) aiquidfeed nee~sitaies use of an injection nozzle'i "flapper-type check valve to restrict gas flow through thewan 'atomizing 0gas ýThe" liquid input system consists of k and (3) a seal leg with a solids flow control valve to-01), C,'sprayi. injection '-nozzle," (2) .a liquid chemical external and internal pressures. In each case...,injectionj ,system, ,.(3) , fluidizing gas 'system.ni ',,obstruction and accumulation as a result of inadequate,'ý'; Injectioný'- nozzles have :'been shown to be a source o, Wdesign may'result in large quantities of holdup material.cake .formation and holdup: Furthermore,*a'..'area .of -the' equipment directly across the'fluidized bed fses are discharged near'the top of a fluidized!, bopposltt,.the vnozzle'also' has'beerin-"shown-,16`b -ýKb ".bed reactor through a variety of components which are'susceptible, sto;u.. .,,. cake formation o An .'of holdup. These include cyclones for dusti, .ýconsequence whch fu propags h orenoval, carbon or sintered metal filters, and cold traps.ter fer enc&c!r4:, f ... .'ck ' ':.`,ihJ,~o m o rt.f e t no a.%operation' t Holdup in these. devices occurs to such an extent that itepositedoat"tthe bottom'of the fluidized necessary to recover the solids which are carried byon ?'the s leaving the fluidized bed.::,-,"fluidization and may~cause localized hot spots. Also, t.',ýproduct ',ýoverflow : tube may-become blocked. ,Typical ..,'. One 'of the advantages of utilizing a fluidized bed iscauisesof this agglomeration are impurities in feed, fines*'iý' 'Its favorable heat transfer performance. Unfortunately"from-"the recovery!system,.poor.gas distri. from "the: viewpoint of mmizig holdup, heat,;spray.ý nozzle operation, generally, poor fluidization, and` exchanger tubes manifolded at the bottom and top orAother unfavorable operating ,k;' '.bayonets internal to the reactor are sometimes used.At ' .-"" S .' uch use should be avoided where possible since it.' The.fluidized bhed ",chemical 'reactor'~syste 'may"' t aggravates the problems of holdup and cleanout.consst"~of~ne1 ot ,:several ,configurations. 'A series of ~ J"~~~~individ ual; reactorSasb i wellas a ngle.reactor.wi th the absence of moving parts contnrbutese ttanbeien iused t r icantgeneral effective: cleanouts. For tall reactors.ubltiple 6ontactig:ta :, nfortusgately,.,iplO however,", or* for:'a mixture of fluidized solids withcompartments as well as the series configuration provideý differentcharacteristics, mechanical mixing may bemultipl l1cations' fodr holdutpii In either casea'.i sufficient.. tiized to reduce segregation. Without adeqate mixing,w 'of 'gasis necessarxy .toq forestall, plugging and ;aigglomeration may. occur because 'of particle fusion orpoor dispersion of adhesive feed solids.'n, ýgenerali~poes~crdtoi~ lec~ fi~ "'con"toa 'atcegrowth 'size reduction inforatzon of holdupVarables whic~h'-yfldized.beds-: contributes" to' the generation andformationtare'xnlet. gas composition ad velocbty;,bedis4,iubsequent deposition 'of fines. Major size reductiontemperature -and depthi ;and feed miterial temperaturil. _Pmechanisms in fluid beds" ar attrition, impact, andand. ._consistiency .'.-The !;generationriof.. f'mes 'esped ally is'.ýYg e rmal decrepitation. All three mechanisms produce.deperident on the ioperating rangedof :process varablet2 & t,`fmes' which may contribute to holdup, especially inBuildup'of fines may. cause bridging of. solids 'inthe;;i filters or components downline from the fluidized bedreactor structure .In, addition,, deposition of substances ._ reactor. Exceptionally fine particles may necessitate anon internal'surfaces."of the --reactor structure. and min._',.-"offgas cleanup system on-line rather than a complete....

shutdown periodically. An alternative is to use aninternal fines filter.nirnintatibn i 'of: fli b ed act 66rs ?largil3~Of. "gtemperature and ...i pressure. snsorIO&Pl,ý..,wells;,.and:1/2ýtressure,,t aps ý.are otenHAwails, covers, or other, means of containment to preventspilling or release of the contents during operation..k The, feed material is properly prepared to~mn-mize the potential for holdup formation within afluldized !bed. Also,- sizing.. operations ::,prior toiAntroduction of particles are evaluated.',The influence of operating variables such as gasand: temperature Jis evaluated to reduceundesirable Tormation of holdup such as caking which.,may be induced by operating in an undesirable range of-'.E pment :.':minimum ",of 'internal.Rcomponents upon which process material can collect;tequipmento'is free from internal structural supports,flanges,;-ý support .. rings,-trays, or devices that are notiessential to operation.'i;.,- ,.v -:>,', ,,b-:Racks.,.carriages,"'conveyor*, guides, or drive-mechanisms that are used to assist or direct the transport.bOf --trays ,through ..a tray drier-are designed so that1'individual ;.trays :..cannot be tipped or ride over one, another. For: example,.: for ,.ý."walking-beam" -.. drives,!vertical travel is constrained to less than one-fourth theei t of.the. tray to minimize the probability of trayoverride; .4- ) are" to2-. .:,Mecharical :agitat6ro are designed to permit01 surfaces to drain freely and present minimum surface forcollection of solids. ! ! '`. .-. .-Sd,'ti Sensing:, devices ' such .as thermocouples aretstalled in a manner that minimizes the amount of solid Athat can be retained by sensing devices.,. 5-2? e.,,t,-Pressure taps projecting into the equipment'the capability for being gas purged.'f...Because of highly abrasive wear in fluldized'beds and potential for SNM retention on horizontal-and protection tubes, installation of internal,telements, are positioned, for example vertical, to reduce'erosion and holdup. Examples of such elements -may be sensing elements, bayonet heaters, mechanicalmixers, and heat transfer fins. ..': tly ,,mounted p~rocess 'equipment'nternals; that -cannot.. be removed for cleaning allow1-'rinsings and normal contents of vessels such as fluidized,'bed .driers ,-to freely;: from the.. bottom: of the,...,h`;,ýTh6 iegt oth~disengaging section above the .->expanded bed is adequate to reduce particle entrainment.'which :' ,contnrbutes ; to! holdup .in :the. recovery.teni'for the fluidized bed operatfons.- .f,.Filters'and/or cyclones are provided above the','Rexpanded.bed to sepaiate elutriated particulates, fines,.and 1dust from'the exhaust gases. These are designed to".accumulated solids to the bed; for example, filters.,.are equipped with an automatic cyclic blowback feature, .or a. n external bin is designed to accumulate solids." ' ..,":;Cyclones,',are ,equipped: with suitable solidsM:. .:.canbe, completely emptied.- fordraindown of the reactor.. ,.', :' ,Flow- ;control valves .and spray nozzles forfeeding solutions to the fluidized bed are designed and!-.A.A '.'... .are1-1imt-u'cted ;'with" 'adeq uate ,strength.topý ,.pclude-breakage and deteoration, r ýMs. in.tray' driersre ad~signednot toi tip spill, thecontents"-durng handling '.and' ope ration.- Fofully :oade distance of the centeogravityifrom 'any .side- is at cast four timies tsdisanofrom the b6ottom f othe "P;9.il le quipment ,in,- hic,* ha.:rat l is:'agitated.4 A anit-Alltr vt 'PneIcvp wiith -td

91to mHnimize cake' ....... on th ozl' ror~ti orhn the equipment. .....S 1;: Components mcias meal lega4 vas"hainnd 'Penings are designed to minimie accuhdob .structo niie .. .. .. .. ... mulation'btrution by particulatamter;g ~~ Holes "drilled in perforated suppr'pe%Tncal, `3o: ',!.'te ZI! f,, ./ir1 .,r 7lca .-o r .edue h'earea of flat surfaces.on.whichmay stagnate and cake during reactýl iTo. .u th operation of~the bedSuiduOf lar-"ge particles that cannot be drained readily,thereactor Is equipped with a gas-jet ve~essel for'the fluidized bed has a taperet Preclude the accumulation of material such-'_sintered SNM in cor'Oa materia,_......ers at the b ottr,, .......... " .. arance ... .:t4 o permit, nextondestructive'assay3:-instruments or internal .pr._ý'detectf the:preencebf:or to "identify 'the loc.:-residual material not.visually sbb."`The2 bodyr,1,of t fluidized'dea(!equipped with' vibrators"or external impactors toprevent packing and adhesion of' '. .,Extended ':heat "transfer" surfaces, .o ,rfins -and'tubesg for :both-heating a"nd coolin' ae"'~ ti g n co l gare e3to th fluidizedibed containment structure.-An altSourcef heat may be preheated inputs.;d..Fgroun idied! r, breed rrn tprevenit build.-b up"0 of. stat ch..case .bdexpasO ndconsequent.:;" rei4 DeSgn f or Cleaou't Where Necessar..... a..-Driers ar. provided with access ports,& removable.i: covers,ý<, or. removable sides for visual?finspectfon of the internai surfaces. ...,,. b.`, tAccess ports or removable panels are provded, for" ccaning internal surfaces by appropriate methods:as b rusing, v au umin& washin ca pn o rSrnsing to remove, dislodge, or dissolve SNM particles.---.... Equipment ,i is provided .with -fittings forTconnections for washdown and rinsing with liquids that1:will remove, dislndge, or dissolve all particulate processresidua. iquid, and condensed vapors that mayI'remtain on internal surfaces after the equipment has been,ý'shutI .N; Provision I"s gma d 'ollectin g,Tnsings In which SNM may be entrained or dissolved. ..-4 multiple." stages .in h'fluidized bed reactorsempOY!surfonaceluha perforated ;gasp owcomersý5t'-Pflanges for'defflestio-baffles ýfor. m&`Ing, and p~late'baffles for, pairtitioning, these are accessible for cleanout.'. ': Supplementa Interalmechanical equipment,..,.otpermanently. mounted such as scrapers,- agitators,rinsers, or atomizers Inside equipment is capable of beingdisassembled and removed for cleaning and inspection .;.ibottom outlet d to facilitateand cleano'ut " .lhaptleanu' .e. "io .. ... or"s:ut packingmaterial is used, to: prevent the bottom outlet frombecoming encrusted, which may hinder the discharge ofluidized bed material during equipment draindown and.'cleanoutv'; -r.... '. ......i :- ,-. .: .....- --:7...M-,4704 :y : :.-, .,.,--.-. ,.. ..: Y. ." ..5.8-5