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 1973 U.S. ATOMIC ENERGY COMMISSION

R EGU LATO.RY GU I D EIDIRECTORATE

i:.OF REGULATORY

STANDARDS Rm Il -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 the special nuclear material (SNM) contain a changes progressively through the four stages i of the applicant's procedures for control: of operation, shutdown, draindown, and inting;.for.:

SNMWTwhichý,will.be, in ..hi! !,Siý,:cleanout..

Simultaneously, the number of sources of ider ;license,':ý!.including'-`procedureso uncertainty and the levels of uncertainty in the physical 4M .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 in quirements,".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 of if.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 the er.; 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 improved 4it 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 h Se:re gulatry i;ide',describes".,ý'acceptable6-,;

icategory of material unaccounted for (MUF) may be sand characteristics

'for. minimizing thel -lessened,:'

and, the influence of uncertainty associated Ipof-.'SNM

'in'dryiri"id -fluidized bed gwith measured holdup on the limit of error on material er shutdown;'.draindown,'

or cleanout unaccounted for (LEMUF) in a material balance may be itate', material 'control and accountability~fll lessened.

Furthermore,.

the extent to which the hese 'features and characteristics are eruncertainty

,;in.. holdup contributes to a physical terfere 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 the q..uantityof holdup directly decreases the uncertainty

..contributed

.. to a physical inventory.

amt~

laton ,,,o ,h b.'A reduction in the quantity of holdup material s'i N which are referred to as residual'

.ts o SNM~ tmustbe

recovered.

following the three stages of..........

..-. ..shutdown, ...drain down.-. and cleanout of process umtlty exceeedla-u an yor tltni, m s o 'equipment decreases the quantity of SNM susceptible to urn'

..plutoniumtor, any..the diversion during sampling, identification, and subsequent uclei"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 qw i6ontained Vurani'combination'" th operation of 'n 50 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 for methds ace 4able oteACRgltrrtf fIpeetn pcfcprso K im'provemnentt In these guides are encouraged and should be gent to the Secretary the 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 to the issuance or continuance of a permit or iicensa by the Commission.

-'1. Power Reacto ' ' '. Producu"" '2. Research and Test Reactors

7. Traneportation

7 ..' 3. Fuels and Materials Facilities I. Occupational Heatth Publishe gudes will be revse perlodcally,*

as appropriate, to aoenmmodate

,; 4. Environmental and Siting 9. Antitf it Review comments andto reflect new Inforrmation or eXperience.

-.. .t :ta n lP.0 en in holdup also can enhance process operations by decreasing the extent and time of., unit, process innterruptions 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 on c an invent o, effort requnIredo"stablish the presence of an :io .remove residual material for a physical inventor" " Ws:ireduced "Consequently, '.the amount of time and thei Thme: 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 is advantageous.

With individual batches it is possible to imaintain batch identity.

However, batch driers require ,,more physical handling of material than continuous driers, may be difficult to automate, and may necessitate iadditional 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 product is very fine. Internal mechanical scrapers constitute an 4additional Impediment to cleaning the drying chamber., 6T ,. ndirectly heated driers inwhich 'the material being 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, some 4:material will be entrained in the gas and may be retained Jin: 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 spill jout. of the drier. Furthermore,-.

the screw, conveyor is idifficult 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 density and 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 be for cleanout or draindown.

Directly heated r'.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 and high porosity.

Consequently, they may be a significant:

~,source of material holdup. .;,.,-, Batch pan driers are completely enclosed and rlsuaUy are equipped with agitators which constitute a source'of material holdup upon emptying.

Agitators also Scauseg~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 components ofa fluidized bed operation that contact the process rmaterial, there 'exists a large number of locations where holdup 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 syste

m. 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..ieriý

lap ea ps % entrainýmifent the':niis'irnimal.'Useý':of agitators in' diictll3 heated driers 1rmay'.:..be:,'.desirable',,,from

".",an "',ioperating standpoint However,'.;'driers with agitators are difficult to empty and clean '.". ',, "" ' i' ' ' ...-high.thioughput ope6ration. '~;arises

.:incrieased

continuity iof 'operation

iand in creased'unimiforrnity!

of fmat erial handling.'

C ontinuous operatioi~.have;the:'

advant ager s of ,firinimaphysica1 r potentia

l. process automation

-'.

input, screw conveyors, pneumatic carriers, dip pipes, connecting lines is sensitive to process variables such as seal legs, injectors, or other means are used to introduce temperature.

Severe .operating conditions such as solids into, the reactor proper. Difficulties with holdup temperature excursions are particularly conducive to arise and :speclal '.techniques become "necessary when , severe segregation and packing of SNM against the base 1 sol:dsare not free flowing, such as filtered precipitates'

.ý'.and interior walls. The existence of holdup may and 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 further be necessary to assist handling by wetting the material t excursion and additional resulting holdup by the i~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 fses and~herby.

acheve~~eterjhanlinRL-Exiting materil consists of product and offgass iand*.-therebyR'(.

a chi eve'ýIcbe tter .*'ihan dlin gqualities.-I

--Prouct may be batch removed by freely draining Nonhomogeneous

--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 through f: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) a iquidfeed nee~sitaies use of an injection nozzle'i "flapper-type check valve to restrict gas flow through the wan '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 dust i, .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 it epositedoat"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 is cauisesof 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 or Aother 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 contnrbutes e ttanbeien iused t r icantgeneral effective:

cleanouts.

For tall reactors.ubltiple 6ontactig:ta

, nfortusgately,.,iplO however,", or* for:'a mixture of fluidized solids with compartments as well as the series configuration provideý differentcharacteristics, mechanical mixing may be multipl 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 or poor dispersion of adhesive feed solids.'n, ýgenerali~poes~crdtoi~

lec~ fi~ "'con"toa

'atcegrowth

'size reduction in foratzon of holdupVarables whic~h'-yfldized.beds-:

contributes" to' the generation and formationtare'xnlet.

gas composition ad velocbty;,bedis4,iubsequent deposition

'of fines. Major size reduction temperature -and depthi ;and feed miterial temperaturil.

_Pmechanisms in fluid beds" ar attrition, impact, and and. ._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 in Buildup'of fines may. cause bridging of. solids 'inthe;;i filters or components downline from the fluidized bed reactor structure .In, addition,, deposition of substances ._ reactor. Exceptionally fine particles may necessitate an on internal'surfaces."of the --reactor structure.

and min._',.-"offgas cleanup system on-line rather than a complete..

..

shutdown periodically.

An alternative is to use an internal fines filter.nirnintatibn i 'of: fli b ed act 66rs ?largil3~Of. "gtemperature and ...i pressure.

snsor IO&Pl,ý..,wells;,.and:1/2ýtressure,,t aps ý.are otenHA wails, covers, or other, means of containment to prevent spilling or release of the contents during operation.

.k The, feed material is properly prepared to~mn-mize the potential for holdup formation within a fluldized

!bed. Also,- sizing.. operations

,prior to iAntroduction of particles are evaluated.', The influence of operating variables such as gas and: temperature Jis evaluated to reduce undesirable 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 not iessential 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 that 1'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 the ei t of.the. tray to minimize the probability of trayoverride;

.4- ) are" to 2-. .:,Mecharical

agitat6ro are designed to permit 01 surfaces to drain freely and present minimum surface for collection of solids. ! ! '`. .-. .-Sd,'ti Sensing:, devices ' such .as thermocouples are tstalled in a manner that minimizes the amount of solid A

that 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, mechanical mixers, and heat transfer fins. ..':

tly ,,mounted p~rocess 'equipment

'nternals;

that -cannot..

be removed for cleaning allow 1-'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 solids M:.

.:.canbe, completely emptied.-

for draindown of the reactor..

,.', : ' ,Flow- ;control valves .and spray nozzles for feeding 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.- Fo fully :oade distance of the cente ogravityifrom

'any .side- is at cast four timies tsdisano from the b6ottom f othe "P;9.il le quipment ,in,- hic,* ha.:rat l is:'agitated

.4 A anit-Alltr vt 'PneIcvp wiith -td

91 to mHnimize cake' ....... on th ozl' ror~ti or hn the equipment.

.....S 1;: Components mcias meal lega4 vas"hain nd 'Penings are designed to minimie accu hdob .structo niie .. .. .. .. ... mulation'btrution by particulatamter;

g ~~ Holes "drilled in perforated suppr'pe%Tncal, `3o: ',!.'te ZI! f,,

./ir 1 .,r 7lca .-o r .edue h'earea of flat surfaces.on.which may stagnate and cake during reactýl iTo. .u th operation of~the bed SuiduOf 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'assay

3:-instruments or internal .pr._ý'detectf the:preencebf:or to "identify

'the loc.:-residual material not.visually sb b."`The2 bodyr,1,of t fluidized'dea(!equipped with' vibrators"or external impactors toprevent packing and adhesion of' '. .,Extended

':heat "transfer" surfaces, .o ,r fins -and'tubesg for :both-heating a"nd coolin' ae

"'~ ti g n co l gare e3 to th fluidizedibed containment structure.-An alt Sourcef heat may be preheated inputs.;d..Fgroun idied! r, breed r rn tprevenit build.-b up"0 of. stat ch..case .bdexpasO

ndconsequent.:;" rei 4 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 r Srnsing to remove, dislodge, or dissolve SNM particles.--

-.... Equipment ,i is provided .with -fittings for Tconnections for washdown and rinsing with liquids that 1:will remove, dislndge, or dissolve all particulate processresidua. iquid, and condensed vapors that may I'remtain on internal surfaces after the equipment has been ,ý'shut I .N; Provision I"s gma d 'ollectin g ,Tnsings In which SNM may be entrained or dissolved.

..-4 multiple." stages .in h'fluidized bed reactors empOY!surfonaceluha perforated

gas p 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 being disassembled and removed for cleaning and inspection

.;.ibottom outlet d to facilitate and cleano'ut " .lhaptleanu' .e. "io .. ... or"s:ut packing material is used, to: prevent the bottom outlet from becoming encrusted, which may hinder the discharge of luidized bed material during equipment draindown and.'cleanoutv'; -r.... '. ......i :- ,-. .: .....- --:7...M-,470 4 :y : :.-, .

,.,--.-. ,.. ..: Y. ." ..5.8-5