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 E

IDIRECTORATE i:.OF REGULATORY STANDARDS

Il - NIPt4 GI1015.8 Rm

!"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

applicationfor Nuclear Material,";requires," indg;

alicenseto design, mode of operation, and type of process material.

possess certainri , :I'This holdup may be decreased as the status of the special nuclear material (SNM) contain a *:equipment changes progressively through the four stages i of the applicant's procedures for control: *consisting, 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 ,':*'q;*,,,.tý. e' ofnventory

"relation ui 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 *A.effectiveness 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'Vy'reducing y uncertainty due to holdup. For example, issociated:

..

. . with

....

.rý',.. determination

. .,'j:

'd. . :of ..residual !* *t the heAcontribution n r b t o off uunmeasured m a u r d hholdup l u material a ei l tto the 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 -fluidized bed gwith 'in'dryiri"id measured holdup on the limit of error on material er shutdown;'.draindown,' or cleanout in'*- 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 not-:*. 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 B.amenableOto**,B

'DISCUSSION being measured. Therefore, reducing the q..uantityof holdup directly decreases the uncertainty

. . ..contributed to a physical inventory.

amt~ pr'esses'*permit~~accu laton ,,,o ,h b.'A reduction in the quantity of holdup material s'iN which are referred to as residual'  ;.

ts o

..........

.*,,1

. SNM~ -....

.. * shutdown, drain down.-.following tmustbe:recovered. and cleanoutthe three of stages processof

• j~j;,This Is a qw umtlty exceeedla-uan yor tltni, mo s 'equipment decreases the quantity of SNM susceptible to i6ontained Vurani urn' f23S'fouraniuim;233*,*,itor plutoniumtor,.. any..the diversion during sampling, identification, and subsequent

'combination'" thuclei"eaciorlicensed pursuaatitoi 1 CtFeRPartan separation .'.or: recycle of this material as may be operation of 'n

50 brfasiealed 3tOur.re*,, !necessary to complete a physical inventor

y. A reduction

'USAEC EGULATOCopUies of published gulidemay 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 oteACRgltrrtf

4able fIpeetn pcfcprso K im'provemnentt In theseguides 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 will be accoptable if they provide a basis for the findings requisite to the issuance or continuance of apermit or iicensa by the Commission.

7. .

-

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'

'1.

'2.

The guides ae IssuedIn the following ton brood visions:

Power Reacto ' '

Research andTest Reactors

'.

3. Fuels and Materials Facilities Producu

7. Traneportation I. Occupational Heatth Publishe gudes will be revse perlodcally,*asappropriate, 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 For certain processes, however, batch drying is decreasing the extent and time of., unit, process advantageous. With individual batches it is possible to innterruptions for material accountabllity. For example. imaintain batch identity. However, batch driers require jfor a process amenable to dynamic inventory techniques, ,,more physical handling of material than continuous icredibility'.In" such .a '.technique may be. increased by; driers, may be difficult to automate, and may necessitate

ducingthe;detrimental effects of residual holdup on iadditional equipment cleanout between batches.

c an invent o, Wy*** SFor' directly heated fluid-bed ders and spray driers, effort requnIredo"stablish the presence of an :io.remove residual material for a physical inventor"" 't re is a tendency for dry product to adhere to the Ws:ireduced "Consequently, '.the amount of time and thei ,walls and bottom of the drier, particularly if the product Thme: of people who need access to SNM are reduced.' is very fine. Internal mechanical scrapers constitute an The,'opp ortunity..rfor unauthorized .individuals to gain 4additional Impediment to cleaning the drying chamber.

Sccess to SNM during this stage of a physical Inventory, l"iso ninv he retplme J.A *) -ski3.

h1, ri~ 'i,'*: , ,. 6T id,,i*I 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 of

ýholdup.-`'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

• £necessary for cleanout or draindown. Directly heated r'.7otary driers also may be accompanied by carry over of

£1

,`,dust or ultrafine powder which becomes a form of

• !!Woven metal 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

inif a' bnt'ainei .';.the ýgas l'steam and must be, rlsuaUy are equipped with agitators which constitute a collected 'ri:filtersand recovered from the process. The, source'of material holdup upon emptying. Agitators also entrainment hcontrbutes'to holdup' of, fines in ductwork" Scauseg~more,,,difficulty,". for a ,*,complete.,, cleanout.

and Wfiters.'!Oný itihie ',bthfer':,,!hand;:,dirctlyv1heated dr..ieriý -Continuous. driers'.with open :static beds such as pans, usinfi*i*i ea lap ps %are:advaniageous'*sin **? .boats, ,or trays. do not contribute significantly to holdup entrainýmifent the':niis'irnimal.'Useý':of agitators in' diictll3 .inless"'they are accidentally tipped -or the contents are'.

heated driers 1rmay'.:..be:,'.desirable',,,from ".",an"',ioperating ,otherwise Spilled.' Trays` pans, and boats also may be.,

standpoint However,'.;'driers with agitators are difficult to j..easy to empty and clean if necessary.:

empty and clean '.". ',, "" ' i' ' ' *. ..-

!3;,Holdupn'iFluidized Bed Operations

! Because. of the numerous interrelated components high.thioughput si".low-holduip.i* ope6ration. :*,This '~;arises ofa fluidized bed operation that contact the process largely*!,from .:incrieased :;continuity iof 'operation :iand in creased'unimiforrnity! offmat erial handling.' Continuous rmaterial, there 'exists a large number of locations where operatioi~.have;the:' advant agers of ,firinimaphysica1 holdup may occur and numerous sources initiating its

,.,occurrence.".'.';,However,' holdup in fluidized beds handlin"'aw*el as'sgreate** r potential. for*,*i process `commonly '.depends upon .the 'size and growth of automation - '. 'ý;%ýparticles throughout the syste

m. I Beginning with the

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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 sol:dsare

1 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.-JIf $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 acheve~~eterjhanlinRL-Exiting materil consists of product and offgass and~herby.

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 dryo* 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

%:Monriginalparticle size subsequintly. tend to segregate outý, :agglomerates may form that do not drain through f:the'bed *rAftera 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.

.a injection '-nozzle," (2) liquid chemical *"equalize external and internal pressures. In each case.

..,injectionj ,system, *and ,.(3) .a* , 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.

'*locahzed. 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' ýKb has'beerin-"shown-,16`b - ".bed reactor through a variety of components which are

'susceptible,

.,,. cake formationo An * . sto;u.. 'of holdup. These include cyclones for dust i,. ýconsequence whch fu q*!iin ter fer enc&c!r4:,... f .'ck propags hm orenoval, carbon or sintered metal filters, and cold traps.

' ':.`,ihJ,~o 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 *:is necessary to recover the solids which are carried by d-'.*d on ?'the *igasýdistribution:.plate'§interferewith 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 performanc

e. 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 conditions.",* ,k;' '.bayonets internal to the reactor are sometimes used.

. At

' -"" . ' Such 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 inddual*pcomacin e ttanbeien t r icantgeneral iused 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 chann*elin 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

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shutdown periodically. An alternative is to use an wails, covers, or other, means of containment to prevent internal fines filter. spilling or release of the contents during operation.

. k The, feed material is properly prepared to nirnintatibni 'of: fli b ed act

66rs ?largil3 ~mn-mize the potential for holdup formation within a

~Of. "gtemperature and ...i pressure. snsor fluldized !bed. Also,- sizing.. operations ::,prior to IO&Pl,ý..,wells;,.and:1/2ýtressure,,t aps ý.are otenHA iAntroduction of particles are evaluated.',

The influence of operating variables such as gas

• low.*.rate 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 *-has :.':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 tray

• override;  ::Zi*.*- )

• are"

.4- to

2-. .:,Mecharical :agitat6ro are designed to permit

01surfaces 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

• !material that can be retained by sensing devices.,. 5

-2? e.,,t,-Pressure taps projecting into the equipment'

• 'hve the capability for being gas purged.'

• f...Because of highly abrasive wear in fluldized

'beds and potential for SNM retention on horizontal-

• ielements and protection tubes, installation of internal

,telements, are positioned, for example vertical, to reduce'

• surface erosion and holdup. Examples of such elements -

may be sensing elements, bayonet heaters, mechanical mixers, and heat transfer fins. .. '

g.*iPermanen 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 *:drain, 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. *offgas recovery

• .teni'for the fluidized bed operatfons.- .*--' .

• f,.Filters'and/or cyclones are provided above the','

Rexpanded.bed to sepaiate elutriated particulates, fines, are1-1imt-u'cted ;'with" 'adeq uate ,strength.topý ,.pclude .and 1dust from'the exhaust gases. These are designed to".

-breakage and deteoration, r ýM *.return accumulated solids to the bed; for example, filters S-*Tray s. in.tray' driersre ad~signednot toi tip np* .,.are equipped with an automatic cyclic blowback feature, .

spill, thecontents"-durng handling '.and' ope ration.- Fo or a.n external bin is designed to accumulate solids." ' .

exam~plew*hen fully :oade d,*the distance of the cente .,":;Cyclones,',are ,equipped: with suitable solids ogravityifrom 'any .side- is at cast four timies tsdisano M:. retum":ines,*;-that .:.canbe, completely emptied.- for from the b6ottom othe f "P;9 draindown of the reactor.. ,.',  :

.il le quipment ,in,- hic,* ha.:rat l is:'agitated ' ,Flow- ;control valves .and spray nozzles for

.4 A anit-Alltr vt 'PneIcvp wiith -td feeding solutions to the fluidized bed are designed and

!-.A.A '.'... .

,*,'Installed to' mHnimize cake'

ror~ti. on...... th ozl or hn the equipment.

91 S 1;: Components mcias meal lega4 nd 'Penings hdob .structo are.. designed

.....

.. .. to.. minimie btrution by particulatamter;

niie

.

vas"hain 4 DeSgn for Cleaou't Where Necessar a..-Driers ar. provided with access ports,

.....

..accumulation' & removable.i: covers,ý<,

?finspectfon of the internai surfaces.

or. removable sides for visual g~~ Holes "drilled Tncal, `3o: ',!.'tein perforated suppr'pe%

..!II'-*?*::-*..:.: .

1 7lca .,r . -oreduce*h,,

r .edue ZI! ,*t~pates f,,

h'earea of flat surfaces.on.which ./ir ,,.b.`,tAccess ports or removable panels are provded appropriatecamethods may stagnate and cake during reactýl

, ccaning for"

• tsuch internalv surfaces

as b rusing, au umin& bywashin pn o r u iTo. .

SuiduOf th operation of~the bed lar-"ge particles that cannot be drained remove, dislodge, or dissolve SNM particles.--

thereactor Isequipped with a gas-jet readily, - .... to Equipment Srnsing

,iis provided .with grInder*,.- Tconnections for washdown and rinsing with-fittings liquids that for

• ..ftorn t Preclude the ve~essel for'the fluidized :will remove, accumulation bedmaterialhas a tapere 1 dislndge, or dissolve all particulate process

-'_sintered

• ...*

• ,**,:SNM..... in*.cm cor'Oa ers at the b ottr,, of;*.'-V.`,_, such materia,_. ......

• material, residua. iquid, and I'remtain on internal surfaces condensed vapors that may after the equipment has been

,ý'shut

'ollectin g

,Tnsings I .N; which SNMI"s InProvision entrained gma maydbe f-*and or dissolved. ..

....l'2i*

"ondestructive'assay

.. ... arance . :t4o permit, next -4 .*:Y.f, multiple."stages .inh'fluidized bed

3 :-instruments or internal reactors

._ý'detectf the:preencebf:or .pr empOY!surfonaceluha

.:-residual material not.visually to "identify sb

'the loc

'defflestio-baffles perforated ;gas p ýfor.m&`Ing, and p~late'

owcomersý5t'-Pflanges b."`The2 bodyr,1,of t for

!equipped with' vibrators"or fluidized'dea( baffles for, pairtitioning, external these are accessible for cleanout.

• 7or prevent packing and adhesion of impactors to

' '. .,Extended ':heat M* '.':Supplementa

,.otpermanently. Interalmechanical equipment,..

mounted fins -and'tubesg "transfer" surfaces,.o ,r rinsers, or atomizers Inside such as scrapers,- agitators,

.'*i~~~i0for :both-heating

"'~ ti g n co l equipment is capable of being to th fluidizedibed containmenta"nd coolin'gare ae e3 disassembled and removed for cleaning and inspection Sourcef heat may be preheated structure.-An alt ibottom .;.

outlet d to facilitate d..Fgroun inputs.; and cleano'ut " .

idied! r,breed r lhaptleanu'.e. "io rn tprevenit build.-b up"0of. stat .. ...or"s:ut ch.. material is used, to: prevent packing becoming encrusted, which may the bottom outlet from case.bdexpasO ndconsequent.:;" hinder bed-rmaterial during equipment the discharge of rei luidized cleanoutv'; draindown and.'

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