Publication Date: 6/1/74
    Pages: 7
    Date Entered: 2/22/84
    Title: DESIGN CONSIDERATIONS FOR MINIMIZING RESIDUAL HOLDUP OF SPECIAL NUCLEAR MATERIAL IN EQUIPMENT FOR WET PROCESS OPERATIONS
    June 1974
    U.S. ATOMIC ENERGY COMMISSION
    REGULATORY GUIDE
    DIRECTORATE OF REGULATORY STANDARDS
    REGULATORY GUIDE 5.25
    DESIGN CONSIDERATIONS FOR MINIMIZING
    RESIDUAL HOLDUP OF SPECIAL NUCLEAR MATERIAL
    IN EQUIPMENT FOR WET PROCESS OPERATIONS
A. INTRODUCTION
    Section 70.22 "Contents of applications," of 10 CFR Part 70,
    "Special Nuclear Material," requires, among other things, that each
    application for a license to possess at any one time more than one
    effective kilogram of special nuclear material (SNM) contain a full
    description of the applicant's program for control of and accounting for
    SNM which will be in his possession under license, including procedures
    for controlling SNM during its processing or use in the facility.
    Section 70.51, "Material balance, inventory, and records requirements,"
    requires, among other things, that certain licensees conduct their
    nuclear material physical inventories in compliance with specific
    requirements set forth in 10 CFR Part 70.
    The control of and material balance accounting for SNM can be made
    more effective by reducing residual holdup in process equipment
    following draindown or following draindown and cleanout. This would
    lessen the severity of problems associated with determination of the
    residual holdup component of a physical inventory and would reduce the
    component of uncertainty contributed by residual holdup to a physical
    inventory.
    This regulatory guide describes design features and
    characteristics acceptable to the Regulatory staff for minimizing the
    residual holdup of SNM after draindown or cleanout of equipment used in
    wet process operations. These features and characteristics are expected
    to facilitate physical inventory measurements and ameliorate material
    balance uncertainties without interfering with process operations. In
    particular, this guide is addressed to operations including (1) liquid
    blending and gas-liquid contacting, (2) liquid transfer and storage, (3)
    precipitation, (4) slurry transfer, and (5) liquid-solid separations.(1)B. DISCUSSION
1. Background
    Past experience and current observation of the unit operations
    used in operating systems at plants for chemical conversion, fuel
    fabrication, scrap recovery, and fuel reprocessing indicate that
    publication of general guidance for equipment design could assist in
    achieving the degree of material control that is essential for
    satisfactory protection of SNM. In processing, SNM may accumulate as a
    sizable deposit which increases during processing, or SNM may accumulate
    only during draindown. For a given process, mode of operation, and type
    of material, the amount of holdup may fluctuate near some characteristic
    value. In other cases, the quantity accumulated may continue to
    increase as operation continues and become apparent as residual holdup
    only upon draindown or cleanout.
    It is often difficult to determine the quantity of SNM holdup with
    sufficient precision and accuracy to meet the MUF and LEMUF requirements
    of Section 70.51. This determination usually includes locating,
    sampling, identifying, and analyzing the SNM. Appropriate design not
    only could assist in reducing residual holdup and consequent need for
    determination, but also could assist in increasing the effectiveness of
    draindown and cleanout, if necessary.
    ----------
    (1) Regulatory Guide 5.8, "Design Considerations for Minimizing
    Residual Holdup of SNM in Drying and Fluidized Bed Operations," is a
    parallel guide.
    ----------
    With unknown or imprecisely known quantities of residual holdup in
    equipment, the effectiveness of a material balance as a control
    mechanism is seriously impaired. Minimizing the quantity of material
    retained in process equipment generally enhances the effectiveness of a
    material protection program in the following ways.
    a. Quality of Physical Inventories
    The extent to which inaccuracy and uncertainty in measured
    residual holdup detracts from a physical inventory depends on the amount
    of holdup and the uncertainty in that amount. Therefore, the influence
    of this uncertainty on the LEMUF (limit of error on material unaccounted
    for) can be reduced directly by reducing residual holdup. By reducing
    the quantity of material that cannot be measured well, the quality of
    the physical inventory(2) is improved. In addition, the contribution of
    unmeasured holdup to the MUF (material unaccounted for) can be reduced.
    In general, one of the influential factors which must be
    controlled to achieve a satisfactory inventory is the presence of
    residual holdup and its influence on inventory uncertainty. For a
    process amenable to dynamic inventory techniques, in particular,
    credibility in the technique itself would be increased by reducing or
    removing the uncertainty of residual holdup.
    b. Susceptibility of SNM to Diversion
    Reduction of the quantity of residual holdup following draindown
    or draindown and cleanout of process equipment decreases the quantity of
    SNM which is susceptible to diversion during sampling and identification
    and subsequent separation, recycle, or recovery as appropriate.
    Decreasing the residual holdup limits the effort necessary to
    establish the presence of residual material and to remove it for a
    physical inventory. Consequently, the amount of time SNM is accessible
    and the number of people who need access to it are reduced, and the
    opportunity for unauthorized individuals to gain access to SNM during
    this stage of a physical inventory is reduced. Where the effects of
    residual holdup are negligible, an in-process or dynamic inventory
    method might be utilized, thereby reducing direct contact (i.e.,
    accessibility) of inventory personnel with SNM.
    Automated processes have the effect of directly limiting personnel
    access to SNM during normal operation. A dynamic or in-process
    inventory may be conducted for an automated process line; this would
    continuously limit access to SNM. Consequently, it is beneficial to
    consider the effects of residual holdup early in the stages of equipment
    design, particularly if shutdown and cleanout could be avoided for an
    entire automated process (e.g., chemical conversion facility) or a
    remotely operated process (e.g., fuel reprocessing plant).
    ----------
    (2) Regulatory Guide 5.13, "Conduct of Nuclear Material Physical
    Inventories," addresses the subject of conducting physical inventories
    for nuclear material.
    ----------
2. Unit Operations
    This guide is addressed to reducing residual SNM holdup in five
    unit operations common to wet chemical processes. These are described
    in the following paragraphs. For purposes of this discussion, the term
    "significant amounts" refers to those quantities which may cause
    difficulty in satisfying the inventory quality requirements of Section
    70.51.
    a. Liquid Blending and Gas-Liquid Contacting
    Gas-liquid contacting refers to the reaction of a gas with a
    liquid to yield a liquid product. An example of a gas-liquid reaction
    is the hydrolysis of uranium hexafluoride to form an aqueous solution of
    uranyl fluoride, which then may pass to a precipitation operation.
    Liquid blending is used, for example, to produce a uniform
    mixture of uranium and plutonium nitrate solutions which subsequently
    may pass to a coprecipitation operation or be transferred to a fluid bed
    drier. To prevent the formation of polymeric species of plutonium
    during mixing, control of the temperature and acidity of plutonium
    nitrate or of mixed nitrates is necessary.
    A distinguishing characteristic of gas-liquid contacting and
    liquid blending is that the bulk material is a liquid; solids normally
    are not expected to be present. In general, draindown of equipment used
    for the operations can be enhanced significantly if accompanied by
    rinsing. To remove residual deposits of any plutonium polymer formed,
    additional cleaning may be necessary.
    b. Liquid Transfer and Storage
    Liquids containing SNM are transferred and stored throughout
    a number of chemical conversion processes and fuel reprocessing steps.
    For example, uranium and plutonium nitrate solutions or uranyl fluoride
    solutions are transferred between vessels. Also, waste solutions may be
    transferred from liquid-solid separating operations to temporary storage
    tanks or evaporating ponds. Tanks are utilized for feed adjustments,
    dissolution, accountability, settling, surge, and product collection.
    In general, a low level of residual holdup can be achieved
    if equipment used for transfer and storage of liquids is flushed out or
    rinsed after draining. However, precipitation of solids or buildup of
    salt on vessel walls may resist meager attempts at rinsing.
    c. Precipitation
    In precipitation reactions, SNM in aqueous solution is
    converted to solid form by the addition of a precipitating agent. The
    resulting solid initially is in suspension but may undergo settling.
    Holding or aging tanks may be used for purposes such as crystal growth,
    chemical adjustment, or buffer storage.
    This type of unit operation is used for the conversion of
    uranyl fluoride to ammonium diuranate (ADU); uranyl nitrate to ADU;
    plutonium nitrate to plutonium peroxide, oxalate, or hydroxide; and
    mixed uranium-plutonium nitrates to mixed ADU-plutonium hydroxide. An
    additional application is the conversion of uranium-containing and/or
    plutonium-containing solutions to sols.
    In general, draindown of equipment used for precipitation
    operations may leave a significant quantity of residual holdup.
    d. Slurry Transfer
    Slurry transfer is the movement of a liquid in which solid
    or semisolid materials are suspended. An example is the transfer of
    slurries from precipitation operations (mentioned in section B.2.c.
    above) to separating or drying operations. Gels or sols containing
    uranium and/or plutonium also may be transferred as slurries.
    Draindown of equipment used for this operation without
    cleanout may leave a significant quantity of material as holdup.
    e. Liquid-Solid Separations
    Unit operations currently utilized to achieve liquid-solid
    separation, including dewatering or solvent removal, are centrifugation,
    filtration, and settling. Liquid-solid separations separate bulk liquids
    from suspensions or slurries of solids and consolidate the solid
    material as a damp cake for subsequent operations. By means of
    liquid-solid separations, SNM-containing material from enrichment or
    fuel reprocessing plants may be converted to a form suitable for fuel
    fabrication. Draindown of the equipment may leave a significant quantity
    of residual holdup.
    Operations that result in a dry solid product (e.g., drying
    and fluidized bed operations) are not included in this unit operation
    and are the subject of a separate regulatory guide.(1)3. Holdup in Liquid Blending and Gas-Liquid Contacting
    Many types of contactors (e.g., mixer-settlers, mixer columns,
    scrubbers, etc.) are used for liquid blending and gas-liquid reactions
    to produce liquid products. Although pulse columns may be preferred for
    liquid-liquid contacting, centrifugal contactors have the advantage of
    low holdup volume. Therefore, a small decrease in inventory error can
    be realized by using centrifugal contactors rather than pulsed columns.
    Disadvantages of centrifugal contactors are thay they are expensive and
    must be constructed to small tolerances. Furthermore, the kinetics of
    some reactions are not favored by the use of centrifugal contactors.
    Liquid holdup in liquid blending and gas-liquid contacting
    equipment can occur at low spots in lines, in pump cavities, and in
    vessels without bottom outlets. Internally mounted equipment such as
    mixers, baffles, and spray rings provide additional surfaces where
    material can collect. However, as is true for most processes in which
    solids are absent, liquid products generally can be readily removed by
    gravity, i.e., by simply draining and flushing.
    More complex problems are encountered when plutonium in solution
    forms polymeric species of a colloidal or gelatinous character that
    makes their removal from equipment difficult. Acidification can, at
    least partially, resolubilize the polymer, but kinetics limit the rate
    at which this occurs. To improve the ability of a facility to meet
    accountability requirements, it may be necessary to provide cleanout
    capability in those units of equipment where polymers could conceivably
    form.
4. Holdup in Liquid Transfer and Storage
    Liquids are stored in various kinds of vessels and are transferred
    to process equipment through piping systems by means of gravity, pumps,
    steam or air jets, air lifts, or vacuum. When liquid is transferred by
    any of the above means, holdup problems can result from the existence of
    stagnant zones, low points in lines, or incomplete draining of
    equipment. As for the previous unit operation, internally mounted
    equipment such as mixers, baffles, and spray rings provide surfaces
    where material can collect. Therefore, equipment design effectively
    could be directed toward improved draining, supplemented by provisions
    for rinsing and flushing.
    Gravity flow of material in a process is beneficial since it
    provides a degree of self-action (automation) for draining and flushing
    operations. Feed solution pumped to the highest point in a process
    would then cascade downward through the process network. Transfer lines
    for the entire process would be sloped for better overall drainage.
    However, even with an entire system designed inherently for free
    drainage, excessively flushing out the wet end of a process to reduce
    the quantity of SNM in the equipment for inventory purposes can produce
    a large quantity of dilute solution that is unsuitable for processing.
    Consequently, vacuum transfer and removal of solutions may be
    preferable.
    More onerous holdup problems include the buildup of sludges in the
    bottoms of tanks used for accountability, transfer, or storage and the
    residual jet heels that remain after such tanks are emptied. Dissolution
    tanks have been constructed of stainless steel and Teflon-coated
    stainless steel (for other than irradiated service); the latter is
    preferred for purposes of reducing surface accumulation. Storage tanks
    and other vessels should be accessible for the installation of sensing
    devices such as dip tubes or inductive and sonic level detectors. This
    recommendation should be considered in view of other factors such as
    shielding and protecting vessels containing SNM from severe weather by
    embedding the vessels in concrete.
5. Holdup in Precipitation
    Slurries and suspensions formed by precipitation can be removed
    readily from vessels by simple draining if settling does not occur.
    Loosely adhering solids on vessel walls can be dislodged by flushing.
    In sustained operations, however, solids may deposit on and adhere to
    surfaces in a manner that makes removal difficult. Agitation, which is
    provided principally to enhance particle agglomeration, reduces but does
    not eliminate this deposition of solids. In the preparation of sols
    using precipitation as a process step, agitation is necessary to
    resuspend the precipitate. The amount of deposited solids usually is
    sufficiently large to necessitate total cleanout for a physical
    inventory.
    Several troublesome problems are related to residual holdup during
    precipitation and digestion. Where internally coated vessels are used
    for processing (e.g., Teflon-coated glass for fuel particle
    preparation), a positive seal should be assured between the lining and
    the vessel walls to prevent accumulation of particles in annular spaces
    between the two surfaces. Another problem can be the oxidation of
    intermediate compounds to undesirable compounds that may be gummy and
    insoluble. This could cause plugging of equipment and process piping if
    not controlled. For example, PuF(3) may oxidize to PuF(4) * 2.5H(2)O.
    In addition, some process intermediates (e.g., PuF(3)) or interferents
    (e.g., polymers of plutonium) have a tendency to deposit on the surfaces
    of vessels used for precipitation and digestion. Plugging can be caused
    when flakes or globules of the deposits break loose from the surface and
    flow to a constriction such as an outlet or other piping. A more
    serious consequence of such deposits of SNM may be the hazard from
    accumulation of large yet unknown quantities.
    Use of antioxidants and efficient agitation can assist in
    preventing these problems of holdup. The composition of the materials
    of construction as well as the condition of the interior surfaces of
    vessels (e.g., roughness or texture) may equally influence residual
    holdup prevention. The differences between these two factors may indeed
    be subtle.
    Where deposits form on equipment surfaces, ultrasonic treatment
    can be effective for removing deposits. Such a cleaning technique may
    be needed if other methods of altering process conditions (e.g., use of
    surfactants) or modifying process equipment (e.g., electrostatically
    charging polyethylene vessels or maintaining polished internal surfaces)
    are ineffective.
    Unfortunately, anomalous situations may arise if it is not
    possible to identify a deposit sufficiently to understand its
    properties. For example, flaking of deposits and consequent plugging of
    piping downstream can be decreased by flushing precipitator vessels with
    acid between batch runs. However, reduced plugging can be a result not
    only of the acid dissolving the deposits but also of the acid causing
    the deposits to be more adherent. More adherent deposits are less
    likely to flake off and plug the equipment, but large quantities may
    accumulate.
6. Holdup in Slurry Transfer
    Slurries are transferred from one process vessel to another by
    methods that are essentially the same as those used to transfer liquids.
    However, holdup problems are more complex for slurry transfer because of
    a tendency of the suspended solids to settle out of the carrier liquid.
    Although different materials exhibit different settling characteristics,
    a critical velocity exists below which particles begin to settle out.
    Such settling is most likely to occur at shutdown or when flow rates are
    reduced because of abnormal operations. In such situations, pumps and
    valves may act as sites in which solids can accumulate or be trapped.
    Cavities and recesses in pumps used to transfer slurries or
    suspended solids can collect significant quantities of solid material
    that are difficult to flush out. Transferring material by jets or gas
    lifts may minimize this difficulty.
    When screw conveyors are used to transfer moist pastes, a coarse
    intermediate cleanout may be necessary for operational reasons, i.e., to
    prevent subsequent plugging of the process line. Additionally, a more
    complete cleanout may be needed at the end of each run. Because
    frequent cleanouts are necessary for operational reasons alone, a paste
    transfer method necessitating less interruption is desirable.
7. Holdup in Liquid-Solid Separations
    Unit operations used for the separation of liquids and solids are
    centrifugation, filtration, and settling of slurries. A wide variety of
    devices is used for this operation. The type selected is dependent on
    the nature of the material being processed, the throughput rate, and the
    liquid content of the feed and product. Holdup problems are discussed
    below in connection with the type of equipment and the characteristics
    of the process material.
    a. Centrifuges
    In facilities with high throughputs, two centrifuges in
    series are typically used for separation. A primary centrifuge for
    separation and recovery of bulk solids is upstream from a clarifying
    centrifuge for removal and recovery of residual trace solids. The
    principal purpose of the primary unit is to produce a concentrated solid
    product having a relatively low water or solvent content. The second
    centrifuge serves principally for clarifying the centrate (i.e., the
    centrifuge effluent) from the first centrifuge. In processes in which
    the centrate from the first unit is not recycled through the fuel
    preparation process, the clarification step serves to recover residual
    SNM before the centrate is transferred to waste treatment.
    Most of the material held up in a centrifuge after draindown
    exists as unremoved solids. In a batch basket-type centrifuge, holdup
    is normally small after unloading by normal procedures. However, in a
    solid-bowl continuous centrifuge equipped with a helical conveyor to
    remove solids, any solids deposited on the surfaces of the flights of
    the conveyor, on bowl surfaces in the clearance space between the
    flights of the conveyor and the bowl, and on surfaces of the
    solids-discharge cavity are difficult to remove. Simple flushing is not
    likely to be effective in dislodging solids, either from surfaces
    contacted by the flush or surfaces inaccessible to the flush.
    Comparable difficulties occur with other types of centrifuges,
    especially continuous centrifuges having complex unloading mechanisms.
    b. Filters
    In facilities having low throughputs or in facilities
    handling highly enriched uranium or plutonium, dewatering may be
    effected by continuous (e.g., rotary) filters or batch filters. For
    reasons of criticality control, this equipment is typically small in
    size. Following draindown, less material may be held up in filters than
    in centrifuges.
    Although batch filters and drum filters have readily exposed
    surfaces that can be cleaned out by simple flushing or mechanical
    removal, it is difficult to clean out other types of filters, e.g.,
    plate-and-frame presses. Leakage and bypassing of material can occur
    around the edges of a filter drum used in a continuous process line; pan
    filters have better cake removal than do drum filters. In filters such
    as those using a metal grid to support a paper filter medium, fines can
    lodge in the interstices of the equipment.
    Parts of the separation system exposed to centrates and
    filtrates usually can be drained readily, but simple flushing probably
    does not remove solids adequately. Cleanout of plate-and-frame filter
    presses in particular can be difficult since centrates and filtrates
    each contain suspended solids, and sustained normal operation results in
    holdup of solids.
C. REGULATORY POSITION
    For purposes of facilitating the measurement and/or recovery of
    residual special nuclear material held up in process equipment and to
    improve the accuracy and reliability of a physical inventory, the amount
    of SNM held up in equipment should be minimized. The design of
    equipment used to carry out physical or chemical changes on special
    nuclear material by wet operations, including liquid blending,
    gas-liquid contacting, liquid transfer and storage, precipitation,
    slurry transfer, and liquid-solid separation, should incorporate
    features that minimize residual holdup. Some appropriate equipment
    design features and characteristics whose use is generally acceptable to
    the Regulatory staff for this purpose are described in the following
    paragraphs. These should be implemented to the extent practicable.
    Usage also should be consistent with quality assurance, health, and
    nuclear safety codes that may be applicable.
1. General Design
    a. Vessels, piping, valves, and accessory equipment should be
    designed to minimize undrained volume and should be free draining where
    practicable.
    b. Inside surfaces of equipment should be free of crevices,
    cracks, protrusions, and other irregularities that could entrap
    material.
    c. Surfaces that contact SNM should be selected and coated,
    polished, or machined to prevent or resist the adherence of liquids or
    solids.
    d. Overlapping metal surfaces in contact with process material
    should be avoided except where sealed by welding; internal welds should
    be ground flush with inner surfaces. Exceptions may be gasketed
    openings such as inspection and cleanout doors or ports.
    e. The internal angles, corners, and recesses should be rounded
    with a radius larger than a minimum radius, for example, one fourth
    inch.
    f. Seams that may promote corrosion should not be used.
    g. Materials of construction that contact SNM in any form
    should be selected to minimize corrosion, dissolution, or erosion of
    surfaces during operation or during contact with rinse solutions used
    for cleaning.
    h. Structural integrity should be adequate to resist formation
    of leaks, cracks, and crevices due to stresses such as thermal and
    vibratory stresses. Accordingly, valves and pumps should be installed
    so as to minimize stresses on attached piping and vessels.
    i. The influence of operating variables such as material flow
    rate, pH, concentration, and temperature should be evaluated to reduce
    undesirable formation of holdup (e.g., caking or sticking) that might be
    induced by operating in an undesirable range of operating conditions.
    j. Flow control valves should have a minimum of internal holdup
    or obstruction to flow and should be installed in a location and
    position that enhances draining of the entire piping network.
    k. Pipe lines for slurries or suspensions should be sized
    according to process flow requirements so that flow velocity is above
    the critical velocity at which settling can occur.
    l. Material that contains solid forms of SNM, e.g., slurries
    and filtrates, should be transferred continuously to avoid settling.
    m. Process units should be closely coupled and sized, with
    minimal intervening holdup tanks.
    n. Equipment design should eliminate as many areas of
    stagnation and residual accumulation of solutions and slurries as
    possible (e.g., in order to facilitate the capability for conducting
    dynamic inventories).
2. Internal Design
    a. Equipment should have a minimum of internal components upon
    which process material can collect. For example, bowls, product
    chambers, and centrate collection chambers of centrifuges should be
    designed to be free of nonessential protrusions and ledges. Additional
    surfaces in the form of helical conveyors, liquid accelerating bars, and
    devices for removing slurries should be kept to a minimum.
    b. The use of internal mechanical agitators in blenders should
    be avoided. If agitators are used, they should be designed to permit
    surfaces to drain freely and to present minimum surface for the
    collection of solid particles. Seals such as self-sealing packing
    glands and cone pressure seals for maintaining a tight seal around
    stirring shafts should necessitate minimal maintenance.
    c. Sensing devices such as thermocouples or level detectors
    should be installed in a manner that minimizes the amount of solid
    material that can be retained on the surfaces of such devices.
    d. Extended surfaces such as packing (e.g., Raschig rings, Berl
    saddles, etc.) should be avoided. Permanently mounted process equipment
    internals that cannot be removed for cleaning should be designed to
    allow rinsings and normal contents of vessels such as liquid blenders to
    drain freely from the bottom of the equipment. If extended surfaces are
    necessary, the licensee should be able to demonstrate that an acceptable
    limit of error can be obtained, either by rinsing or by removal of
    packing.
    e. All lower portions of vessels such as liquid blenders and
    storage tanks should be sloped (e.g., tanks may have conical or dished
    bottoms) to allow liquids to drain freely.
    f. Equipment such as product and centrate collection vessels or
    chambers of centrifuges should be designed to contain material without
    loss by foaming, splatter, or formation of sprays in wet processes.
3. External Design
    a. Visual access should be provided to all surfaces or spaces
    where material is likely to accumulate; alternatively, clearance should
    be provided to permit external use of nondestructive assay instruments
    or internal probes to detect the presence of SNM or to identify the
    location of residual material not visually accessible.(3) b. Liquid transfer systems or vessels should have drains and
    valves installed at the lowest points to permit draining by gravity or
    other means. The stagnant volumes that may collect in drain lines and
    between tees and drain valves should be kept to a minimum. Transfer
    lines should have adequate slope to permit draining of process solutions
    after shutdown. If a pump is used, a drain equipped with a valve should
    be installed at the low point of the transfer line.
    c. Equipment used to transfer solutions from storage tanks
    should be provided with adequate check valves to prevent siphoning or
    suction of process solutions into the steam or air supply lines. This
    equipment includes steam jets, steam lines, air lifts, gas purge lines,
    and vacuum relief valves.
    d. If vacuum transfer of liquids is used, the vacuum pumps
    should be protected from corrosive vapors or SNM-containing liquids by
    suitable traps and filters. If other transfer methods such as liquid
    piston pumps are employed, these should also be protected.
    e. Although seals and drain valves should be designed to be
    leaktight under normal conditions and to be free of crevices and
    cavities, provision should be made for the collection of material
    leaking through seals and valve seats when abnormal conditions exist.
    f. Gravity transfer of liquid slurries from one vessel to
    another and of wet solids, centrates, and filtrates from centrifuges and
    filters should be used in preference to the use of transfer containers.
    If pumping of liquids or slurries is necessary, gas lifts should be
    used, provided the disengagement of gas does not result in excessive
    foaming or entrainment. Pumps should be designed to minimize cavities
    and stagnant volumes. All pumps should be mounted for maximum drainage
    and designed for minimal cavities and undrained volumes.
    g. Equipment should be arranged so that connecting piping
    follows the shortest practical route with the fewest number of bends and
    fittings.
    h. The piping network should be designed to allow free drainage
    to accumulation points.
    ----------
    (3) Regulatory Guide 5.23, "In Situ Assay of Plutonium Residual
    Holdup," provides additional methods and procedures regarding
    measurements.
    ----------
4. Design for Accomodating Cleanout
    a. Equipment such as precipitators and digestors should be
    provided with access ports, removable covers, or removable sides to
    allow visual inspection of the internal surfaces.
    b. Access ports or removable panels should be provided to allow
    cleaning of internal surfaces by appropriate methods such as brushing,
    vacuuming, washing, scraping, or rinsing to remove, dislodge, or
    dissolve SNM particles.
    c. Equipment should be provided with fittings for connections
    for washdown and rinsing of internal surfaces of vessels and pipes.
    Steam, water, or appropriate chemical solutions should be used to
    dislodge, dissolve, or otherwise remove all particulate process
    material, residual liquid, and condensed vapors remaining on internal
    surfaces of the equipment. Quick-connect (-disconnect) couplings should
    be utilized in process lines where frequent cleanout is necessary.
    d. Provisions should be made for flushing and draining and for
    removing and collecting rinsings in which SNM may be entrained or
    dissolved. Removal of material from centrifuge bowls, product
    collection chambers, and transfer lines should be facilitated by designs
    that permit disassembly. Also, distribution devices for flush solutions
    should be designed and arranged to allow flush solutions to contact the
    interior surfaces and cavities of the process equipment and of auxiliary
    devices inside the equipment. Flush lines to plutonium-containing
    vessels and equipment should be connected only to acidified solution
    sources.
    e. Supplementary internal mechanical equipment not permanently
    mounted such as scrapers, agitators, atomizers, and rinsers should be
    capable of being disassembled and removed for cleaning and inspection.
    f. Bottom outlets and drain plugs should be selectively located
    to facilitate draindown and cleanout. This is particularly important for
    vessels handling plutonium-containing solutions that can form polymeric
    compounds which may settle.
    g. Wash or flush lines and spray rings should be connected at
    high points of transfer lines and piping networks or upper zones of
    interconnected vessels to permit flushing of accumulated solids.
    h. Drain valves should be installed at low points in vessels
    and piping systems. Pumps for the piping network should be designed to
    facilitate disassembly for complete cleanout.
    i. Interconnecting piping and pumps should be capable of being
    cleaned by flushing with clean drainings from storage or process
    vessels. Where necessary, separate flushing lines should be connected
    to transfer lines to assist in cleaning.
    j. Storage vessels should be provided with separate bottom
    drain valves that permit their contents or wash solutions to be removed
    without affecting interconnected vessels.
    k. Provisions (e.g., instrumentation) should be made to permit
    verification that all material has been removed from transfer lines.
    l. Jets should be installed so as to completely empty vessels
    such as liquid storage tanks. To further decontaminate vessels, air and
    steam sparges should be installed as necessary.
    m. The use of filters whose components must be disassembled for
    recovery of solids (for example, plate-and-frame filter presses) should
    be avoided.
    n. Filter media should be removable or be capable of being
    backwashed in situ. Removable filter media should be treated by
    leaching or by combustion and leaching for the recovery and
    determination of SNM.
    o. The composition of flush solutions for equipment containing
    residual plutonium should be controlled to avoid polymerization or
    precipitation (e.g., adequate acidification).
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