Note: Descriptions are shown in the official language in which they were submitted.
~1727~
Description
Solidification of Radioactive Waste Effluents
TECHNICAL FIELD
This invention relates to processes and installations for the
solidification of liquid radioactive effluents from nuclear
facilities, and more particularly to the solidification and
encapsulation of low and medium level radioactive liquid wastes
from nuclear power plants, nuclear research laboratories and
reprocessing plants. The invention is especially useful for the
concentration and solidification of relatively dilute, low level
liquid wastes from light water reactors (LWR) such as pressurized
water reactors (PWR) and boiling water reactors (BWR).
BACKGROUND ART
It is well-known to concentrate liquid wastes from LWR's to a
limited extent and then to encapsulate such concentrates in various
types of matrices such as cement, bitumen or synthetic resin
polymers. The waste and matrix mix is then stored in containers.
In order to further reduce the quantity of waste and the corres-
ponding number of storage containers, it has been proposed more
recently to completely dry the waste and ~ncapsulate a dry product
in the matrix material. The techniques being used and developed in
an effort to reach a dry product before encapsulation are referred
to broadly as "volume reduction'`.
In this specification, the terms "dried waste" and "dry
product" mean waste solids which contain substantially no free
water. Combined water, such as water of hydration or crystalli-
zation, may be present.
Because of the problems encountered with volume reduction as
discussed below, many of the present waste treatment facilities
still encapsulate some form of liquid concentrate. This practice
leads to a large number of radwaste storage drums which must be
stored temporarily above ground and then permanently disposed of,
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either at sea or by land burial. With present practices for
encapsulating concentrate, waste from a 1,000 megawatt (MWe) PWR
can lead to the accumulation of more than 2,000 standard drums for
each year of normal operation. The waste from BWR's of the same
power may require more than 3,000 standard drums per year.
Extending such figures to the hundreds of existing and planned
nuclear power plants, hundreds of thousands of drums will have to
be stored and disposed of each year. For this reason, efforts to
develop a satisfactory volume reduction technique have intensified
in recent years.
Effective volume reduction can result in considerable savings,
both in money and manpower, for a number of reasons. The amount of
matrix material needed for encapsulation of a given quantity of
waste is reduced where the waste is in dry solid form. Similarly,
the quantity of wzste that can be placed in each container is
increased so that the number of containers necessary is also
reduced. Either matrix encapsulation or placement of the waste in
containers is considered to be a way of enveloping the waste in a
protective envelope ior purposes oi this specification. As a
conservative estimate) the final volume of enveloped waste arising
from low level power plant effluents can be reduced by a factor of
at ~east 5 to 15 by volume reduction techniques. A reduction in
the number of storage containers produces a corresponding reduction
$n the capacities of the facilities needed for interim storage,
container handling, container transportation, and ultimate di~posal
and ln the manpower required for all such operationsc ~s an
example of the estimated cost savings for a 1,000 MWe PWR power
station, considering the overall costs for conditioning,
encapsulation and ultimate disposal at sea of 1 cubic meter of
waste effluent containing 12 weight percent dissolved solids~-the
savings achievable with effective volume reduction is estimated to
be in the range of $500 to $1,500 per cubic meter of effluent.
~ost savings in this range will compensate within just a few years
for the additional capital investment required for installation of
a volume reduction system.
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A furthe~ advantage of volume reduction is that lt enhances
safe handling and disposal of the waste material. Since smaller
quantities of waste can be handled, stored, transported and
ultimately disposed of permanently, there is realized a correspon-
ding reduction ln the hazards to personnel and a corresponding
~ncrease in the useful life of equipment. Safety to the environ-
ment is enhanced both by the smaller number of waste containers and
the avoidance of any danger of a releasable water fraction
containing radioactive ions.
Notwithstanding the known advantages of volume reduction, a
number of difficulties have been encountered in developing an
effective volume reduction technique. Attempts have been made to
use thin-film evaporators as the drying apparatus in volume reduc-
tion systems. However, prior to reaching a dry state, waste
concentrate becomes a heavy paste at solids levels in excess of 60
weight percent. This paste dries relatively slowly as a fllm, and
in order to reach dryness in a thin-film evaporator a vacuum is
used along with a relatively slow rate of material advancement
along the heated surface. Such lnstallations therefore require
expensive auxiliary equipment to create the vacuum and the
evaporator is not operated at an efficient throughput because the
feed rate is limited by the drying rate of the film. In addition,
the feed rate must be closely monitored and controlled so that
drying occurs at or very near the evaporator outlet. Premature
drying will causP blockage of transport passages and jaMming of ~he
evaporator rotor. ~otwithstanding such control, blockage frequent-
ly occurs anyway after relatively short periods of operation due to
the gradual buildup of hardened layers of concentrate at the heated
wall surface, a condition which is aggravated by the slow rate of
material advancement. Therefore, such equipment operates much more
efficiently as a concentrator rather than as a dryer.
Other types of dryers have also been proposed for use in
vol~me reduction systems, such as spray dryers and drum dryers.
These types of dryers create large amounts of dust particles ~hich
are difficult to remove from exiting gas streams and can rapidly
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11~2~
erode or jam gas treatment equipment. Spray dryers are further
deficient in that solids can buildup in-and around the spray
nozzles and lead to blockage. Drum dryers are further deficient in
that the dry layers formed on the heated drum can be difficult to
scrape off or otherwise remove.
Although some of the foregoiug problems might be alleviated by
incomplete drying of the concentrate, significant moisture content
in the waste product leads to problems in the characteristics of
the encapsulated product. The presence of water in the radioactive
fraction makes it difficult to control the quality of the final
waste and matrix product. In other words, the amount of dry cement
to be used to make up the final composition depends upon the total
water in the waste and matrix mix and the amount of water in wet
concentrate or partially dried solids can fluctuate and is
difficult to control with any degree of accuracy. As a result,
past practices often have led to either too much or too little
water in the encapsulated product. Water is also extremely
detrimental in a bitumen matrix as this matrix must be heated and
the heated matrix causes water vapor to form which interferes with
the encapsulating process. ~ater is also detrimental in most
resin polymer matrices as it inhibits the polymerization reaction.
For these reasons, the presence of water in the waste fraction
results in a product having poor water resistance (because of the
presence of non-fixed, leachable radioactive ions), poor chemical
resistance, and inferior structural integrity (mechanical
strength).
DISCLOSURE OF THE INVENTION
The present invention overcomes the foregoing deficiencles of
the prior art and produces thoroughly dry solid waste particles at
an unusually high rate. The particles may be encapsulated in a
high integrity matrix. The chemical composition of the waste
eff~uent to be solidified will vary depending upon its origin. The
effluent is subjected to chemical treatment to adjust the pH to a
basic range (greater than 7,0~ and/or to form insoluble compounds
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by nucleation and/or precipitation of waste solids, which may be
either suspended or dissolved ln the waste liquid. If generated by
a PWR, the effluent will usually contain boric acid and lime is the
preferred chemical reagent for adjusting the pH and insolubilizing
S the solids of such effluents. Other metal hydroxides may also be
used, such as the hydroxides of other alkaline-earth metals. If
generated by a BWR, the effluent usually contains sulfates and the
preferred chemical reagent is barium nitrate. Other effluents
which can be treated by the present invention include those from
the drains of laboratory facilities and other nuclear industry
complexes and from other types of nuclear reactor facilities, such
as low and medium, liquid wastes from spent fuel reprocessing
plants.
Many effluents are rirst subjected to at least some degree of
initial concentration in the was~e treatment system of the
generating facility, such as in a large capacity evaporator, prior
to delivery for interim storage in a hold-up tank. The resulting
hold-up tank liquid may contain dissolved and/or suspended solids
in amounts up to about 10 to 25 weight percent. Chemical treatment
of this hold-up tank waste may take place either in a mixing vessel
to which is added the chemical reagent or by adding the chemical
reagent directly to the special ~rying unit described below.
Sufficient reagent is added to raise pH above 7.0, preferably inco
the~range of 10 to 12. This reagent is added in a water solution
state or in a dry state. The chemical reagent selected should bind
radioactive ions into a solid mass upon drying and preferably
produce insoluble salts capable of precipitating from solution as
susRended solids either immediately or upon further concentration
of the waste. In addition, the dried waste solids must be
compatible with the matrix material and produce an encapsulated
product having good mechanical strength and chemical resistance and
a substantially non-leachable solid mix. Where the reagent is
lime~ it is preferahly added in an amount between 30 and 100 weight
percent relative to total solids in the effluent. Where barium
nitrate is the reagent it is preferably added in an amount between
20 and 50 weight percent relative to total solids. If the reagent
is added via a separate mixing vessel, the waste liquid is
preferably stirred for approximately 30 to 6Q minutes to
thourol~ghly intermix the chemical reagent with the waste liquid.
- This hold-up time for reagent mixing is eliminated where`the
chemical reagent is introduced directly into the special dryer
unit. --
Either after or before chemical ereatment~ the waste is sent
to a concentrator which is preferably of the thin-film evaporator
type and may have either a vertical or horizontal configuration.
Such evaporators have vanes or paddles mounted on a rotor which
revolves at relatively high speed in the range of 400 to 1,000
rpm. D~e to this rotational speed, the centrifugal force imparted
to the waste material by the action of the vanes produces a rela-
tively thin film upon an opposing wall which is heated to vaporize
moisture from the film. Although wall temperatures may vary over a
wide-range, they are preferably between 15Q and 300 C. Such
evaporators perform most efficiently with an outlet concentration
in the range of 30 to 70 weight percent solids> prefPrably 40 to 60
weight percent, and most preferably at about 50 weight percent.
The retained liquid in such concentrates allows the rotor to ope-
rate at its optimum speed, optimizes concentrate throughput, and
avoids drying of the concentrate film to levels that can cause
blockage of discharge passageways and ~a~ming of the rotor.
Following concentration of the waste liquid in the evaporator,
the resulting concentrate is sent to a special drying unit
comprised of a mixing apparatus having a heated wall and a rotor
with rugged paddles for both working a heavy concentrate and posi-
tively advancing this concentrate and the resulting dry material.
Because the energy input levels required to perform such functions
per revolution of the rotor are much greater than those of a thin-
film evaporator, the mixer rotor revolves at a significantly slower
3~ rate, generally in the range of 25 to 75 rpm, preferably 40 to 5Q
rpm. In the mixer/dryer, ~he concentrate is heated to a tempe-
rat~re above 100 C, preferably in the range of 150 to 300 C. At
these temperatures and at solids concentrations above about 50
percent, the concentrate forms a hardened layer or crust on heated
surfaces and the strength of the paddles and the rotatlonal speed
o~ the rotor are such that ~his crust can be broken up into
particles and dried without jamming the rotor or blocking transport
passages. The working means also includes ohe
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or more helical members carried by the rotor to
positively advance the concentrate and resulting dry
particles. The helical member may be comprised of a
blade or paddle canted relative to the axis of the
rotor so as to impart positive axial thrust toward the
mixer outlet. m e mixing apparatus may further include
self-cleaning means for removing hardened concentrate
from the internal mixer element.
In accordance with an embodiment of the
invention, there is provided a,process for enveloping
a radioactive waste liquid containing solids which
comprises: ,
chemically treating said waste liquid by adding
at least one~chemical reagent to adjust the pH of said
liquid to greater than 7.0,
pretreating said waste liquid so as to form
a concentrate containing a greater concentration of
solids than in said waste liquid,
feeding said concentxate to a mixing apparatus
having a heated wall and rotor means for evaporating
said liquid from said concentrate so as to form dry
solid material, said rotor means including advancing
means for positively advancing said dry solid material
toward an outlet of said mixing apparatus.
By discharging the concentrate from the thin-
film evaporator before it reaches a heavy, hard to work
state, much higher rates of throughput can be employed.
Instead, the heavy concentrate is dried in a heated
mixing apparatus of rugged structural design capable of
a vigorous mixing and shearing action at high throughput
rates. In addition, the mixing and advancing elements
cooperate with each other and with stationary surfaces
and other elements in a self-cleaning action which shears,
removes and grinds up the hardening layers of solids so as
to prevent buildup on those elements and surfaces and
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produce a fine, powdery product. m e mixer is thus used
both Eor drying the concentrate and for breaking up
hardened and caked layers of dried concentrate. Commer- -
cially available mixers of relatively small size are
capable of performing these functions and completing the
drying process at optimum efficiency. The heat capacity
and length of the mixer is such that the dried waste
solids reach the mixer outlet in the form of a free-
flowing powder. It follows that the present invention
makes optimum use of the operating characteristics of
both a thin-film evaporator and a heated mixer to
produce an optimum dried waste'product.
me term "thin-film evaporator" also includes
drying equipment which can be used as a concentrator,
15 ' provided a relatively thin film is coated on the heated
surfaces, and the film is not allowed to dry but is
removed before the moisture content is reduced to
below 30%. me thin-film concentrator may be of either
a horizontal or vertical type. The preferred thickness
of the concentrate film in the evaporator is in the range
of 0.5 to 5 millimeters, preferably 1 to 3. '
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The moisture vapor produced both in the concentrating section
of the evaporator and in the drying section of the mixer is passed
through a separator apparatus for removing entrained solids and any
carry over of liquid droplets. Although the separator may be
separate from either the evaporator or the mixer/dryer, the
evaporator preferably includes an integral par~icle separator
section, and vapor from the mixer/dryer is vented back to the
evaporator where it passes through the integral separator along
with evaporator vapdr. After removal of particulates, the overhead
vapor stream, which may also contain non-condensable gases, is sent
to an auxiliary gaseous treatment system of conventional design.
There the condensable portion is cooled to produce condensate and
the non-condensable gases are fil~ered and discharged to a
controled ventilation system.
The dry particles of waste product discharged from the
mixer/dryer may then be stored as such in a container envelope,
such as a steel drum, or first mixed with a matrix material to
encapsulate the dry particles. Encapsulation in effect provides a
dual envelope for the radioactive solids, the first envelope being
solidified matrix material and the second envelope being the
container for receiving the waste and matrix mixture prior to
matrix solidification. However, it is to be understood that a
single envelope comprised of either the matrix material or the
container may be sufficient, depending upon the regulations for
handling and storage established by appropriate authority.
The mixing of dry waste powder with encapsulating matrix may
be done either in a discontinùous manner (batch) or in a continuous
manner. For batch mixing, a separate mixer receives a measured
portion of dry waste particles and a measured portion of matrix
material, the mixture being agltated until a substantially homoge-
neous mass is obtained. For a continuous encapsulation step, a
separate mixer of the continuous ~ype may be used in place of the
separate batch mixer. However, an important feature of the present
invention is that the last or downstream portion of the mixer/dryer
may ~e used as an integ~al mixing section for continuously
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encapsulating the dry waste particles in an enveloping matrix. For
this purpose, the heated jacket around the wall of the dryer is
split into two parts or jackets, the upstream ~acket providing
normal heating over the first 50 to 70 percent of the mixer/dryer
length. The downstream ~acket surrounds a matrix mixing sectionO
~ereafter in this specification a heated mixer without a matrix
~ixing section will be referred to as a "dryer" and the upstream
heated section of a mixer with an integral matrix mixing section
will be referred to as a "drying section".
10The matrix mixing section may be heated only as necessary for
incorporation of solid waste in a bitumen matrix. In this connec-
tion, the matrix mixing section may be cooled instead of heated so -
as to control the temperature of copolymerization of a resin
polymer matrix as may be appropriate to improve the characteristics
of the final encapsulated product. The incorporation of the waste
in cement can usually be made at ambient temperature or below.
Because of the substantially moisture free nature of the dry
particles exiting from the dryer or dryer section, it is possible
to encapsulate an unusually high percentage of dry waste particles
in the final matrix encapsulated product. The weight percentage of
solid waste relative to the total mixture preferably falls within
the range of 40 to 70 percent and may go as high as 75 percent
without adversely affecting the characteristics of the encapsulated
product. The maximum conceneration of radioactive ions in the
~5 final dried product produced from L~ effluents usually falls
within the range of 1 to 100 curies per cubi~ meter. However,
higher radioactive concentrations may be realized where the waste
treated is from other sources 9 such as reprocessing plants.
Although all of these wastes can be solidified by the present
invention, provided the chemical composition of the waste forms a
`dry, non-sticky solid compatible with the matrix materiall
encapsulation of high activity solids may be limited by radiation
level requirements at the surface of the drum or other envelope.
A number of different materials may be used as the matrix for
encapsulating such high percentages of solids. These include
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bitumen, resin polymers, and cement. The bitumen preferably has a
penetration point in the range of 40 to 50 The preferred resin
polymers are thermosetting polyester resins. Most preferably, they
are thermosetting resins formed by polymerization reactions of
unsaturated glycol monomers such as propylene glycol with
orthopthalic acid and a vinyl monomer such as styrene.
The most economical matrix ma~erial per cubic meter of dry
waste is believed to be cement which is estimated to cost less than
bitumen by a factor of approximately 2 and less than synthetic
resin polymers by a factor of about 10 to 15. Cement also has the
advantage of permitting solids encapsulation to take place at
ambient temperatures. The preferred cement is one having a high
alumina content and a low conten~ of calcium oxide and silicon
dioxide (silica). The most preferred cement composition contains
alumina in amounts equal to or greater than about 35 weight
percent. Such cement gives after setting principally monocalcium
aluminate; other constituents include the compound consisting of 12
parts calcium oxide and 7 parts aluminum oxide and the compound
consisting of 2 parts calcium oxide and 1 part silicon dioxide.
There may also be smaller quantities of the compound consisting of
2 parts calcium oxide, 1 part aluminum oxide and 1 part silicon
dioxide and the compound consisting of 1 part calciuT~ oxlde and 2
parts aluminum oxide~
Another important feature of the invention is the incorpora-
- 25 tion of dry waste particles in cement ant water. Previous volume
- reduction techniques resulted in a ~aste slurry that was only
partially concentrated and contaiTIed a relatively high percentage
of ~ater. This water had to be taken into account in ad~ing cement
either in dry form or as a previously mixed slurry. Because of
the relatively unknown and variable quantity of water ln the waste~
prior to adding the rement~ it was extremely difficult to control
the quality of the final produrt and the ratio of added cement to
total waste had to fàll within a relatively narrow range to take
into account the range of water that might or might not be
present in the waste.
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This is a particular problem where the pas~ practice has been to
mix cement matrix material with a wet concentrate which provides at
least part of the water needed for the final cement matrix. There
have been occurrences in the past where the water fraction of the
~aste concentrate was not adequately fixed during the encapsulation
process leaving dissolved radioactive ions free for possible
release during subsequent handling or storage. Contrary to past
practices, the solid waste particles of the present invention are
in a dry state when mixed with the inorganic cement and the water.
This step yields a much superior product over that where a portion
of the water is provided by the waste material. The relative
proportions of added cement and added water may vary over a rela-
tively wide range without adversely affecting the superiority of
this product.
The water may be added to the dried waste either with the
ce~ent as a premixed slurry or separately as an independent
ingredient. Where there is continuous mixing of the matrix
materials, such as when using the integral dryer/mixer, it is
preferable to add the cement and water separately with the point of
water addition spaced circumferentially and/or axially downstream
from the point at which dry cement is added. The spacing between
the points of addition is sufficient to avoid plugging of the line
for cement addition. Where cement ànd water are premixed, the
resulting slurry may solidify prematurely in the slurry addition
line. For some applications, it may also be desirable to first form
a dry mixture of dried waste particles and dry çement particles
before free w~ter is introduced into the mixing chamber. After
introduction of the water, there ls preferably sufficient
downstream mixing to achieve a substantially homogeneous mixture of
~0 water, waste and cement.
Separate points of addition to the matrix mixing section may
also be desirable when mixing the dried waste particles with the
fngredients for forming a resin polymer matrix. For example,
dlfferent monomers and vther ingredients, such as hardeners,
accelerators and/or catalysts, may be introduced ~nto the mixin~
chamber at different points around the circumference and/or along
the axis of the integral dryer/mixer.
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Yet another important feature of the invention is the ability
to eliminate or bypass a separate chemical treatment vessel and
instead use an upstream portion of the special drying unit as a
reagent mixing section for mlxing with waste concentrate the
chemical reagent for adjusting pH and/or insolubilizing the waste
solids. This mode of operation is particularly advantageous where
lime is used in the processing of PWR borates. Calcium borate is a
thixotropic (gel-like) material and may stick to the walls of
transfer lines if added upstream of the thin-film evaporator,
particularly where space considerations make it desirable to have
relatively long transfer lines between the evaporator oulet and the
special drying unit.
The lime or other chemical reagent is preferably added in the
dry state rather than as a water solution, thereby avoiding an
increase in the quantity of water that must be evaporated by the
solidification system, This elimination of the need for chemical
solutions may improve the efficiency of the equlpment and/or allow
the use of smaller capacity equipment, with a resulting savings in
operating and equipment costs.
The absence of moisture in the dry waste allows effective use
of bitumen and resin polymers, Where the waste concentrate i5
poorly dried, the excess moisture generates water vapor when it
contacts hot bitumen and the resulting boiling mass produces an
undesirable foaming action. The effective use of resin polymers
also prefers a well dried waste product as any excess moisture
might interfere with the polymerization of the resin monomers and
prevent the formation of an effective encapsulating mAterial.
The invention allows also the further treatment of the dry
waste particles in view of their final storage. Such treatment can
include a calcination and/or encapsulation in a glass matrix.
The invention also lends itself to relatively easy and
flexible operational control, which is preferably based on a water
mass balance between the feed flow to the evaporator and the
accumulated condensate from the combined vapor streams of the
evaporator and the dryer. By differential comparisons between the
weight of feed and the weight of accumulated condensate per unit
time, the amount of dry solids per uni~ time can be determined
knowing the average salts and/or solids content of the feed.
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- The amount of dried product being discharged from the dryer
can also be measured continuously so that the amount of e-lcap-
sulating matrix can be determined on a continuous basis. In the
alternatlve, for batch operations, metering hoppers may be used to
5 measure the ueight of both the dry waste and the matrix material to
be subsewuently fed to the separate matrix mixing apparatus.
It can thus be seen that the present invention optimizes the rate
at which relatively dilute solids solutions can be dried, enhances
continuous operation of process equipment without breakdown or flow
10 interruption, and produces a dry and powdery solid waste product.
T~e characteristics of the dry ~aste product are such that it may
be encapsulated at high solids levels by matrix mixing operations
which are relatively flexible and easy to control, and which are
relatively insensitive to the proportions of water and cement in
15 the matrix material.
The encapsulation by any of the three matrices mentioned
produces a solid waste product having unusually chemical and leach
resistance. Where the matrix is cement or a polymer, the solid
~aste product has also exceptional high mechanical resistance. In
20 sddition, where the ~atrix Is cement of the type specified, the
matrix and waste mixture has the advantage of quick hardening at
ambient temperature to yield a final product with high resistance
to compressive failure (crushing) and thermal degradation (fire
resistance),
Other advantages of the invention include the ability to treat
radioactive solids either in solution or in suspension; to carry
ou~ and control each process step independently, providing flexibi-
lity and ease in adjusting process parameters in case of changing
feed composition or changes in ambient conditions; to package dry
30 waste solids alone for interim or permanent storage if encapsula-
tion is not required by future waste handling techniques; to
accommodate changes in encapsulating materials and equipment
downstream of the dryer or the drying section; to carry out all
process steps except chemical treatment and matrix encapsulation
- 35 without ~onstant operator surveillance; and to provide remote
operation and control without particular problems, normal
operations requirlng no operator actions inside shielded areas.
. The process allows the use of commercially available equipment with
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minor adaptations and modifications. Process equipment is easily
maintained, minimizing access requirements to shielded cells for
maintenance or repair work. Equipment design and arrangements do
not impose any unusual lay-out difriculties or hamper required
accessibility, and decontamination of all equipment can be achieved
~hrough rinsing of internal surfaces with commercially available
decontamination complexes and/or solvents. The present invention
also has all of the previously discussed advantages inherent in
volume reduction techniques.
BRIEF DESCRIPT~ON OF THE DRAWINGS
Figure 1 is a schematic flow diagram illustrating the compo-
nents for carrying out the invention and the conduits conveying
materials between those components.
Figure 2 is a schematic flow diagram illustrating a modifica-
tion of the invention wherein a portion of the drying apparatus is
used for mixing dried solid waste with an encapsulating matrix
material instead of employing a separate mixer as in Figure 1.
Figure 3 is a diagrammatic view in sectional elevation of a
preferred evaporator component.
Figure 4 is a cross-sectional view of the evaporating section
of the evaporator component taken along line 4-4 of Figure 3.
Figure 5 is a cross-sectional view of the particle separator
section of the evaporator component taken along line 5-5 of
Figure 3.
Figure 6 is a cross-sectional view of the dryer component
taken along lines 6-6 of Figure 1 and showing a preferred type of
rotor and mixing blade arrangement.
Figure 7 is a side elevation view of the mixing blades of one
of the rotors of the dFyer component of Figure 6.
Figure 8 is a perspective view of another preferred type of
rotor and mixing blade arrangement for the dryer component o
Figures 1 and 2.
Figure 9 is a longitudinal cross-sectional view of the dryer
component taken along line 9-9 of Figure 3.
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Figure 10 is a diagrammatic side view in elevation of the
rotor and blade elements of the dryer component of Fi~ure 8.
Figure 11 is a diagrammatic sectional view taken along line
11-11 of Figure 10. -
5BEST MODE FOR CARRYING OUT_THE INVENTION
One preferred embodiment of the invention is illustrated dia-
grammatically in Figure 1. Radioactive liquid effluent from a
nuclear facility is fed by line 20 to a high capacity evaporator 22
before being discharged by line 24 to a hold up tank 2S. Purified
water decontaminated by evaporator 22 is discharged through line 28
to a controlled discharge system for releasing purified water to
the environment. Evaporator 22 reduces the quantity of liquid
waste that must be stored on site and this evaporator and hold up
tank 26 may form part of the conventional waste treatment system
for a nuclear power plant or other nuclear facility. If the amount
of dissolved solids in the effluent is already relatively high
(greater than about 10 weight percent), the high capac~ty evapora-
tor may be,omitted.
The liquid waste in hold up tank 26 will contain radioactive
ions which may be in the form of dissolved solids, suspended
solids, or a mixture of both. This waste liquid may be sent by
line 30 to a chemical treatment vessel 32 having a stirrer 34 and a
chemical reagent preparation tank 36.' A sample line 38 provides a
means for withdrawing a liquid sample to measure the quantity of
dissolved and/or suspended solids in vessel 32. Upon the addition
of lime or other appropriate chemical reagent to the liquid waste
in ~essel 32, suspended particles of insoluble salts may orm, if
not already present, and coagulate into a precipitate which can be
allowed to settle to the bottom of the vessel by turnin~ off the
stirrer 34 for the desired settling time. A decanted liquid can
then be separated from the settled layer of more concentrated
solids by drawing off and recycling an upper liquid layer 40 to
evaporator feed line 20. The preferred operating times for mixing
a chemical reagent with the liquid waste are in the range of
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30 to 60 min~tes, and if decanting of the waste liquid is deslred,
6ettling times of 30 to 60 minutes are usually sufficientO
~ hen the pH of the liquid waste has been adjusted into the
preferred range for solidification, the waste is then transferred
to a vertical thin-film evapora~or 50 by means of a slurry pump 52
in connecting line 54. The thin-film evaporator has a rotor 56 and
a cylindrical wall 58 which is heated by a jacket 60 to which a
heating medium, such as steam, is supplied by line 62 and dischar-
ged through a line 64. In the thin-film evaporator, moisture is
removed from the waste liquid to increase the concentration of
dissolved and/ar suspended solids so as to form a concentrate
having a relatively high percentage of solids as compared to the
feed liquid in line 54. However, the heat input and feed rate to
this evaporator is controlled so that the level of solids at outlet
66 remains below 70 weight percent as previously explained.
The concentrate from the thin-film evaporator is discharged
through a conduit 68 to a heated dryer 70 having internal mixing
elements as described in greater detail below. The dryer has a
~acket 72 so that at least a portion of the lnternal wall may be
heated by the heating medium introduced into the ~acket through
line 74 and removed from the jacket by line 76. In the preferred
embodiment~ the heat flux and heated surface area are sufficient to
completely remove moisture from the concentrate and form 8 dry
solid waste material. The temperature vf the heated surface area
of bo~h the dryer and the thin-film evaporator is chosen so as to
~ achieve the desired level of moisture removal in the respectlve
units for the optlmum range of feed rates provided by pump 52.
It is also within the contemplation of the invention to
separately vary the heat input to the evaporator and to the dryer
30 by means of remote control valves 78 and 80 in lines 74 and 829
respectively. In~addition, the heating medium for the evaporator
and the dryer may be different, such as oil instead of steam, and
may be supplied from different sources at different temperatures.
Electrical heating means may also be used.
The operational flexibility of the system ~s further enhanced
.
- 17 -
by a bypass line 84 contalning a remote control valve 86. This
.
line may be used to bypass evaporator S0 so as to transfer some or
all of the waste liquid dlrectly from chemical treatment vessel 32
to dryer 7~. Bypass line 84 can be used to increase the flow of
liquid waste through the dryer without increasing the flow sent to
the evaporator S0, as may be desirable where a relatively high
level of solids are already present in the waste liquid from
treatment vessel 32, such as a solid content in the range of ~0 to
S0 weight percent or even higher. Bypassing the evaporator with a
portion of the waste liquid may also be desirable if the dryer
employed has a significantly greater heated surface area or
operates at a significantly higher temperature than the
evaporator.
The drying and throughput capacity of the dryer may also be
selected so as to be capable of producing a dry waste product
directly from waste feed containing solids concentrations of about
35 % or greater. Accordingly, where upstream evaporation or
upstream settling is especially effective and results in high
solids concentrations in line 54, the entire dlscharge of pump 52
may be sent directly to the dryer without going through the thin-
~ilm evaporator. However, such solids concentrations in the waste
liquid are unusual and are difficult to achieve without special
concentration or settling techniques. .91so, feed rates directly to
the dryer are relatively low so that this component would not be
2~ u~ed in its most e~icient manner. Furthermore, the vapor stream
exiting ~he dryer contains a high level of entrained dry particles
and would require the use of a separate mulei-stage particle
separator, which is one of the less preferred alternatives as
discussed below. A ehin-film evaporator with an internal particle
separator and a downsteam dryer o~ the type described is therefore
considered the best mode for practicing the present inventlon.
... . . .
A further embodiment of the invention is represented by a
bypass line 87 between hold up tank 26 and pump 52, and a chemical
addition line 88 for introducing chemical reagents, preferably in a
dry state 9 directly into the dryer 70 as shown in fig.l. A similar
5 chemical addition line 88a may also be used to introduce chemical
reagents directly into the drying section of the integral
dryer/mixer 160 of fig. 2. Valve 89 in line 87 and valve 89a in the
line between mixing vessel 32 and pump 52 permit liquid effluent
from hold up tank 26 to bypass the separate chemical addition
10 vessel 32 and be ~ed directly to thin-film evaporator 50 prior Eo
the chemical treatment as provided by lines 88 and 88a. The lines
88 and 88a may feed into the same dryer inlet as line 68 or through
a separate inlet peripherially or axially spaced from the inlet
receiving concentrate from evaporator 50.
The dry, powdery waste material produced in the dryer 70 is
sent to a metering hopper 90 controlled by a valve 92. When the
metering hopper is at least partly filled with dry particles, this
waste m terial is then discharged batchwise and with a
predetermined weight to a separate mixer 94 having an agitator 96
774
Depending Oll the weight of dry material to be discharged, the
desired quantity of matrix material is made up in a matrix mix~ng
tank 98 and sent to mixer 94 through a line 100. The matrix mixing
apparatus may also include a metering hopper 102 arranged to auto-
matically actuate a control valve 104 in line 100 so as to automa-
tically discharge a predetermined amount of mixed matrix material
to the matrix mixer. The dry solids and matrix material are then
mixed for a sufficient period of time to intimately mix the dry
particles and the matrix material and form a substantially homoge-
neous mixture which is then discharged to a storage container 106in conventional fashion. It is to be understood that container 106
may be optional and that the mixture may instead be poured into a
mold of desired configuration and hardened into a block capable of
being handled without enclosure in a container. In this
connection, the matrix material envelops practically all of the
radioactive particles in a leach-resistant and chemical resistant
envelope. Even though the particles at the surface of the
solidified matrix may not be completely enclosed in matrix
material, they may be sufficiently fi~ed, depending upon the matrix
composition, to allow subsequent handling within applicable
regulations. As another alternative, the mixer 94 may be omitted
and the dry particles enveloped directly in the container 106 which
may then be provided with a sealed cover and stored.
The moisture removed from the concentrate in the dryer 70 is
vented as a vapor back to evaporator 50 through conduit 68 in
countercurrent relation to the concentrate. ~apor from the dryer
combines with vapor from the thin-film evaporator and then passes
thro~gh an integral separator section 110 within the evaporator for
removal of entrained particles of dry solids and moisture droplets.
The overhead vapor from ~hich entrained particles have been removed
ls sent by line 114 to a condenser 116 for separating moisture from
noncondensable gases. Line 114 may optionally contain a particu-
late filter 118 for catching any particles which have not been
removed by the particle separator. Noncondensable gases from
condenser 116 pass through a line 120 to a conventional venrilation
. , .. .
.
.
~ 1 ~2~
- 20 -
system for controlled discharge to the atmosphere. The ventilation
. . .
system includes a bank of high efficiency ~llters 122 and a
discharge stack 125.
The particle separating function performed for both the dryer
and the evaporator by the evaporator separator 110 reduces the
entrainment of dry particles and water droplets in the vapor
leav~ng the dryer from about 50 to 100 grams per cubic meter in
conduit 68 to less than about .001 grams per cubic meter in
evaporator vapor line 114. Although an entirely separate particle
separator unit may be employed as previously indicated, a
relatively expensive multiple stage unit would be necessary to
achieve the same removal factor.
Condensate from condenser 116 is sent to a condensate tank 130
for providing a suction head to a pump 132 for removing the low
activlty level condensate from the waste solidification system.
Depending on its activity levels, the condensate may be transferred
to the controlled release system for decontaminated fluids or
recycled through a line 134 to feed line ~0 of the high capacity
evaporator 22.
For purposes of operational control, the flow of condensate
from pump 132 is measured by a flow sensor 136 and thc flow of
chemically treated liquid waste feed is measured by a flow sensor
138 in line 54. The respective flow signals are transmitted by
instrument lines 140 and 142, respectively, to a recorder and
control unit 144 which compares the signa]s and generates a control
cignal for regulating the speed of pump 52 through an instrument
line 146.
With reference to Figure 2, there is shown a modification
employing components for continuous encapsulation of the dry waste
powder. In this embodiment, the heating ~acket 158 of a modified
dryer 160 extends only part way along the actual length of the
dryer, preferably S0 to 70 percent of the operative length of the
rotor of this component. The remaining 30 to 50 percent of the
rotor length comprises a downstream matrix mixing section 162
commencing approximately at an imaginary line 163 which marks the
~ ~7~ ¢
- 21 -
end of the heated drying section 164. Matrix material is conti-
nuously fed to mixing section 162 fro~ a preparation tank 165 by a
screw conveyor 166 or some other type of positive displacement
conveying mechanism for matrix ma~erials. ~t is to be understood
S that the waste concentrate has been completely dried and formed
Into dry, powdery particles by the time the waste solids reach the
mixing section 162 as defined by imaginary line 163. The rotor
components of the matrix mixing section are preferably of the same
configuratlon as the rotor components of the upstrea~ dryer
section, although the rotor and mixing elements of these sections
may be different. While the dryer se~tion jacket 158 is heated,
the matrix mixing section preferably has an independent heat
exchange jacket 168 which may be either heated or cooled as
appropriate to give the optimum temperature for matrix mixing.
Thus, the jacket 168 can be used to heat the mixing section wall
when using b~tumen to main~ain its fluidity, or to cool the mixing
section wall when using cement or resin polymers to prevent
premature setting which might otherwise result from heat
transferred from the dryer section.
A heating or cooling medium, such as steam or water
respectively, is supplied to jacket 168 through a line 171.
Where the matrix material is cement, dry cement and water may
be premixed in tank 165 and fed as a slurry through line 167 for
single stage mixing wlth dried waste in mixing section 162,
However, in another preferred embodiment~ dry cement is fed
directly to the mixing section 162 through line 167 separately fro~
; the water. Water is also fed directly to the mixing section 162
through a separate water feed line 169, me outlet of line 169 into
the mixing chamber of dryer 160 is spaced axially downstream and~or
circumferentially from the outlet of line 167.
Similarly, separate systems for the sequential addition of dry
cement first and then water may be used with the batchwise encap-
~ulation arrangement of Fig. 1. In this arrangement, matrix tank 98
and related compounds are modified so as to introduce dry cemen~
batchwise into separate mixer 94. A separate metering hopper and
~ water addition line to mixer 94 are provided for the batchwise
addition of water (not shown).
- 22 -
Separate sySeems for the addition of different chemical
^ ingredients may also be employed with the embodiments of both
Figures 1 and 2 where the matrix material Is a resin polymer.
One preferred embodiment of the thin-film evaporator 50 is
shown in Figure 3. Waste liquid is fed through an inlet 170 to a
distributor 172 which dlstributes the liquid around the inside
perimeter of heated wall 58. The liquid then flows by gravity down
the heated wall where it is wiped against the wall surface as a
thin-film by the paddles or vanes 174 of rotor 1760 The vanes may
be integral with the rotor as illustrated in Figure 4. Wall 58 is
heated by jacket 60 to which steam is fed by a steam inlet 178 and
from which steam is discharged by outlet 180. Rotor 176 is
mounted at its lower end by bearing assembly 182 and at its upper
end by a second bearing assembly 184. As the liquid waste travels
downwardly as a thin-film spread over the cylindrical wall,
moisture is driven off so that the solids are concentrated to for~
a concentrate which collects in a lower chamber 186 and is dischar-
ged therefrom through concentrate outlet 66. Concentrate outlet 66
ls connected by conduit 68 to the concentrate inlet of the dryer as
previously described. For an effective rate of drying compatible
with the other components of the system, ~he thin-film evaporator
used should preferably produce film thicknesses in the range of 0.5
to 5.0 millimeters, preferably 1.0 to 3.0 milli~eters. Some
c~mmercially available units include means for controllably varying
the film thickness and such units may be adjusted to produce film
thic~nesses in the foregoing ranges.
- Although a separate par~icle separator may be employed, the
pre$erred thin-~ilm evaporator includes a particle separator
section 110 ~or removing both liquid and solid par~icles entrained
with the released moisture vapor, The hot vapor rises vertically
betueen the padd~es and exits the evaporator through a vapor outlet
190. In the e~bodiment shown, the particle separator includes an
extension of the rotor 176 having vanes 192 which cooperate with
wall ~ounted baffled 194 as illustrated in Figure 5 to remove
entrained particles.
.
~7~
- 23 -
Centrifugal thin-film evaporators of the type illustrated
in Figure 3 are manufactured by De Dietrich Company located
in Niederbronn-~es-Bains, France, a preferred model being
Type M. Other types of thin-film evaporators may also be
employed, such as that illustrated and described in Defensive
Publication No. T939,005 published in the Official Gazette
of the Uni~ed States Patent & Trademark Office ~n October 7,
1975, the application of which is identified by N.S. 452,857
(series of 1970). Other manufacturers include Sybron in Leven,
Scotland, Luwa in Zurich, Switzerland, Kontro in Athol, Massa-
chussetts, USA, Chemetron in Jeffersontown, Kentucky, USA and
Artisan Industries in Waitham, Massachussetts, USA.
One preferred embodiment of the rotor and the mixing and
advancing elements of the dryer 70 is illustrated in Figures
6 and 7. A pair of cooperating rotors 200-200 is arranged
for clockwise rotation in the direction of arrows R within
dual mixing chambers 202 and 204 defined by a housing 206.
The housing includes heating medium passages 208, 209, 210
and 211 forming the heating jacket 72 of Figure 1. As illus-
trated in Figure 6, there is on:Ly a small clearance between
the crests of the mixing paddles, generally designated 214,
and the walls of chambers 202 and 204 and between the closest
approach of the intermeshed paddles to each other.
The mixing paddles of the rotor may becomprised of a
variety of blades and paddles as illustrated best in Figure
7. In the arrangement illustrated, helicalblade or paddle
216 positively advances the concentrate and the dry particles
resulting therefrom toward the dryer outlet. This blade is
followed by a pair of mixing paddles 218-218 each having crests
220 extending parallel to the longitudinal axis of the rotor.
~ext are a pair of mixing and advancing paddles 223-223 which
both mix and advance the material being worked and for that
purpose have crests 224 extending at an angle of approximately
45 to the rotor axis with the leading edge displaced so as to
- 24 -
advance the material. lhere may also be included along
the length of the rotor a fourth pair of paddles 226-2~6
having crests extending at an angle to the rotor axis
with the leading edge displaced in the opposite direction
to that of the advancing b~ades and paddles so as to
both mix and retard the advance of the worked material.
Such retard paddles cooperate with the remaining
paddles to produce a backward and forward working motion
to efficiently shear and subdivide the hardening material.
me displacement angle of any retard paddles used should
be chosen so as to cause some back mixing without interfer-
ing with the overall advance of the material being worked
in the dryer. It is to be understood that various com-
binations of the helical blades and mixing paddles may be
employed. Thus, all straight paddles may be employed when
a high retention time is desired and all helical paddles
may be employed when a low retention time is desired. To
suit different requirements o shearing and retention time,
the relative number of straight, canted advance and canted
retard paddles and advance blades and their arrangement
can be suitable varied.
One suitable mixer having blades and mixing
elements of the foregoing type is described in U.S. Patent
No. 3.195.868 to Loomans, et al, issued July 20, 1965.
Mixing apparatus of the type illustrated in Figures 6 and
7 are manufactured by Baker Perkins, Inc. of Saginaw,
Michigan, USA and by Teledyne--Readco in York, Pennsylvania,
USA. The baker Perkins model is known as the Multi-Purpose
Mixer and the Teledyne-Readco model is known as the con~
tinuous Processor.
Figure 8 discloses another preferred embodiment
of the dryer rotor and the mi~ing and advancing elements.
~n this embodiment, a single rotor 230 is employed and
rotates within a single chamber defined by a housing 232
which has a heating jacket similar to that shown in Figure
6. With reference to Figures 8 and 9, the mixing elements
~7~7~
,
- 24a -
carried by the rotor are comprised of transverse paddles
234 and axial bars or paddles 236. Paddle cleaning
elements or scrapers 238 are mounted on the housing 232.
m e scrapers have an enlarged base 240 by which they are
mounted within apertures in the housing wall so as to
project radially inward as best shown in Figure 9. m e
projecting portion of the scrapers include a shank 242
and a C-shaped scraping element 244. The shank 242
passes between the ends of the bars 236 on adjacent
paddles and surfaces of the scrapers cooperate with
opposing surfaces of the bars, the transverse paddles
and the rotor so as to shear crusty layers of dried
concentrate from those surfaces.
7~
- 25 -
~ . .
At the same time, outer surfaces 246 of the bars and outer
_ surfaces 24~ of the paddles cooperate with the opposing surface 250
of the heated housing wall to similarly shear crusty material from
~hose surfaces.
With reference to Figures 10 and 11, it can be seen that each
of the bars 236 is displaced at an angle to the rotor axis and
that the bars on consecutive paddles cooperate to form an elongated
helical member or screw, generally designated 260, which has an
over-all cant relative to the rotor axis. In the preferred
embodiment shown, there are three rows of paddles 234, each
carrying bar elements 236 so as to provide three composite helical
members spaced uniformly around the rotor periphery. ~hen the
rotor revolves in the counterclockwise direction as shown by arrow
S, the helical members 260 cause the material being worked to
positively advance in the direction of arrow F toward the outlet of
the dryer component 70. Mixing apparatus of the type illustrated
in Figures 8 through 11 are manufactured by the List Company in
Pratteln, Switzerland, the preferred model being of the type
Discotherm B.
Other rotor and mixing element combinations may be employed in
addition to those described above, provided they perform the same
functions in a similar fashion. Thus, the rotor and mixing element
combination should include means for positively advancin~ both the
concentrate and the resulting dry particles from the inlet to the
outlet of the mixing apparatus to be used as the dryer of the
present invention. In addition, the clearances and cooperation
between both fixed and movin~ elements and surfaces should generate
sufficient shearing action to remove hardened or crusty layers of
concentrate without jamming the rotor. Small clearances of~the
magnitudes previously discussed are therefore preferred between thP
cooperating surfaces of mating elements. The relative movement of
components mating at such clearances also subdivides solid masses
of dried concentrate into fine, powdery particles of waste
material. These finely sheared or ground particles have proven
especially beneficial when employed with subsequent matrix
encapsulation techniques and result in a final waste produce with
excellent characteristics.
~ .~7~4
-- 26 -
INDUSTRIAL APPLICABILITY
The process and apparatus of the present invention provide a
highly efficient and useful volume reduction technique for conver-
ting relatively dilute radioactive waste liquids into a substan-
tially dry, pow~ery solid waste material. The characteristics ofthis solid waste material are such that it can be easily incorpo-
rated by known encapsulation techniques into a variety of matrix
materials to yield an encapsulated waste product having excellent
characteristics compatible with long term storage in the encapsu-
lated state. The process and apparatus may employ conventionalequipement and is capable of utilizing each piece of equipment
within the range of its optimum operating parameters. The
invention is capable of unusual operating flexibility and can be
used to solidify a wide variety of liquid waste effluents,
including those from both light water and breeder nuclear power
plants, from industrial laboratories and other nuclear industrial
complexes, and from the waste treatment plants of nuclear
reprocessing facilities.
