CA1079055A - Low-level radioactive waste disposal process and apparatus - Google Patents

Abstract

LOW-LEVEL RADIOACTIVE WASTE DISPOSAL
PROCESS AND APPARATUS
Abstract of the Disclosure A remotely operated, fully automated, fluid-ized bed system for converting dilute or concentrated low-level nuclear power plant waste evaporator liquor into conveniently handled, anhydrous solids, reusable water, and innocuous gases. The apparatus of the sys-tem includes a pre-concentrator, a processor having a fluid bed, and a packaging unit. In the processing unit, liquid wastes are calcined and reduced in vol-ume to dry particulate material. In the packaging unit, the dry wastes are placed in packages and the spaces between the particles in each package are fill-ed with a solidifying agent to cause the packaged con-tents to be converted to a monolithic block having a volume of approximately 10% of that of the liquid wastes from which the waste solids in the block were derived. The effluent from the processing unit can be treated to form reusable water and innocuous gases.
This invention relates to improvements in the disposal of low-level liquid radioactive wastes from nuclear power plant facilities and the like and, more particularly, to an improved process and apparatus for reducing the volume of such wastes and for packaging the same as a monolithic solid block.

Description

10'~90SS

This application is a division of co-pending Canadian Patent Application No. 207,793 filed August 26, 1974.
BACKGROUND OF THE INVENTION
Nuclear-fueled generating plants comprise the fastest growing segment of the electrical power-generating industry.
But, because of the environmental impact of additional nuclear power plant facilities, plant design is progressing in the direction of closed release plants, i.e., plants which contain and internally process substantially all radioactive wastes to thereby prevent atmospheric or water-borne releases. The impact of closed release plants will result in large quantities of liquid wastes of low-level radioactivity being generated by each power plant and the wastes will require processing, transporting and burial at commercial burial sites. The con-sequence of large processing requirements increases capital investments and operating costs as well as higher annual waste disposal costs. Also, such larger quantities of wastes required to be shipped annually will increase the probability of accidents, risks of public exposure, risks of environmental releases, consumption of burial space at a faster rate, and increased capital investment in equipment required to transport, handle and bury the waste materials. All of these costs will in all likelihood be reflected in higher overall cost of electrical power to the consumer.
Low-level radioactive liquid wastes, exclusive of wastes due to spent fuel elements, are generated in the primary reactor water coolant loop of light water reactors. These wastes result from fission products released from the fuel elements and/or corrosion products which become radioactive after passing through the coolant passages of the reactor core.

~ ~ -2-` 10'79()5S
Acceptable levels of radioactive contamination in the primary coolant are achieved by processing the coolant water through ion exchange media and filtration systems. These units initially concentrate radioactive material and, upon either regeneration or back-washing, result in liquid radioactive wastes which require further processing before disposal.
In disposal techniques now usually practiced by the nuclear power industry, dilute radioactive wastes are further concentrated by means of an evaporator. The liquid concentrates from the evaporator, which are made richer in chemical and radio-isotopes, are conventionally diluted when mixed with solidifying material~ such as cement and vermiculite, and packaged in a container as a solid. However, the portion of the concentrated wastes in each such container has a relatively small volume when compared with the solidifying material added ~ -thereto. Thus, it is desirable to increase the volume of the wasteC added to each container yet still be able to comply with safety requirements in storing, transporting and burying the packaged wastes.
By 1969, application of the present state of the art of radioactive liquid waste handling produced in the order of 4,000 fifty-five gallon barrels for burial. Without improvements in process methods, this volume can be expected to increase to more than 200,000 fifty-five gallon barrels per year by 1980.
Burial site limitations, perpetual surveillance and costs clearly emphasize the desirability of improved processes ~or radioactive waste volume reduction.

~ 3-. ~ .
.

~790SS
SUMMARY OF THE INVENTION
The improved process and apparatus of this invention operates to effect approximately a ten-fold reduction in volume of radioactive wastes generated during power plant operation.
The resultant minimum volume waste product is solid and may be stored on site indefinitely in a recoverable form, thereby maximizing radioactive decay prior to shipment. Alternatively, it can be removed to an off-site burial facility by conventional means. The unit cost for transport and burial of the waste product in accordance with the present invention may be higher than the corresponding unit costs incurred with the use of conventional disposal techniques. Nonetheless, a volume reduction to at least one-tenth of the original volume, as achieved with the present invention, results in a significant overall cost reduction for disposal of such wastes.
The apparatus of the invention includes a processing unit in the form of a calciner-dryer which drives off the liquid portion of the wastes, leaving the wastes in the form of anhydrous, particulate solids containing substantially all the chemicals and radioactivity of the incoming wastes. The solids, which have a volume of approximately five times less than that of the input wastes to the processing unit, are directed to a packaging station at which a number of containers or packages are filled with the solids. In each package, a solidifying agent is placed to fill the spaces between the particles and, upon curing, solidification occurs, resulting in a monolithic block suitable for storage or transit to a burial site. The volume of solids placed ~ 4-1079~55 in each package is 2 to 2.5 times the volume of liquid wastes placed in such a package using conventional packaging techniques since, conventionally, the package is filled only to 40% or 50%
of its capacity, following which cement or vermiculite is added to fill the package. Thus, the overall volume reduction that can be realized by carrying out the teachings of the process of this invention is at least of the order of ten to one depending upon the feed composition of the input radioactive wastes to the processor. The off-gas drawn off from the processor can be further processed, such as by filtering, scrubbing and cooling, to form reusable water and innocuous gases which can be passed to the atmosphere.
The primary object of this invention is to provide an improved process and apparatus for disposing of low-level liquid radioactive wastes wherein the volume of the wastes can be reduced by a factor of at least ten to thereby minimize costs of storage and ultimate disposal of the wastes yet the wastes will be in a form more suitable for storage, transit and burial while still conforming to safety codes with respect thereto.
Another object of this invention is to provide a process of disposing of low-level liquid radioactive wastes wherein the wastes are reduced in volume to a mass of free-flowing, solid particles by calcining and the particles are packaged in the presence of a solidifying agent or the particles are compressed, sintered or fused to cause the wastes to be con-verted to a monolithic product capable of being readily and more safely stored or moved to a burial site.

~ -5-- . : . ~ . - . , - :

In one particular aspect the present application, which _s a division of aforementioned Canadian Application No.
207,793, is concerned with the provision of a process for handling radioactive wastes in theform of a dilute salt solution comprising: directing the solution into a first region having a fluidized bed; applying heat to the bed particles from a location externally of the fluidized bed to evaporate the liquid fraction of said solution to form an effluent and to cause the solid fraction to adhere to the bed particles, moving at least a portion of the bed particles and solid fraction adhering thereto out of said first region-and to a second region for containerization; directing the effluent through a third region to remove the fines therefrom and to decontaminate the -effluent; and returning the effluent after it has been decon-taminated from said third region to said first region as fluidizing medium for fluidizing said fluidized bed, whereby said first, second and third regions form parts of a closed fluid path.
Other ob~ects of this invention will become apparent as the following specification progresses, reference being had to the accompanying drawings for an illustration of the apparatus for carrying out the process of the invention.
In the drawings:
Fig. 1 is a schematic view of the apparatus;
Fig. 2 is a representative plot of waste volume versus liquid waste feed composition showing direct volume reductions achieved with the invention; and Fig. 3 is a representative plot similar to Fig. 2 showing - relative volume reductions for various liquid waste compositions and a number of relative packaging efficiencies.

jl/ 6 ~0790S5 .

A preerred embodiment of the apparatus for carrying out the process of this invention is denoted by the numeral 10 and includes a liquid waste processor 12 comprised of a calciner-drier unit which operates at but is not limited to temperatures between 400F and 1200F depending upon the composition of the waste feed solutions directed to it for processing. Processor 12 has a lower portion 13 for containing a fluidized bed, portion 13 having a source of incoming fluidizing gas. Portion 13 also has a waste feed inlet 18 for in-bed atomization through single or multiple injection nozzles (not shown). Processor 12 can also have another waste inlet 20 coupled to nozzles (not shown) for overhead spraying of wastes onto the fluidized bed.
An effluent outlet 21 is coupled by a tube 22 to the input of a particulate separator 24. The separator 24 may be of any of the types commonly known to those skilled in the art, e.g., cyclone, filter, settling chamber, etc. Portion 13 has an outlet 26 from which solids in the form of anhydrous partlcles can be discharged therefrom to a waste product storage container 28. The particles can be batchwise or continuously withdrawn either by mechanical, pneumatic or gravity transfer. Separator 24 also communicates with container 28 to teliver particles separated from the processor effluent.
The energy required for calcining the wastes can be supplied in any suitable manner, such as by any or all of the following sources: external or immersion resistance heating;
induction heating; infrared heating; microwave heating; internal immersion heat exchangers; in-bed combustion of fuel; and over-bed combustion of fuels. For purposes of illustration, an external resistance heater 30 is shown surrounding portion 13.
:-:

- .-: .

lV79~)S5 The rate of energy required by processor 12 is precisely controlled from a remote console (not shown) as are the volume - of fluidizing gas entering portion 13 through gas inlet 14 and the volume of the input liquors.
The liquid, low-level radioactive wastes are directed from a storage tank 31 to processor 12 in any suitable manner. If the incomlng feed liquor is dilute in dissolved solids, the liquor can be further concentrated by directing the same through an evaporator 32 which can be fired in any suitable manner, such as by fuel combustion, waste heat, steam or electrical resistance heating. The evaporator operates to pre-concentrate the feed liquor to a form having a desired percentage of waste solids therein for greater operating efficiencies. The effluent from evaporator 32 is passed by a tube 33 to a condensor 34 from which the condensate is collected, evaluated for chemical and radioactive contamination and recycled back for reuse in the power plant by a pump 36. The liquid feed stream from evaporator 32 is directed by a pump 38 to a waste storage tank 40 from which the liquid wastes are directed by a pump 42 to either or both of inlets 18 and 20 of processor 12.
Separator 24 can be of various types commonly known to those skilled in the art and is designed such that the largest diameter dust particles entrained therein are removed and transferred either by, but not limited to, gravity or by pneumatic conveyance to storage container 28. Alternatively, such dust particles can be returned to the fluid bed in portion 13 of processor 12 to form nuclei for bed growth. Dust returned to the :.

~07~S5 fluid bed can be accomplished in any manner, such as by fluidiæed weir pots with dipleg seals, a screw conveyor or by pneumatic injection.
The exhaust from separator 24 is passed by a tube 44 to a scrubber sys~em 46 or directly to condensor 68 if the scrubber system 46 is not required to effect the desired degree of gaseous effluent decontamination. The scrubber system 46 may include various components commonly known to those skilled in the art of gas scrubbing such as a pre-cooler 48, a high-energy venturi scrubber 50, a deentrainment section 52 and a low-energy scrubber 54. In the sample scrubber system 46 shown, all of the liquor from the scrubbers are cooled and collected, evaluated for solid concentration and adjusted for pH, and recycled for further scrubbing duty. A portion of the scrub liquor is bled ~-via a line 57 to tank 31 or tank 40 to control scrub liquor composition. A scrub solution sump 56 receives the scrubbed liquor from pre-cooler 48 and deentrainment section 52 and a pump 58 returns such liquor through a heat exchanger 60 back to pre-cooler 48 over path 62, back to scrubber 50 over path 64 and to section 52 and scrubber 54 over path 66. During the scrubbing operations, the effluent gases are cooled to their dew point and a small amount of condensation occurs to assure essentially complete particle elimination from the effluent.
In excess of 50% of the water in the effluent issuing from final scrubber 54 or separator 24, if scrubber system 46 is not required, is condensed by a condensor 68 coupled by a tube 70 to the outlet of scrubber 54 or by tube 44 to the outlet of separator 24. Condensor 68 may be a surface-type condensor or a barometric condensor. The quantity of water extracted therefrom is limited only by the condensing temperature. The condensate is collected in a tank 72 and returned by means of a pump 74 back to the nuclear power plant for reuse therein.
As a safety measure, the condensate is evaluated continuously for quality and if the quality reaches a predetermined lower limit, the condensate will automatically be diverted to tank 31 for return to processor 12. The condensate may also be used as make-up water for the scrub solution and/or as wash-down water for the scrub system components.
Exhaust gases from condensor 68 pass through a heater 76 to raise the dry-bulb temperature of the gases and to prevent further condensation before entering the high efficiency filter and adsorbers denoted by the numeral 78. Radiation monitors (not shown) audit the gas quality during this time and, after being monitored, the gases are exhausted to the atmosphere through a stack 80.
The foregoing explanation relates to an open loop cycle of processing the effluent Lrom processor 12. If the gas discharge quality becomes more restrictive due to Federal or State regulatory changes, such open loop cycle can be changed to a closed loop cycle by directing the exhaust gases from condensor 68 to the input of blower 16. In this way, the exhaust gases can be directed into the fluid bed for fluidizing purposes. Moreover, waste heat from the effluent from processor 12 may be used to pre-heat the fluidizing air from blower 16 or for providing part 10'79055 of heat energy for evaporator 32. Moreover, such waste heat may be used in other energy-recovery systems or in providing temperature control of storage container 28 of tank 31.
The solid particulate matter received in storage container 28 is packaged in the containers or packages approved by the U.S. Department of Transportation. The packaging of the solid wastes from container 28 is done at a packaging station denoted by the numeral 82 and the waste solids are directed thereto in any suitable manner, such as by gravity or by mechanical or pneumatic means. At the packaging station, the solids are directed into each package so as to fill the same to its maximum capacity. Then, a solidifying agent is directed into each package, specifically into the spaces between the particles to fill such spaces. After a predetermined curing time, solidification occurs to cause the particles to form a monolithic block, thereby maintaining maximum loading of the package while assuring a minimal environmental contamination in the event that ~-the package ruptures during transit and burial.
The solid particulate matter from container 28 is directed along a line 29 into a container at station 82 through a dust-type hood 81. Generally, the container will be elevated into tight engagement with the hood and after being filled, will be lowered away from the hood. The container at station 82 is maintained at a constant temperature by a heater 87.
The solidifying agent is contained in a solidification mix tank 83 and heated by an external heater 86. The agent is pumped out of tank 83 by a metering pump 84 and directed through line 85 into hood 81 and then into the container engaging the hood.

1079~S~
The solidifying agent can be of any suitable composition.
For instance, the agent can be a mixture of polyethylene and paraffin or can be a styrene copolymer with a polyester resin added thereto. Generally, the agent will be in solid form and can be packaged and stored ready for use. When the agent is to be used, it is heated to a flowable condition and poured or injected into the package either before or after the waste solids are directed into the package. In order to obtain the maximum volume reduction of the waste product,-the solid particulate matter recieved in storage container 28 may be -compressed, sintered or fused into a monolithic structure such that the theoretical density of the compounds are nearly obtained.
Apparatus 10 incorporates other structure (not shown) for usual maintenance problems. For instance, the apparatus includes decontamination process piping, valving and controls to periodically wash internally all process equipment. In this way, the major portion of radioactive contamination is removed, thereby permitting manned access to the various components of the apparatus. Moreover, the apparatus can be constructed so that it forms a self-contained unit which is of minimum size and can be moved to an operating site with a minimum of difficulty.

1~'79~55 OPERATION
Waste feed liquor from a nuclear power plant is directed into tank 31 for storage until ready for use. If an evaporator is used, the waste liquor in tank 31 is pumped to evaporator 32 by a pump 35 and the output of the evaporator is directed to tank 40 from which the liquor is pumped to either or both inlets 18 and 20 of processor 12. The input liquor to processor 12 will vary generally from 2% to 50% of solids concentration.
In the processor, the heat energy therein immediately vaporizes all of the incoming water in the liquid wastes. Thus, processor 12 functions as a drier. It also operates to oxidize all carbonaceous materials, if present, and the resultant solid residue is deposited in the fluidized bed in portion 13. The bed particles, which are spheroidal in shape and free-flowing, are withdrawn from outlet 26 to storage container 28. The solids from container 28 are then directed to packaging station 82 where various packages are filled with the solids, following which or before which the solidifying agent is directed into each package. The result is that the soIids in each package - become a monolithic block which can be stored or moved easily and safely to a burial site.
The effluent from processor 12 is directed through separator 24 to remove the dust particles therefrom. The exhaust gases from separator 24 are effectively cleaned by scrubbing and/or filtering to purify the same so that the ~-exhaust gases can ultimately be vented to the atmosphere if the gas quality meets regulatory standards.

jl/b~ 13 ; ~

SS
Processor 12 utilizes differential pressures to measure fluidizing efficiency and fluid bed height. The processor also operates to regulate the rate at which solids are discharged from the fluid bed. Fluidizing gas entering inlet 14 of processor 12 is controlled at, but not limited to, superficial bed velocities in the range of .5 to 4 feet per second. Various instruments (not shown) are used to maintain and monitor feed solution quality, scrubber solution concentration, condensate quality and flow rate, and to maintain pipeline temperatures of incoming feed liquor well above normal crystallization temperatures.
The results of utilizing the process of the present invention can be shown graphically in Figs. 2 and 3. For instance, in Fig. 2, curve A represents the approximate reduction in waste volume for liquid waste feeds of various dissolved solid concentrations as compared with curve B
representing the raw liquid waste. Curve A shows that a significant reduction in volume occurs when the waste is calcined and also indicates that such volume reduction varies as a function of the solids in the input liquor. The achievable direct-volume reductions are impressive and the improved physical characteristics of the solid wastes, when compared with those of the liquid wastes, allow more efficient packaging thereof, thereby permitting an even greater volume reduction.
In Fig. 3, the packaged volumes of calcined-dried liquid wastes compared with those of unprocessed liquid wastes for various liquid waste compositions and relative packaging jl/\`~i"-~ 14 ~079055 efficiencies are shown. Relative packaging efficiency is the ratio of the volume of the packaged product obtained from the process of the present invention placed in a given package configuration compared with the volume of raw liquid waste placed in the same package according to conventional techniques of radioactive liquid waste disposal. Because raw liquid wastes are conventionally packed with large quantities of additives, such as cement and vermiculite which act as a diluent, the relative packaging efficiency will typically be near 2 to 1.
If, in addition, the solid particulate matter is compressed, sintered or fused, the value of the relative packaging efficiency will typically be near 4 to 1.
Essentially all of the radioactive material present in i~
the liquid radioactive waste feed stream are ultimately removed in the form of solid products.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for handling radioactive wastes in the form of a dilute salt solution comprising: directing the solution into a first region having a fluidized bed; applying heat to the bed particles from a location externally of the fluidized bed to evaporte the liquid fraction of said solution to form an effluent and to cause the solid fraction to adhere to the bed particles, moving at least a portion of the bed particles and solid fraction adhering thereto out of said first region and to a second region for containerization;
directing the effluent through a third region to remove the fines therefrom and to decontaminate the effluent; and returning the effluent after it has been decontaminated from said third region to said first region as fluidizing medium for fluidizing said fluidized bed, whereby said first, second and third regions form parts of a closed fluid path.