US2897049A - Treatment of fission product waste - Google Patents

Description

July 28, 1959 J. B. HUFF TREATMENT OF FISSION PRODUCT WASTE Filed Oct. 28, 1957 B m n TREATh [ENT OF EISSION PRODUCT WAS'DE John B. Huff, Blackfoot, Idaho, assignor to the United States of America 'as represented by the United States Atomic Energy Commission Application October 28, 1957, Serial No. 692,989

3 Claims. (Cl. 23-21) V i This invention relates to a method of treating aqueous nuclear wastes to separate radioactive from non-radioactive constituents thereof and an apparatus therefor.

The inadequacy of existing methods for the disposal of high activity level, aqueous wastes has been generally recognized. Since it is not possible to disperse high activity level wastes to the environment, it is necessary that they be stored. It is, of course, possible to simply store the liquids in underground tanks but the constant hazard arising from the possibility of leakage .and the tremendous cost of the tanks necessary makes this course of action undesirable.

Concentration of the solution to remove the water therefrom is difficult when the solution contains large quantities of nitric acid and aluminum nitrate. Such :solutions become syrupy and partial decomposition takes place. As the temperature is raised, corrosion and heat transfer problems become more serious. Removal .of water, however, .is essential for .a long-term solution to the problem. v V

In addition, the nitrates present should be destroyed to leave the radioactive wastes in the form .ofoxides of the elements contained therein. These materials may be stored with a reasonable degree of ease. However, com- ;plete separation of the radioactive constituents of the waste solution from the nonradioactive constituents would be the most desirable answer to the problem of storage uof aqueous wastes since the physical volume to be stored would be reduced thereby by a factor of some ,ten thousand times.

A complete segregation of radioactive from nonradioactive constituentsis a problem of great chemical .com- ;plexity complicated by the high level of radioactivity encountered. Since aluminum nitrate makes up the great bulk of the nonradioactive constituents ofthe typical waste solution obtained from treatment of spent uranium :aluminum fuel elements, removal of aluminum and ni .trate ions from the solution would reduce the storage problem to a more reasonable level and, in addition, WOllld remove a primary source-of interference-in the sepaeration of fission products. I a

It is accordingly an object .of the present invention :to develop a method of concentrating radioactive waste :solutions.

It is a-further object of the present invention to develop :a method of handling radioactive waste solutions to sep- :arate radioactive from nonradioactive constituents thereof.

It is a more specific object of the present invention to rilevelop a method of vaporizing aluminum ions and :nitrate ions from the radioactive waste solution obtained lfI'OlTl processing spent aluminum-uranium fuel elements.

Another object of the present invention is to provide an apparatus by which the above objectives may be accom- 'plished.

These and other objects of my invention are attained by my novel method wherein the highly radioactive waste solution is fed to the top of a burning mass of coke and 2,897,049 Patented July 28, 1959 chlorine is passed through the resulting mixture of oxides and ash to remove aluminum as the chloride.

The invention will now be described with reference to to accompanying drawing wherein the single figure illustrates a schematic view of a furnace in which my method can be carried out.

The drawing shows a retort 10 formed of firebrick. Retort 10 is located in apit 11 in the ground. Pit 11 is lined with concrete walls '12 having cooling coils 13 located therein. Insulating material 14, such as magnesia bricks, is located in the space between retort 10 and concrete walls 12.

Pit 11 is enlarged at 15 to provide room for two flue chambers 16 and 17 which are provided with flues 18 and 19, respectively. The enlarged portion 15 of pit 11 is also lined with firebrick. Flue chambers 16 and 17 may alternatively be placed in communication with retort 10 by operation of butterfly valves 20 and 21. Flue chambers 16 and 17 each contain a brick checkerwork heat reservoir 22 and 23, respectively. The bottom of retort 10 slopes toward a discharge pipe 24.

Chlorination product recovery lines 25 and 26 afford communication between the bottom of retort 10 and the exterior and are provided with valves 27 and 28. Conduits 29 and 30 extend between flue chambers 16 and 17, respectively, and lines 25 and 26, respectively, and are provided with valves 31 and 32.

One or more liquid waste feed lines 33 penetrates the retort 10 near the top thereof and terminates in a sprinkler head 34 which is recessed in'the firebrick. The sprinkler head 34' may be protected'by a water jacket (not shown). One or more oil feed lines 35 penetrates the retort 10 just below the liquid waste feed lines 33. p

A coke supply bin 36 is located above the top of the retort 10. Coke is fed by screw conveyor 37 through an opening in the firebrick into retort 10. p The retort is operated in a two-stage cycle. In the first stage water and nitrates are expelled from the waste solution, and in the second stage aluminum chloride is volatilized. The drawing shows the apparatus set for is set on fire. Air is directed into the retort '10 through flue 18, past brick heat reservoir 22 in flue chamber 16, through conduit 29 and line 25 while olf-gas passes out past brick heat reservoir 23 in flue chamber 17 to flue 19. After a short period of time, on the order of 15 minutes, the flow of air is reversed and the air in passing over the brick heat reservoir 23 becomes preheated. The off-gas passes over brick heat reservoir 22 heating it.

An air-oil mixture is passed through line 35 and the mixture burns as it enters the retort. Liquid waste solution is then introduced into the retort through line 33 and sprayed on to the top of the coke fire through sprinkler heads 34. The solution is thereby evaporated andcalcined. 'Oxides of nitrogen obtained as a product of the calcination constitute a potentially dangerous explosion and corrosion hazard. The slightly smoky flame from the burning of the air-oil mixture reduces these to elemental nitrogen which proceeds, along with excess air and other products of combustion, through one of the brick checkerwork heat reservoirs to its associated flue.

This procedure is continued until a considerable mass of aluminum oxide interspersed in coke and ash is obtained. The chlorination stage is then started.

In the chlorination stage chlorine is passed into the mass from one of the fiues 18 or 19' and aluminum chloride taken off through one of lines 25 and 26. Valves 31 and 32 are kept closed. A temperature of between 900 C. and 1000" C. is suitable for the chlorination stage. This temperature will be originally attained from the burning coke and will be maintained by the heat of reaction of chlorine with aluminum oxide.

The aluminum chloride is permitted to condense and is recovered. Fission product chlorides and ash remain in the retort. If any unburned coke remains it may be burned out with oxygen. The residue is removed through ash discharge pipe 24, and the retort is then ready for a new charge of coke.

Petroleum coke is used by preference since it is almost 100% carbon. Only a relatively small quantity of ash will be obtained therefore from a complete cycle of operations.

The mixture of ash and fission products can be stored since it is of relatively small volume or it can be processed to recover individual fission products or fractions of fission products therefrom for use as radiation sources or as tracer elements.

An elevated pressure of air or nitrogen may be supplied to the insulated space surrounding the furnace and to the coke supply bin so that any leakage will be into the system.

A furnace which is 2 meters in diameter and 6 meters in height will incinerate 1570 liters of 2.2 M aluminum nitrate solution per hour. As a chlorinator it will have a capacity of 40,000 pounds per day, equivalent to 5,670 gram mols of aluminum chloride per hour. If operated 14.9 hours as an incinerator and 9.1 hours as a chlorinator, 23,400 liters of waste solution will be processed in 24 hours. The consumption of petroleum coke for incineration is approximately 725 kilograms per hour of incineration. Chlorine is used during the chlorination cycle at a rate of about 500 kilograms per hour.

Life of the furnace is estimated at 100 days during which time it should be able to process over 2,000,000 liters of waste solution. At the end of this time it is proposed that the furnace will be filled up and abandoned as its radioactivity is so high to make it useless to salvage any of the components.

The off-gas from the incineration stage will contain all those elements volatilized in that step. This will include some radioactive material-rnostly rutheniumwhich must be removed from the off-gas before it is passed to the atmosphere. This is accomplished by passing the off-gas through an oil scrubber at a temperature slightly over 100 C. The oil used can then be evaporated from the residue. Other means of decontaminating the off-gas are available. These include filtration through glass wool filters and redistillation in the presence of carbon. One or any combination of these procedures may be employed to obtain an olfgas which can be passed to the atmosphere.

The stream containing aluminum chloride can not be so simply treated. The condensate obtained by cooling this stream is highly contaminated by Nb and Zr It is at present contemplated that this activity be handled by temporary storage of the aluminum chloride. After storage of from 2 to 4 years the activity should decrease to the point where the aluminum chloride may be released for unrestricted use.

Decontamination to comparable levels of course would be more desirable and a number of possible methods are indicated such as fractional sublimation, reduction separation, filtration, and resublimation. The invention however is not concerned with this problem and the field is left open to other workers to develop an economically satisfactory process.

It will be noted that I have outlined a process which results in the transformation of aluminum nitrate wastes into three fractions. That portion vaporized in the incineration stage is, as described, easily decontaminated. The aluminum chloride fraction may be stored for a few years before it can be used unrestrictedly. A few years storage represents a practical solution to the problem as opposed to storage of the entire waste solution which may have to be for centuries and therefore represents only a stopgap solution.

The fission product fraction will have to be separated into its components or stored indefinitely. This can be done since the volume is relatively small and the material is in an easily stored form.

It will also be noted that difiiculty in evaporating aluminum nitrate solutions is obviated according to this invention. The solution is sprinkled onto burning coke and is immediately evaporated and calcined to the oxide. The oxides appear as fine particles interspersed with ash which make them easily susceptible to chlorination in the chlorination stage.

Details of a preliminary laboratory expericent will now be described. A retort was fabricated from Vycor glass and connected to a Pyrex condenser by a ball joint. The retort was heated to 900 C. by a furnace having Nichrome heating elements.

To 10 grams carbon black was added 4.5 grams liquid waste. This liquid waste was obtained by dissolving in nitric acid a uranium fuel element clad with aluminum. The fuel element had been irradiated in a neutronic reactor and had been allowed to undergo radioactive decay for several weeks to reduce its radioactivity. The solution had a specific gravity of 1.29 and was 1.6 M in aluminum, about 5.8 M in nitrates and 1 M in ammonium ion. It had an activity equivalent to 67 curies/liter. The solution was heated in the retort to 900 C. in a nitrogen atmosphere. The off-gas was condensed at 0 C. and the condensate redistilled in the presence of carbon. A series of these redistillations indicated that ruthenium could be removed in this fashion. A number of tests on the off-gas indicated that it can be scrubbed to pass maximum permissible concentration.

Chlorine gas was passed at 900 C. through the carbon-aluminum oxide mixture remaining in the retort. The vaporized constituents were permitted to condense and consisted primarily of aluminum chloride. Tests. on this condensate indicated that complete decontamination of the aluminum chloride would be very diflicult. Since it is economically possible to hold the material for long enough so that the material becomes decontaminated by the passage of time, these tests were not continued.

It was thus shown that the process of this invention is technically possible. Additional tests were made in a furnace which had an opening 10 cm. square and 46 cm. high and was lined with firebrick. A coke fire was started in the furnace and compressed air supplied through a hose. An aluminum nitrate solution was fed at 5 liters per hour. The best efiiciency obtained in these experiments showed that .97 liter of 2.2 M aluminum nitrate was converted toash for each pound of coke burned. Cost of fuel is therefore estimated to be on the order of one cent per liter.

It will be understood that this invention is not to be limited to the details given herein but that it may be modified within the scope of the appended claims.

What is claimed is:

1. A method for concentrating aqueous waste solutions obtained from processing uranium-aluminum fuel elements comprising spraying the solution on a bed of burning coke, whereby the water and other volatile constituents in the solution including ruthenium are heat-vaporized and the nitrates in the solution are heatdecomposed into the volatile gaseous oxides of nitrogen, conducting away the resulting mixture of gases into a first separate vessel, and thereafter passing chlorine through said bed whereby the aluminum, niobium-95, and zirconiumdecomposition products are converted to chlorides, conducting away said chlorides in an admixed gaseous state and condensing them in a second separate vessel.

2. The method of claim 1 where the coke is petroleum coke.

3. The method of claim 1 where the waste solution is sprayed on the bed of burning coke in the proportion of about 1570 liters of 2.2 M aluminum nitrate per hour for 14.9 hours, where oxides of nitrogen are reduced by a reducing flame and the resulting mixture of gases is thereafter scrubbed until free of fission products, where the chlorine is passed through the bed for 9.1 hours, where the vaporized, condensed chlorides are thereafter stored for 2-4 years, and the residual ash and commingled fission products are stored until their radioactivity ceases.

References Cited in the file of this patent UNITED STATES PATENTS 821,190 Potter May 22, 1906 1,350,877 Mayer et a1. Aug. 24, 1920 1,503,648 Hall Aug. 5, 1924 1,619,022 Hall Mar. 1, 1927 2,084,290 McAfee June 15, 1937 2,591,188 Nilsson Apr. 1, 1952 OTHER REFERENCES Proceedings of the International Conference on the Peaceful Uses of Atomic Energy, vol. 9, p. 635, Aug. 8-20, 1955, United Nations (1956). 7

ABC Document, IDO-14392, Nov. 1, 1956, pp. 3-16.

Claims (1)

1. A METHOD FOR CONCENTRATING AQUEOUS WASTE SOLUTIONS OBTAINED FROM PROCESSING URANIUM-ALUMINUM FUEL ELEMENTS COMPRISING SPRAYING THE SOLUTION ON A BED OF BURNING COKE, WHEREBY THE WATER AND OTHER VOLATILE CONSTITUENTS IN THE SOLUTION INCLUDING RUTHENIUM ARE HEAT-VAPORIZED AND THE NITRATED IN THE SOLUTION ARE HEAT-DECOMPOSED INTO THE VOLATILE GASEOUS OXIDES OF NITROGEN, CONDUCTING AWAY THE RESULTING MIXTURE OF GASES INTO A FIRST SEPARATE VESSEL, AND THEREAFTER PASSING CHLORINE THROUGH SAID BED

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