I
0.F. HILL and V. R. COOPER Hanford Laboratories Operation, General Electric
Co.,Richland, Wash.
Scale-up Problems in
The Plutonium Separations Program EDITOR'S NOTE. Scientists and engineers working on atomic proiects early during World War II found the days both exciting and hearthelp make a bomb as breaking. Everyone had only one job-to soon as possible. This article describes solutions to several engineering problems encountered, some of which are still the best answer.
A F T E R WORLD WAR11, the large scale experimental program for building a chemical separations facility a t Hanford declined to an insurance-type effort. In 1947, however, it again was accelerated and today it is a large undertaking. In 1942 when the first process for isolating plutonium on a commercial scale was begun, the objectives were simple : Process. Adaptability to remote operations and control, simple chemistry, and standard engineering. Safety. Critical mass control, compatibility of chemicals; and contamination containment and control. Equipment. Control of corrosiveness, adaptable to remote operation and control; standard chemical process equipment adaptable to changes. Product quality. Decontamination factor of lo7 (contact levels), and adequate purity.
Economically, however, the objectives were unusual. Enough plutonium (more than 50%) had to be recovered to make a bomb befo;e the enemy did. This was a primary factor in selecting a process-plant construction had to precede the final process.
fact, most processes conceived today for treating irradiated uranium and plutonium were in some measure under study then : Exploratory Processes, 1942-43. Precipitation, solvent extraction, adsorption, volatility, and pyroprocessing. Proved Methods, 1957. Precipitation, solvent extraction, and ion exchange. Under Active Investigation, 1957. Volatility, pyrochemistry, pyrometallurgy, and homogeneous reactors. Emphasis was placed on the precipitation process. Engineering know-how for such methods in general was well established, and the process met the criteria of simplicity necessary for remote operation and control. Laboratory and pilot plant programs were quickly set up for adapting the lanthanum fluoride process on a large scale. However, problems such as corrosion, remote operations, and handling of the gelatinous precipitate were encountered. Consequently, research for an alternative precipitation procedure paralleled this effort.
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BiPOa
Flowsheet Development
The first plutonium, prepared by cyclotron bombardment, was isolated by a combination of two processes: solvent extraction of uranyl nitrate hexahydrate with ether, which separated the bulk of uranium from the plutonium, and coprecipitation of reduced plutonium on lanthanum fluoride. These processes ensured that plutonium could be isolated on a commercial scale; however, engineering to a remote operation was complicated by safety hazards associated with ether extraction and problems in materials of constructio& b r the fluoride process. Many proposed unit operations were investigated. In
Extraction
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F i r s t Cycle Bi PO4 Product PPTN
A procedure for separating plutonium was developed, based on alternately precipitating zirconium phosphate from reduced and oxidized solutions. This, however, had three major disadvantages : its gelatinous nature, difficulty in dissolving the precipitate, and the inconsistency and lack of reproducibility of carrying. Early in 1943, S. G. Thompson observed that bismuth phosphate would carry plutonium(1V) phosphate but not plutonium in the oxidized state. Since bismuth phosphate is crystalline and readily soluble in acid, effort was concentrated on this approach. Bismuth phosphate is alternatively precipitated from reduced [plutonium(IV)] and oxidized [plutonium(VI)] solutions to permit a stepwise separation of plutonium from uranium and fission products, or fission products from plutonium. Plutonium is concentrated by carrying on lanthanum fluoride, where the amount of carrier required is small compared to that of bismuth phosphate. This permits concentrating the plutonium to 5 to 20 grams per liter where it can be isolated from the carrier, lanthanum, by direct precipitation. Plutonium peroxide was used for this step because lanthanum is not coprecipitated. Variables studied included solution concentrations of uranyl nitrate hexahydrate, nitric acid, bismuth, phosphoric acid, sulfuric acid, and iron(II), together with methods of strike, strike
by ProdGct PPTY I
Second C y c l e u1 PO4 ny P r o d u c t PPTN
S c ( ond Cy( Ir 131 PO4 Product PPTV
Cross-Over BiP04 by P r o d u c t PPTN
Lross-OvPr La F j by P r o d u c t PPTN
Cross-Ovcr La F g Product PPTN
Metathosis and
Plutonium PPr o x i d e PPTN
HNO < Dissoln and
HNO? D i s s o l n
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Pu Nitrate
I w1< e) I Many months of experimentation resulted in a bismuth phosphate batch-wise process for separating plutonium
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A prefabricated jumper ready for installation and digestion times, temperature, agitation, and solids removal. Laboratory results were later verified in pilot plant tests at Oak Ridge. Mechanical problems and process details Ivere established by these tests and later, at Hanford, by production tests. Extrapolation from laboratory ultramicrochemical and beaker studies to full scale production was surprisingly accurate. Not only was it necessary to establish concentration of the carrier (bismuth) required, for example, but also the optimum concenrration of the precipitating agent, phosphoric acid. Decontamination in the by-product precipitation steps was improved by using supplemental scavenging precipitates for specific fission products. Finally, to remove major phosphate fission products, precipitation of zirconium and cerium(1V) phosphates along with bismuth phosphate in the first decontamination cycle by-product step was adapted to the process. Furthermore, adding Ruorosilicate to complex zirconium during the bismuth phosphate product
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precipitation steps of the decontamination cycles was necessary to attain the required decontamination. Dissolution and its kinetics of bismuth phosphate were studied to complete the process design. The data in Figure 1 are typical of those required for process development and engineering. This figure was chosen because it illustrates a property of bismuth phosphate that caused considerable concern-it precipitated in two crystalline forms, alpha (normal hexagonal) and beta (monoclinic). The alpha form is easily soluble in 10M nitric acid to 70 to 100 grams per liter, whereas the beta form is difficultly soluble to only 25 to 35 grams per liter. Thus, it is desirable to promote formation of the alpha form. Considerable time was spent in studying this problem, in both the laboratory and pilot plant. Fortunately, in the extraction step, uranyl nitrate hexahydrate inhibits formation of the beta form and, in the product precipitation step of the decontamination cycle, iron(11) also inhibits this formation. I n the by-product steps where the formation is more probable, precipitation conditions are more critical; however, complete dissolution of the by-product precipitate is less important because, unless recovery is necessary, it normally goes to waste. Plant Design To house the process, a "canyon" consisting of thick concrete walls was constructed, which for versatility was laid out in identical sections, each containing two cells. Versatility was necessary because the building design and construction were proceeding concurrently with process development. Precipitators, catch tanks, and precipitate dissolvers are simply stainless steel tanks. Centrifuges, designed for remote operation, were adaptable for solid separation portions of the process.
Each section of the canyon contains these four pieces of equipment. Decontamination prior to contact maintenance necessitates fairly long down times for any major equipment conversion and maintenance. O n the other hand, this has been done successfully in pilot plants and small scale production plant operation. For the bismuth phosphate plants, the remote connector head, essentially the same equipment used in today's plants, was developed to facilitate replacement of such items as leaky jumpers or failed pieces of equipment. A remote connector head is placed on each end of a jumper which is carefully laid out to dimensions in a mock-up shop and equipped with a hook at the exact center of gravity. The jumper is then easily dropped into place by a skilled crane operator and fastened into place by a remote power wrench controlled and operated by cables. Items such as tanks and centrifuges are also mocked up with jumper connections carefully controlled as to dimensional placement and also equipped with hooks for ready handling and remote replacement by the crane operator. Feed Preparation Dissolvers for the nitric acid dissolution of the irradiated uranium metal were easily designed to prepare solutions for the process. The aluminum slug jackets are removed in a sodium hydroxidesodium nitrate solution. An alternate to this jacket removal process used in the pilot plant studies at Oak Ridge is the mercury-catalyzed nitric acid dissolution of the aluminum jackets. The sodium hydroxide-sodium nitrate procedure was chosen because the Hanford production slugs are bonded to the aluminum jacket with an aluminumsilicon alloy. It was desirable to remove as much of this bond as possible to avoid complicating factors in the feed solution.
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Typical arrangement of a process section showing flow of solutions; tained four pieces of equipment
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INDUSTRIAL AND ENGINEERING CHEMISTRY
each con-
SMALL SCALE E N G I N E E R I N G D A T A Caustic removes it fairly effectively. Sodium nitrate was added to the caustic solution to promote formation of ammonia in the off-gas instead of hydrogen and thus avoid a potential explosion hazard. After transferring a coating solution to underground storage, the dissolver and its contents are washed. Uranium metal is dissolved in nitric acid. The dissolvers are usually operated with a large uranium heel to promote shorter dissolving cycles since the rate of dissolution of uranium metal is dependent on the total surface area. The time cycle requirements and thus the size and number of pieces of equipment required were estimated from laboratory data. This basic process is still employed for feed preparation, although design of the dissolvers has been improved to permit larger batch sizes. Downdraft scrubbing of the off-gases to conserve nitric acid consumption is now used.
Typical cell arrangement. to process tank
Remote jumpers extend from wall nozzles
Off-Gas Problems
Some years after the bismuth phosphate plants were put into operation, it was found that the surrounding area was receiving detectable amounts of radioactivity which were traced to two sources -particulate matter and radioiodine, escaping from the 200-foot stack. Most of the radioiodine was emitted from dissolvers during the dissolution step. The particulate matter was emitted via the ventilation air of the canyon and dissolver off-gas. Pilot plant equipment was set up to establish the engineering parameters of gas filters. Within 6 months, it was established that a carefully graded bed of sand would remove about 99.9% of the particulate matter. A bed, 48 by 100 feet, was constructed through which the ventilation air was filtered prior to discharge to the environs through the stack. This bed gave an efficiency of 99.7%. An estimated life of 5 years was predicted on the basis of one grain per 1000 cubic feet. After 7 years of operation the pressure drop remained below that allowable. Particulate matter, as well as radioiodine, was also emitted from the dissolver operation, and the piping did not permit the dissolver off-gases to be routed through the sand filter; therefore, another method was required for this stream. Because of space limitations within the dissolver cell, a smaller unit was required, than could be designed with a sand filter. Chemical Warfare Service filter studies suggested the use of glass fibers for which data were obtained: measures of filtration characteristics (pressure drop and collection efficiencies) as a function of air velocity,
bed depth, and packing density. A compact unit was designed, using several layers of fibers of varying fiber diameters, layer thicknesses and compactness, and capable of operating a t high superficial air velocities, low flow resistance, and long life expectancy. Measured efficiencies of installed units have been greater than 99.9% and probably near 99.99%. Instead of using a sand filter, the most recent plant conQtructed a t Hanford filters canyon ventilation air with a large scale glass fiber unit. T o remove radioiodine from the dissolver off-gas, which contained significant concentrations of nitrogen oxides, silver nitrate deposited on a carrier packing was efficient. In this development the first tests were attempted with inert iodine. Since anomalies were observed in the data, equipment was installed to withdraw plant off-gas streams through laboratory scale equipment. Fast reactions were observed and a 2-inch bed depth removed more than 99.3% of the iodine from the gas. Plant scale units, designed from these data, were equally efficient, but the reactions are not completely understood and the life of the reactors has not been as long as initially anticipated. However, regenerating procedures have been developed as a relatively cheap means of reconstituting an active bed. Only about one half of the radioiodine is evolved during the dissolving step. Under conditions in the process where significant concentrations of nitrite are added to control pluto&um oxidation, iodine is released in the free form. During jetting operations some
of this iodine can then be released to the ventilation air. Iodine so released can be minimized by complexing the iodide with mercury(I1) added as the nitrate. The basic data to provide the process conditions for this step were obtained by sparging experiments in the laboratory. Improvements and Testing
When the bismuth phosphate plants were operating smoothly, attention was focused on making the processes more economical. With the exception of a few minor start-up problems, the process operated well and smoothly from the start. Decontamination and recovery performance exceeded that expected from laboratory and semiworks studies; plutonium yields were greater than 90%, and decontamination factors immediately approached and later exceeded the lo7 design bases. Although only a fraction of the laboratory data available was essential in designing the original flowsheet, knowing the effect of changing variables was valuable for establishing flowsheet control limits. With further plant operating experience, plant observations could be combined with laboratory knowledge to effect substantial improvements with assurance and promptness. Reducing volumes was one of the first obvious means of economizing. The tank farms for storing wastes were filled rapidly and new tanks had to be constructed. Some wastes were low in activity and contained no long-lived fission products; consequently, the first expedient was to allow solids to settle from the neutralized solutions in tanks VOL. 50, NO. 4
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and scavenge most of the radioactivity from the solutions. The supernatants were then allowed to overflow-the soil has a high specific retention of liquid. An even more fruitful approach was to reduce the volume of solutions necessary to be handled in the first place. The original p r o c w was designed so that moderate variations from the process conditions would not complicate product decontamination or product recovery. By designing the process so that operations were carried out and controlled more closely to the limiting conditions, reductions to less than 50% of the initial volumes appeared possible. Many laboratory data were available and process flowsheets were drawn u p which appeared feasible. Conditions in the production plant were then systematically varied to test these process variables for their effect on product recovery or decontamination. By the proper combination of variables, volumes were reduced more than 50% and time cycles were substantially reduced to permit significant increases in plant capacity. The initial plant was conservatively designed to assure wide process Aexibility, and extra equipment which had been installed was used. By properly routing streams, the same process step could be carried out simultaneously in more than one piece of equipment. This permitted higher throughputs and attendant economies. Furthermore, routine rework procedures were devised to recover plutonium from the waste streams containing high waste losses and the process was improved to allow 77 to 78% recovery, routinely.. All technical laboratory data employed as the basis for these tests were obtained prior to or concurrent with plant startup. Solvent Extraction Processes
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The wisdom in selecting a precipitation process was apparent when a solvent extraction method, called the Redox process, was developed for plutonium recovery. This approach, chosen for normal economic reasons including lower unit costs, high yields, and uranium recovery, involves solvent extraction of uranium and plutonium from a nitrate solution using hexone (metal isobutyl ketone) as the solvent. The extensive laboratory development required a complete definition of the physical and chemical properties of the system to permit economic and safe design of flowsheets. Certain fission products, particularly ruthenium, zirconium, and niobium, were troublesome, and extensive laboratory investigations were carried out to assure their adequate separation from the product streams. In the process engineering area, extensive pilot plant studies were required to develop the necessary process equipment design information €or a solvent extraction plant,
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including information on packed extrac- . tion towers. Most data had to be developed from scratch because related technology was unavailable in the literature. Effects of factors such as column diameter, and size and kind of packing on extraction efficiency and column throughputs had to be developed. Furthermore, since the process conceived is continuous, equipment adaptable to remote operation had to be developed. Because more than one solvent extraction cycle was required to complete decontamination from the fission products, design information had to be developed on concentrators, strippers, and other related equipment. Even though the basic process chemistry had been worked out in the wartime metallurgical laboratory of the University of Chicago in 1745-46, further investment of more than $1,000,000 was still needed for pilot plant test equipment and building modifications. Manpower for this program included more than 100 scientists and engineers for longer than 3 years. A major shortcoming of the bismuth phosphate process was lack of uranium recovery. The uranium waste stream from the extraction step, containing the bulk of the fission products, was neutralized with caustic and carbonate and stored underground. Caustic was used to neutralize the excess acid, including sulfuric and phosphoric acid: and carbonate was added to complex the uranium to reduce the viscosity for underground pipe transfer to the waste tanks. As this waste solution digested in the tanks at high temperatures resulting from fission product decay, uranium partially precipitated as uranyl phosphate; and double salts of uranyl carbonate. Since this uranium had an intrinsic value, exploratory tests of recovery processes were carried out in 1944-46. However, it was not until 1948 that a process for its recovery was developed. At first, a precipitation process to clean up the uranium from phosphate and sulfate was tried. This uranium could then be placed in a nitric acid solution and fed to the Redox process for recovery by hexone extraction. However, since tributyl phosphate was found to be a good solvent for uranium, even in the presence of complexants such as phosphate and sulfate, a solvent extraction process was installed in an unused bismuth phosphate canyon. The sludge containing the insoluble phosphates and carbonates was slurried into supernatant or water and the resultant slurry acidified with nitric acid to prepare a feed for the solvent extraction system. The efforts devoted to developing this metalrecovery process were approximately equal to that for the Redox process. A significant development of this program was the pulse column as a solvent extraction contactor.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Because tributyl phosphate will also extract plutonium, the Purex process for recovering both plutonium and uranium was later developed around this extractant using nitric acid as a volatile, recoverable salting agent. Conclusions
Pilot plant data, obtained with both “hot” and “cold” semiworks, contributed substantially to the early successful operation of the full scale plant. The major contributions of the pilot plant were mechanical data-e.g., agitation, digestion, centrifugation, instrumentation, and jet requirements-which were difficult or impossible to duplicate in the laboratory. hieans of duplicating plant characteristics in laboratory scale equipment were unknown. On the other hand, the process chemistry was fully demonstrated in laboratory apparatus. This experience has shown that radiochemical precipitation processes can be scaled up with a good degree of confidence from laboratory data. In fact, the concept of test tubes to production plant scale has been carried out and demonstrated for other precipitation processes-e.g., manganese dioxide scavenging processes for feed preparation steps to the solvent extraction process, and nickel ferrocyanide and calcium or strontium phosphate precipitation for removing long-li\ ed fission products from waste streams. Engineering design and construction were carried out with a minimum of pilot plant studies. Of course, final process design and details of operation were worked out by production tests, but the important feature of this experience is that laboratory scale data were nearly sufficient for process engineering. Precipitation methods are now being developed for recovering certain specific fission products such as cesium, strontium, and cerium. This work is being carried out with confidence that the precipitation process can be readily adapted to the equipment existing in the old bismuth phosphate canyons. I t is now the rule rather than the exception that plant design and improvements can successfully be based on a two-step procedure, procurement of chemical data from the laboratory and equipment performance data from the semiworks. The latter program is conducted without fission products. Accordingly, the most expensive phase of the development program is bypassed except in rare instances. Acknowledgment This work has resulted from the combined efforts of hundreds of scientists and engineers. The authors gratefully acknowledge the contributions of these men and women RECEIVED for review January 4, 1958 ACCEPTED January 9, 1958
