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University of Chicago Met Lab

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University of Chicago Metallurgical Laboratory (Met Lab)

Pile Design, Experimental Plutonium Production & Extraction

Web Master's Notes: We have included the Met Lab at this point in our story because as the Manhattan Project moved forward into the design and construction phase, the Met Lab also moved from being a primary research facility into one of being a supporting laboratory. Later on, scientists of the Met Lab originated the idea of demonstrating the newly developed atomic weapon prior to its military use, through its issuance of the "Franck Report".The Metallurgical Laboratory at the University of Chicago played a prominent role in forging ahead with breaking technology during the years leading up to the official formation of the Manhattan Project. Not only did Fermi successfully carry out his pile experiments here, but Glenn Seaborg carried out his research on Plutonium separation and concentration, while other brilliant physicists such as Leo Szilard, James Franck, Eugene Wigner, and Walter Zinn worked tirelessly on theoretical studies contributing to the formal approval of the Project.

However, once the Project moved from the conceptual stage to the design and construction stage, the role of the Met Lab changed to that of a supporting laboratory. In fact many of the prominent scientists relocated to other locations for the duration of the project. Some, such as Fermi, worked at Oak Ridge, Hanford and eventually Los Alamos as scientific expertise requirements constantly changed.

One of the most important branches of the far-flung Manhattan Project was the Metallurgical Laboratory at the University of Chicago. Known as the Met Lab, its primary role was to design a production pile to produce plutonium. Here again the job was to design equipment for a technology that was not well understood even in the laboratory. The Fermi pile, important as it was historically, provided little technical guidance other than to suggest a lattice arrangement of graphite and uranium. Any pile producing more power than the few watts generated by Fermi's famous experiment (CP-1) would require elaborate controls, radiation shielding, and a cooling system. These engineering features would all contribute to a reduction in neutron multiplication (neutron multiplication being represented by "k"); so it was imperative to determine which pile design would be safe and controllable and still have a k high enough to sustain a chain reaction. 

A group headed by Compton's chief engineer, Thomas V. Moore, began designing the production pile in June 1942. Moore's first goals were to find the best methods of extracting plutonium from the irradiated uranium and for cooling the pile.

Webmaster's Note: The entire process of a chain reaction in uranium producing plutonium is of course a very complex procedure. However, for the "everyday" person trying to better understand, I offer the following simplified explanation:

Blocks of uranium and a moderator, such as graphite, are assembled (stacked) in a "pile" until enough neutrons are emitted to sustain a chain reaction.

This chain reaction continues unabated and essentially "cooks" the uranium, producing energy. As a by-product, this cooking also transmutes some of the uranium atoms to form another element: plutonium.

After a certain number of days "cooking", the irradiated uranium blocks (now containing some plutonium) are removed from the pile.

This irradiated uranium now undergoes a chemical extraction process where the plutonium is removed and purified.

One major hurdle: The extracted plutonium is highly radioactive. Therefore the removal of the irradiated uranium and the extraction of the plutonium would have to accomplished using remote control equipment.

It quickly became clear that a production pile would differ significantly in design from Fermi's experimental reactor (CP-1), possibly by extending uranium rods into and through the graphite next to cooling tubes and building a radiation and containment shield. Although experimental reactors like Fermi's did not produce enough power to need a cooling system, piles built to produce plutonium would operate at high power levels and require coolants. The Met Lab group considered the full range of gases and liquids to isolate the substances with the best nuclear characteristics, with hydrogen and helium standing out among the gases and water, even with its tendency to corrode uranium, as the best liquid.

During the summer, Moore and his group began planning a helium-cooled pilot pile for the Argonne Forest Preserve near Chicago, built by Stone & Webster, and on September 25 they reported to Compton. The proposal was for a 460-ton cube of graphite to be pierced by 376 vertical columns each containing twenty-two cartridges of uranium and graphite. Cooling would be provided by circulating liquid helium from from top to bottom through the pile. A wall of graphite surrounding the reactor would provide radiation containment, while a series of spherical segments that gave the design the nickname Mae West would make up the outer shell.

By the time Compton received Moore's report, he had two other pile designs to consider. One was a water-cooled model developed by Eugene Wigner and Gale Young, a former colleague of Compton's. Wigner and Young proposed a twelve-foot by twenty-five foot cylinder of graphite with pipes of uranium extending from a water tank above, through the cylinder, and into a second tank of water underneath. Coolant would circulate continuously through the system, and corrosion would be minimized by coating interior surfaces or lining the uranium pipes.

A second alternative to Mae West was more daring. Szilard thought that liquid metal would be such an efficient coolant that, in combination with an electromagnetic pump having no moving parts (adapted from a design he an Einstein had invented), it would be possible to achieve high power levels in a considerably smaller pile. Szilard had trouble obtaining supplies for his experiment, primarily because bismuth, the metal he preferred as the coolant, was very rare.

October of 1942 found General Groves in Chicago ready to force a showdown on pile design. Szilard was noisily complaining that decisions had to be made so that design could move to procurement and construction. Compton's delay reflected uncertainty of the superiority of the helium pile and awareness that, engineering studies could not be definitive until the precise value of "k" had been established. Some scientists at the Met Lab urged that a full production pile be built immediately, while others advocated a multi-step process, perhaps beginning with an externally cooled reactor as proposed by Enrico Fermi. 

The situation was tailor-made for a man with Groves' temperament. On October 5, 1942, Groves exhorted the Met Lab to decide on final pile design within a week. Even wrong decisions were better than no decisions, Groves claimed, and since time was more valuable than money, more than one approach should be pursued if no single design stood out. While Groves did not mandate a specific decision, his imposed deadline forced the Met Lab scientists to reach a consensus.

As expected, Compton decided on compromise. Fermi would study the fundamentals of pile operation on a small experimental unit to be completed and in operation by the end of the year. Hopefully he would be able to determine the precise value of "k" and make a significant advance in pile engineering. An intermediate pile with external cooling would be built at Argonne and operated until June 1, 1943, when it would be dismantled for plutonium extraction. The helium-cooled Mae West, designed to produce 100 grams of plutonium a day, would be built and operating by March 1944. Studies on liquid-cooled reactors would continue, including Szilard's work with liquid metals.

While the Met Lab labored to make headway on pile design, Glenn Seaborg and his co-workers tried to gain enough information about transuranium chemistry to insure that plutonium produced could be successfully extracted from the irradiated uranium. Using lanthanum fluoride as a carrier, Seaborg isolated a weighable sample of plutonium in August of 1942. At the same time, Isadore Perlman and William J. Knox explored the peroxide method of extraction; John E. Willard studied various materials to determine which best absorbed plutonium; Theodore T. Magel and Daniel K. Koshland researched solvent-extraction processes; and Harrison S. Brown and Orville F. Hill performed experiments into volatility reactions. Basic research on plutonium's chemistry continued as did work on radiation and fission products.

Seaborg's discovery and subsequent isolation of plutonium were major events in the history of chemistry, but, like Fermi's achievement, it remained to be seen whether they could be translated into a production process useful to the bomb effort. In fact, Seaborg's challenge seemed even more daunting, for while "piles" had to be scaled up ten to twenty times, a plutonium separation plant would involve a scale-up of the laboratory experiment on the order of a billion-fold.

Collaboration with DuPont's Charles M. Cooper and his staff on plutonium separation facilities began even before Seaborg succeeded in isolating a sample of plutonium. Seaborg was reluctant to drop any of the approaches then under consideration, and Cooper agreed. The two decided to pursue all four methods of plutonium separation but put first priority on the lanthanum fluoride process Seaborg had already developed. Cooper's staff ran into problems with the lanthanum fluoride method in late 1942, but by then Seaborg had become interested in phosphate carriers. Work led by Stanley G. Thompson found that bismuth phosphate retained over ninety-eight percent plutonium in a precipitate. With bismuth phosphate as a backup for the lanthanum fluoride, Cooper moved forward on a semi-works near Stagg Field at the University of Chicago. 

Compton's original plans to build the experimental pile and chemical separation plant on the University of Chicago campus changed during the fall of 1942. The S-1 Executive Committee concurred that it would be safer to put Fermi's pile at Argonne and build the semi-works (pilot plant) and separation facilities at Oak Ridge than to place these experiments in a populous area. On October 3rd, DuPont agreed to design and build the chemical separation plant. Groves tried to entice further DuPont participation at Oak Ridge by having the firm prepare an appraisal of the pile project and by placing three DuPont staff members on the Lewis Committee. Because DuPont was sensitive about its public image (the company was still smarting from charges that it profiteered during World War I), Groves ultimately obtained the services of the giant chemical company for the sum of one dollar over actual costs. In addition, DuPont vowed to stay out of the bomb business after the war and offered all patents to the United States government.

Groves had done well in convincing DuPont to join the Manhattan Project. DuPont's proven administrative structure assured excellent coordination (Crawford Greenwalt was given the responsibility of coordinating all DuPont and Met Lab planning), and Groves and Compton welcomed the company's demand that it be put in full charge of the Oak Ridge plutonium project. DuPont had a strong organization and had studied every aspect of the Met Lab's program thoroughly before accepting the assignment. While deeply involved in the overall war effort, DuPont expected to be able to divert personnel and other resources from explosives work in time to throw its full weight into the Oak Ridge project.

The fall 1942 planning sessions at the Met Lab led to the decision to build a second Fermi experimental pile at Argonne as soon as his experiments on the first (CP-1) were completed and to proceed on design of the Mae West helium-cooled unit. When DuPont engineers assessed the Met Lab's plans in the late fall, they agreed that helium should be given first priority. They placed heavy water second and urged an all-out effort to produce more of this highly effective moderator. Bismuth and water were ranked third and fourth in DuPont's analysis.

Priorities changed when Fermi's calculations demonstrated a higher value of "k" than anyone had anticipated. Met Lab scientists concluded that a water-cooled pile was now feasible, while DuPont shifted its interest to air cooling. Since a helium-cooled unit shared important design characteristics with an air-cooled one, Greenwalt thought that an air-cooled semi-works at Oak Ridge would contribute significantly to designing the full-scale facilities at Hanford.

DuPont established the general specifications for the air-cooled semi-works and chemical separation facilities in early 1943. A massive graphite block, protected by several feet of concrete, would contain hundreds of horizontal channels filled with uranium slugs surrounded by cooling air. New slugs would be pushed into the channels on the face of the pile, forcing the irradiated ones at the rear to fall into an underwater bucket. The buckets of irradiated slugs would undergo radioactive decay for several weeks, then be moved by an underground canal into the separation facility where the plutonium would be extracted using remote control equipment.

Met Lab activities focused on designing a water-cooled pile for the full-scale plutonium plant. Taking their cue from the DuPont engineers, who utilized a horizontal design for the air-cooled semi-works, Met Lab scientists abandoned the vertical arrangement with water tanks, which had posed serious engineering difficulties. Instead they proposed to place uranium slugs sealed in aluminum cans inside aluminum tubes. The tubes, laid horizontally through a massive graphite block, would cool the pile with water injected into each tube, The pile, containing 200 tons of uranium and 1,200 tons of graphite, would need 75,000 gallons of fresh water per minute for cooling.

Pushing for a Decision on Pile Design

Greenwalt's initial response to the water-cooled design was guarded. He worried about pressure problems that might lead to boiling water in individual tubes, corrosion of the slugs and tubes, and the one-percent margin of safety for "k". But he was even more worried about the proposed helium-cooled model. He feared that the giant compressors would not be ready in time for Hanford to come on-line, that the shell could not be made vacuum-tight, and that the pile would be extremely difficult to operate. DuPont engineers conceded that Greenwalt's fears were well-founded. Late in February 1943, Greenwalt reluctantly concluded that the Met Lab's model, while it had its problems, was superior to DuPont's own helium-cooled design and decided to adopt the water-cooled approach.

The Met Lab's victory in the pile design competition came as its status within the Manhattan Project was changing. Still an exciting place intellectually, the Met Lab occupied a less central place in the bomb project as Oak Ridge and Hanford rose to prominence. Fermi continued to work on the Stagg Field pile (CP-1), hoping to determine the exact value of "k". Subsequent experiments at the Argonne site using CP-2, built with material from CP-1, focused on neutron capture probabilities, control systems, and instrument reliability. Once the production facilities at Oak Ridge and Hanford were underway, however, Met Lab research became increasingly unimportant in the race for the bomb and the scientists found themselves serving primarily as consultants to DuPont.

A Decision on Chemical Extraction

While the Met Lab physicists chafed under DuPont domination, a smoother and quieter relationship existed between the chemists and DuPont. Seaborg and Cooper continued to work well together, and enough progress was made in the semi-works for the lanthanum fluoride process in late 1942 that DuPont moved into the plant design stage and converted the semi-works for the bismuth phosphate method. DuPont pressed for a decision on plutonium extraction methods in late May 1943. Greenwalt chose bismuth phosphate, though even Seaborg admitted he could find little to distinguish between the two. Greenwalt based his decision on the corrosiveness of lanthanum fluoride and on Seaborg's guarantee that he could extract at least fifty percent of the plutonium using bismuth phosphate. DuPont began constructing the chemical separation pilot plant at Oak Ridge, while Seaborg continued refining the bismuth phosphate method.

It was now Cooper's job to design the pile as well as the plutonium extraction facilities at Clinton, both complicated engineering tasks made even more difficult by high levels of radiation produced by the process. Not only did Cooper have to oversee the design and fabrication of parts for yet another new Manhattan Project technology, he had to do so with an eye toward planning the Hanford facility. Safety was a major consideration because of the hazards of working with plutonium, which was highly radioactive. Uranium, a much less active element than plutonium, posed far fewer safety problems.

In July of 1942 Compton had established a new health division at the Met Lab and put Robert S. Stone in charge. Stone established emission standards and conducted experiments on radiation hazards, providing valuable planning information for the Oak Ridge and Hanford facilities.

 
 
 
 

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