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
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
- 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
- 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.