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PRODUCTION REACTOR (PILE) DESIGN (Met Lab, 1942)
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The Plutonium Path to the Bomb, 1942-1944
By 1942, scientists had established that some of the
uranium exposed to
radioactivity in a
reactor (pile) would eventually decay
into plutonium, which could then be
separated by chemical means from the
uranium. Important theoretical research on this was
ongoing, but the work was scattered at various
universities from coast to coast. In early 1942,
Arthur Compton arranged for all pile
research to be moved to the Met Lab at
the University of Chicago.
Two distinct but related tasks faced the scientists of the
Met Lab in early 1942: the construction of a small
experimental pile to explore the physics of the
fission chain reaction (hence the term
"reactor") and planning for a much larger
reactor that could produce plutonium on an industrial
scale. Although the two were in theory similar, in
practice the much larger production pile would require
elaborate controls, radiation shielding, and a cooling
system. In addition, the theory behind the
generation and control of chain reactions was still poorly
understood, which was why Enrico Fermi's
small experimental pile was necessary in the first
place.
Planning for the experimental reactor -- dubbed "CP-1" for "Chicago Pile
No. 1" -- began even before Fermi's team from
Columbia University arrived in
Chicago. One of the main goals in building CP-1 was
to determine the precise value of the neutron reproduction
factor "k" for a theoretical
reactor of infinite size. Early experiments leading
to a chain-reacting pile were conducted on a squash
racquet court under the abandoned west stands of the
University of Chicago's football stadium, Stagg
Field. Arthur Compton made plans to build the first
pile at a site in the Argonne Forest Preserve, about
twenty-five miles southwest of Chicago, "where the
hazards would be minimized." Labor and other
difficulties, however, delayed construction at the Argonne
site. Convinced by Fermi that calculations were
reliable enough to preclude a catastrophic run-away chain
reaction, Compton authorized construction of the pile at
the Stagg Field site. Fearing rejection, Compton sought
approval for this decision from neither General Groves nor
the University of Chicago administration. Fermi was
confident that the world's
first nuclear chain reaction could be
produced before the end of 1942.
But planning for industrial-scale production piles had to
proceed even before the theoretical questions could be
explored by CP-1. The Fermi pile, important as it
was, would provide little technical guidance when it came
to the complicated cooling, control, and shielding systems
required of a large reactor. Just as would happen
with uranium enrichment at
Oak Ridge, the job was to design
equipment for a technology that was not yet well
understood even in the laboratory.
In June 1942, a group headed by Arthur Compton's chief
engineer, Thomas V. Moore, began designing the first
production reactor (pile). It quickly became clear
that a production pile would differ significantly in
design from Enrico Fermi's planned experimental reactor
(CP-1). Radiation and containment shielding would be
necessary, as would a cooling system. Although
experimental piles like Fermi's did not generate enough
power to need cooling systems, any reactor large enough to
produce non-trivial amounts of plutonium would have to
operate at high power levels and require coolants of some
kind. The Met Lab group considered the full range of
gases and liquids as potential coolants in a search to
isolate the substances with the best nuclear
characteristics, with hydrogen and helium standing out
among the gases and water -- even with its marginal
nuclear properties and tendency to corrode uranium -- as
the best liquid. In addition, a method was
needed for removing the irradiated uranium, preferably
without destroying the reactor. One obvious option
was to extend uranium rods into and through the graphite
next to cooling tubes.
During the summer, Moore and his group began planning a
helium-cooled pilot pile to be built by Stone &
Webster in the Argonne Forest Preserve near Chicago. On
September 25, they reported to Compton. The proposal
was for a 460-ton cube of graphite to be pierced by 376
vertical columns containing twenty-two cartridges of
uranium and graphite. Cooling would be provided by
circulating helium 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 (left) 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. 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 water
tank 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.
Leo 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 and Albert Einstein had
created), 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 rare.
October 1942 found Leslie Groves (right)
in Chicago ready to force a showdown on pile design.
Szilard had been complaining that decisions had to be made
so that design could move to procurement and
construction. Compton's delay reflected uncertainty
regarding the superiority of the helium pile and awareness
that engineering studies could not be definitive until
uncertainties surrounding the neutron reproduction factor
k had been cleared up, which would not happen
until experiments with CP-1 began. 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
proposed by Fermi. The situation was tailor-made for
a man with Groves's temperament. On October 5,
Groves gave the Met Lab one week to decide. 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.
Compton decided on compromise. Fermi would explore the
precise value of k and study the fundamentals of
pile operation in CP-1, to be completed and in operation
by the end of the year. An intermediate pile with
external cooling would be built at Argonne and operated
until June 1, 1943, when it would be taken down for the
extraction of its resulting plutonium. The
100,000-kilowatt helium-cooled Mae West, designed to
produce 100 grams of plutonium a day, would be built at
Oak Ridge and operating by March 1944. Compton and
the pile researchers hoped that this pile would function
as both a test facility and the first unit of the
full-scale production plant. Studies on liquid-cooled
reactors, meanwhile, would continue, including Szilard's
work on liquid metals. Once again, in the absence of one
clearly preferable approach, the urgency of the Manhattan
Project required that every possibility be explored
simultaneously.
Next
Sources and notes for this page.
The text for this page was adapted from, and portions
were taken directly from the
Office of History and Heritage Resources
publications:
F. G. Gosling,
The Manhattan Project: Making the Atomic Bomb
(DOE/MA-0001; Washington: History Division, Department
of Energy, January 1999), 26-27, and Richard G. Hewlett and Oscar E. Anderson,
Jr., The New World, 1939-1946: Volume I,
A History of the United States
Atomic Energy Commission
(Washington: U.S. Atomic Energy Commission, 1972),
108-9, 174-82. Also used were Jack M. Holl,
Argonne National Laboratory, 1946-96 (Urbana,
IL: University of Illinois Press), 13-16, and Vincent C.
Jones, Manhattan: The Army and the Atomic Bomb,
United States Army in World War II (Washington: Center
of Military History, United States Army, 1988), 190-91.
The terms "atomic pile" and "nuclear
reactor" refer to the same thing. The term
"pile" was more common during early atomic
research, and it was gradually replaced by
"reactor" in the later years of the Manhattan
Project and afterwards. In this web site, the
phrase "pile (reactor)" is
used to refer to early, experimental piles, and
"reactor (pile)" is used to refer to later
production reactors, which had more elaborate controls
and in general more closely resembled post-war
reactors. Much as the term "pile"
gradually gave way to "reactor,"
"atomic" was gradually replaced by
"nuclear." The schematic drawing of
X-10 is reproduced from Hewlett and
Anderson, The New World, 195. The drawing
of CP-1 is courtesy the
National Archives. The Hanford reactor schematic
is reproduced from the
Department of Energy
report
Linking Legacies: Connecting the Cold War Nuclear
Weapons Production Processes to their Environmental
Consequences
(Washington: Center for Environmental Management
Information, Department of Energy, January 1997), 164. The photograph of
Vannevar Bush and
Arthur Compton is courtesy the
Lawrence Berkeley National Laboratory. The portrait of
Leslie Groves is courtesy the
Los Alamos National Laboratory.
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