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FINAL REACTOR DESIGN AND X-10 (Met Lab and Oak Ridge [Clinton], 1942-1943)
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The Plutonium Path to the Bomb, 1942-1944
Before any plutonium could be
chemically separated from uranium for a
bomb, however, that uranium would first have to be
irradiated in a production pile.
CP-1 had been a success as a scientific
experiment, but the pile was built on such a small scale
that recovering any significant amounts of plutonium from
it was impractical. In the fall of 1942, scientists
of the Met Lab had decided to build a
second Fermi pile at Argonne as soon as his experiments on
the first were completed and to proceed with the "Mae
West" design for a helium-cooled production pile as
well. 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
began to change when Enrico Fermi's CP-1
calculations demonstrated a higher value for the neutron
reproduction factor k (for a theoretical reactor
of infinite size) than anyone had anticipated. Met
Lab scientists concluded that a water-cooled pile was now
feasible. Crawford Greenewalt, head of the DuPont
effort, continued, however, to support helium
cooling.
The higher value for k also seemed to make
possible an experimental pile using air cooling.
Since a helium-cooled unit shared important design
characteristics with an air-cooled one, and an air-cooled
unit would be easier to design and quicker to build,
Greenewalt thought that first constructing an experimental
air-cooled production pile at
Oak Ridge would help significantly in
designing the full-scale helium-cooled reactors for
Hanford. Thus, the
X-10 Graphite Reactor (left) was
born. In early 1943, DuPont established the general
specifications for this experimental production reactor at
Oak Ridge, as well as its accompanying chemical separation
facilities. X-10 would be a massive graphite block,
protected by several feet of concrete, containing 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
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 underground canal into the chemical separation facility
where the plutonium would be extracted with remote control
equipment. DuPont broke ground at the X-10
complex at Oak Ridge in February 1943, and on November 4,
1943, X-10 went critical for the first time. By the
end of the month, it was producing small but
experimentally-valuable samples of plutonium.
Met Lab activities focused, meanwhile, on the design of
the proposed full-scale water-cooled piles to be built at
Hanford (left). Taking their cue from the DuPont
engineers, who utilized a horizontal design for the
air-cooled X-10, 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 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 water per minute for cooling.
Greenewalt's initial response to the water-cooled design
for Hanford was guarded. He worried about pressure
problems that might lead to boiling water in individual
tubes, corrosion of 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 compressors would not be ready in time,
that the shell could not be made vacuum-tight, and that
the pile would be extremely difficult to operate.
DuPont engineers conceded that Greenewalt's fears were
well-grounded. Late in February, Greenewalt
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 (right), built with material from
CP-1, focused on neutron capture
probabilities, control systems, and instrument
reliability. Once the X-10 production reaction
projects were underway, however, Met Lab research became
increasingly unimportant in the race for the bomb and the
scientists found themselves serving primarily as
consultants for DuPont's work at Oak Ridge and Hanford.
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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), 30, 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),
193-201. 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
photograph of X-10 is courtesy the
Oak Ridge National Laboratory. 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 CP-2 is
courtesy the
Argonne National Laboratory.
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