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PILES AND PLUTONIUM (1939-1942)
Events
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Early Government Support, 1939-1942
The Uranium Committee's first report,
issued on November 1, 1939, recommended that, despite the
uncertainty of success, the government should immediately
obtain four tons of graphite and fifty tons of uranium
oxide. This recommendation led to the first outlay
of government funds -- $6,000 in February 1940 -- and
reflected the importance attached to the Fermi-Szilard
pile (reactor) experiments already
underway at Columbia University.
Building upon the
work performed in 1934 demonstrating the value of
moderators in producing slow neutrons,
Enrico Fermi thought that a mixture of
the right moderator and natural uranium could produce a
self-sustaining
fission chain reaction. Fermi and
Leo Szilard increasingly focused their
attention on carbon in the form of graphite. Perhaps
graphite could slow down, or moderate, the
neutrons coming from the fission
reaction, increasing the probability of their causing
additional fissions in sustaining the chain
reaction. A pile containing a large amount of
natural uranium could then produce enough secondary
neutrons to keep a reaction going.
There was, however, a large theoretical gap between
building a self-generating pile and building a bomb.
Although the pile envisioned by Fermi and Szilard could
produce large amounts of power and might have military
applications (powering naval vessels, for instance), it
would be too big for a bomb. It would take
separation of uranium-235 or substantial enrichment of
natural uranium with uranium-235 to create a fast neutron
reaction on a small enough scale to build a usable
bomb. While certain of the chances of success in his
graphite power pile, Fermi in 1939 thought that there was
"little likelihood of an atomic bomb, little proof that we
were not pursuing a chimera."
Experiments conducted in early 1941 at
the Radiation Laboratory at the
University of California, Berkeley,
finally completed the link between pile research and bomb
construction. Edwin M. McMillan and Philip H.
Abelson had been studying uranium fission fragments
produced in a cyclotron there
(above). Their research led to the chemical
identification of element 93, neptunium, while research by
Glenn T. Seaborg (left) revealed that an
isotope of neptunium decayed to yet another transuranium
(man-made) element. In February, Seaborg identified
this as element 94, which he later named plutonium.
By May, he had proven that plutonium-239 was 1.7 times
more likely than uranium-235 to fission. This
finding made the Fermi-Szilard experiment more important
than ever, as it suggested the possibility of producing
large amounts of the fissionable plutonium in a uranium
pile using plentiful uranium-238, and then separating it
chemically. Surely this would be less expensive and
simpler than building isotope-separation plants. A
second, perhaps easier, path to the atomic bomb now seemed
possible.
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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
publication:
F. G. Gosling,
The Manhattan Project: Making the Atomic Bomb
(DOE/MA-0001; Washington: History Division, Department
of Energy, January 1999), 6-8. The "chimera" comment is from Laura Fermi,
Atoms in the Family: My Life With Enrico Fermi
(Chicago: University of Chicago Press, 1954), 164.
The terms "atomic pile" and "nuclear reactor" refer to
the same thing. The term "pile" was more common
during early atomic research but gradually was 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 photograph of
Enrico Fermi is courtesy the
Department of Energy
(via the
National Archives). The
fission chain reaction graphic is
adapted from a graphic originally produced by the
Washington State Department of Health; modifications are original to the Department of
Energy's Office of History and Heritage Resources.
The photographs of the cyclotron and of
Glenn Seaborg are courtesy the
Lawrence Berkeley National Laboratory.
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