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IMPLOSION BECOMES A NECESSITY
(Los Alamos: Laboratory, 1944)
Events >
Bringing It All Together, 1942-1945
Because the gun-type bomb design seemed
so simple and practical, Deke Parsons had assigned implosion studies a
low priority and placed the emphasis on the more familiar artillery
method. Consequently, Seth H. Neddermeyer performed his early
implosion tests in relative obscurity. Neddermeyer found it difficult
to achieve symmetrical implosions at the low velocities he had
achieved. When the Princeton mathematician John von Neumann, a
Hungarian refugee, visited Los Alamos late in
1943, he suggested that high-speed assembly and high velocities would
prevent predetonation and achieve more symmetrical explosions. A
relatively small, subcritical mass could be placed under so much pressure
by a symmetrical implosion that an efficient detonation would occur.
Less fissionable material would be required, bombs could be ready earlier,
and extreme purification of plutonium
would be unnecessary. Von Neumann's theories excited Robert
Oppenheimer, who assigned Parsons's deputy, George B. Kistiakowsky,
the task of perfecting implosion techniques. (Kistiakowsky would
later become President Dwight D. Eisenhower's science adviser.)
Because Parsons and Neddermeyer did not get along, it was Kistiakowsky who
worked with the scientists on the implosion project.
While experiments on the
gun and implosion methods continued, Parsons directed much of his effort
toward developing bomb hardware, including arming and wiring mechanisms and
fusing devices. Working with the Army Air Force, Parsons's group
developed two bomb models by March 1944 and began testing them with
B-29s. "Thin Man," named for President
Roosevelt, utilized the plutonium gun design, while "Fat
Man" (right), named after Winston Churchill, was an implosion
prototype. (Emilio Segrè's lighter, smaller uranium
design became "Little Boy," Thin Man's brother).
In the summer of 1944, however, it became clear that, because of the
plutonium-240 problem, a gun-type design would not work for the plutonium
bomb. The implosion method was now transformed from an intriguing
possibility into a difficult necessity. Glenn Seaborg had
warned that when plutonium-239 was irradiated for a length of time it was
likely to pick up an additional neutron,
transforming it into plutonium-240 and increasing the danger of
predetonation, i.e., the bullet and target in the plutonium weapon would
melt before coming together. Measurements taken at Oak Ridge confirmed the
presence of plutonium-240 in the plutonium produced in their experimental
pile (X-10).
On July 17, the difficult decision was made to cease work on the plutonium
gun method -- there would be no "Thin Man." Plutonium could
be used only in an implosion device, but in the summer of 1944 an implosion
weapon looked like a long shot.
Abandonment
of the plutonium gun project eliminated a shortcut to the bomb. This
necessitated revision of the estimates of weapon delivery Vannevar Bush
had given the President in 1943. The new timetable, presented to
General George Marshall by Leslie Groves on
August 7, 1944 -- two months after "D-Day," the Allied invasion
of France -- promised small implosion weapons of uranium or plutonium in
the second quarter of 1945 if experiments proved satisfactory. More
certain was the delivery of a uranium gun-type bomb by August 1, 1945, and
the delivery of one or two more by the end of that year. Marshall and
Groves agreed that Germany might well surrender by the summer of 1945, thus
making it probable that Japan would be the target of any atomic bombs ready
by that time.
Oppenheimer acted quickly to maximize the laboratory's efforts to master
implosion. Only if the implosion method could be perfected would the
plutonium produced at Hanford
come into play. Without either a plutonium gun bomb or implosion
weapon, the burden would fall entirely on uranium and the less efficient
gun method. Oppenheimer directed a major reorganization of Los Alamos
in July 1944 that prepared the way for the final development of an implosion
bomb. Robert Bacher took over G Division (for "Gadget") to
experiment with implosion and design a bomb; Kistiakowsky led X Division
(for "explosives") in work on the explosive components; Hans
Bethe continued to head up theoretical studies; and Parsons now focused on
overall bomb construction and delivery.
Field tests
performed with uranium-235 prototypes in late 1944 eased doubts about the
gun-type method to be employed in the uranium bomb. It was clear that the
uranium-235 from Oak Ridge could be used in a gun-type nuclear device to
meet the August 1 deadline Groves had given General Marshall and the Joint
Chiefs of Staff. The plutonium produced at such expense and effort at
Hanford (right), however, would not fit into wartime planning unless a
breakthrough in implosion technology could be found.
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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), 40,
42. The diagram illustrating implosion 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), 13.
The photograph of the implosion experiment is courtesy the Los Alamos National Laboratory; it is
reprinted in Rachel Fermi and Esther Samra, Picturing the Bomb:
Photographs from the Secret World of the Manhattan Project (New York:
Harry N. Abrams, Inc., Publishers, 1995), 111, 116. The photograph of
Fat Man is courtesy the U.S. Army Corps of Engineers (via the National Archives). The photograph
of Leslie Groves
with Robert
Oppenheimer is courtesy the Department of Energy. Click here for more
information on the Hanford B Reactor photograph.
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