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CP-1 GOES CRITICAL (Met Lab, December 2, 1942)
Events
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The Plutonium Path to the Bomb, 1942-1944
While arrangements were proceeding for the construction of
full-size plutonium production reactors,
critical questions remained about their basic
design. The Italian physicist
Enrico Fermi hoped to answer some of
these questions with CP-1, his
experimental "Chicago Pile #1" at the
University of Chicago. On December
2, 1942, after a series of frustrating delays, CP-1 first
achieved a self-sustaining
fission chain reaction. After the
end of the war, Leslie Groves, commander
of the Manhattan Project, described the first time CP-1
went critical as the single most important scientific
event in the development of atomic power.
An outsider viewing preparations for this historic moment
would have been greeted by a strange sight. In an
abandoned squash court under the west grandstand of the
University of Chicago's Stagg Field there lay a huge
oblong pile of black bricks and wooden timbers, shrouded
on all sides but one by gray balloon material.
(Security regulations had forbidden the engineers from
explaining to Goodyear what the Army wanted with a giant
square balloon.) Workers machined bricks for the
pile until their faces were so covered with graphite dust
that they looked like coal miners. They sang
together to pass the time during their 12-hour shifts, and
afterwards it took them half an hour to remove the
graphite dust from their skin. The dust also made
the cement floor dangerously slippery.
What an outsider would not have understood -- but what the
men and women who would operate it certainly did -- was
how dangerous this pile of wood and bricks really
was. The wooden timbers supported a lattice
structure that contained over six tons of pure
uranium metal, along with 34 more tons of
uranium oxide. The almost 400 tons of black bricks in the
assembly were graphite, placed there to serve as
moderators; the bricks in two of every three layers had a
nodule of uranium inside each of them. The presence
of so much "moderating" material might have sounded
comforting to outsiders until they learned that the
moderators were there to increase the amount of
fission
produced by the uranium. The only things preventing
a fission chain reaction from growing within the pile were
a series of cadmium rods inserted into the pile's side to
absorb the free neutrons emitted by the
radioactive uranium. Unlike most
reactors that have been built since, this first one had no
radiation shielding and no cooling system of any
kind. Fermi had convinced
Arthur Compton that his calculations were
reliable enough to rule out a runaway chain reaction or an
explosion, but, as the official historians of the Atomic
Energy Commission later noted, the "gamble" remained in
conducting "a possibly catastrophic experiment in one of
the most densely populated areas of the nation!"
Daily the pile grew, brick by brick. Tests on the
early afternoon of December 1st indicated that it was very
close to being ready. By that evening, the
scientists present were convinced that if they withdrew
the cadmium control rods the fission chain reaction in the
pile would be self-sustaining. Final preparations
for the first test began. The next morning most of
the observers found themselves crowded together onto a
balcony where squash spectators had once stood, ten feet
above the floor on the north end of the room. Fermi,
Compton, Walter H. Zinn, and Herbert L. Anderson were
grouped around an instrument console at one end of the
balcony; from there they could operate one set of control
rods. The only person on the floor of the squash
court was George Weil, the man who would physically
withdraw the final control rod. If the reaction
threatened to grow out of control Weil could re-insert his
control rod, and an automatic control rod would also
insert itself if the reaction reached a certain pre-set
level. In case of emergency, such as Weil becoming
incapacitated or failure of the automatic control rod,
Norman Hilberry stood on the balcony with an improbable
nuclear safety device: an axe. In an emergency, he
would cut a rope that ran up to the balcony, releasing
another emergency control rod into the pile. The
last line of defense consisted of a "liquid-control squad"
that stood on a platform, ready to flood the pile with a
cadmium-salt solution. Taken together, these safety
precautions were a strange combination of the high-tech
and the ad hoc.
After rehearsals, Fermi at 9:54 a.m. ordered the
electrically-operated control rods removed. All eyes
turned to the array of instruments indicating the pace of
the fission reaction within the pile. Shortly after
10:00, Fermi ordered the emergency control rod removed and
tied to its rope. At 10:37, Fermi ordered Weil to
pull all but thirteen feet of the final rod out of the
pile. The pace of the audible clicking from the
neutron counters (similar to Geiger counters)
increased. Over the next few hours, the pile inched
its way toward criticality, Weil gradually removing more
and more of the final rod while Fermi monitored his array
of instruments. William Overbeck continued to call
out the neutron count over a speaker system while Leona
Marshall, William Sturm, and Anderson (see the photograph
above) recorded the readings from the instruments.
(Marshall, a physics graduate student, was the only woman
present.) At 11:25, Fermi ordered the automatic and
emergency control rods reinserted for a final safety
check. Ten minutes later these were both removed in
order for the experiment to resume. The neutron
counters immediately resumed their clicking, the pace
growing and growing until a sudden "whrrrump!" filled the
room. The automatic control rod had slammed home
into the pile, having been set too low during the safety
check. While everyone present took a few deep
breaths, Fermi calmly called for lunch.
By 2:00 p.m., everyone had resumed their places.
Fermi resumed the slow process of inching toward
criticality, more and more of the control rod appearing as
Weil slowly withdrew it from the pile. Finally Fermi
said to Compton "this is going to do it. Now it will
become self-sustaining." Everyone waited as Fermi
ran through some final calculations on his slide rule,
turning it over occasionally to jot down some figures on
its ivory back. By this time the clickety-click of
the neutron counter had become a steady hum, too fast for
the ear to count. At 3:25 p.m., Weil slid the rod
back one more time. As Fermi completed one final
calculation his face broke into a broad smile and he
announced "the reaction is self-sustaining." There
was a quiet ripple of applause in the room. For the
first time in history, humans had unleashed and controlled
the power of the atom. The
reactor was generating about half a watt, barely enough to
power a small light bulb.
Following 28 minutes of operation, at 3:53 p.m. Fermi
ordered the emergency control rod replaced. The
neutron counter abruptly slowed; the chain reaction was
over. The pile had achieved a reproduction factor
k of 1.0006. Eugene Wigner then produced a
bottle of Chianti wine (right) from behind his back, and
paper cups were passed around for everyone to drink.
The scientists held up their cups in a silent toast.
All of them but Wigner signed the bottle's label. A
report was quickly dispatched to Groves, and Compton gave
James Conant a call to tell him of their
success. No code had been prearranged so they had to
make one up on the spot. Compton told Conant "the
Italian navigator has landed in the New World." "How
were the natives?" asked Conant. "Very friendly" was
the reply. Leo Szilard wrote
later that his view of the day's events was very different
-- Szilard lingered on the balcony until most people had
left, then turned to Fermi, shook his hand, and said that he
thought the day would go down as a "black day in the history
of mankind.
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Sources and notes for this page.
The text for this entry is based upon, and portions
were taken directly from, a press release, written by
the Press Relations Section of the Manhattan Project,
November 26, 1946 (to be released December 1, 1946)
entitled "Background Material for Use in Connection with
Observance of the Fourth Anniversary, December Second,
of the Scientific Event of Outstanding Significance in
the United States Program of Development of Atomic
Energy"; this release is available on the University
Publications of America microfilm collection
President Harry S. Truman's Office
Files, 1945-1953
(Frederick, MD: 1989), Part 3, reel #41/42; the press
release itself is a government document. See also
John F. Hogerton, ed., "Chicago Pile No. 1 (CP-1),"
The Atomic Energy Deskbook (New York: Reinhold
Publishing Corporation, 1963, prepared under the
auspices of the Division of Technical Information, U.S.
Atomic Energy Commission), 97-98.
For "gamble" quote, see the History Office publication:
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), 109.
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 painting
of CP-1 going critical and the drawing
of the pile by itself are both courtesy the
National Archives. The photograph of the construction of CP-1 is
courtesy
Argonne National Laboratory (ANL); 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),
103. Click
here for more information on the photograph of "Met
Lab" alumni. The photograph of
Enrico Fermi is courtesy the
Department of Energy
(via the
National Archives). The data printout is reproduced from Hewlett
and Anderson, The New World, between pages 112
and 113. The photograph of the Chianti is courtesy
ANL.
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