The Human Plutonium Injection Experiments

was to be directed from the University

of Chicago by Arthur H. Compton

under the classified wartime name of

the Plutonium Project.  In January

1942, Compton consolidated the effort

by moving many of the separate re-

search projects to the University of

Chicago under the cryptic title of the

Metallurgical Laboratory.  The Met

Lab’s goals were to demonstrate a nu-

clear chain reaction using natural urani-

um and to develop chemical procedures

for isolating the plutonium that would

be produced in the reactor fuel.  From

the group of scientists at Berkeley who

had worked to discover plutonium (see

“The Making of Plutonium-239”),

Glenn Seaborg moved from Berkeley to

Chicago in April 1942 to head the plu-

tonium chemical-separation effort.

Joseph Kennedy, Arthur Wahl, and

Emilio Segrè continued their research

on the chemistry and nuclear properties

of plutonium at Berkeley and then

transferred to the Site Y Laboratory at

Los Alamos in early 1943.  Their col-

league, Ed McMillan, was already

there, having helped set up the new

Laboratory.

The Manhattan Project.  As the

weapon programs grew in size and

complexity, it was decided that the mil-

itary should coordinate the effort, in-

cluding spearheading the huge construc-

tion projects needed to supply the raw

weapons materials.  In August 1942,

the Army Corps of Engineers formed

the Manhattan Engineer District, or

Manhattan Project, and took over con-

trol of all research on atomic weapons.

In September, General Leslie R. Groves

was assigned to direct the Project.

At that time, even before the demon-

stration of a chain reaction at Chicago,

plans were already being made for con-

struction of larger reactors to produce

plutonium in the kilogram quantities

needed for weapons.  A pilot reactor

would be built in Clinton, Tennessee,

and production reactors would be built

at the Hanford Engineer Works, a site

in southern Washington adjacent to the

Columbia River.  The Clinton and Han-

ford facilities would also perform chem-

ical separation of “product” (plutonium)

from the reactor fuel pellets; Clinton

would develop the process, Hanford

would use it on a large scale with auto-

mated state-of-the-art facilities.  

Right from the start, plutonium was a

secret topic, and the Manhattan Project

used the code words “product” or “49”

to refer to plutonium (“49” was arrived

at by taking the final digits in the atom-

ic number, 94, and the atomic mass,

239).  During the period from 1941

through 1944, documents discussing

“product” were classified Secret Limit-

ed.  Only personnel with authorization

to know were permitted knowledge of

plutonium.

In March 1943, the Los Alamos Project

became operational under the direction

of J. Robert Oppenheimer.  The respon-

sibility of this laboratory was the de-

sign of the uranium-235 and plutonium-

239 weapons.  Two months later, Los

Alamos was also assigned responsibility

for the final purification of plutonium

and its reduction to metal.  

Health protection.  To protect the

thousands of workers at the various

sites who would soon be working to

produce kilogram amounts of this new

element, a Health Division at Chicago

was authorized in July 1942, and a team

of personnel knowledgeable about the

The Human Plutonium Injection Experiments

Number 23  1995  Los Alamos Science  

179

 

The Making of Plutonium-239

In 1940, Edwin McMillan and Philip Abelson demonstrated with the cy-

clotron at the University of California Radiation Laboratory in Berkeley that

when uranium-238 was bombarded with neutrons, a new element was pro-

duced (neptunium-239) that was chemically distinct from the uranium.  In

1941, Glenn Seaborg, Joseph Kennedy, Arthur Wahl, and Emilio Segrè,

building on the earlier work, isolated the daughter of neptunium-239, an el-

ement, also of mass 239, that had been predicted theoretically by Louis

Turner.  The chemical properties of this material were different than those

of neptunium or uranium, and its presence was identified by its alpha activ-

ity (about 130,000 alpha disintegrations per minute per microgram, which

corresponded to a half-life of about 30,000 years).  They then demonstrat-

ed that the isotope had the properties predicted by Turner—it underwent

fission with slow neutrons with a greater cross-section than uranium-235,

making it a potentially favorable material for an explosive chain reaction.

The new element was named plutonium by its discoverers in 1942.

The next important step was to demonstrate how to produce plutonium-

239 in the quantities needed for a weapon.  The key was the construction

of a “nuclear pile” that could sustain a chain reaction.  In such a reactor,

the predominant uranium-238 isotope in the fuel would absorb neutrons

from the chain reaction to create uranium-239.  This isotope would then

decay by two beta emissions to plutonium-239.  By December 1942, Enri-

co Fermi achieved a controlled chain reaction in a graphite-uranium pile

under the west stands of Stagg Field at the University of Chicago, thereby

completing the first goal of the Met Lab and demonstrating in principle that

plutonium-239 could be produced in quantity.  It was then up to the Man-

hattan Project to construct the production reactors and for Seaborg’s team

at the Met Lab to perfect the chemical techniques that would separate the

plutonium from the uranium fuel and the radioactive fission products.  

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