5

The

Manhattan Project

National Security History Series 
5

Visit our Manhattan Project web site:

http://www.cfo.doe.gov/me70/manhattan/index.htm
5

DOE/MA-0002 Revised

F. G. Gosling

Office of History and Heritage Resources

Executive Secretariat

Office of Management

Department of Energy

January 2010

The

Manhattan Project

National Security History Series
5

National Security History Series

Volume I: 

The Manhattan Project: Making the Atomic Bomb

Volume II: 

Building the Nuclear Arsenal: Cold War Nuclear Weapons Development 

and Production, 1946-1989 (in progress)

Volume III: 

Nonproliferation and Stockpile Stewardship: 

The Nuclear Weapons Complex in the Post-Cold War World (projected)

The National Security History Series is a joint project of the Office of History and 

Heritage Resources and the National Nuclear Security Administration.
5

Foreword to the 2010 edition

In a national survey at the turn of the millennium, journalists and historians ranked the dropping of the 

atomic bomb and the surrender of Japan to end the Second World War as the top story of the twentieth 

century. The advent of nuclear weapons, brought about by the Manhattan Project, not only helped 

bring an end to World War II but ushered in the atomic age and determined how the next war—the 

Cold War—would be fought. The Manhattan Project also became the organizational model behind the 

impressive achievements of American “big science” during the second half of the twentieth century, which 

demonstrated the relationship between basic scientific research and national security.

This edition of the Office of History’s perennial “bestseller” is part of a joint project between the Office of 

History and Heritage Resources and the National Nuclear Security Administration to produce a threevolume National Security History Series documenting the Department of Energy’s role in developing, 

testing, producing, and managing the Nation’s nuclear arsenal. There will be hard copy and online 

versions of each volume as well as complementary websites. The first website–The Manhattan Project: An 

Interactive History–is available on the Office of History and Heritage Resources website, http://www.cfo.

doe.gov/me70/history. The Office of History and Heritage Resources and the National Nuclear Security 

Administration hope that the National Security History Series will provide the Department and the public 

with reliable and useful information about the national security policies and programs of the Department 

and its predecessor agencies.

Erica De Vos

Director

Office of the Executive Secretariat

F. G. Gosling

Chief Historian

Office of History and Heritage Resources

Thomas P. D’Agostino

Administrator

National Nuclear Security Administration
5
5

Table of Contents

Introduction: The Einstein Letter--------------------------------------------------vii

Part I: Physics Background, 1890-1939--------------------------------------------1

Part II: Early Government Support------------------------------------------------6

Part III: The Manhattan Engineer District----------------------------------------15

Part IV: The Manhattan Engineer District in Operation-------------------------20

Manhattan Project Photo Gallery--------------------------------------------- 36 - 73

Part V: The Atomic Bomb and American Strategy-------------------------------87

Part VI: The Manhattan District in Peacetime------------------------------------99

Manhattan Project Chart-----------------------------------------------------------103

Notes-------------------------------------------------------------------------------- 104

Select Bibliography------------------------------------------------------------------106

Manhattan Project Chronology----------------------------------------------------107

v
5
5

Introduction

Introduction: The Einstein Letter

On October 11, 1939, Alexander Sachs, Wall 

Street economist and longtime friend and 

unofficial advisor to President Franklin Delano 

Roosevelt, met with the President to discuss a 

letter written by Albert Einstein the previous 

August. Einstein had written to inform Roosevelt 

that recent research on chain reactions utilizing 

uranium made it probable that large amounts of 

power could be produced by a chain reaction and 

that, by harnessing this power, the construction 

of “extremely powerful bombs” was conceivable.1

Einstein believed the German government 

was actively supporting research in this area 

and urged the United States government to do 

likewise. Sachs read from a cover letter he had 

prepared and briefed Roosevelt on the main 

points contained in Einstein’s letter. Initially 

the President was noncommittal and expressed 

concern over locating the necessary funds, but at 

a second meeting over breakfast the next morning 

Roosevelt became convinced of the value of 

exploring atomic energy.

Einstein drafted his famous letter with the help 

of the Hungarian émigré physicist Leo Szilard, 

one of a number of European scientists who had 

fled to the United States in the 1930s to escape 

Nazi and Fascist repression. Szilard was among 

the most vocal of those advocating a program 

to develop bombs based on recent findings in 

nuclear physics and chemistry. Those like Szilard 

and fellow Hungarian refugee physicists Edward 

Teller and Eugene Wigner regarded it as their 

responsibility to alert Americans to the possibility that German scientists might win the race 

to build an atomic bomb and to warn that Hitler 

would be more than willing to resort to such a 

weapon. But Roosevelt, preoccupied with events 

in Europe, took over two months to meet with 

Sachs after receiving Einstein’s letter. Szilard and 

his colleagues interpreted Roosevelt’s inaction as 

unwelcome evidence that the President did not 

take the threat of nuclear warfare seriously.

Roosevelt wrote Einstein back on October 

19, 1939, informing the physicist that he had 

set up a committee consisting of civilian and 

military representatives to study uranium.2

Events proved that the President was a man 

of considerable action once he had chosen a 

direction. In fact, Roosevelt’s approval of uranium 

research in October 1939, based on his belief 

that the United States could not take the risk of 

allowing Hitler to achieve unilateral possession 

of “extremely powerful bombs,” was merely the 

first decision among many that ultimately led to 

the establishment of the only atomic bomb effort 

that succeeded in World War II—the Manhattan 

Project.

The French, who did important research on 

fission and the feasibility of chain reactions 

using uranium in 1939 and early 1940, fell under 

German occupation in June 1940. The British, 

who made significant theoretical contributions 

early in the war, did not have the resources to 

pursue a full-fledged atomic bomb research 

program while fighting for their survival. 

Consequently, the British acceded, reluctantly, 

to American leadership and sent scientists to 

every Manhattan Project facility. The Germans, 

despite Allied fears that were not dispelled until 

the ALSOS mission in 1944,3 were little nearer to 

producing atomic weapons at the end of the war 

than they had been at the beginning of the war. 

German scientists pursued research on fission, 

but the government’s attempts to forge a coherent 

strategy met with little success.4

The Russians built a program that grew increasingly active as the war drew to a conclusion, but 

the first successful Soviet test was not conducted 

vii
5

until 1949. The Japanese managed to build several 

cyclotrons by war’s end, but the atomic bomb 

research effort could not maintain a high priority 

in the face of increasing scarcities. Only the 

Americans, late entrants into World War II and 

protected by oceans on both sides, managed to 

take the discovery of fission from the laboratory 

to the battlefield and gain a shortlived atomic 

monopoly.

viii

Introduction
5

Part I:

Physics Background, 1890-1939

The Atomic Solar System

The modern effort to uncover the inner structure 

of the atom began with the discovery of the 

electron by the English physicist J. J. Thomson 

in 1897. Thomson proved that cathode rays were 

not some sort of undefined process occurring in 

“ether” but were in fact composed of extremely 

small, negatively charged particles. Dubbed 

electrons, their exact charge and mass were 

soon determined by John Townsend and Robert 

Millikan.

At the same time, discoveries relating to the 

curious phenomenon of radioactivity had also 

begun to propel atomic research forward. In 1896, 

the French physicist Antoine Becquerel detected 

the three basic forms of radioactivity, which were 

soon named alpha, beta, and gamma by Ernest 

Rutherford, a student of Thomson from New 

Zealand. Also in 1896, the husband-and-wife 

team of Marie and Pierre Curie began work in 

Paris on the emission of radiation by uranium 

and thorium. The Curies soon announced their 

discoveries of radium and polonium. They also 

proved that beta particles were negatively charged. 

In 1900, Becquerel realized that beta particles and 

electrons were the same things.

In the first decade of the 20th century, Rutherford 

began to pull all of this information into a 

coherent whole. In 1903, he proposed that 

radioactivity was caused by the breakdown of 

atoms.” In 1908, he correctly identified alpha 

particles as being the nucleus of helium; and in 

1911, along with the German physicist Hans 

Geiger, Rutherford postulated that electrons orbit 

an atom’s nucleus, much as the planets orbit the 

sun. The second fundamental atomic particle, the 

proton, was “identified by Rutherford in 1919.”

The Danish physicist Neils Bohr combined 

Rutherford’s atomic concepts with Max Planck’s 

quantum theory to produce the first modern 

model of the atom. In 1913, Bohr demonstrated 

that electrons moved around an atom’s nucleus in 

certain discrete energy “shells” and that radiation 

is emitted or absorbed when an electron moves 

from one shell to another. The following year, 

Henry Moseley, an English physicist, showed that 

each element could be identified by its “unique 

atomic number.” 

By the 1910s, then, scientists investigating the 

inner structure of the atom had come to believe, 

among other things, that energy exists within the 

atom. Considered in light of Albert Einstein’s 1905 

theoretical formula E=mc2 (energy equals mass 

times the square of the velocity of light) stating 

that matter and energy were equivalent, this belief 

held breathtaking possibilities. For if Einstein 

were correct that matter and energy were different 

forms of the same thing, it followed that anyone 

unlocking the secrets of how these minute particles were held together–and how they could be 

broken apart–could produce a massive release of 

energy. By the late 1910s, then, the stage was set 

to begin attempting to artificially transmute one 

atom into another. And if Einstein was correct 

that matter and energy were different forms of the 

same thing, it followed that the conversion of a 

significant quantity of matter into energy should 

result in a massive release of energy.

The Road to the Bomb

The road to the atomic bomb began in 1919 when 

Rutherford, working in the Cavendish Laboratory 

at Cambridge University in England, achieved the 

first artificial transmutation of an element when 

1
5

he changed several atoms of nitrogen into oxygen. 

At the time of Rutherford’s breakthrough, the 

atom was conceived as a miniature solar system, 

with extremely light negatively charged particles, 

called electrons, in orbit around the much 

heavier positively charged nucleus. In the process 

of changing nitrogen into oxygen, Rutherford 

detected a high-energy particle with a positive 

charge that proved to be a hydrogen nucleus. The 

proton, as this subatomic particle was named, 

joined the electron in the miniature solar system. 

Another addition came in 1932 when James 

Chadwick, Rutherford’s colleague at Cambridge, 

identified a third particle, the neutron, so-named 

because it had no charge. 

By the early 1930s, the atom was thought to 

consist of a positively charged nucleus, containing 

both protons and neutrons, circled by negatively 

charged electrons equal in number to the protons 

in the nucleus. The number of protons determined the element’s atomic number. Hydrogen, 

with one proton, came first and uranium, with 

ninety-two protons, last on the periodic table. 

This simple scheme became more complicated 

when chemists discovered that many elements 

existed at different weights even while displaying 

identical chemical properties. It was Chadwick’s 

discovery of the neutron in 1932 that explained 

this mystery. Scientists found that the weight 

discrepancy between atoms of the same element 

resulted because they contained different numbers 

of neutrons. These different classes of atoms of the 

same element but with varying numbers of neutrons were designated isotopes. The three isotopes 

of uranium, for instance, all have ninety-two 

protons in their nuclei and ninety-two electrons 

in orbit. But uranium-238, which accounts for 

over ninety-nine percent of natural uranium, has 

146 neutrons in its nucleus, compared with 143 

neutrons in the rare uranium-235 (.7 percent of 

natural uranium) and 142 neutrons in uranium234, which is found only in traces in the heavy 

metal. The slight difference in atomic weight 

between the uranium-235 and uranium-238 

isotopes figured greatly in nuclear physics during 

the 1930s and 1940s.

The year 1932 produced other notable events in 

atomic physics. The Englishman J. D. Cockroft 

and the Irishman E. T. S. Walton, working 

jointly at the Cavendish Laboratory, were the 

first to split the atom when they bombarded 

lithium with protons generated by a particle 

accelerator and changed the resulting lithium 

nucleus into two helium nuclei. Also in that 

year, Ernest O. Lawrence and his colleagues M. 

Stanley Livingston and Milton White successfully 

operated the first cyclotron on the Berkeley 

campus of the University of California.

Moonshine

Lawrence’s cyclotron, the Cockroft-Walton 

machine, and the Van de Graaff electrostatic 

generator, developed by Robert J. Van de Graaff 

at Princeton University, were particle accelerators 

designed to bombard the nuclei of various 

elements to disintegrate atoms. Attempts of the 

early 1930s, however, required huge amounts of 

energy to split atoms because the first accelerators 

used proton beams and alpha particles as sources 

of energy. Since protons and alpha particles are 

positively charged, they met substantial resistance 

from the positively charged target nucleus when 

they attempted to penetrate atoms. Even highspeed protons and alpha particles scored direct 

hits on a nucleus only approximately once in a 

million tries. Most simply passed by the target 

nucleus. Not surprisingly, Ernest Rutherford, 

Albert Einstein, and Niels Bohr regarded particle 

bombardment as useful in furthering knowledge 

of nuclear physics but believed it unlikely to meet 

public expectations of harnessing the power of the 

atom for practical purposes anytime in the near 

future. In a 1933 interview Rutherford called such 

expectations “moonshine.”5 Einstein compared 

particle bombardment with shooting in the dark 

at scarce birds, while Bohr agreed that the chances 

of taming atomic energy were remote.6

2

Part I
5

From Protons to Neutrons: Fermi

Rutherford, Einstein, and Bohr proved to be 

wrong in this instance, and the proof was not

long in coming. Beginning in 1934, the Italian 

physicist Enrico Fermi began bombarding 

elements with neutrons instead of protons, 

theorizing that Chadwick’s uncharged particles 

could pass into the nucleus without resistance. 

Like other scientists at the time, Fermi paid little 

attention to the possibility that matter might 

disappear during bombardment and result in the 

release of huge amounts of energy in accordance 

with Einstein’s formula, E=mc2, which stated that 

mass and energy were equivalent. Fermi and his 

collegues bombarded sixty-three stable elements 

and produced thirty-seven new radioactive ones.7

They also found that carbon and hydrogen proved 

useful as moderators in slowing the bombarding 

neutrons and that slow neutrons produced the 

best results since neutrons moving more slowly 

remained in the vicinity of the nucleus longer and 

were therefore more likely to be captured.

One element Fermi bombarded with slow 

neutrons was uranium, the heaviest of the known 

elements. Scientists disagreed over what Fermi 

had produced in this transmutation. Some 

thought that the resulting substances were new 

“transuranic” elements, while others noted 

that the chemical properties of the substances 

resembled those of lighter elements. Fermi was 

himself uncertain. For the next several years, 

attempts to identify these substances dominated 

the research agenda in the international scientific 

community, with the answer coming out of Nazi 

Germany just before Christmas 1938.

The Discovery of Fission:

Hahn and Strassmann

The radiochemists Otto Hahn and Fritz 

Strassmann were bombarding elements with 

neutrons in their Berlin laboratory when they 

made an unexpected discovery. They found 

that while the nuclei of most elements changed 

somewhat during neutron bombardment, 

uranium nuclei changed greatly and broke into 

two roughly equal pieces. They split and became 

not the new transuranic elements that some 

thought Fermi had discovered but radioactive 

barium isotopes (barium has the atomic number 

56) and fragments of the uranium itself. The 

substances Fermi had created in his experiments, 

that is, did more than resemble lighter elements; 

they were lighter elements. Importantly, the 

products of the Hahn-Strassmann experiment 

weighed less than that of the original uranium 

nucleus, and herein lay the primary significance 

of their findings. For it followed from Einstein’s 

equation that the loss of mass resulting from the 

splitting process must have been converted into 

energy in the form of kinetic energy that could in 

turn be converted into heat. Calculations made by 

Hahn’s former colleague, Lise Meitner, a refugee 

from Nazism then staying in Sweden, and her 

nephew, Otto Frisch, led to the conclusion that so 

much energy had been released that a previously 

undiscovered kind of process was at work. Frisch, 

borrowing the term for cell division in biology—

binary fission—named the process fission.8 For 

his part, Fermi had produced fission in 1934 but 

had not recognized it.

Chain Reaction

It soon became clear that the process of fission 

discovered by Hahn and Strassmann had another 

important characteristic besides the immediate 

release of enormous amounts of energy. This was 

the emission of neutrons. The energy released 

when fission occurred in uranium caused several 

neutrons to “boil off ” the two main fragments 

as they flew apart. Given the right set of 

circumstances, perhaps these secondary neutrons 

might collide with other atoms and release more 

neutrons, in turn smashing into other atoms and, 

at the same time, continuously emitting energy. 

Beginning with a single uranium nucleus, fission 

could not only produce substantial amounts of 

energy but could also lead to a reaction creating 

3

Physics Background, 1919-1939 
5

ever-increasing amounts of energy. The possibility 

of such a “chain reaction” completely altered the 

prospects for releasing the energy stored in the 

nucleus. A controlled self-sustaining reaction 

could make it possible to generate a large amount 

of energy for heat and power, while an unchecked 

reaction could create an explosion of huge force.

Fission Comes to America: 1939

News of the Hahn-Strassmann experiments 

and the Meitner-Frisch calculations spread 

rapidly. Meitner and Frisch communicated their 

results to Niels Bohr, who was in Copenhagen 

preparing to depart for the United States via 

Sweden and England. Bohr confirmed the validity 

of the findings while sailing to New York City, 

arriving on January 16, 1939. Ten days later Bohr, 

accompanied by Fermi, communicated the latest 

developments to some European émigré scientists 

who had preceded him to this country and to 

members of the American scientific community at 

the opening session of a conference on theoretical 

physics in Washington, D.C.

Uranium-235 Fission Chain Reaction. Department of Energy.

American physicists quickly grasped the 

importance of Bohr’s message, having by the 

1930s developed into an accomplished scientific 

community. While involved in important 

theoretical work, Americans made their most 

significant contributions in experimental physics, 

where teamwork had replaced individualism 

in laboratory research. No one epitomized the 

“can do” attitude of American physicists better 

than Ernest O. Lawrence, whose ingenuity and 

drive made the Berkeley Radiation Laboratory 

the unofficial capital of nuclear physics in the 

United States. Lawrence staked his claim to 

American leadership when he built his first 

particle accelerator, the cyclotron, in 1930. Van de 

Graaff followed with his generator in 1931, and 

from then on Americans led the way in producing 

equipment for nuclear physics and high-energy 

physics research later.

Early American Work on Fission

American scientists became active participants 

in attempts to confirm and extend Hahn’s and 

Part I

4
5

Strassmann’s results, which dominated nuclear 

physics in 1939. Bohr and John A. Wheeler 

advanced the theory of fission in important 

theoretical work done at Princeton University, 

while Fermi and Szilard collaborated with Walter 

H. Zinn and Herbert L. Anderson at Columbia 

University in investigating the possibility of 

producing a nuclear chain reaction. Given that 

uranium emitted neutrons (usually two) when 

it fissioned, the question became whether or not 

a chain reaction in uranium was possible, and, 

if so, in which of the three isotopes of the rare 

metal it was most likely to occur. By March 1940 

John R. Dunning and his colleagues at Columbia 

University, collaborating with Alfred Nier of 

the University of Minnesota, had demonstrated 

conclusively that uranium-235, present in only 1 

in 140 parts of natural uranium, was the isotope 

that fissioned with slow neutrons, not the more 

abundant uranium-238 as Fermi had guessed. This 

finding was important, for it meant that a chain 

reaction using the slightly lighter uranium-235 was 

 5

possible, but only if the isotope could be separated 

from the uranium-238 and concentrated into 

a critical mass, a process that posed serious 

problems. Fermi continued to try to achieve a 

chain reaction using large amounts of natural 

uranium in a pile formation.

Dunning’s and Nier’s demonstration promised 

nuclear power but not necessarily a bomb. It 

was already known that a bomb would require 

fission by fast neutrons; a chain reaction using 

slow neutrons might not proceed very far before 

the metal would blow itself apart, causing little, 

if any, damage. Uranium-238 fissioned with fast 

neutrons but could not sustain a chain reaction 

because it required neutrons with higher energy. 

The crucial question was whether uranium-235 

could fission with fast neutrons in a chainreacting manner, but without enriched samples 

of uranium-235, scientists could not perform the 

necessary experiments.

Physics Background, 1919-1939 
5

Part II:

Early Government Support

Schematic Diagram of Flow of Process Gas in Gaseous 

Diffusion Cascade. Reprinted from Richard G. Hewlett 

and Oscar E. Anderson, Jr., The New World, 1939-1946,

Volume I of A History of the United States Atomic Energy 

Commission (University Park: Pennsylvania State 

University Press, 1962).

The Uranium Committee

President Roosevelt responded to the call for 

government support of uranium research quickly 

but cautiously. He appointed Lyman J. Briggs, 

director of the National Bureau of Standards, head 

of the Advisory Committee on Uranium, which 

met for the first time on October 21, 1939. The 

committee, including both civilian and military 

representation, was to coordinate its activities 

with Sachs and look into the current state of 

research on uranium to recommend an appropriate role for the federal government. In early 1940 

the Uranium Committee recommended that the 

government fund limited research on isotope 

separation as well as Fermi’s and Szilard’s work on 

chain reactions at Columbia.

Isotope Separation

Scientists had concluded that enriched samples of 

uranium-235 were necessary for further research 

and that the isotope might serve as a fuel source 

for an explosive device; thus, finding the most 

effective method of isotope separation was a high 

priority. Since uranium-235 and uranium-238 

were chemically identical, they could not be separated by chemical means. And with their masses 

differing by less than one percent, separation by 

physical means would be extremely difficult and 

expensive. Nonetheless, scientists pressed forward 

on several complicated techniques of physical 

separation, all based on the small difference in 

atomic weight between the uranium isotopes.

The Electromagnetic Method

The electromagnetic method, pioneered by Alfred 

O.C. Nier of the University of Minnesota, used 

a mass spectrometer, or spectrograph, to send a 

stream of charged particles through a magnetic 

field. Atoms of the lighter isotope would be 

deflected more by the magnetic field than those 

of the heavier isotope, resulting in two streams 

that could then be collected in different receivers. 

The electromagnetic method as it existed in 

1940, however, would have taken far too long to 

6
5

separate quantities sufficient to be useful in the 

current war. In fact, twenty-seven thousand years 

would have been required for a single spectrometer to separate one gram of uranium-235.9

Gaseous Diffusion

Gaseous diffusion appeared more promising. 

Based on the well-known principle that molecules 

of a lighter isotope would pass through a porous 

barrier more readily than molecules of a heavier 

one, this approach proposed to produce by myriad 

repetitions a gas increasingly rich in uranium-235 

as the heavier uranium-238 was separated out in 

a system of cascades. Theoretically, this process 

could achieve high concentrations of uranium235 but, like the electromagnetic method, would 

be extremely costly. British researchers led the 

way on gaseous diffusion, with John R. Dunning 

and his colleagues at Columbia University joining 

the effort in late 1940.

Centrifuge

Many scientists initially thought the best hope for 

isotope separation was the high-speed centrifuge, 

a device based on the same principle as the 

cream separator. Centrifugal force in a cylinder 

spinning rapidly on its vertical axis would 

separate a gaseous mixture of two isotopes since 

the lighter isotope would be less affected by the 

action and could be drawn off at the center and 

top of the cylinder. A cascade system composed 

of hundreds, perhaps thousands, of centrifuges 

could produce a rich mixture. This method, 

being pursued primarily by Jesse W. Beams at the 

University of Virginia, received much of the early 

isotope separation funding.10

Liquid Thermal Diffusion

The Uranium Committee briefly demonstrated 

an interest in a fourth enrichment process during 

1940, only to conclude that it would not be worth 

pursuing. This process, liquid thermal diffusion, 

was being investigated by Philip Abelson at the 

Carnegie Institution. Into the space between 

two concentric vertical pipes Abelson placed 

pressurized liquid uranium hexafluoride. With 

the outer wall cooled by a circulating water jacket 

and the inner heated by high-pressure steam, the 

lighter isotope tended to concentrate near the hot 

wall and the heavier near the cold. Convection 

would in time carry the lighter isotope to the top 

of the column. Taller columns would produce 

more separation. Like other enrichment methods, 

liquid thermal diffusion was at an early stage of 

development.11

Limited Government Funding: 1940

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 experiments 

already underway at Columbia University. 

Building upon the work performed in 1934 

demonstrating the value of moderators in 

producing slow neutrons, Fermi thought that 

a mixture of the right moderator and natural 

uranium could produce a self-sustaining 

chain reaction. Fermi and 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 

 7

Early Government Support
5

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.”12

The National Defense Research Committee

Shortly after World War II began with the 

German invasion of Poland on September 1, 

1939, Vannevar Bush, president of the Carnegie 

Foundation, became convinced of the need 

for the government to marshall the forces of 

science for a war that would inevitably involve 

the United States. He sounded out other science 

administrators in the nation’s capital and agreed 

to act as point man in convincing the Roosevelt 

administration to set up a national science 

organization. Bush struck an alliance with 

Roosevelt’s closest advisor, Harry Hopkins, and 

after clearing his project with the armed forces 

First Mass Spectrograph Components in 37-Inch Cyclotron Tank. Reprinted from Richard G. Hewlett and Oscar E. Anderson, Jr., 

The New World, 1939-1946, Volume I of A History of the United States Atomic Energy Commission (University Park: Pennsylvania 

State University Press, 1962).

and science agencies, met with the President 

and Hopkins. With the imminent fall of France 

undoubtedly on Roosevelt’s mind, it took less than 

ten minutes for Bush to obtain the President’s 

approval and move into action.13

Roosevelt approved in June 1940 the 

establishment of a voice for the scientific 

community within the executive branch. The 

National Defense Research Committee, with Bush 

at its head, reorganized the Uranium Committee 

into a scientific body and eliminated military 

membership. Not dependent on the military for 

funds, as the Uranium Committee had been, the 

National Defense Research Committee would 

have more influence and more direct access to 

money for nuclear research. In the interest of 

security, Bush barred foreign-born scientists 

from committee membership and blocked 

the further publication of articles on uranium 

research. Retaining programmatic responsibilities 

8

Part II
5

for uranium research in the new organizational 

setup (among the National Defense Research 

Committee’s early priorities were studies on radar, 

proximity fuzes, and anti-submarine warfare), the 

Uranium Committee recommended that isotope 

separation methods and the chain reaction work 

continue to receive funding for the remainder of 

1940. Bush approved the plan and allocated the 

funds.

A Push From Lawrence

During 1939 and 1940, most of the work done 

on isotope separation and the chain reaction 

pile was performed in university laboratories by 

academic scientists funded primarily by private 

foundations. While the federal government began 

supporting uranium research in 1940, the pace 

appeared too leisurely to the scientific community 

and failed to convince scientists that their work 

was of high priority. Certainly few were more 

inclined to this view than Ernest O. Lawrence, 

director of the Radiation Laboratory at the 

University of California in Berkeley. Lawrence 

was among those who thought that it was merely 

a matter of time before the United States was 

drawn into World War II, and he wanted the 

government to mobilize its scientific forces as 

rapidly as possible.

Specifically what Lawrence had on his mind in 

early 1941 were experiments taking place in his 

own laboratory using samples produced in the 

cyclotron. Studies on uranium fission fragments 

by Edwin M. McMillan and Philip H. Abelson 

led to the chemical identification of element 93, 

neptunium, while research by Glenn T. Seaborg 

revealed that an isotope of neptunium decayed to 

yet another transuranium (man-made) element. In 

Ernest Lawrence, Arthur Compton, Vannevar Bush, James Conant, Karl Compton. and Alfred Loomis. Reprinted from Richard 

G. Hewlett and Oscar E. Anderson, Jr., The New World, 1939-1946, Volume I of A History of the United States Atomic Energy 

Commission (University Park: Pennsylvania State University Press, 1962).

 9

Early Government Support
5

February, Seaborg identified this as element 94, 

which he later named plutonium. By May he had 

proven that plutonium-239 was 1.7 times as likely 

as 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 isotopeseparation plants.

Lawrence, demonstrating his characteristic energy 

and impatience, launched a campaign to speed 

up uranium research. He began by proposing to 

convert his smaller cyclotron into a spectrograph 

to produce uranium-235. Since both the cyclotron 

and the spectrograph used a vacuum chamber 

and electromagnet, this conversion would be 

relatively uncomplicated. Lawrence then took his 

case to Karl T. Compton and Alfred L. Loomis at 

Harvard University, both doing radar work for 

the National Defense Research Committee and 

benefiting from Lawrence’s advice in staffing their 

laboratories. Infected by Lawrence’s enthusiasm, 

Compton forwarded Lawrence’s optimistic 

assessment on uranium research to Bush, warning 

that Germany was undoubtedly making progress 

and that Briggs and the Uranium Committee were 

moving too slowly. Compton also noted that the 

British were ahead of their American colleagues, 

even though, in his opinion, they were inferior in 

both numbers and ability.

Program Review: Summer 1941

Bush and Lawrence met in New York City. 

Though he continued to support the Uranium 

Committee, Bush recognized that Lawrence’s 

assessment was not far off the mark. Bush 

shrewdly decided to appoint Lawrence as an 

advisor to Briggs—a move that quickly resulted 

in funding for plutonium work at Berkeley and 

for Nier’s mass spectrograph at Minnesota—and 

also asked the National Academy of Sciences 

to review the uranium research program. 

Headed by Arthur Compton of the University of 

Chicago and including Lawrence, this committee 

submitted its unanimous report on May 17. 

Compton’s committee, however, failed to provide 

the practical-minded Bush with the evidence he 

needed that uranium research would pay off in 

the event the United States went to war in the near 

future. Compton’s group thought that increased 

uranium funding could produce radioactive 

material that could be dropped on an enemy 

by 1943, a pile that could power naval vessels 

in three or four years, and a bomb of enormous 

power at an indeterminate point, but certainly 

not before 1945. Compton’s report discussed 

bomb production only in connection with slow 

neutrons, a clear indication that much more 

scientific work remained to be done before an 

explosive device could be detonated.14

Bush reconstituted the National Academy of 

Sciences committee and instructed it to assess the 

recommendations contained in the first report 

from an engineering standpoint. On July 11, the 

second committee endorsed the first report and 

supported continuation of isotope separation 

work and pile research for scientific reasons, 

though it admitted that it could promise no 

immediate applications. The second report, like 

the first, was a disappointing document from 

Bush’s point of view.15

The Office of Scientific Research 

and Development

By the time Bush received the second National 

Academy of Sciences report, he had assumed 

the position of director of the Office of Scientific 

Research and Development. Established by an 

executive order on June 28, 1941—six days after 

German troops invaded the Soviet Union—the 

Office of Scientific Research and Development 

strengthened the scientific presence in the federal 

government. Bush, who had lobbied hard for the 

new setup, now reported directly to the President 

10

Part II
5

and could invoke the prestige of the White 

House in his dealings with other federal agencies. 

The National Defense Research Committee, 

now headed by James B. Conant, president of 

Harvard University, became an advisory body 

responsible for making research and development 

recommendations to the Office of Scientific 

Research and Development. The Uranium 

Committee became the Office of Scientific 

Research and Development Section on Uranium 

and was codenamed S-l (Section One of the Office 

of Scientific Research and Development).

Turning the Corner: The MAUD Report

Bush’s disappointment with the July 11 National 

Academy of Sciences report did not last long. Several days later he and Conant received a copy of a 

draft report forwarded from the National Defense 

Research Committee liaison office in London. 

The report, prepared by a group codenamed the 

MAUD Committee and set up by the British in 

spring 1940 to study the possibility of developing 

a nuclear weapon, maintained that a sufficiently 

purified critical mass of uranium-235 could fission even with fast neutrons.16 Building upon 

theoretical work on atomic bombs performed by 

refugee physicists Rudolf Peierls and Otto Frisch 

in 1940 and 1941, the MAUD report estimated 

that a critical mass of ten kilograms would be 

large enough to produce an enormous explosion. 

A bomb this size could be loaded on existing aircraft and be ready in approximately two years.17

Americans had been in touch with the MAUD 

Committee since fall 1940, but it was the July 1941 

MAUD report that helped the American bomb 

effort turn the corner. Here were specific plans for 

producing a bomb, produced by a distinguished 

group of scientists with high credibility in the 

United States, not only with Bush and Conant but 

with the President.18 The MAUD report dismissed 

plutonium production, thermal diffusion, the 

electromagnetic method, and the centrifuge and 

called for gaseous diffusion of uranium-235 on a 

massive scale. The British believed that uranium 

research could lead to the production of a bomb 

in time to effect the outcome of the war. While 

the MAUD report provided encouragement to 

Americans advocating a more extensive uranium 

research program, it also served as a sobering 

reminder that fission had been discovered in Nazi 

Germany almost three years earlier and that since 

spring 1940 a large part of the Kaiser Wilhelm 

Institute in Berlin had been set aside for uranium 

research.

Bush and Conant immediately went to work. 

After strengthening the Uranium Committee, 

particularly with the addition of Fermi as head 

of theoretical studies and Harold C. Urey as head 

of isotope separation and heavy water research 

(heavy water was highly regarded as a moderator), 

Bush asked yet another reconstituted National 

Academy of Sciences committee to evaluate the 

uranium program. This time he gave Compton 

specific instructions to address technical 

questions of critical mass and destructive 

capability, partially to verify the MAUD results.

Bush Reports to Roosevelt

Without waiting for Compton’s committee to 

finish its work, Bush went to see the President. 

On October 9 Bush met with Roosevelt and 

Vice President Henry A. Wallace (briefed on 

uranium research in July). Bush summarized the 

British findings, discussed cost and duration of 

a bomb project, and emphasized the uncertainty 

of the situation. He also received the President’s 

permission to explore construction needs with 

the Army. Roosevelt instructed him to move as 

quickly as possible but not to go beyond research 

and development. Bush, then, was to find out if a 

bomb could be built and at what cost but not to 

proceed to the production stage without further 

presidential authorization. Roosevelt indicated 

that he could find a way to finance the project 

and asked Bush to draft a letter so that the British 

government could be approached “at the top.”19

11

Early Government Support
5

Compton reported back on November 6, just one 

month and a day before the Japanese attack on 

Pearl Harbor on December 7, 1941, brought the 

United States into World War II (Germany and 

Italy declared war on the United States three days 

later). Compton’s committee concluded that a 

critical mass of between two and 100 kilograms 

of uranium-235 would produce a powerful 

fission bomb and that for $50-100 million 

isotope separation in sufficient quantities could 

be accomplished. Although the Americans were 

less optimistic than the British, they confirmed 

the basic conclusions of the MAUD committee 

and convinced Bush to forward their findings to 

Roosevelt under a cover letter on November 27. 

Roosevelt did not respond until January 19, 1942; 

when he did, it was as commander in chief of a 

nation at war. The President’s handwritten note 

read, “V. B. OK—returned—I think you had best 

keep this in your own safe FDR.”20

Moving Into Action

By the time Roosevelt responded, Bush had set 

the wheels in motion. He put Eger V. Murphree, 

a chemical engineer with the Standard Oil 

Company, in charge of a group responsible for 

overseeing engineering studies and supervising 

pilot plant construction and any laboratory-scale 

investigations. And he appointed Urey, Lawrence, 

and Compton as program chiefs. Urey headed 

up work including diffusion and centrifuge 

methods and heavy-water studies. Lawrence took 

electromagnetic and plutonium responsibilities, 

and Compton ran chain reaction and weapon 

theory programs. Bush’s responsibility was to 

coordinate engineering and scientific efforts and 

make final decisions on recommendations for 

construction contracts. In accordance with the 

instructions he received from Roosevelt, Bush 

removed all uranium work from the National 

Defense Research Committee. From this point 

forward, broad policy decisions relating to 

uranium were primarily the responsibility of the 

Top Policy Group, composed of Bush, Conant, 

Vice President Wallace, Secretary of War Henry 

L. Stimson, and Army Chief of Staff George C. 

Marshall.21 A high-level conference convened by 

Wallace on December 16 put the seal of approval 

on these arrangements. Two days later the S-l 

Committee gave Lawrence $400,000 to continue 

his electromagnetic work.

With the United States now at war and with the 

fear that the American bomb effort was behind 

Nazi Germany’s, a sense of urgency permeated 

the federal government’s science enterprise. Even 

as Bush tried to fine-tune the organizational 

apparatus, new scientific information poured in 

from laboratories to be analyzed and incorporated 

into planning for the upcoming design and 

construction stage. By spring 1942, as American 

naval forces slowed the Japanese advance in the 

Pacific with an April victory in the battle of the 

Coral Sea, the situation had changed from one of 

too little money and no deadlines to one of a clear 

goal, plenty of money, but too little time. The race 

for the bomb was on.

Continuing Efforts on 

Isotope Separation

During the first half of 1942, several routes to a 

bomb were explored. At Columbia, Urey worked 

on the gaseous diffusion and centrifuge systems 

for isotope separation in the codenamed SAM 

(Substitute or Special Alloy Metals) Laboratory. At 

Berkeley, Lawrence continued his investigations 

on electromagnetic separation using the 

mass spectrograph he had converted from his 

thirty-seven-inch cyclotron. Compton patched 

together facilities at the University of Chicago’s 

Metallurgical Laboratory for pile experiments 

aimed at producing plutonium. Meanwhile 

Murphree’s group hurriedly studied ways to 

move from laboratory experiments to production 

facilities.

Research on uranium required uranium ore, and 

obtaining sufficient supplies was the responsibility 

12

Part II
5

of Murphree and his group. Fortunately, enough 

ore was on hand to meet the projected need of 

150 tons through mid-1944. Twelve hundred 

tons of high-grade ore were stored on Staten 

Island, and Murphree made arrangements to 

obtain additional supplies from Canada and the 

Colorado Plateau, the only American source. 

Uranium in the form of hexafluoride was also 

needed as feed material for the centrifuge and 

the gaseous and thermal diffusion processes. 

Abelson, who had moved from the Carnegie 

Institution to the Naval Research Laboratory, was 

producing small quantities, and Murphree made 

arrangements with E. I. du Pont de Nemours and 

Company and the Harshaw Chemical Company 

of Cleveland to produce hexafluoride on a scale 

sufficient to keep the vital isotope separation 

research going.

Lawrence was so successful in producing enriched 

samples of uranium-235 electromagnetically with 

his converted cyclotron that Bush sent a special 

progress report to Roosevelt on March 9, 1942. 

Bush told the President that Lawrence’s work 

might lead to a short cut to the bomb, especially 

in light of new calculations indicating that the 

critical mass required might well be smaller than 

previously predicted. Bush also emphasized that 

the efficiency of the weapon would probably 

be greater than earlier estimated and expressed 

more confidence that it could be detonated 

successfully. Bush thought that if matters were 

expedited a bomb was possible in 1944. Two days 

later the President responded: “I think the whole 

thing should be pushed not only in regard to 

development, but also with due regard to time. 

This is very much of the essence.”22

In the meantime, however, isotope separation 

studies at Columbia quickly confronted serious 

engineering difficulties. Not only were the 

specifications for the centrifuge demanding, but, 

depending upon rotor size, it was estimated that 

it would require tens of thousands of centrifuges 

to produce enough uranium-235 to be of value. 

Gaseous diffusion immediately ran into trouble as 

well. Fabrication of an effective barrier to separate 

the uranium isotopes seemed so difficult as to 

relegate gaseous diffusion to a lower priority (the 

barrier had to be a corrosion-resistant membrane 

containing millions of submicroscopic holes per 

square inch). Both separation methods demanded 

the design and construction of new technologies 

and required that parts, many of them never 

before produced, be finished to tolerances not 

previously imposed on American industry.

In Chicago, Compton decided to combine all pile 

research by stages. Initially he funded Fermi’s pile 

at Columbia and the theoretical work of Eugene 

Wigner at Princeton and J. Robert Oppenheimer 

at Berkeley. He appointed Szilard head of 

materials acquisition and arranged for Seaborg 

to move his plutonium work from Berkeley to 

Chicago in April 1942. Compton secured space 

wherever he could find it, including a racket court 

under the west grandstand at Stagg Field, where 

Samuel K. Allison began building a graphite and 

uranium pile. Although it was recognized that 

heavy water would provide a moderator superior 

to graphite, the only available supply was a small 

amount that the British had smuggled out of 

France. In a decision typical of the new climate 

of urgency, Compton decided to forge ahead 

with graphite, a decision made easier by Fermi’s 

increasingly satisfactory results at Columbia 

and Allison’s even better results in Chicago. In 

light of recent calculations that cast doubt on the 

MAUD report’s negative assessment of plutonium 

production, Compton hoped that Allison’s pile 

would provide plutonium that could be used as 

material for a weapon.

By May 1942, Bush decided that production 

planning could wait no longer, and he instructed 

Conant to meet with the S-l section leaders and 

make recommendations on all approaches to the 

bomb, regardless of cost. Analyzing the status 

13

Early Government Support
5

of the four processes then under consideration 

for producing fissionable materials for a 

bomb—the gaseous diffusion, centrifuge, and 

electromagnetic uranium isotope separation 

methods and the plutonium producing pile—the 

committee decided on May 23 to recommend 

that all four be pushed as fast as possible to the 

pilot plant stage and to full production planning. 

This decision reflected the inability of the 

committee to distinguish a clear front-runner 

and its consequent unwillingness to abandon any 

method. With funds readily available and the 

outcome of the war conceivably hanging in the 

balance, the S-l leadership recommended that all 

four methods proceed to the pilot plant stage and 

to full production planning.

Enter the Army

The decision to proceed with production planning led directly to the involvement of the Army, 

specifically the Corps of Engineers. Roosevelt had 

approved Army involvement on October 9, 1941, 

and Bush had arranged for Army participation at 

S-l meetings beginning in March 1942. The need 

for security suggested placing the S-l program 

within one of the armed forces, and the construction expertise of the Corps of Engineers made it 

the logical choice to build the production facilities 

envisioned in the Conant report of May 23.

By orchestrating some delicate negotiations 

between the Office of Scientific Research 

and Development and the Army, Bush was 

able to transfer the responsibility for process 

development, materials procurement, engineering 

design, and site selection to the Corps of 

Engineers and to earmark approximately sixty 

percent of the proposed 1943 budget, or $54 

million, for these functions. An Army officer 

would be in overall command of the entire 

project. This new arrangement left S-l, with a 

budget of approximately $30 million, in charge 

of only university research and pilot plant 

studies. Additional reorganization created an 

S-l Executive Committee, composed of Conant, 

Briggs, Compton, Lawrence, Murphree, and Urey. 

This group would oversee all Office of Scientific 

Research and Development work and keep abreast 

of technical developments that might influence 

engineering considerations or plant design.23

With this reorganization in place, the nature of 

the American atomic bomb effort changed from 

one dominated by research scientists to one in 

which scientists played a supporting role in the 

construction enterprise run by the United States 

Army Corps of Engineers.

14

Part II
5

Part III:

The Manhattan Engineer District

Initial Problems

Summer 1942—during which the American 

island-hopping campaign in the Pacific began at 

Guadalcanal—proved to be a troublesome one 

for the fledgling bomb project. Colonel James C. 

Marshall received the assignment of directing 

the Laboratory for the Development of Substitute 

Metals, or DSM. Marshall immediately moved 

from Syracuse to New York City, where he set up 

the Manhattan Engineer District, established by 

general order on August 13. Marshall, like most 

other Army officers, knew nothing of nuclear 

physics. Furthermore, Marshall and his Army 

superiors were disposed to move cautiously. In 

one case, for instance, Marshall delayed purchase 

of an excellent production site in Tennessee pending further study, while the scientists who had 

been involved in the project from the start were 

pressing for immediate purchase. While Bush had 

carefully managed the transition to Army control, 

there was not yet a mechanism to arbitrate disagreements between S-l and the military. The resulting lack of coordination complicated attempts 

to gain a higher priority for scarce materials and 

boded ill for the future of the entire bomb project.

Reorganization of the Manhattan 

Engineer District:

Groves and the Military Policy Committee

Decisions made in September provided 

administrative clarity and renewed the project’s 

sense of urgency. Bush and the Army agreed that 

an officer other than Marshall should be given 

the assignment of overseeing the entire atomic 

project, which by now was referred to as the 

Manhattan Project. On September 17, the Army 

appointed Colonel Leslie R. Groves (promoted 

to Brigadier General six days later) to head the 

effort. Groves was an engineer with impressive 

credentials, including building of the Pentagon, 

and, most importantly, had strong administrative 

abilities. Within two days Groves acted to 

obtain the Tennessee site and secured a higher 

priority rating for project materials. In addition, 

Groves moved the Manhattan Engineer District 

headquarters from New York to Washington. 

He quickly recognized the talents of Marshall’s 

deputy, Colonel Kenneth D. Nichols, and 

arranged for Nichols to work as his chief aide and 

troubleshooter throughout the war.

Bush, with the help and authority of Secretary 

of War Henry L. Stimson, set up the Military 

Policy Committee, including one representative 

each from the Army, the Navy, and the Office of 

Scientific Research and Development. Bush hoped 

15

General Leslie R. Groves. Reprinted from Vincent C. Jones, 

Manhattan: The Army and the Atomic Bomb (Washington, 

D.C.: U.S. Government Printing Office, 1985).
5

that scientists would have better access to decision 

making in the new structure than they had enjoyed when DSM and S-l operated as parallel but 

separate units. With Groves in overall command 

(Marshall remained as District Engineer, where 

his cautious nature proved useful in later decision 

making) and the Military Policy Committee in 

place (the Top Policy Group retained broad policy 

authority), Bush felt that early organizational 

deficiencies had been remedied.24

During summer and fall 1942, technical and 

administrative difficulties were still severe. 

Each of the four processes for producing 

fissionable material for a bomb remained under 

consideration, but a full-scale commitment 

to all four posed serious problems even with 

the project’s high priority. When Groves took 

command in mid-September, he made it clear that 

by late 1942 decisions would be made as to which 

process or processes promised to produce a bomb 

in the shortest amount of time. The exigencies 

of war, Groves held, required scientists to move 

from laboratory research to development and 

production in record time. Though traditional 

scientific caution might be short-circuited in the 

process, there was no alternative if a bomb was to 

be built in time to be used in the current conflict. 

As everyone involved in the Manhattan Project 

soon learned, Groves never lost sight of this goal 

and made all his decisions accordingly.

Producing Fissionable Materials: Fall 1942

Groves made good on his timetable when he 

scheduled a meeting of the Military Policy 

Committee on November 12 and a meeting of the 

S-l Executive Committee on November 14. The 

scientists at each of the institutions doing isotope 

separation research knew these meetings would 

determine the separation method to be used in 

the bomb project; therefore, the keen competition 

among the institutions added to the sense of 

urgency created by the war. Berkeley remained 

a hotbed of activity as Lawrence and his staff 

pushed the electromagnetic method into the lead. 

The S-l Executive Committee even toyed with the 

idea of placing all its money on Lawrence but was 

dissuaded by Conant. Throughout the summer 

and fall, Lawrence refined his new 184-inch 

magnet and huge cyclotron to produce calutrons, 

as the tanks were called in honor of the University 

of California, capable of reliable beam resolution 

and containing improved collectors for trapping 

the enriched uranium-235. The S-l Executive 

Committee visited Berkeley on September 13 and 

subsequently recommended building both a pilot 

plant and a large section of a full-scale plant in 

Tennessee.

The centrifuge being developed by Jesse Beams 

at the University of Virginia was the big loser 

in the November meetings. Westinghouse had 

Part III

J. Robert Oppenheimer. Reprinted by Permission of the 

J. Robert Oppenheimer Memorial Committee.

16
5

been unable to overcome problems with its 

model centrifuge. Parts failed with discouraging 

regularity due to severe vibrations during trial 

runs; consequently, a pilot plant and subsequent 

production stages appeared impractical in the 

near future. Conant had already concluded that 

the centrifuge was likely to be dropped when he 

reported to Bush on October 26. The meetings of 

November 12 and 14 confirmed his analysis.

Gaseous diffusion held some promise and 

remained a live option, although the Dunning 

group at Columbia had not yet produced any 

uranium-235 by the November meetings. The 

major problem continued to be the barrier; nickel 

was the leading candidate for barrier material, but 

there was serious doubt as to whether a reliable 

nickel barrier could be ready in sufficient quantity 

by the end of the war.

While the centrifuge was cancelled and gaseous 

diffusion received mixed reviews, optimism 

prevailed among the pile proponents at the 

Metallurgical Laboratory in Chicago. Shortages 

of uranium and graphite delayed construction 

of the Stagg Field pile—CP-1 (Chicago Pile 

Number One)—but this frustration was tempered 

by calculations indicating that a completed pile 

would produce a chain reaction. With Fermi’s 

move to Chicago in April, all pile research 

was now being conducted at the Metallurgical 

Laboratory as Compton had planned, and Fermi 

and his team anticipated a successful experiment 

by the end of the year. Further optimism stemmed 

from Seaborg’s inventive work with plutonium, 

particularly his investigations on plutonium’s 

oxidation states that seemed to provide a way to 

separate plutonium from the irradiated uranium 

to be produced in the pile. In August Seaborg’s 

team produced a microscopic sample of pure 

plutonium, a major chemical achievement and 

one fully justifying further work on the pile. 

The only cloud in the Chicago sky was the 

scientists’ disappointment when they learned that 

construction and operation of the production 

facilities, now to be built near the Clinch River 

in Tennessee at Site X, would be turned over to 

a private firm. An experimental pile would be 

built in the Argonne Forest Preserve just outside 

Chicago, but the Metallurgical Laboratory 

scientists would have to cede their claim to 

pile technology to an organization experienced 

enough to take the process into construction and 

operation.

The Luminaries Report From Berkeley

While each of the four processes fought to 

demonstrate its “workability” during summer 

and fall 1942, equally important theoretical 

studies were being conducted that greatly 

influenced the decisions made in November. 

Robert Oppenheimer headed the work of a 

group of theoretical physicists he called the 

luminaries, which included Felix Bloch, Hans 

Bethe, Edward Teller, and Robert Serber, while 

John H. Manley assisted him by coordinating 

nationwide fission research and instrument and 

measurement studies from the Metallurgical 

Laboratory in Chicago. Despite inconsistent 

experimental results, the consensus emerging 

at Berkeley was that approximately twice as 

much fissionable material would be required 

for a bomb than had been estimated six months 

earlier. This was disturbing, especially in light 

of the military’s view that it would take more 

than one bomb to win the war. The goal of 

mass-producing fissionable material, which still 

appeared questionable in late 1942, seemed even 

more unrealistic given Oppenheimer’s estimates. 

Oppenheimer did report, with some enthusiasm, 

that fusion explosions using deuterium (heavy 

hydrogen) might be possible. The possibility of 

thermonuclear (fusion) bombs generated some 

optimism since deuterium supplies, while not 

abundant, were certainly larger and more easily 

supplemented than were those of uranium and 

plutonium. S-l immediately authorized basic 

research on other light elements.

17

The Manhattan Engineer District
5

Input From DuPont

Final input for the November meetings of the 

Military Policy Committee and the S-l Executive 

Committee came from DuPont. One of the 

first things Groves did when he took over in 

September was to begin courting DuPont, hoping 

that the giant chemical firm would undertake 

construction and operation of the plutonium 

separation plant to be built in Tennessee. He 

appealed to patriotism, informing the company 

that the bomb project had high priority with the 

President and maintaining that a successful effort 

could affect the outcome of the war. DuPont 

managers resisted but did not refuse the task, and 

in the process they provided an objective appraisal 

of the pile project. Noting that it was not even 

known if the chain reaction would work, DuPont 

stated that under the best of circumstances 

plutonium could be mass-produced by 1945, 

and it emphasized that it thought the chances 

of this happening were low. This appraisal did 

not discourage Groves, who was confident that 

DuPont would take the assignment if offered.

Time for Decisions

The Military Policy Committee met on November 

12, 1942, and its decisions were ratified by the 

S-l Executive Committee two days later. The 

Military Policy Committee, acting on Groves’s 

and Conant’s recommendations, cancelled the 

centrifuge project. Gaseous diffusion, the pile, 

and the electromagnetic method were to proceed 

directly to full-scale, eliminating the pilot plant 

stage. The S-l Executive Committee approved 

these recommendations and agreed that the 

gaseous diffusion facility was of lower priority 

than either the pile or the electromagnetic 

plant but ahead of a second pile. The scientific 

committee also asked DuPont to look into 

methods for increasing American supplies of 

heavy water in case it was needed to serve as a 

moderator for one of the new piles.

Part III

A Brief Scare

Anxious as he was to get moving, Groves 

decided to make one final quality control check 

before acting on the decisions of November 

12 and 14. This decision seemed imperative 

after a brief scare surrounding the pile project. 

While Fermi’s calculations provided reasonable 

assurance against such a possibility, the vision 

of a chain reaction running wild in heavilypopulated Chicago arose when the S-l Executive 

Committee found that Compton was building the 

experimental pile at Stagg Field, a decision he had 

made without informing either the committee 

or Groves. In addition, information from British 

scientists raised serious questions about the 

feasibility of deriving plutonium from the pile. It 

took several days for Groves and a committee of 

scientists including Lawrence and Oppenheimer 

to satisfy themselves that the pile experiment 

posed little danger, was justified by sound theory, 

and would in all probability produce plutonium as 

predicted.

One Last Look: The Lewis Committee

On November 18, Groves appointed Warren 

K. Lewis of the Massachusetts Institute of 

Technology to head a final review committee, 

comprised of himself and three DuPont 

representatives. During the next two weeks, the 

committee traveled from New York to Chicago 

to Berkeley and back again through Chicago. 

It endorsed the work on gaseous diffusion at 

Columbia, though it made some organizational 

recommendations; in fact, the Lewis committee 

elevated gaseous diffusion to first priority and 

expressed reservations about the electromagnetic 

program despite an impassioned presentation by 

Lawrence in Berkeley. Upon returning to Chicago, 

Crawford H. Greenewalt, a member of the Lewis 

committee, was present at Stagg Field when, at 

3:20 p.m. on December 2, 1942, Fermi’s massive 

lattice pile of 400 tons of graphite, six tons of 

uranium metal, and fifty tons of uranium oxide 

achieved the first self-sustaining chain reaction, 

18
5

operating initially at a power level of one-half 

watt (increased to 200 watts ten days later).25

As Compton reported to Conant, “the Italian 

navigator has just landed in the new world.” To 

Conant’s question, “Were the natives friendly?” 

Compton answered, “Everyone landed safe and 

happy.”26 Significant as this moment was in 

the history of physics, it came after the Lewis 

committee had endorsed moving to the pilot 

stage and one day after Groves had instructed 

DuPont to move into design and construction on 

December 1.27

No Turning Back: Final Decisions and 

Presidential Approval

The S-l Executive Committee met to consider the 

Lewis report on December 9, 1942, just weeks 

after Allied troops landed in North Africa. Most 

of the morning session was spent evaluating the 

controversial recommendation that only a small 

electromagnetic plant be built. Lewis and his 

colleagues based their recommendation on the 

belief that Lawrence could not produce enough 

uranium-235 to be of military significance. 

But since the calutron could provide enriched 

samples quickly, the committee supported the 

construction of a small electromagnetic plant. 

Conant disagreed with the Lewis committee’s 

assessment, believing that uranium had more 

weapon potential than plutonium. And since he 

knew that gaseous diffusion could not provide 

any enriched uranium until the gaseous diffusion 

plant was in full operation, he supported the 

one method that might, if all went well, produce 

enough uranium to build a bomb in 1944. During 

the afternoon, the S-l Executive Committee 

went over a draft Groves had prepared for Bush 

to send to the President. It supported the Lewis 

committee’s report except that it recommended 

skipping the pilot plant stage for the pile. After 

Conant and the Lewis committee met on 

December 10 and reached a compromise on 

the electromagnetic method, Groves’s draft was 

amended and forwarded to Bush.28

19

The Manhattan Engineer District

On December 28, 1942, President Roosevelt 

approved the establishment of what ultimately 

became a government investment in excess of 

$2 billion, $.5 billion of which was itemized in 

Bush’s report submitted on December 16. The 

Manhattan Project was authorized to build fullscale gaseous diffusion and plutonium plants 

and the compromise electromagnetic plant, as 

well as heavy water production facilities. In his 

report, Bush reaffirmed his belief that bombs 

possibly could be produced during the first half 

of 1945 but cautioned that an earlier delivery 

was unlikely. No schedule could guarantee that 

the United States would overtake Germany in 

the race for the bomb, but by the beginning of 

1943 the Manhattan Project had the complete 

support of President Roosevelt and the military 

leadership, the services of some of the nation’s most 

distinguished scientists, and a sense of urgency 

driven by fear. Much had been achieved in the year 

between Pearl Harbor and the end of 1942.

No single decision created the American atomic 

bomb project. Roosevelt’s December 28 decision 

was inevitable in light of numerous earlier ones 

that, in incremental fashion, committed the 

United States to pursuing atomic weapons. In fact, 

the essential pieces were in place when Roosevelt 

approved Bush’s November 9, 1941, report on 

January 19, 1942. At that time, there was a science 

organization at the highest level of the federal 

government and a Top Policy Group with direct 

access to the President. Funds were authorized, 

and the participation of the Corps of Engineers 

had been approved in principle. In addition, the 

country was at war and its scientific leadership—

as well as its President—had the belief, born of 

the MAUD report, that the project could result 

in a significant contribution to the war effort. 

Roosevelt’s approval of $500 million in late 

December 1942 was a step that followed directly 

from the commitments made in January of that 

year and stemmed logically from the President’s 

earliest tentative decisions in late 1939.
5

Part IV:

The Manhattan Engineer 

District in Operation

The Manhattan Project

In many ways the Manhattan Engineer District 

operated like any other large construction 

company. It purchased and prepared sites, let 

contracts, hired personnel and subcontractors, 

built and maintained housing and service 

facilities, placed orders for materials, developed 

administrative and accounting procedures, and 

established communications networks. By the 

end of the war Groves and his staff had spent 

approximately $2.2 billion on production facilities 

and towns built in the states of Tennessee, 

Washington, and New Mexico, as well as on 

research in university laboratories from Columbia 

to Berkeley. What made the Manhattan Project 

unlike other companies performing similar 

functions was that, because of the necessity of 

moving quickly, it invested hundreds of millions 

of dollars in unproven and hitherto unknown 

processes and did so entirely in secret. Speed and 

secrecy were the watchwords of the Manhattan 

Project.

Secrecy proved to be a blessing in disguise. 

Although it dictated remote site locations, 

required subterfuge in obtaining labor and 

supplies, and served as a constant irritant to 

the academic scientists on the project, it had 

one overwhelming advantage: Secrecy made 

it possible to make decisions with little regard 

for normal peacetime political considerations. 

Groves knew that as long as he had the backing 

of the White House money would be available 

and he could devote his considerable energies 

entirely to running the bomb project. Secrecy in 

the Manhattan Project was so complete that many 

people working for the organization did not know 

what they were working on until they heard about 

the bombing of Hiroshima on the radio. The need 

for haste clarified priorities and shaped decision 

making. Unfinished research on three separate, 

unproven processes had to be used to freeze 

design plans for production facilities, even though 

it was recognized that later findings inevitably 

would dictate changes. The pilot plant stage was 

eliminated entirely, violating all manufacturing 

practices and leading to intermittent shutdowns 

and endless troubleshooting during trial runs 

in production facilities. The inherent problems 

of collapsing the stages between the laboratory 

and full production created an emotionally 

charged atmosphere with optimism and despair 

alternating with confusing frequency.

Despite Bush’s assertion that a bomb could 

probably be produced by 1945, he and the other 

principals associated with the project recognized 

the magnitude of the task before them. For any 

large organization to take laboratory research 

into design, construction, operation, and product 

delivery in two-and-a-half years (from early 

1943 to Hiroshima) would be a major industrial 

achievement. Whether the Manhattan Project 

would be able to produce bombs in time to affect 

the current conflict was an open question as 1943 

began. (Obvious though it seems in retrospect, 

it must be remembered that no one at the time 

knew that the war would end in 1945 or who the 

remaining contestants would be if and when the 

atomic bomb was ready for use.)

Clinton Engineer Works (Oak Ridge)

By the time President Roosevelt authorized 

the Manhattan Project on December 28, 1942, 

work on the east Tennessee site where the first 

production facilities were to be built was already 

underway. The final quarter of 1942 saw the 

acquisition of the roughly ninety-square-mile 

parcel (59,000 acres) in the ridges just west 

of Knoxville, the removal of the relatively few 

families on the marginal farmland, and extensive 

20
5

Clinton Engineer Works. Reprinted from Vincent C. Jones, Manhattan: The Army and the Atomic Bomb (Washington, D.C.: 

U.S. Government Printing Office, 1985).

site preparation to provide the transportation, 

communications, and utility needs of the town 

and production plants that would occupy the 

previously underdeveloped area. Original plans 

called for the Clinton Engineer Works, as the 

military reservation was named, to house approximately 13,000 people in prefabricated housing, 

trailers, and wood dormitories. By the time the 

Manhattan Engineer District headquarters were 

moved from Washington to Tennessee in the 

summer of 1943 (Groves kept the Manhattan 

Project’s office in Washington and placed Nichols 

in command in Tennessee), estimates for the 

town of Oak Ridge had been revised upward to 

40-45,000 people. (The name Oak Ridge did not 

come into widespread usage until after World War 

II but will be used here to avoid confusion.) At 

the end of the war, Oak Ridge was the fifth largest 

town in Tennessee, and the Clinton Engineer 

Works was consuming one-seventh of all the 

power being produced in the nation.29 While the 

Army and its contractors tried to keep up with the 

rapid influx of workers and their families, services 

always lagged behind demand, though morale 

remained high in the atomic boomtown.

The three production facility sites were located 

in valleys away from the town. This provided 

security and containment in case of explosions. 

The Y-12 area, home of the electromagnetic 

plant, was closest to Oak Ridge, being but one 

ridge away to the south. Farther to the south and 

The Manhattan Engineer District in Operation

21
5

Y-12 Alpha Racetrack at Clinton. Spare vacuum tanks in Left Foreground. Reprinted from Richard G. Hewlett and Oscar E. 

Anderson, Jr., The New World, 1939-1946, Volume I of A History of the United States Atomic Energy Commission (University 

Park: Pennsylvania State University Press, 1962).

west lay both the X-10 area, which contained 

the experimental plutonium pile and separation 

facilities, and K-25, site of the gaseous diffusion 

plant and later the S-50 thermal diffusion plant. 

Y-12 and X-10 were begun slightly earlier in 1943 

than was K-25, but all three were well along by the 

end of the year.

The Y-12 Electromagnetic Plant:

Final Decisions

Although the Lewis report had placed gaseous 

diffusion ahead of the electromagnetic approach, 

many were still betting in early 1943 that 

Lawrence and his mass spectrograph would 

eventually predominate. Lawrence and his 

laboratory of mechanics at Berkeley continued 

to experiment with the giant 184-inch magnet, 

trying to reach a consensus on which shims, 

sources, and collectors to incorporate into Y-12 

design for the Oak Ridge plant. Research on 

magnet size and placement and beam resolution 

eventually led to a racetrack configuration of 

two magnets with forty-eight gaps containing 

two vacuum tanks each per building, with ten 

buildings being necessary to provide the 2,000 

sources and collectors needed to separate 100 

grams of uranium-235 daily. It was hoped that 

improvements in calutron design, or placing 

Part IV

22
5

Y-12 Electromagnetic Plant Under Construction at Clinton. Reprinted from Richard G. Hewlett and Oscar E. Anderson, 

Jr., The New World, 1939-1946, Volume I of A History of the United States Atomic Energy Commission (University Park: 

Pennsylvania State University Press, 1962).

multiple sources and collectors in each tank, 

might increase efficiency and reduce the number 

of tanks and buildings required, but experimental 

results were inconclusive even as Stone & Webster 

of Boston, the Y-12 contractor at Oak Ridge, 

prepared to break ground.

At a meeting of Groves, Lawrence, and John 

R. Lotz of Stone & Webster in Berkeley late in 

December 1942, Y-12 plans took shape. It was 

agreed that Stone & Webster would take over 

design and construction of a 500-tank facility, 

while Lawrence’s laboratory would play a 

supporting role by supplying experimental data. 

By the time another summit conference on Y-12 

took place in Berkeley on January 13 and 14, 

Groves had persuaded the Tennessee Eastman 

Corporation to sign on as plant operator and 

arranged for various parts of the electromagnetic 

equipment to be manufactured by the Westinghouse Electric and Manufacturing Company, the 

Allis-Chalmers Manufacturing Company, and 

the Chapman Valve Manufacturing Company. 

General Electric agreed to provide electrical 

equipment.

On January 14, after a day of presentations and 

a demonstration of the experimental tanks in 

The Manhattan Engineer District in Operation

23
5

the cyclotron building, Groves stunned the Y-12 

contractors by insisting that the first racetrack 

of ninety-six tanks be in operation by July 1 and 

that 500 tanks be delivered by year’s end. Given 

that each racetrack was 122 feet long, 77 feet wide 

and 15 feet high; that the completed plant was 

to be the size of three two-story buildings; that 

tank design was still in flux; and that chemical 

extraction facilities also would have to be built, 

Groves’s demands were little less than shocking. 

Nonetheless, Groves maintained that his schedule 

could be met.30

For the next two months Lawrence, the 

contractors, and the Army negotiated over the 

final design. While all involved could see possible 

improvements, there simply was not enough time 

to incorporate every suggested modification. 

Y-12 design was finalized at a March 17 meeting 

in Boston, with one major modification—the 

inclusion of a second stage of the electromagnetic 

process. The purpose of this second stage was 

to take the enriched uranium-235 derived 

from several runs of the first stage and use it 

as the sole feed material for a second stage of 

racetracks containing tanks approximately half 

the size of those in the first. Groves approved this 

arrangement and work began on both the Alpha 

(first-stage) and Beta (second-stage) tracks.

Construction of Y-12

Groundbreaking for the Alpha plant took place 

on February 18, 1943. Soon blueprints could 

not be produced fast enough to keep up with 

construction as Stone & Webster labored to meet 

Groves’s deadline.The Beta facility was actually 

begun before formal authorization. While 

laborers were aggressively recruited, there was 

always a shortage of workers skilled enough to 

perform jobs according to the rigid specifications. 

(A further complication was that some tasks 

could be performed only by workers with special 

clearances.) Huge amounts of material had to 

be obtained (38 million board feet of lumber, 

for instance), and the magnets needed so much 

copper for windings that the Army had to borrow 

almost 15,000 tons of silver bullion from the 

United States Treasury to fabricate into strips 

and wind on to coils as a substitute for copper.31

Treasury silver was also used to manufacture the 

busbars that ran around the top of the racetracks.

Replacing copper with silver solved the 

immediate problem of the magnets and busbars, 

but persistent shortages of electronic tubes, generators, regulators, and other equipment plagued 

the electromagnetic project and posed the most 

serious threat to Groves’s deadline. Furthermore, 

last-minute design changes continued to frustrate 

equipment manufacturers. Nonetheless, when 

Lawrence toured with Y-12 contractors in May 

1943, he was impressed by the scale of operations. 

Lawrence returned to Berkeley rededicated to the 

“awful job” of finishing the racetracks on time.32

Design Changes at Y-12

Lawrence and his colleagues continued to look 

for ways to improve the electromagnetic process. 

Lawrence found that hot (high positive voltage) 

electrical sources could replace the single cold 

(grounded) source in future plants, providing 

Y-12 Beta Racetrack at Clinton. Reprinted from Richard G. 

Hewlett and Oscar E. Anderson, Jr., The New World, 

1939-1946, Volume I of A History of the United States 

Atomic Energy Commission (University Park: Pennsylvania 

State University Press, 1962).

Part IV

24
5

more efficient use of power, reducing insulator 

failure, and making it possible to use multiple 

rather than single beams.33 Meanwhile, receiver 

design evolved quickly enough in spring and 

summer 1943 to be incorporated into the Alpha 

plant. Work at the Radiation Laboratory picked up 

additional speed in March with the authorization 

of the Beta process. With Alpha technology far 

from perfected, Lawrence and his staff now had to 

participate in planning for an unanticipated stage 

of the electromagnetic process.

While the scientists in Berkeley studied changes 

that would be required in the down-sized Beta 

racetracks, engineering work at Oak Ridge 

prescribed specific design modifications. For 

a variety of reasons, including simplicity of 

maintenance, Tennessee Eastman decided that the 

Beta plant would consist of two tracks of thirtysix tanks each in a rectangular, rather than oval, 

arrangement. Factoring this configuration into 

their calculations, Lawrence and his coworkers 

bent their efforts to developing chemical 

processing techniques that would minimize the 

loss of enriched uranium during Beta production 

runs. To make certain that Alpha had enough 

feed material, Lawrence arranged for research 

on an alternate method at Brown University and 

expanded efforts at Berkeley. With what was left of 

his time and money in early 1943 Lawrence built 

prototypes of Alpha and Beta units for testing 

and training operating personnel. Meanwhile 

Tennessee Eastman, running behind schedule, 

raced to complete experimental models so that 

training and test runs could be performed at Oak 

Ridge.

Warning From Los Alamos

But in the midst of encouraging progress in 

construction and research on the electromagnetic 

process in July came discouraging news from 

Oppenheimer’s isolated laboratory in Los Alamos, 

New Mexico, set up in spring 1943 to consolidate 

work on atomic weapons. Oppenheimer warned 

that three times more fissionable material would 

be required for a bomb than earlier estimates had 

indicated. Even with satisfactory performance 

of the racetracks, it was possible that they might 

not produce enough purified uranium-235 

in time. Lawrence responded to this crisis in 

characteristic fashion: He immediately lobbied 

Groves to incorporate multiple sources into the 

racetracks under construction and to build more 

racetracks. Groves decided to build the first four 

as planned but, after receiving favorable reports 

from both Stone & Webster and Tennessee 

Eastman, allowed a four-beam source in the 

fifth. Convinced that the electromagnetic process 

would work and sensing that estimates from Los 

Alamos might be revised downward in the future, 

Groves let Lawrence talk him into building a 

new plant—the Y-12 Extension—doubling the 

size of the electromagnetic complex. The Alpha 

component of the Y-12 Extension was designated 

as Alpha II and would consist of two buildings, 

each with two rectangular racetracks of ninety-six 

tanks operating with four-beam sources. Also 

authorized was a second Beta building containing 

two racetracks. Improvisation remained the key 

work at Oak Ridge.

Shakedown at Y-12

During summer and fall 1943, the first electromagnetic plant began to take shape. The huge 

building to house the operating equipment 

was readied as manufacturers began delivering 

everything from electrical switches to motors, 

valves, and tanks. While construction and 

outfitting proceeded, almost 5,000 operating and 

maintenance personnel were hired and trained. 

Then, between October and mid-December, 

Y-12 paid the price for being a new technology 

that had not been put through its paces in a pilot 

plant. Vacuum tanks in the first Alpha racetrack 

leaked and shimmied out of line due to magnetic 

pressure, welds failed, electrical circuits malfunctioned, and operators made frequent mistakes. 

Most seriously, the magnet coils shorted out 

because of rust and sediment in the cooling oil.

The Manhattan Engineer District in Operation

25
5

Groves arrived on December 15 and shut the 

racetrack down. The coils were sent to AllisChalmers with hope that they could be cleaned 

without being dismantled entirely, while measures 

were taken to prevent recurrence of the shorting 

problem. The second Alpha track now bore the 

weight of the electromagnetic effort. In spite of 

precautions aimed at correcting the electrical 

and oil-related problems that had shut down 

the first racetrack, the second Alpha fared little 

better when it started up in mid-January 1944. 

While all tanks operated at least for short periods, 

performance was sporadic and maintenance 

could not keep up with electrical failures and 

defective parts. Like its predecessor, Alpha 2 was a 

maintenance nightmare.

Alpha 2 produced about 200 grams of twelvepercent uranium-235 by the end of February, 

enough to send samples to Los Alamos and 

feed the first Beta unit but not enough to satisfy 

estimates of weapon requirements. The first 

four Alpha tracks did not operate together until 

April, a full four months late. While maintenance 

improved, output was well under previous 

expectations. The opening of the Beta building on 

March 11 led to further disappointment. Beam 

resolution was so unsatisfactory that complete 

redesign was required. To make matters worse, 

word spread that the K-25 gaseous diffusion 

process was in deep trouble because of its ongoing 

barrier crisis. K-25 had been counted upon to 

provide uranium enriched enough to serve as 

feed material for Beta. Now it would be producing 

such slight enrichment that the Alpha tracks 

would have to process K-25’s material, requiring 

extensive redesign and retooling of tanks, doors, 

and liners, particularly in units that would be 

wired to run as hot, rather than as cold, electrical 

sources.34

Reworking the Racetracks

It became clear to Groves that he would have to 

find a way for a combination of isotope separation 

processes to produce enough fissionable material 

for bombs. This meant making changes in 

the racetracks so that they could process the 

slightly enriched material produced by K-25. He 

then concentrated on further expansion of the 

electromagnetic facilities. Lawrence, seconded by 

Oppenheimer, believed that four more racetracks 

should be built to accompany the nine already 

finished or under construction. Groves agreed 

with this approach, though he was not sure that 

the additional racetracks could be built in time.

As K-25 stock continued to drop and plutonium 

prospects remained uncertain, Lawrence lobbied 

yet again for further expansion of Y-12, arguing 

that it provided the only possible avenue to 

a bomb by 1945. His plan was to convert all 

tanks to multiple beams and to build two more 

racetracks. By this time even the British had given 

up on gaseous diffusion and urged acceptance of 

Lawrence’s plan.

Time was running out, and an element of 

desperation crept into decisions made at a 

meeting on July 4, 1944. Groves met with the 

Oak Ridge contractors to consider proposals 

Lawrence had prepared after assessing once 

again the resources and abilities of the Radiation 

Laboratory. There was to be no change in the 

completed racetracks; there simply was not 

enough time. Some improvements were to be 

made in the racetracks then under construction. 

In the most important decision made at the 

meeting, Lawrence was to throw all he had into a 

completely new type of calutron that would use 

a thirty-beam source. Technical support would 

come from both Westinghouse and General 

Electric, which would cease work on four-beam 

development. It was a gamble in a high-stakes 

game, but sticking with the Alpha and Beta 

racetracks might have been an even greater 

gamble.

Part IV

26
5

The K-25 Gaseous Diffusion Plant

Eleven miles southwest of Oak Ridge on the 

Clinch River was the site of the K-25 gaseous 

diffusion plant upon which so much hope had 

rested when it was authorized in late 1942. 

Championed by the British and placed first by the 

Lewis committee, gaseous diffusion seemed to be 

based on sound theory but had not yet produced 

samples of enriched uranium-235.

At Oak Ridge, on a relatively flat area of about 

5,000 acres, site preparation for the K-25 powerplant began in June. Throughout the summer, 

contractors contended with primitive roads as 

they shipped in the materials needed to build 

what became the world’s largest steam electric 

plant. In September work began on the cascade 

building, plans for which had changed dramatically since the spring. Now there were to be fifty 

four-story buildings (2,000,000 square feet) in 

a U-shape measuring half a mile by 1,000 feet. 

Innovative foundation techniques were required 

to avoid setting thousands of concrete piers to 

support load-bearing walls.

Since it was eleven miles from the headquarters at 

Oak Ridge, the K-25 site developed into a satellite 

town. Housing was supplied, as was a full array of 

service facilities for the population that reached 

15,000. Dubbed Happy Valley by the inhabitants, 

K-25 Gaseous Diffusion Plant Under Construction at Clinton. 

Reprinted from Richard G. Hewlett and Oscar E. Anderson, Jr., 

The New World, 1939-1946, Volume I of A History of the United 

States Atomic Energy Commission (University Park: 

Pennsylvania State University Press, 1962).

K-25 from Opposite End. White Building in Center of 

Previous Picture Discernible at Far End. Reprinted from 

Richard G. Hewlett and Oscar E. Anderson, Jr., The New 

World, 1939-1946, Volume I of A History of the United States 

Atomic Energy Commission (University Park: Pennsylvania 

State University Press, 1962).

The Manhattan Engineer District in Operation

27
5

the town had housing similar to that in Oak 

Ridge, but, like headquarters, it too experienced

chronic shortages. Even with a contractor camp 

with facilities for 2,000 employees nearby, half of 

Happy Valley’s workers had to commute to the 

construction site daily.

Downgrading K-25

In late summer 1943, it was decided that K-25 

would play a lesser role than originally intended. 

Instead of producing fully enriched uranium-235, 

the gaseous diffusion plant would now provide 

around fifty percent enrichment for use as feed 

material in Y-12. This would be accomplished by 

eliminating the more troublesome upper part of 

the cascade. Even this level of enrichment was 

not assured since a barrier for the diffusion plant 

still did not exist. The decision to downgrade 

K-25 was part of the larger decision to double 

Y-12 capacity and fit with Groves’s new strategy 

of utilizing a combination of methods to produce 

enough fissionable material for bombs as soon as 

possible.

There was no doubt in Groves’s mind that gaseous 

diffusion still had to be pursued vigorously. Not 

only had major resources already been expended 

on the program, but there was also the possibility 

that it might yet prove successful. Y-12 was in 

trouble as 1944 began, and the plutonium pile 

projects were just getting underway. A workable 

barrier design might put K-25 ahead in the race 

for the bomb. Unfortunately, no one had been able 

to fabricate barrier of sufficient quality. The only 

alternative remaining was to increase production 

enough to compensate for the low percentage 

of barrier that met specifications. As Lawrence 

prepared to throw everything he had into a thirtybeam source for Y-12, Groves ordered a crash 

barrier program, hoping to prevent K-25 from 

standing idle as the race for the bomb continued.

Help From the Navy

As problems with both Y-12 and K-25 reached 

crisis proportions in spring and summer 1944, 

the Manhattan Project received help from an 

unexpected source—the United States Navy. 

President Roosevelt had instructed that the 

atomic bomb effort be an Army program and that 

the Navy be excluded from deliberations. Navy 

research on atomic power, conducted primarily 

for submarines, received no direct aid from 

Groves, who, in fact, was not up-to-date on the 

state of Navy efforts when he received a letter on 

the subject from Oppenheimer late in April 1944.

Oppenheimer informed Groves that Philip 

Abelson’s experiments on thermal diffusion at the 

Philadelphia Naval Yard deserved a closer look. 

Abelson was building a plant to produce enriched 

uranium to be completed in early July. It might be 

possible, Oppenheimer thought, to help Abelson 

complete and expand his plant and use its slightly 

enriched product as feed for Y-12 until problems 

with K-25 could be resolved.

The liquid thermal diffusion process had been 

evaluated in 1940 by the Uranium Committee, 

when Abelson was at the National Bureau of 

Standards. In 1941 he moved to the Naval 

Research Laboratory, where there was more 

support for his work. During summer 1942 Bush 

and Conant received reports about Abelson’s 

research but concluded that it would take too long 

for the thermal diffusion process to make a major 

contribution to the bomb effort, especially since 

the electromagnetic and pile projects were making 

satisfactory progress. After a visit with Abelson 

in January 1943, Bush encouraged the Navy 

to increase its support of thermal diffusion. A 

thorough review of Abelson’s project early in 1943, 

however, concluded that thermal diffusion work 

should be expanded but should not be considered 

as a replacement for gaseous diffusion, which was 

better understood theoretically. Abelson continued 

his work independently of the Manhattan Project. 

He obtained authorization to build a new plant at 

Part IV

28
5

the Philadelphia Naval Yard, where construction 

began in January 1944.

Groves immediately saw the value of 

Oppenheimer’s suggestion and sent a group to 

Philadelphia to visit Abelson’s plant. A quick 

analysis demonstrated that a thermal diffusion 

plant could be built at Oak Ridge and placed in 

operation by early 1945. The steam needed in the 

convection columns was already at hand in the 

form of the almost completed K-25 powerplant. 

It would be a relatively simple matter to provide 

steam to the thermal diffusion plant and produce 

enriched uranium, while providing electricity 

for the K-25 plant when it was finished. Groves 

gave the contractor, H. K. Ferguson Company of 

Cleveland, just ninety days from September 27 

to bring a 2,142-column plant on line (Abelson’s 

plant contained 100 columns). There was no time 

to waste as Happy Valley braced itself for a new 

influx of workers.

The Metallurgical Laboratory

One of the most important branches of the farflung Manhattan Project was the Metallurgical 

Laboratory (Met Lab) in Chicago, which was 

counted on to design a production pile for 

plutonium. Here again the job was to design 

equipment for a technology that was not well 

understood even in the laboratory. The Fermi 

pile, important as it was historically, provided 

little technical guidance other than to suggest a 

lattice arrangement of graphite and uranium. Any 

pile producing more power than the few watts 

generated in Fermi’s famous experiment would 

require elaborate controls, radiation shielding, 

and a cooling system. These engineering 

features would all contribute to a reduction in 

neutron multiplication (neutron multiplication 

being represented by k); so it was imperative to 

determine which pile design would be safe and 

controllable and still have a k high enough to 

sustain an ongoing reaction.35

Pile Design

A group headed by Compton’s chief engineer, 

Thomas V. Moore, began designing the 

production pile in June 1942. Moore’s first goals 

were to find the best methods of extracting 

plutonium from the irradiated uranium and for 

cooling the uranium. It quickly became clear that 

a production pile would differ significantly in 

design from Fermi’s experimental reactor, possibly 

by extending uranium rods into and through 

the graphite next to cooling tubes and building 

a radiation and containment shield. Although 

experimental reactors like Fermi’s did not generate 

enough power to need cooling systems, piles built 

to produce plutonium would operate at high power 

levels and require coolants. The Met Lab group 

considered the full range of gases and liquids in 

a search to isolate the substances with the best 

nuclear characteristics, with hydrogen and helium 

standing out among the gases and water—even 

with its marginal nuclear properties and tendency 

to corrode uranium—as the best liquid.

During the summer, Moore and his group 

began planning a helium-cooled pilot pile for 

the Argonne Forest Preserve near Chicago, 

built by Stone & Webster, and on September 25 

they reported to Compton. The proposal was 

for a 460-ton cube of graphite to be pierced by 

376 vertical columns containing twenty-two 

cartridges of uranium and graphite. Cooling 

would be provided by circulating helium from 

top to bottom through the pile. A wall of graphite 

surrounding the reactor would provide radiation 

containment, while a series of spherical segments 

that gave the design the nickname Mae West 

would make up the outer shell.

By the time Compton received Moore’s report, 

he had two other pile designs to consider. One 

was a water-cooled model developed by Eugene 

Wigner and Gale Young, a former colleague of 

Compton’s. Wigner and Young proposed a twelvefoot by twenty-five-foot cylinder of graphite with 

pipes of uranium extending from a water tank 

The Manhattan Engineer District in Operation

29
5

above, through the cylinder, and into a second 

water tank underneath. Coolant would circulate 

continuously through the system, and corrosion 

would be minimized by coating interior surfaces 

or lining the uranium pipes.

A second alternative to Mae West was more 

daring. Szilard thought that liquid metal would 

be such an efficient coolant that, in combination 

with an electromagnetic pump having no moving 

parts (adapted from a design he and Einstein 

had created), it would be possible to achieve high 

power levels in a considerably smaller pile. Szilard 

had trouble obtaining supplies for his experiment, 

primarily because bismuth, the metal he preferred 

as the coolant, was rare.

Groves Steps In

October 1942 found Groves in Chicago ready 

to force a showdown on pile design. Szilard was 

noisily complaining that decisions had to be 

made so that design could move to procurement 

and construction. Compton’s delay reflected 

uncertainty of the superiority of the helium pile 

and awareness that engineering studies could not 

be definitive until the precise value of k had been 

established. Some scientists at the Met Lab urged 

that a full production pile be built immediately, 

while others advocated a multi-step process, 

perhaps beginning with an externally cooled 

reactor proposed by Fermi. The situation was 

tailor-made for a man with Groves’s temperament. 

On October 5 Groves exhorted the Met Lab to 

decide on pile design within a week. Even wrong 

decisions were better than no decisions, Groves 

claimed, and since time was more valuable 

than money, more than one approach should 

be pursued if no single design stood out. While 

Groves did not mandate a specific decision, his 

imposed deadline forced the Met Lab scientists to 

reach a consensus.

Compton decided on compromise. Fermi would 

study the fundamentals of pile operation on a 

small experimental unit to be completed and in 

operation by the end of the year. Hopefully he 

could determine the precise value of k and make 

a significant advance in pile engineering possible. 

An intermediate pile with external cooling would 

be built at Argonne and operated until June 1, 

1943, when it would be taken down for plutonium 

extraction. The helium-cooled Mae West, 

designed to produce 100 grams of plutonium a 

day, would be built and operating by March 1944. 

Studies on liquid-cooled reactors would continue, 

including Szilard’s work on liquid metals.

Seaborg and Plutonium Chemistry

While the Met Lab labored to make headway on 

pile design, Seaborg and his coworkers tried to 

gain enough information about transuranium 

chemistry to insure that plutonium produced 

could be successfully extracted from the 

irradiated uranium. Using lanthanum fluoride 

as a carrier, Seaborg isolated a weighable sample 

of plutonium in August 1942. At the same time, 

Isadore Perlman and William J. Knox explored the 

peroxide method of separation; John E. Willard 

studied various materials to determine which 

best adsorbed plutonium;36 Theodore T. Magel 

and Daniel K. Koshland, Jr., researched solventextraction processes; and Harrison S. Brown 

and Orville F. Hill performed experiments into 

volatility reactions. Basic research on plutonium’s 

chemistry continued as did work on radiation and 

fission products.

Seaborg’s discovery and subsequent isolation 

of plutonium were major events in the history 

of chemistry, but, like Fermi’s achievement, 

it remained to be seen whether they could be 

translated into a production process useful to the 

bomb effort. In fact, Seaborg’s challenge seemed 

even more daunting, for while piles had to be 

scaled up ten to twenty times, a separation plant 

for plutonium would involve a scale-up of the 

laboratory experiment on the order of a billionfold.

Part IV

30
5

Collaboration with DuPont’s Charles M. Cooper 

and his staff on plutonium separation facilities 

began even before Seaborg succeeded in isolating 

a sample of plutonium. Seaborg was reluctant 

to drop any of the approaches then under 

consideration, and Cooper agreed. The two 

decided to pursue all four methods of plutonium 

separation but put first priority on the lanthanum 

fluoride process Seaborg had already developed. 

Cooper’s staff ran into problems with the 

lanthanum fluoride method in late 1942, but by 

then Seaborg had become interested in phosphate 

carriers. Work led by Stanley G. Thompson found 

that bismuth phosphate retained over ninety-eight 

percent plutonium in a precipitate. With bismuth 

phosphate as a backup for the lanthanum fluoride, 

Cooper moved ahead on a semiworks near Stagg 

Field.

DuPont Joins the Team

Compton’s original plans to build the experimental pile and chemical separation plant on 

the University of Chicago campus changed 

during fall 1942. The S-l Executive Committee 

concurred that it would be safer to put Fermi’s 

pile in Argonne and build the pilot plant and 

separation facilities in Oak Ridge than to place 

these experiments in a populous area. On October 

3 DuPont agreed to design and build the chemical 

separation plant. Groves tried to entice further 

DuPont participation at Oak Ridge by having the 

firm prepare an appraisal of the pile project and 

by placing three DuPont staff members on the 

Lewis committee. Because DuPont was sensitive 

about its public image (the company was still 

smarting from charges that it profiteered during 

World War I), Groves ultimately obtained the 

services of the giant chemical company for the 

sum of one dollar over actual costs. In addition, 

DuPont vowed to stay out of the bomb business 

after the war and offered all patents to the United 

States government.

Groves had done well in convincing DuPont to 

join the Manhattan Project. DuPont’s proven 

administrative structure assured excellent 

coordination (Crawford Greenewalt was given 

the responsibility of coordinating DuPont and 

Met Lab planning), and Groves and Compton 

welcomed the company’s demand that it be 

put in full charge of the Oak Ridge plutonium 

project. DuPont had a strong organization and 

had studied every aspect of the Met Lab’s program 

thoroughly before accepting the assignment. 

While deeply involved in the overall war effort, 

DuPont expected to be able to divert personnel 

and other resources from explosives work in time 

to throw its full weight into the Oak Ridge project.

Moving the pilot plutonium plant to Oak Ridge 

left too little room for the full-scale production 

plant at the X-10 site and also left too little 

generating power for yet another major facility. 

Furthermore, the site was uncomfortably close to 

Knoxville should a catastrophe occur. Thus the 

search for an alternate location for the full-scale 

plutonium facility began soon after DuPont 

joined the production team. Compton’s scientists 

needed an area of approximately 225 square 

miles. Three or four piles and one or two chemical 

separation complexes would be at least a mile 

apart for security purposes, while nothing would 

be allowed within four miles of the separation 

complexes for fear of radioactive accidents. 

Towns, highways, rail lines, and laboratories 

would be several miles further away.

Hanford

December 16, 1942, found Colonel Franklin 

T. Matthias of Groves’s staff and two DuPont 

engineers headed for the Pacific Northwest 

and southern California to investigate possible 

production sites. Of the possible sites available, 

none had a better combination of isolation, long 

construction season, and abundant water for 

hydroelectric power than those found along the 

Columbia and Colorado Rivers. After viewing six 

locations in Washington, Oregon, and California, 

the group agreed that the area around Hanford, 

Washington, best met the criteria established by 

The Manhattan Engineer District in Operation

31
5

Hanford Engineer Works. Reprinted from Vincent C. Jones, Manhattan: The Army and the Atomic 

Bomb (Washington, D.C.: U.S. Government Printing Office, 1985).

Part IV

32
5

the Met Lab scientists and DuPont engineers. 

The Grand Coulee and Bonneville Dams offered 

substantial hydroelectric power, while the flat but 

rocky terrain would provide excellent support 

for the huge plutonium production buildings. 

The ample site of nearly one-half million acres 

was far enough inland to meet security requirements, while existing transportation facilities 

could quickly be improved and labor was readily 

available. Pleased with the committee’s unanimous 

report, Groves accepted its recommendation 

and authorized the establishment of the Hanford 

Engineer Works, codenamed Site W.

Now that DuPont would be building the 

plutonium production complex in the Northwest, 

Compton saw no reason for any pile facilities 

in Oak Ridge and proposed to conduct Met 

Lab research in either Chicago or Argonne. 

DuPont, on the other hand, continued to support 

a semiworks at Oak Ridge and asked the Met 

Lab scientists to operate it. Compton demurred 

on the grounds that he did not have sufficient 

technical staff, but he was also reluctant because 

his scientists complained that their laboratory 

was becoming little more than a subsidiary of 

DuPont. In the end, Compton knew the Met Lab 

would have to support DuPont, which simply 

did not have sufficient expertise to operate the 

semiworks on its own. The University of Chicago 

administration supported Compton’s decision in 

early March.

Pile Design: Changing Priorities

The fall 1942 planning sessions at the Met Lab 

led to the decision to build a second Fermi pile at 

Argonne as soon as his experiments on the first 

were completed and to proceed on design of the 

Mae West helium-cooled unit. 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 changed when Fermi’s calculations 

demonstrated a higher value for k than anyone 

had anticipated. Met Lab scientists concluded 

that a water-cooled pile was now feasible, while 

DuPont remained interested inhelium.” Since 

a helium-cooled unit shared important design 

characteristics with an air-cooled one, Greenewalt 

thought that an air-cooled semiworks at Oak 

Ridge would contribute significantly to designing 

the full-scale facilities at Hanford.

DuPont established the general specifications for 

the air-cooled semiworks and chemical separation 

facilities in early 1943. A massive graphite block, 

protected by several feet of concrete, would 

contain 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.

Met Lab activities focused on designing a watercooled pile for the full-scale plutonium plant. 

Taking their cue from the DuPont engineers, who 

utilized a horizontal design for the air-cooled 

semiworks, 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.

The Manhattan Engineer District in Operation

33
5

Decision on Pile Design

Greenewalt’s initial response to the water-cooled 

design 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 heliumcooled model. He feared that the compressors 

would not be ready in time for Hanford, 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 

Air-Cooled Pile Built in X-10 Area at Clinton. Reprinted from Richard G. Hewlett and Oscar E. Anderson, Jr., The New 

World, 1939-1946, Volume I of A History of the United States Atomic Energy Commission (University Park: Pennsylvania 

State University Press, 1962).

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 (CP1), hoping to determine the exact value of k. 

Subsequent experiments at the Argonne site using 

CP-2, built with material from CP-1, focused on 

neutron capture probabilities, control systems, 

Part IV

34
5

and instrument reliability. Once the production 

facilities at Oak Ridge and Hanford 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.

Decision on Chemical Extraction

While the Met Lab physicists chafed under 

DuPont domination, a smoother and quieter 

relationship existed between the chemists and 

DuPont. Seaborg and Cooper continued to work 

well together, and enough progress was made in 

the semiworks for the lanthanum fluoride process 

in late 1942 that DuPont moved into the plant 

design stage and converted the semiworks for the 

bismuth phosphate method. DuPont pressed for 

a decision on plutonium extraction methods in 

late May. Greenewalt chose bismuth phosphate, 

though even Seaborg admitted he could find little 

to distinguish between the two. Greenewalt based 

his decision on the corrosiveness of lanthanum 

fluoride and on Seaborg’s guarantee that he could 

extract at least fifty percent of the plutonium 

using bismuth phosphate. DuPont began 

constructing the chemical separation pilot plant at 

Oak Ridge, while Seaborg continued refining the 

bismuth phosphate method.

It was now Cooper’s job to design the pile as well 

as the plutonium extraction facilities at Clinton, 

both complicated engineering tasks made 

even more difficult by high levels of radiation 

produced by the process. Not only did Cooper 

have to oversee the design and fabrication of 

parts for yet another new Manhattan Project 

technology, he had to do so with an eye toward 

planning the Hanford facility. Safety was a major 

consideration because of the hazards of working 

with plutonium, which was highly radioactive. 

Uranium, a much less active element than 

plutonium, posed far fewer safety problems.

In July 1942, Compton set up a health division at 

the Met Lab and put Robert S. Stone in charge. 

Stone established emission standards and 

conducted experiments on radiation hazards, 

providing valuable planning information for the 

Oak Ridge and Hanford facilities.

Construction at Oak Ridge

DuPont broke ground for the X-10 complex at 

Oak Ridge in February 1943. The site would 

include an air-cooled experimental pile, a pilot 

chemical separation plant, and support facilities. 

Cooper produced blueprints for the chemical 

separation plants in time for construction to 

begin in March. A series of huge underground 

concrete cells, the first of which sat under the pile, 

extended to one story above ground. Aluminum 

cans containing uranium slugs would drop into 

the first cell of the chemical separation facility 

and dissolve and then go through the extraction 

Workers Loading Uranium Slug Into Face of Air-Cooled Pile. 

Reprinted from Richard G. Hewlett and Oscar E. Anderson, 

Jr., The New World, 1939-1946, Volume I of A History of the 

United States Atomic Energy Commission (University Park: 

Pennsylvania State University Press, 1962).

The Manhattan Engineer District in Operation

35
5

Aerial View of the town of Oak Ridge looking east

In September 1942, General Leslie Groves, U.S. Army Corps of Engineers, designated a 59,000-acre parcel 

of land between Black Oak Ridge to the north and the Clinch River to the south as the first federal reserve for 

producing nuclear materials for the atomic bomb. Framed by the foothills of the Appalachian Mountains, the site 

consisted of family farms and small communities. About 3,000 residents vacated their homes within weeks. 

One of them later observed: “What do you do? The government needed your land to win the war. Who could 

refuse such a request as that?”

The first phase of Oak Ridge community planning evolved early in 1943 for a town of around 13,000 residents. 

This and subsequent planning estimates proved too low. The peak population in September 1945 was 75,000, 

occupying 10,000 family dwelling units, 13,000 dormitory spaces, 5,000 trailers, and more than 16,000 hutment 

and barracks accommodations. From 1942 to 1949, Oak Ridge operated as a closed city on the northern edge of 

the reservation.

Manhattan Project

Photo Gallery Oak Ridge

36
5

Oak Ridge

Gallaher Ferry

Employees of the Manhattan Engineer District cross the Clinch River on the hand-pulled Gallaher Ferry in June 1943.

K-25 steel-frame construction

The gaseous diffusion plant, designed by the 

Kellex Corporation, bore the name K-25. Because 

the number 25 was used throughout the project to 

designate uranium-235, it was added to the title.

Y-12 Plant

As part of the wartime secrecy effort, the designation 

Y-12 given the electromagnetic plant had no meaning 

except to confuse enemy agents.

37
5

Oak Ridge

Y-12 Alpha Racetrack under construction X-10 Pile

Y-12 Alpha Racetrack

38
5

Oak Ridge

Y-12 worker at Beta-3 control panel

Management cautioned male co-workers 

not to talk to female Y-12 workers as they 

monitored the control panels.

Worker on lunch break

War workers were drawn from across the nation. 

Engineers and scientists, clerks and stenographers, technical personnel and general laborers 

poured into Oak Ridge to work on the project.

War workers commuting by bus

The strict security at Oak Ridge meant that 

commuters confined their conversations to 

lighter topics.

39
5

Oak Ridge

Army Post Exchange

Jackson Square

The town’s original shopping center.

Lining up for cigarettes at Williams Drug Store, 

Jackson Square

40
5

Oak Ridge

K-25 counterpart of “Rosie-the-Riveter” X-10 in operation

World’s first radioactive isotope produced for medical use, 

withdrawn from the Graphite Reactor, August 1946.

Y-12 control room

Many women graduates from East Tennessee high schools tended the Y-12 control panels. Each operator 

monitored two control panels.

41
5

Oak Ridge

Happy Valley service station

The isolation of the K-25 site forced the Jones 

Construction Company to build housing facilities for 

workers. Jones camp, nicknamed “Happy Valley” by its 

inhabitants, eventually had a total population of about 

15,000.

Boom Town

Living conditions were likened to a boom town 

atmosphere as families struggled to live with uncertain 

electricity and water supplies. One resident commented: 

“We weren’t born soon enough for the Gold Rush, but 

we made up for it at Oak Ridge.”

Trailer area

Trailers were obtained from the Federal Public Housing Authority. As operating and technical personnel moved 

into the area, more government trailers were obtained and accommodations provided for privately-owned trailers.

42
5

Oak Ridge

East Village area, 1945 Cemesto housing, 1944

Aerial view of trailer park, 1945

43
5

Oak Ridge

The “Jolly Old Elf” encounters tight 

security at Oak Ridge

Military police at the Elza Gate

Along the perimeter of Oak Ridge stood a 

settlement known as Elza. Here, the Manhattan 

Engineer District set up a guarded entrance to 

control access.

Little boy being badged

Heightened security at Oak Ridge pervaded 

every aspect of life. Even children wore identification badges.

44
5

Oak Ridge

Roller skating

The Oak Ridge Recreation and Welfare Association 

organized social and recreational activities including 

music, art, drama, dancing, and other events like 

roller skating.

Teen dance

Dancing was very popular. Segregation laws meant 

separate social and recreational events like this 

teen dance.

The “Rhythm Engineers” entertain

45
5

Oak Ridge

A photograph reveals the veil of secrecy at Oak Ridge

A disabled veteran addresses a bond rally at the K-25 plant and urges the workers to stay on the job. 

Army Intelligence cut the negative of this photograph taken December 13, 1944. The left section was published, but 

the right side, showing the gaseous diffusion plant, remained classified until after the war.

The isolation of East Tennessee made it ideally suited for the secrecy the Manhattan Project required. 

One K-25 worker recounted that a military policeman cautioned her that if she ever noticed an odor that she 

had never smelled before, she and her co-workers should flee immediately. The MP added that he could not 

tell her more.

Workers and their families accepted the security requirements at Oak Ridge as a routine part of daily living. 

“It’s all right, son,” one mother wrote, “if you can’t tell me what you’re doing in Oak Ridge, but I do hope 

it’s honorable.”

46
5

Oak Ridge

K-25 workers attend to the words of the speaker at the bond rally

The K-25 Plant was a major construction effort. When first built, the building was one of the largest roofed 

structures in the world, covering nearly 43 acres. Completed at a cost of $500 million and operated by 12,000 

workers, the K-25 plant separated uranium-235 from uranium-238.

Construction of the K-25, Y-12, and X-10 plants took place almost simultaneously in 1943. Recruiters were 

sent throughout the region since there was a shortage of workers. Machinists, plumbers, and other skilled 

workers found well-paying jobs. For the first time, many East Tennessee women had an opportunity to work 

outside the home.

47
5

Oak Ridge

“Sunday Punch”

K-25 workers pooled their wages for three Sundays to purchase a North American B-25 Mitchell bomber as a 

gift to the U.S. Army Air Forces. The workers were hourly workers, or “clockpunchers,” thus the name, “Sunday 

Punch.”

Thousands attended the christening of the plane at the Knoxville airport in April 1945. It was assigned to the 12th

Bomber Group, 81st Squadron, known as the Battering Rams. A member of the squadron, Tom Evans, a 1941 

graduate of Knoxville High School (where his father was principal) learned from a friend about the plane. The 

commander assigned the plane to Evans. Reminiscing years later, Evans said: “I was in my glory. I had my own 

airplane--and one bought by hometown folks.” 

48
5

Oak Ridge

Y-12 Workers at Shift Change

This photograph taken in August 1945 vividly demonstrates the role women played at Oak Ridge during the war. 

Early in May 1943, E.O. Lawrence visited Oak Ridge. He looked down on the Y-12 plant. On his return to Berkeley, 

he told his colleagues: “Just from the size of the thing, you can see that a thousand people would just be lost in this 

place, and we’ve got to make a definite attempt to just hire everybody in sight and somehow use them, because it’s 

going to be an awful job to get those racetracks into operation on schedule.”

The racetracks operated around the clock. Women who previously worked as clerks or waitresses found that they 

could work for the unheard of wage of seventy-five cents an hour. Word of the recruitment effort went out in local 

papers and friends told friends. Operators worked three shifts over a twenty-four hour period, rotating every seven 

days. By spring 1945, almost 22,000 workers were required to keep Y-12 in operation.

49
5

Oak Ridge

East Tennesseans celebrate the end of World War II

Relief and joy mingle on the faces of the celebrants at Oak Ridge. At war’s end, Oak Ridgers could at last talk about 

the project.

50
5

Oak Ridge

The first vote in the town of Oak Ridge, August 1946

Early in 1943, the Tennessee Legislature, alarmed by the growing number of Federal projects in the State and the 

subsequent loss of state lands and taxes from such lands, declined to cede sovereignty to the Federal government 

over the land taken over by the War Department for the Oak Ridge project. Oak Ridge residents came under the 

legal jurisdiction of Roane and Anderson Counties as well as the State of Tennessee. After a year’s residency, Oak 

Ridgers could vote in state and county elections.

51
5

Hanford Manhattan Project

Photo Gallery

Hanford Reach 

Of the potential sites investigated, none had a better 

combination of isolation, abundant water, and surplus 

hydroelectric power.

Wagon tracks date to pioneer days 

The earliest recorded immigration into the area was in 

1853, when settlers turned north from the Oregon trail 

to pass through the Yakima Valley.

Hanford

Five hundred sixty square miles of shrub steppe, sand, and sagebrush located on the Columbia River in 

southeastern Washington State. Selected to serve as the central location for the plutonium plant for the Manhattan 

Project, the site was given the name Hanford Engineer Works.

52
5

Hanford

Groves at Hanford

General Groves visited Hanford in early 1944 to 

speak with workers regarding safety issues. Originally 

the work week consisted of six 8-hour days, but by 

September 1943 it had expanded to six 9-hour days.

Hanford workers in the HC-11 bank

HC-2 hutments

Both men and women lived in (separate) 16’ by 40’ hutments in the Hanford Construction Camp. A total of 880 

units were built, each housing ten workers. 

53
5

Hanford

Aerial view of Richland 

Richland 

Temporary quarters for construction workers were built 

in Hanford while permanent facilities for other personnel 

were located down the road in Richland. Here a neighborhood of mixed housing styles takes shape.

Trailer park at the Hanford townsite

54
5

Hanford

Church

One of five churches built in nearby Richland for 

Hanford employees and their families. 

Library

This library served Hanford employees and their families.

Liberty Theatre at Pasco

The Liberty had a stage for live performances and a full basement for social events. At one end of the basement 

was a raised platform for the dance bands that often played there. Pasco was the oldest community in the Hanford 

area.

55
5

Hanford

All the comforts (almost) of home

A Hanford family relaxes on their patio. The pervasive construction excavations slit the fine, ashy soil in so many 

places that blowing dust and sand buried newly planted lawns, trees, and shrubs.

Shoe repair Sears Roebuck

56
5

Hanford

Waiting in line at the clothing store Barber shop

This family owned and operated shop is still in business 

today.

Sending money home

57
5

Hanford

Batter-up!

America’s favorite pastime was taken very seriously by Hanford workers, and the baseball games were a major 

social occasion.

Game plan Catcher

58
5

Hanford

The children of Hanford

Hanford had one of the highest birth rates in the nation. 

The rapidly expanding families at Manhattan Project sites gave General 

Groves concern for the unexpected burden on the medical facilities. 

One anonymous poet put it thus: 

The General’s in a stew

He trusted you and you

He thought you’d be scientific

Instead you’re just prolific

And what is he to do?

Santa pays a visit to Hanford Christmas 1943

59
5

Hanford

Lining up to buy war bonds

Day’s Pay

Hanford workers contributed a day’s pay to purchase a 

Boeing B-17 Flying Fortress that they presented to the 

U.S. Army Air Forces in July 1944. The bomber was 

appropriately named “Day’s Pay.”

Conserving for the war effort

60
5

Hanford

T Plant - world’s first large-scale plutonium separation facility

The chemical separation buildings, nicknamed “Queen Marys” because of their size and shape, are massive 

canyon-like structures, 800 feet long, 65 feet wide, and 80 feet high.

T Plant under construction

Facility was completed in December 1944. 

T Plant 

The white discharge from the stack was sulfur dioxide, 

a fogging agent used in early meteorological testing 

at Hanford.

61
5

Hanford

F Reactor looking north 

One of three reactors (along with B and D) built about 

six miles apart on the south bank of the Columbia 

River. 

U Plant

The third process separations facility, U-Plant, 

was identical to B (East Area) and to T (West Area).

Face of B Reactor under construction

B Reactor was the world’s first large scale plutonium production reactor. Built on a much larger scale than the X-10 

at Oak Ridge, the B Reactor used water as a coolant. 

62
5

Hanford

241-T tank farm under construction, early 1944

High-level radioactive wastes from the chemical separations plants were placed in the finished tanks.

184-B Power House under construction

Provided steam for heating and processing to the B 

Reactor and surrounding support facilities.

Columbia River Water Pumping Station

Furnished water to the Hanford and White Bluffs 

living areas.

63
5

Los Alamos

Security Patrol at Los Alamos

General Groves welcomed the isolation offered by Los Alamos-—high desert mesa cut by deep canyons into long 

finger-like extensions, populated by only a few homesteaders.

“The Big House” 

Once a dormitory and classroom for the 

Los Alamos Ranch School, this building housed 

some of the first scientists to arrive at Los Alamos 

in 1943.

Homestead cabin near Pajarito Site

64

Manhattan Project

Photo Gallery
5

Los Alamos

Main Hill Road, looking west, leading to Los Alamos

Early residents described the drive to Los Alamos as nerve-wracking.

Highway to the “secret city”

Road construction on State Highway 4, one of two 

alternative routes into Los Alamos, was begun in 

September 1943.

A bumpy ride

Winter snows and summer rains turned roads into 

muddy tracks.

65
5

Los Alamos

A resident shows his pass at the Main Gate

Life as you knew it ended at Project Y’s Main Gate 

where your identity was exchanged for a number.

Technical Area 1

Guards stationed at the Main Gate checked cars and 

passengers for proper identification. Technical area 

personnel went through another set of gates to enter 

the laboratory.

Los Alamos parking space

66
5

Los Alamos

Home sweet home

New arrivals to Project Y were struck by the eclectic housing mix: rows of barracks, apartments, Quonset huts, 

government trailers, and prefabricated units lined row upon row of nameless, unpaved streets.

Wartime general store Kitchen and dinette in a Morgan housing unit

Houses were supplied with modern refrigerators but had 

antiquated stoves, nicknamed Black Beauties. Many 

residents gave up trying to learn how to operate these 

“beauties” and used hotplates instead.

67
5

Los Alamos

Main Cafeteria, Fuller Lodge, 1946 Hanford-style housing unit, used as self-help 

laundry, 1945

Laundry day

Water, or the lack of it, was a constant problem. Soldiers hand delivered bulletins to Los Alamos residents with 

precise instructions on water use.

68
5

Los Alamos

Slotin criticality accident, Pajarito site 

Within a year of the Trinity test, Louis Slotin perished 

when a plutonium core started an unintended fission 

chain reaction. 

Ernest Lawrence, Enrico Fermi, and Isidor Rabi, 1945

Handle with care

Pulling a radioactive source from a storage building. Careful handling of large radioactive sources was 

accomplished by crude methods in the early days. There were a number of female technicians at Los Alamos.

69
5

Los Alamos

Trinity base camp

The base camp included barracks, a mess hall, warehouses, and an explosive magazine. 

At the time of the test, there were over 200 camp residents.

The road to Trinity

Military Police examine special credentials at an 

intersection designated Broadway and Pennsylvania 

Avenues at the Trinity test site.

McDonald ranch house, Trinity Site

The McDonald ranch house was used for the final 

assembly of the vital components.

70
5

Los Alamos

Jumbo’s desert journey

A specially designed 64-wheel trailer began the journey 

of carrying Jumbo across the desert from a nearby 

railroad siding to the Trinity Site. Originally intended to 

be a part of the Trinity test, the 200-ton container was 

eliminated during final planning.

South Yard Bunker

Three observation bunkers located 10,000 yards 

north, west, and south of the Trinity Tower were used 

to determine the symmetry of the implosion and the 

amount of energy released.

Valued Cargo

Vital components for the device are brought into the McDonald ranch house in a shock-proof case. 

71
5

Los Alamos

The Trinity fireball, 0.053 seconds after

detonation, as it shook the desert near

the town of San Antonio, New Mexico,

on July 16, 1945

The fast-rising incandescent cloud produced 

by the explosion illuminated the skies

15 seconds into detonation, the Trinity 

fireball rises into the air above the desert

“The whole country was lighted by a searing 

light with the intensity many times that of the 

midday sun,” recalled General Thomas Farrell.

72
5

Los Alamos

J. Robert Oppenheimer and General

Leslie Groves and the remains of the

Trinity Test Tower, September 1945 

The Tower Marker at Trinity

At war’s end, the Laboratory received the 

Army/Navy E (for excellence) award

73
5

process. The pile building went up during the 

spring and summer, a huge concrete shell seven 

feet thick with hundreds of holes for uranium 

slug placement. Slugs were to plutonium piles 

what barrier was to gaseous diffusion; that is, an 

obstacle that could shut down the entire process. 

The Aluminum Company of America (Alcoa) was 

the only firm left working on a process to enclose 

uranium-235 in aluminum sheaths, and it was 

still having problems. Initial production provided 

mixed results, with many cans failing vacuum 

tests because of faulty welds.

X-10 in Operation: Fall 1943

The moment everyone had been waiting for 

came in late October when DuPont completed 

Aerial View of Hanford Community. Reprinted from Richard G. Hewlett and Oscar E. Anderson, Jr., The New World, 1939-

1946, Volume I of A History of the United States Atomic Energy Commission (University Park: Pennsylvania State University 

Press, 1962).

construction and tests of the X-10 pile at Clinton 

Engineer Works. After thousands of slugs were 

loaded, the pile went critical in the early morning 

of November 4 and produced plutonium by the 

end of the month. Criticality was achieved with 

only half of the channels filled with uranium. 

During the next several months, Compton 

gradually raised the power level of the pile 

and increased its plutonium yield. Chemical 

separation techniques using the bismuth 

phosphate process were so successful that Los 

Alamos received plutonium samples beginning in 

the spring. Fission studies of these samples at Los 

Alamos during summer 1944 heavily influenced 

bomb design.

Part IV

74
5

Pile D at Hanford. Pile in Foreground, Water Treatment Plant in Rear. Reprinted from Richard G. Hewlett and Oscar E. 

Anderson, Jr., The New World, 1939-1946, Volume I of A History of the United States Atomic Energy Commission (University 

Park: Pennsylvania State University Press, 1962).

Hanford Takes Shape

Colonel Matthias returned to the Hanford area to 

set up a temporary office on February 22, 1943. 

His orders were to purchase half a million acres in 

and around the Hanford-Pasco-White Bluffs area, 

a sparsely populated region where sheep ranching 

and farming were the main economic activities. 

Many of the area’s landowners rejected initial 

offers on their land and took the Army to court 

seeking more acceptable appraisals. Matthias 

adopted a strategy of settling out of court to save 

time, time being a more important commodity 

than money to the Manhattan Project.

Matthias received his assignment in late March. 

The three water-cooled piles, designated by the 

letters B, D, and F, would be built about six miles 

apart on the south bank of the Columbia River. 

The four chemical separation plants, built in 

pairs, would be nearly ten miles south of the piles, 

while a facility to produce slugs and perform tests 

would be approximately twenty miles southeast of 

the separation plants near Richland. Temporary 

quarters for construction workers would be put 

up in Hanford, while permanent facilities for 

other personnel would be located down the road 

in Richland, safely removed from the production 

and separation plants.

During summer 1943, Hanford became the 

Manhattan Project’s newest atomic boomtown. 

Thousands of workers poured into the town, 

The Manhattan Engineer District in Operation

75
5

and install the top shield and two more months 

to wire and pipe the pile and connect it to the 

various monitoring and control devices.

At Hanford, irradiated uranium slugs would drop 

into water pools behind the piles and then be 

moved by remote-controlled rail cars to a storage 

facility five miles away for transportation to 

their final destination at one of the two chemical 

separation locations, designated 200-West and 

200-East. The T and U plants were located at 

200-West, while a single plant, the B unit, made 

up the 200-East complex (the planned fourth 

chemical separation plant was not built). The 

Hanford chemical separation facilities were 

massive scaled-up versions of those at Oak Ridge, 

each containing separation and concentration 

buildings in addition to ventilation (to eliminate 

radioactive and poisonous gases) and waste 

storage areas. Labor shortages and the lack of final 

blueprints forced DuPont to stop work on the 200 

areas in summer 1943 and concentrate its forces 

on 100-B, with the result that 1943 construction 

progress on chemical separation was limited to 

digging two huge holes in the ground.37

The Chemical Separation Buildings 

(Queen Marys)

Both 221T and 221U, the chemical separation 

buildings in the 200-West complex, were finished 

by December 1944. 221B, their counterpart 

in 200-East, was completed in spring 1945. 

Nicknamed Queen Marys by the workers 

who built them, the separation buildings were 

awesome canyon-like structures 800 feet long, 

65 feet wide, and 80 feet high containing forty 

process pools. The interior had an eerie quality as 

operators behind thick concrete shielding manipulated remote control equipment by looking 

through television monitors and periscopes from 

an upper gallery. Even with massive concrete 

lids on the process pools, precautions against 

radiation exposure were necessary and influenced 

all aspects of plant design.38

many of them to leave in discontent. Well situated 

from a logistical point of view, Hanford was a sea 

of tents and barracks where workers had little 

to do and nowhere to go. DuPont and the Army 

coordinated efforts to recruit laborers from all 

over the country for Hanford, but even with a 

relative labor surplus in the Pacific Northwest, 

shortages plagued the project. Conditions 

improved significantly during the second half 

of the year, with the addition of recreational 

facilities, higher pay, and better overall services 

for Hanford’s population, which reached 50,000 

by summer 1944. Hanford still resembled the 

frontier and mining towns once common in the 

west, but the rate of worker turnover dropped 

substantially.

Groundbreaking for the water-cooling plant for 

the 100-B pile, the westernmost of the three, took 

place on August 27, less than two weeks before 

Italy’s surrender to the Allies on September 8. 

Work on the pile itself began in February, with 

the base and shield being completed by mid-May. 

It took another month to place the graphite pile 

Chemical Separation Plant (Queen Mary) Under 

Construction at Hanford. Reprinted from Richard G. Hewlett 

and Oscar E. Anderson, Jr., The New World, 1939-1946, 

Volume I of A History of the United States Atomic Energy 

Commission (University Park: Pennsylvania State University 

Press, 1962).

Part IV

76
5

Completed Queen Mary at Hanford. Reprinted from Richard G. Hewlett and Oscar E. Anderson, Jr., The New World, 1939-

1946, Volume I of A History of the United States Atomic Energy Commission (University Park: Pennsylvania State University 

Press, 1962).

Construction of the chemical concentration 

buildings (224-T, -U, and -B) was a less daunting 

task because relatively little radioactivity was 

involved, and the work was not started until very 

late 1944. The 200-West units were finished in 

early October, the East unit in February 1945. In 

the Queen Marys, bismuth phosphate carried the 

plutonium through the long succession of process 

pools. The concentration stage was designed 

to separate the two chemicals. The normal 

relationship between pilot plant and production 

plant was realized when the Oak Ridge pilot plant 

reported that bismuth phosphate was not suitable 

for the concentration process but that Seaborg’s 

original choice, lanthanum fluoride, worked quite 

well. Hanford, accordingly, incorporated this 

suggestion into the concentration facilities. The 

final step in plutonium extraction was isolation, 

performed in a more typical laboratory setting 

with little radiation present. Here Perlman’s earlier 

research on the peroxide method paid off and 

was applied to produce pure plutonium nitrate. 

The nitrate would be converted to metal in Los 

Alamos, New Mexico.

Los Alamos

The final link in the Manhattan Project’s farflung 

network was the Los Alamos Scientific Laboratory 

in Los Alamos, New Mexico. The laboratory 

that designed and fabricated the first atomic 

bombs, codenamed Project Y, began to take 

shape in spring 1942 when Conant suggested to

Bush that the Office of Scientific and Research 

Development and the Army form a committee 

to study bomb development. Bush agreed and 

forwarded the recommendation to Vice President 

Wallace, Secretary of War Stimson, and General 

Marshall (the Top Policy Group). By the time of 

his appointment in late September, Groves had 

orders to set up a committee to study military 

applications of the bomb. Meanwhile, sentiment 

was growing among the Manhattan scientists 

that research on the bomb project needed to be 

better coordinated. Oppenheimer, among others, 

The Manhattan Engineer District in Operation

77
5

Los Alamos Site. Reprinted from Vincent C. Jones, Manhattan: The Army and the Atomic Bomb

(Washington, D.C.: U.S. Government Printing Office, 1985).

advocated a central facility where theoretical and 

experimental work could be conducted according 

to standard scientific protocols. This would insure 

accuracy and speed progress. Oppenheimer 

suggested that the bomb laboratory operate 

secretly in an isolated area but allow free exchange 

of ideas among the scientists on the staff. Groves 

accepted Oppenheimer’s suggestion and began 

seeking an appropriate location.

The search for a bomb laboratory site quickly 

narrowed to two places in northern New Mexico, 

Jemez Springs and the Los Alamos Boys Ranch 

School, locations Oppenheimer knew well since 

Part IV

78
5

he had a ranch nearby in the Pecos Valley of the 

Sangre de Cristo Mountains. In mid-November, 

Oppenheimer, Groves, Edwin M. McMillan, and 

Lieutenant Colonel W. H. Dudley visited the 

two sites and chose Los Alamos. Located on a 

mesa about thirty miles northwest of Santa Fe, 

Los Alamos was virtually inaccessible. It would 

have to be provided with better water and power 

facilities, but the laboratory community was 

not expected to be very large. The boys’ school 

occupying the site was eager to sell, and Groves 

was equally eager to buy. By the end of 1942 the 

district engineer in Albuquerque had orders 

to begin construction, and the University of 

California had contracted to provide supplies and 

personnel.

Oppenheimer and Groves

Oppenheimer, selected to head the new 

laboratory, proved to be an excellent director 

despite initial concerns about his administrative 

inexperience, leftist political sympathies, and lack 

of a Nobel Prize when several scientists he would 

be directing were prizewinners. Groves worked 

well with Oppenheimer although the two were 

fundamentally different in temperament. Groves 

was a practical-minded military man, brusque 

and goal-oriented. His aide, Colonel Nichols, 

characterized his heavyset boss as ruthless, 

egotistical, and confident, “the biggest S.O.B. I 

have ever worked for. He is most demanding. 

He is most critical. He is always a driver, never 

a praiser. He is abrasive and sarcastic.” Nichols 

admitted, however, that if he had it to do over 

again, he would once again “pick General Groves 

[as his boss]” because of his unquestioned 

ability.39 Groves demanded that the Manhattan 

Project scientists spend all their time on the bomb 

and resist the temptation, harmless enough in 

peacetime, to follow lines of research that had 

no direct applicability to immediate problems. 

In contrast to Groves, Oppenheimer was a 

philosophical man, attracted to Eastern mysticism 

and of a decidedly theoretical inclination and 

sensitive nature. A chainsmoker given to long 

working hours, Oppenheimer appeared almost 

emaciated. The Groves-Oppenheimer alliance, 

though not one of intimacy, was marked by 

mutual respect and was a major factor in the 

success of the Manhattan Project.

Oppenheimer insisted, with some success, 

that scientists at Los Alamos remain as much 

an academic community as possible, and he 

proved adept at satisfying the emotional and 

intellectual needs of his highly distinguished 

staff. Hans Bethe, head of the theoretical division, 

remembered that nobody else in that laboratory 

“even came close to him. In his knowledge. There 

was human warmth as well. Everybody certainly 

had the impression that Oppenheimer cared 

what each particular person was doing. In talking 

to someone he made it clear that that person’s 

work was important for the success of the whole 

project.”40

Groves and Oppenheimer. Reprinted from Leslie R. Groves, 

Now It Can Be Told (New York: Harper & Row, 1962).

The Manhattan Engineer District in Operation

79
5

Oppenheimer had a chance to display his 

persuasive abilities early when he had to convince 

scientists, many of them already deeply involved 

in war-related research in university laboratories, 

to join his new organization. Complicating his 

task were the early plans to operate Los Alamos 

as a military laboratory. Oppenheimer accepted 

Groves’s rationale for this arrangement but soon 

found that scientists objected to working as 

commissioned officers and feared that the military 

chain of command was ill-suited to scientific 

decision making. The issue came to a head when 

Oppenheimer tried to convince Robert F. Bacher 

and Isidor I. Rabi of the Massachusetts Institute 

of Technology’s Radiation Laboratory to join the 

Los Alamos team. Neither thought a military 

environment was conducive to scientific research. 

At Oppenheimer’s request, Conant and Groves 

wrote a letter explaining that the secret weaponrelated research had presidential authority and 

was of the utmost national importance. The 

letter promised that the laboratory would remain 

civilian through 1943, when it was believed that 

security would require militarization of the final 

stages of the project (in fact, militarization never 

took place). Oppenheimer would supervise all 

scientific work, and the military would maintain 

the post and provide security.

Recruiting the Staff

Oppenheimer spent the first three months of 

1943 tirelessly crisscrossing the country in an 

attempt to put together a first-rate staff, an effort 

that proved highly successful.41 Even Bacher 

signed on, though he promised to resign the 

moment militarization occurred; Rabi, though 

he did not move to Los Alamos, became a 

valuable consultant. As soon as Oppenheimer 

arrived at Los Alamos in mid-March, recruits 

began arriving from universities across the 

United States, including California, Minnesota, 

Chicago, Princeton, Stanford, Purdue, Columbia, 

Iowa State, and the Massachusetts Institute of 

Technology, while still others came from the 

Met Lab and the National Bureau of Standards. 

Virtually overnight Los Alamos became an ivory 

tower frontier boomtown, as scientists and their 

families, along with nuclear physics equipment, 

including two Van de Graaffs, a Cockroft-Walton 

accelerator, and a cyclotron, arrived caravan 

fashion at the Santa Fe railroad station and 

then made their way up to the mesa along the 

single primitive road. It was a most remarkable 

collection of talent and machinery that settled this 

remote outpost of the Manhattan Project.

Theory and the “Gadget”

The initial spartan environment of “the Hill” 

(which included box lunches and temporary 

housing) was without doubt quite a contrast to the 

comfortable campus settings so familiar to many 

on the staff. But the laboratory’s work began even 

as the Corps of Engineers struggled to provide 

the amenities of civilized life. The properties of 

uranium were reasonably well understood, those 

of plutonium less so, and knowledge of fission 

explosions entirely theoretical. That 2.2 secondary 

neutrons were produced when uranium-235 

fissioned was accepted, but while Seaborg’s 

team had proven in March 1941 that plutonium 

underwent neutron-induced fission, it was not 

known yet if plutonium released secondary 

neutrons during bombardment. The theoretical 

consensus was that chain reactions took place 

with sufficient speed to produce powerful releases 

of energy and not simply explosions of the critical 

mass itself, but only experiments could test the 

theory. The optimum size of the critical mass 

remained to be established, as did the optimum 

shape. When enough data were gathered to 

establish optimum critical mass, optimum 

effective mass still had to be determined. That is, 

it was not enough simply to start a chain reaction 

in a critical mass; it was necessary to start one in 

a mass that would release the greatest possible 

amount of energy before it was destroyed in the 

explosion.

In addition to calculations on uranium and 

plutonium fission, chain reactions, and critical 

Part IV

80
5

and effective masses, work needed to be done on 

the ordnance aspects of the bomb, or “gadget” as 

it came to be known. Two subcritical masses of 

fissionable material would have to come together 

to form a supercritical mass for an explosion to 

occur. Furthermore, they had to come together 

in a precise manner and at high speed. Measures 

also had to be taken to insure that the highly 

unstable subcritical masses did not predetonate 

because of spontaneously emitted neutrons or 

neutrons produced by alpha particles reacting 

with lightweight impurities. The chances of 

predetonation could be reduced by purification of 

the fissionable material and by using a high-speed 

firing system capable of achieving velocities of 

3,000 feet per second. A conventional artillery 

method of firing one subcritical mass into the 

other was under consideration for uranium-235, 

but this method would work for plutonium only 

if absolute purification of plutonium could be 

achieved.

A variation of the artillery method was designed 

for uranium. Bomb designers, unable to solve 

the purification problem, turned to the relatively 

unknown implosion method for plutonium. With 

implosion, symmetrical shockwaves directed 

inward would compress a subcritical mass of 

plutonium packed in a nickel casing (tamper), 

releasing neutrons and causing a chain reaction.

Always in the background loomed the hydrogen 

bomb, a thermonuclear device considerably more 

powerful than either a uranium or plutonium 

device but one that needed a nuclear fission bomb 

as a detonator. Research on the hydrogen bomb, 

or Super, was always a distant second in priority 

at Los Alamos, but Oppenheimer concluded that 

it was too important to ignore. After considerable 

thought, he gave Teller permission to devote 

himself to the Super. To make up for Teller’s 

absence, Rudolf Peierls, one of a group of British 

scientists who reinforced the Los Alamos staff 

at the beginning of 1944, was added to Bethe’s 

theory group in mid-1944. Another member of 

the British contingent was the Soviet agent Klaus 

Fuchs, who had been passing nuclear information 

to the Russians since 1942 and continued doing so 

until 1949 when he was caught and convicted of 

espionage (and subsequently exchanged).42

Another Lewis Committee

The first few months at Los Alamos were occupied 

with briefings on nuclear physics for the technical 

staff and with planning research priorities and 

organizing the laboratory. Groves called once 

again on Warren Lewis to head a committee, 

this time to evaluate the Los Alamos program. 

The committee’s recommendations resulted in 

the coordinated effort envisioned by those who 

advocated a unified laboratory for bomb research. 

Fermi took control of critical mass experiments 

and standardization of measurement techniques. 

Plutonium purification work, begun at the Met 

Lab, became high priority at Los Alamos, and 

increased attention was paid to metallurgy. The 

committee also recommended that an engineering 

division be organized to collaborate with 

physicists on bomb design and fabrication. The 

laboratory was thus organized into four divisions: 

theoretical (Hans A. Bethe); experimental physics 

(Robert F. Bacher); chemistry and metallurgy 

(Joseph W. Kennedy); and ordnance (Navy 

Captain William S. “Deke” Parsons). Like other 

Manhattan Project installations, Los Alamos soon 

began to expand beyond initial expectations.

As director, Oppenheimer shouldered burdens 

both large and small, including numerous 

mundane matters such as living quarters, mail 

censorship, salaries, promotions, and other 

“quality of life” issues inevitable in an intellectual 

pressure-cooker with few social amenities. 

Oppenheimer relied on a group of advisers to 

help him keep the “big picture” in focus, while a 

committee made up of Los Alamos group leaders 

provided day-to-day communications between 

divisions.

The Manhattan Engineer District in Operation

81
5

Early Progress

Early experiments on both uranium and 

plutonium provided welcome results. Uranium 

emitted neutrons in less than a billionth of a 

second—just enough time, in the world of nuclear 

physics, for an efficient explosion. Emilio Segrè 

later provided an additional cushion with his 

discovery in December 1943 that, if cosmic rays 

were eliminated, the subcritical uranium masses 

would not have to be brought together as quickly 

as previously thought; nor would the uranium 

have to be as pure. Muzzle velocity for the 

scaled-down artillery piece could be lower, and 

the gun could be shorter and lighter.43 Segrè’s tests 

on the first samples of plutonium demonstrated 

that plutonium emitted even more neutrons 

than uranium due to the spontaneous fission of 

plutonium-240. Both theory and experimental 

data now agreed that a bomb using either 

element would detonate if it could be designed 

and fabricated into the correct size and shape. 

But many details remained to be worked out, 

including calculations to determine how much 

uranium-235 or plutonium would be needed for 

an explosive device.

Bacher’s engineering division patiently generated 

the essential cross-sectional measurements 

needed to calculate critical and efficient mass. 

(The cross section is a measurement that indicates 

the probability of a nuclear reaction taking place.) 

The same group utilized particle accelerators to 

produce the large numbers of neutrons needed 

for its cross-sectional experiments. Bacher’s 

group also compiled data that helped identify 

tamper materials that would most effectively 

push neutrons back to the core and enhance the 

efficiency of the explosion. Despite Los Alamos’s 

postwar reputation as a mysterious retreat where 

brilliant scientists performed miracles of nuclear 

physics, much of the work that led to the atomic 

bombs was extremely tedious.

The chemists’ job was to purify the uranium-235 

and plutonium, reduce them to metals, and 

process the tamper material. Only highly purified 

uranium and plutonium would be safe from 

predetonation. Fortunately purification standards 

for uranium were relatively modest, and the 

chemical division was able to focus its effort on 

the lesser known plutonium and make substantial 

progress on a multi-step precipitation process 

by summer 1944. The metallurgy division had to 

turn the purified uranium-235 and plutonium 

into metal. Here, too, significant progress was 

made by summer as the metallurgists adapted a 

stationary-bomb technique initially developed at 

Iowa State University.

Parsons, in charge of ordnance engineering, 

directed his staff to design two artillery pieces 

of relatively standard specifications except for 

their extremely light barrels—one for a uranium 

weapon and one for a plutonium bomb. The 

weapons needed to achieve high velocities, but 

they would not have to be durable since they 

would only be fired once. Here again early efforts 

centered on the more problematic plutonium 

weapon, which required a higher velocity due to 

its higher risk of predetonation. Two plutonium 

guns arrived in March and were field-tested 

successfully. In the same month, two uranium 

guns were ordered.

Early Implosion Work

Parsons 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. Neddemeyer 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 

Part IV

82
5

critical material would be required, bombs could 

be ready earlier, and extreme purification of 

plutonium would be unnecessary. Von Neumann’s 

theories excited Oppenheimer, who assigned 

Parsons’s deputy, George B. Kistiakowsky, the 

task of perfecting implosion techniques. Because 

Parsons and Neddemeyer did not get along, it 

was Kistiakowsky who worked with the scientists 

on the implosion project. While experiments 

on implosion and explosion continued, Parsons 

directed much of his effort toward developing 

bomb hardware, including arming and wiring 

mechanisms and fuzing 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, named after Winston Churchill, 

was an implosion prototype. (Segrè’s lighter, 

smaller uranium gadget became Little Boy, Thin 

Man’s brother.)

Elimination of Thin Man

Thin Man was eliminated four months later 

because of the plutonium-240 contamination 

problem. 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 (the bullet 

and target in the plutonium weapon would melt 

before coming together). Measurements taken at 

Clinton confirmed the presence of plutonium-240 

in the plutonium produced in the experimental 

pile. On July 17 the difficult decision was made 

to cease work on the plutonium gun method. 

Plutonium could be used only in an implosion 

device, but in summer 1944 an implosion weapon 

looked like a long shot.

Abandonment of the plutonium gun project 

eliminated a shortcut to the bomb. This necessitated a revision of the estimates of weapon 

delivery Bush had given the President in 1943. 

The new timetable, presented to General Marshall 

by Groves on August 7, 1944—two months after 

the Allied invasion of France began at Normandy 

on June 6—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 bomb 

by August 1, 1945, and the delivery of one or two 

more by the end of that year. Marshall and Groves 

acknowledged that German surrender might take 

place by summer 1945, thus making it probable 

that Japan would be the target of any atomic 

bombs ready at that time.

Question Marks: Summer 1944

It was still unclear if even the August 1 deadline 

could be met. While expenditures reached 

Section of S-50 Liquid Thermal Diffusion Plant at Clinton. 

Reprinted from Richard G. Hewlett and Oscar E. Anderson, 

Jr., The New World, 1939-1946, Volume I of A History of the 

United States Atomic Energy Commission (University Park: 

Pennsylvania State University Press, 1962).

The Manhattan Engineer District in Operation

83
5

$100 million per month by mid-1944, the 

Manhattan Project’s goal of producing weapons 

for the current war was not assured. Operational 

problems plagued the Y-12 electromagnetic 

facility just coming on line. The K-25 gaseous 

diffusion plant threatened to become an expensive 

white elephant if suitable barriers could not be 

fabricated. And the Hanford piles and separation 

facilities faced an equally serious threat as not 

enough of the uranium-containing slugs to 

feed the pile were available. Even assuming that 

enough uranium or plutonium could be delivered 

by the production facilities built in such great 

haste, there was no guarantee that the Los Alamos 

laboratory would be able to design and fabricate 

weapons in time. Only the most optimistic in 

the Manhattan Project would have predicted, 

as Groves did when he met with Marshall, that 

a bomb or bombs powerful enough to make a 

difference in the current war would be ready by 

August 1, 1945.

Progress at Oak Ridge

During winter 1944-45, there was substantial 

progress at Oak Ridge, thanks to improved 

performance in each of the production facilities 

and Nichols’s work in coordinating a complicated 

feed schedule that maximized output of enriched 

uranium by utilizing the electromagnetic, thermal 

diffusion, and gaseous diffusion processes in 

tandem. Nine Alpha and three Beta racetracks 

were operational and, while not producing up 

to design potential, were becoming significantly 

more reliable because of maintenance improvements and chemical refinements introduced by 

Tennessee Eastman. The S-50 thermal diffusion 

plant being built by the H. K. Ferguson Company 

was almost complete and was producing small 

amounts of enriched material in the finished 

racks, and the K-25 gaseous diffusion plant, 

complete with barriers, was undergoing final 

leak tests. By March 1945, Union Carbide had 

worked out most of the kinks in K-25 and had 

started recycling uranium hexafluoride through 

the system. S-50 was finished at the same time 

that the Y-12 racetracks were demonstrating 

increased efficiency. The Beta calutrons at the 

electromagnetic plant were producing weapongrade uranium-235 using feed from the modified 

Alpha racetracks and the small output from the 

gaseous diffusion and thermal diffusion facilities. 

Oak Ridge was now sending enough enriched 

uranium-235 to Los Alamos to meet experimental 

needs. To increase production, Groves proposed 

an additional gaseous diffusion plant (K-27) for 

low-level enrichment and a fourth Beta track for 

high-level enrichment, both to be completed by 

February 1946, in time to contribute to the war 

against Japan, which many thought would not 

conclude before summer 1946.

Hanford’s Role

With the abandonment of the plutonium gun 

bomb in July 1944, planning at Hanford became 

more complicated. Pile 100-B was almost 

complete, as was the first chemical separation 

plant, while pile D was at the halfway point. Pile 

F was not yet under construction. If implosion 

devices using plutonium could be developed 

at Los Alamos, the three piles would probably 

produce enough plutonium for the weapons 

required, but as yet no one was sure of the amount 

needed.

Pile Operation

Excitement mounted at Hanford as the date for 

pile start-up approached. Fermi placed the first 

slug in pile 100-B on September 13, 1944. Final 

checks on the pile had been uneventful. The 

scientists could only hope they were accurate, 

since once the pile was operational the intense 

radioactivity would make maintenance of many 

components impossible. Loading slugs and 

taking measurements took two weeks. From just 

after midnight until approximately 3:00 a.m. on 

September 27, the pile ran without incident at 

a power level higher than any previous chain 

reaction (though only at a fraction of design 

capacity). The operators were elated, but their 

excitement turned to astonishment when the 

Part IV

84
5

power level began falling after three hours. It 

fell continuously until the pile ceased operating 

entirely on the evening of the 28th. By the next 

morning the reaction began again, reached the 

previous day’s level, then dropped.

Xenon Poisoning

Hanford scientists were at a loss to explain 

the pile’s failure to maintain a chain reaction. 

Only the foresight of DuPont’s engineers made 

it possible to resolve the crisis. The cause of 

the strange phenomenon proved to be xenon 

poisoning. Xenon, a fission product isotope with 

a mass of 135, was produced as the pile operated. 

It captured neutrons faster than the pile could 

produce them, causing a gradual shutdown. With 

shutdown, the xenon decayed, neutron flow 

began, and the pile started up again. Fortuitously, 

despite the objections of some scientists who 

complained of DuPont’s excessive caution, the 

company had installed a large number of extra 

tubes. This design feature meant that pile 100-B 

could be expanded to reach a power level sufficient to overwhelm the xenon poisoning. Success 

was achieved when the first irradiated slugs were 

discharged from pile 100-B on Christmas Day, 

1944. The irradiated slugs, after several weeks 

of storage, went to the chemical separation and 

concentration facilities. By the end of January 

1945, the highly purified plutonium underwent 

further concentration in the completed chemical 

isolation building, where remaining impurities 

were removed successfully. Los Alamos received 

its first plutonium on February 2.44

Reorganizing for the Final Push

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; 

George Kistiakowsky led X Division (for explosives) in work on the explosive components; Hans 

Bethe continued to head up theoretical studies; 

and “Deke” Parsons now focused on overall bomb 

construction and delivery.

Field tests performed with uranium-235 prototypes in late 1944 eased doubts about the artillery 

method to be employed in the uranium bomb. It 

was clear that the uranium-235 from Oak Ridge 

would 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 would not fit into wartime planning 

unless a breakthrough in implosion technology 

occurred.

At the same time, Los Alamos shifted from 

research to development and production. Time 

was of the essence, though laboratory research 

had not yet charted a clear path to the final 

product. Army Air Force training could wait 

no longer, and in September at Wendover Field 

in western Utah, Colonel Paul Tibbets began 

drilling the 393rd Bombardment Squadron, 

the heart of the 509th Composite Wing, in test 

drops with 5,500-pound orange dummy bombs, 

nicknamed pumpkins. In June 1945, Tibbets 

and his command moved to Tinian Island in the 

Marianas, where the Navy SeaBees had built the 

world’s largest airport to accommodate Boeing’s 

new B-29 Superfortresses.

Taking Care of Business

Personnel shortages, particularly of physicists, 

and supply problems complicated Oppenheimer’s 

task. The procurement system, designed to 

protect the secrecy of the Los Alamos project, led 

to frustrating delays and, when combined with 

persistent late war shortages, proved a constant 

headache. The lack of contact between the remote 

The Manhattan Engineer District in Operation

85
5

laboratory and its supply sources exacerbated the 

problem, as did the relative lack of experience the 

academic scientists had with logistical matters.

Groves and Conant were determined not to let 

mundane problems compromise the bomb effort, 

and in fall 1944 they made several changes to 

prevent this possibility. Conant shipped as many 

scientists as could be spared from Chicago and 

Oak Ridge to Los Alamos, hired every civilian 

machinist he could lay his hands on, and arranged 

for Army enlisted men to supplement the work 

force (these GIs were known as SEDs, for Special 

Engineering Detachment). Hartley Rowe, an 

experienced industrial engineer, provided help in 

easing the transition from research to production. 

Los Alamos also arranged for a rocket research 

team at the California Institute of Technology to 

aid in procurement, test fuzes, and contribute to 

component development. These changes kept Los 

Alamos on track as weapon design reached its 

final stages.

Freezing Weapon Design

Weapon design for the uranium gun bomb was 

frozen in February 1945. Confidence in the 

weapon was high enough that a test prior to 

combat use was seen as unnecessary. The design 

for an implosion device was approved in March 

with a test of the more problematic plutonium 

weapon scheduled for July 4. Oppenheimer 

shifted the laboratory into high gear and 

assigned Allison, Bacher, and Kistiakowsky to the 

Cowpuncher Committee to “ride herd” on the 

implosion weapon. He placed Kenneth T. BainPart IV

bridge in charge of Project Trinity, a new division 

to oversee the July test firing. Parsons headed 

Project Alberta, known as Project A, which had 

the responsibility for preparing and delivering 

weapons for combat.

During these critical months, much depended 

upon the ability of the chemists and metallurgists 

to process the uranium and plutonium into 

metals and craft them into the correct shape and 

size. Plutonium posed by far the greater obstacle. 

It existed in different states, depending upon 

temperature, and was extremely toxic. Working 

under intense pressure, the chemists and metallurgists managed to develop precise techniques 

for processing plutonium just before it arrived in 

quantity beginning in May.

As a result of progress at Oak Ridge and metallurgical and chemical refinements on plutonium 

that improved implosion’s chances, the nine 

months between July 1944 and April 1945 saw the 

American bomb project progress from doubtful 

to probable. The August 1 delivery date for the 

Little Boy uranium bomb certainly appeared more 

likely than it had when Groves briefed Marshall. 

There would be no implosion weapons in the first 

half of 1945 as Groves had hoped, but developments in April boded well for the scheduled 

summer test of the Fat Man plutonium bomb. 

And recent calculations provided by Bethe’s 

theoretical group gave hope that the yield for the 

first weapon would be in the vicinity of 5,000 

tons of TNT rather than the 1,000-ton estimate 

provided in fall 1944.

86
875

Part V:

The Atomic Bomb and 

American Strategy

With the Manhattan Project on the brink of 

success in spring 1945, the atomic bomb became 

an increasingly important element in American 

strategy. A long hoped-for weapon now seemed 

within reach at a time when hard decisions were 

being made, not only on ending the war in the 

Pacific, but also on the shape of the postwar 

international order.

From Roosevelt to Truman

On April 12, only weeks before Germany’s 

unconditional surrender on May 7, President 

Roosevelt died suddenly in Warm Springs, 

Georgia, bringing Vice President Harry S. 

Truman, a veteran of the United States Senate, to 

the presidency. Truman was not privy to many of 

the secret war efforts Roosevelt had undertaken 

and had to be briefed extensively in his first 

weeks in office. One of these briefings, provided 

by Secretary of War Stimson on April 25, 

concerned S-l (the Manhattan Project). Stimson, 

with Groves present during part of the meeting, 

traced the history of the Manhattan Project, 

summarized its status, and detailed the timetable 

for testing and combat delivery. Truman asked 

numerous questions during the forty-five-minute 

meeting and made it clear that he understood 

the relevance of the atomic bomb to upcoming 

diplomatic and military initiatives.

By the time Truman took office, Japan was near 

defeat. American aircraft were attacking Japanese 

cities at will. A single firebomb raid in March 

killed nearly 100,000 people and injured over a 

million in Tokyo. A second air attack on Tokyo 

in May killed 83,000. Meanwhile, the United 

States Navy had cut the islands’ supply lines. But 

because of the generally accepted view that the 

Japanese would fight to the bitter end, a costly 

invasion of the home islands seemed likely, 

though some American policy makers held that 

successful combat delivery of one or more atomic 

bombs might convince the Japanese that further 

resistance was futile.

The Interim Committee Report

On June 6, Stimson again briefed Truman on S-l. 

The briefing summarized the consensus of an 

Interim Committee meeting held on May 31. The 

Interim Committee was an advisory group on 

atomic research composed of Bush, Conant, Karl 

Compton, Under Secretary of the Navy Ralph 

A. Bard, Assistant Secretary of State William L. 

Clayton, and future Secretary of State James F. 

Byrnes. Oppenheimer, Fermi, Arthur Compton, 

and Lawrence served as scientific advisors (the 

Scientific Panel), while Marshall represented 

the military. A second meeting on June 1 with 

Walter S. Carpenter of DuPont, James C. White 

of Tennessee Eastman, George H. Bucher of 

Westinghouse, and James A. Rafferty of Union 

Carbide provided input from the business side. 

The Interim Committee was charged with recommending the proper use of atomic weapons in 

wartime and developing a position for the United 

States on postwar atomic policy. The May 31 

meeting concluded that the United States should 

try to retain superiority of nuclear weapons in 

case international relations deteriorated.45 Most 

present at the meeting thought that the United 

States should protect its monopoly for the present, 

though they conceded that the secrets could not 

be held long. It was only a matter of time before 

another country, presumably Russia, would be 

capable of producing atomic weapons.

There was some discussion of free exchange of 

nuclear research for peaceful purposes and the 
5

Part V

Trinity Test Site. Reprinted from Vincent C. Jones, Manhattan: The Army and the Atomic Bomb (Washington, D.C.: 

U.S. Government Printing Office, 1985).

88
5

The Atomic Bomb and American Strategy

89

international inspection system that such an 

exchange would require. Lawrence’s suggestion 

that a demonstration of the atomic bomb might 

possibly convince the Japanese to surrender was 

discussed over lunch and rejected. The bomb 

might be a dud, the Japanese might put American 

prisoners of war in the area, or shoot down the 

plane, and the shock value of the new weapon 

would be lost. These reasons and others convinced 

the group that the bomb should be dropped 

without warning on a dual target—a war plant 

surrounded by workers’ homes. The meeting with 

the industrialists on June 1 further convinced the 

Interim Committee that the United States had a 

lead of three to ten years on the Soviet Union in 

production facilities for bomb fabrication.

On June 6, Stimson informed the President that 

the Interim Committee recommended keeping 

S-l a secret until Japan had been bombed. The 

attack should take place as soon as possible and 

without warning. Truman and Stimson agreed 

that the President would stall if approached about 

atomic weapons in Berlin, but that it might be 

possible to gain concessions from Russia later 

in return for providing technical information. 

Stimson told Truman that the Interim Committee 

was considering domestic legislation and that 

its members generally held the position that 

international agreements should be made in 

which all nuclear research would be made public 

and a system of inspections would be devised. 

In case international agreements were not 

forthcoming, the United States should continue to 

produce as much fissionable material as possible 

to take advantage of its current position of 

superiority.

Planning for Surrender

Strategies for forcing Japanese capitulation 

occupied center stage in June. Truman gained 

Chinese concurrence in the Yalta agreements by 

assuring T. V. Soong, the Chinese foreign minister, 

that Russia’s intentions in the Far East were 

benevolent, smoothing the way for the entrance 

of the Red Army. Joseph C. Grew, acting secretary 

of state, clarified the definition of unconditional 

surrender. Japan need not fear total annihilation, 

Grew stated. Once demilitarized, Japan would 

be free to choose its political system and would 

be allowed to develop a vibrant economy. Grew 

hoped that a public statement to Japan would 

lead to surrender before a costly invasion would 

have to be launched. The Joint Chiefs of Staff 

continued to advocate the invasion of Kyushu, a 

plan identified as Operation Olympic. Stimson 

hoped that an invasion could be avoided, either 

by redefining the surrender terms or by using the 

atomic bomb.

Indicative of the wide range of his responsibilities 

was Groves’s position as head of a bomb 

target selection group set up in late April, a 

responsibility he shared with General Thomas 

Farrell, appointed Groves’s military aide in 

February 1945. In late May, the committee of 

scientists and Army Air Force officers listed 

Kokura Arsenal, Hiroshima, Niigata, and Kyoto 

as the four best targets, believing that attacks on 

these cities—none of which had yet been bombed 

by Curtis LeMay’s Twentieth Air Force (which 

planned to eliminate all major Japanese cities 

by January 1, 1946)—would make a profound 

psychological impression on the Japanese and 

weaken military resistance. Stimson vetoed 

Kyoto, Japan’s most cherished cultural center, 

and Nagasaki replaced the ancient capital in the 

directive issued to the Army Air Force on July 

25.46

The Franck Report and Its Critics

Meanwhile the Met Lab was beginning to stir. 

The Scientific Panel of the Interim Committee 

was the connection between the scientists and 

the policy makers, and Compton was convinced 

that there must be a high level of participation in 

the decision-making process. His June 2 briefing 

of the Met Lab staff regarding the findings of the 

Interim Committee led to a flurry of activity. The 

Met Lab’s Committee on the Social and Political 
5

Part V

Tower For Trinity Test. Department of Energy.

Implications of the Atomic Bomb, chaired by 

James Franck and including Seaborg and Szilard, 

issued a report advocating international control 

of atomic power as the only way to stop the arms 

race that would be inevitable if the United States 

bombed Japan without first demonstrating the 

weapon in an uninhabited area.

The Scientific Panel disagreed with the Franck 

Report, as the Met Lab study was known, and 

concluded that no technical test would convince 

Japan to surrender. The Panel concluded that 

such a military demonstration of the bomb 

might best further the cause of peace but held 

that such a demonstration should take place only 

after the United States informed its allies. On 

June 21, the Interim Committee sided with the 

position advanced by the Scientific Panel. The 

bomb should be used as soon as possible, without 

warning, and against a war plant surrounded 

by additional buildings. As to informing allies, 

the Committee concluded that Truman should 

mention that the United States was preparing to 

use a new kind of weapon against Japan when he 

went to Berlin in July. On July 2, 1945, President 

Truman listened as Stimson outlined the peace 

terms for Japan, including demilitarization and 

prosecution of war criminals in exchange for 

economic and governmental freedom of choice. 

Stimson returned on July 3 and suggested that 

Truman broach the issue of the bomb with Stalin 

and tell the Soviet leader that it could become a 

force for peace with proper agreements.47

Trinity Device Being Readied. Reprinted from Richard G. 

Hewlett and Oscar E. Anderson, Jr., The New World, 1939-

1946, Volume I of A History of the United States Atomic 

Energy Commission (University Park: Pennsylvania State 

University Press, 1962).

90
5

The Atomic Bomb and American Strategy

91

The Trinity Test

Meanwhile, the test of the plutonium weapon, 

named Trinity by Oppenheimer (a name inspired 

by the poems of John Donne), was rescheduled 

for July 16 at a barren site on the Alamogordo 

Bombing Range known as the Jornada del 

Muerto, or Journey of Death, 210 miles south of 

Los Alamos. A test explosion had been conducted 

on May 7 with a small amount of fissionable 

material to check procedures and fine-tune 

equipment. Preparations continued through May 

and June and were complete by the beginning of 

July. Three observation bunkers located 10,000 

yards north, west, and south of the firing tower 

at ground zero would attempt to measure critical 

aspects of the reaction. Specifically, scientists 

would try to determine the symmetry of the 

implosion and the amount of energy released. 

Additional measurements would be taken to 

determine damage estimates, and equipment 

would record the behavior of the fireball. The 

biggest concern was control of the radioactivity 

the test device would release. Not entirely content 

to trust favorable meteorological conditions to 

carry the radioactivity into the upper atmosphere, 

the Army stood ready to evacuate the people in 

surrounding areas.

Remains of Trinity Test Tower Footings. Oppenheimer and 

Groves at Center. Department of Energy.

On July 12, the plutonium core was taken to 

the test area in an army sedan. The non-nuclear 

components left for the test site at 12:01 a.m., 

Friday the 13th. During the day on the 13th, 

final assembly of the gadget took place in the 

McDonald ranch house. By 5:00 p.m. on the 15th, 

the device had been assembled and hoisted atop 

the one-hundred-foot firing tower. Groves, Bush, 

Conant, Lawrence, Farrell, Chadwick (head of the 

British contingent at Los Alamos and discoverer 

of the neutron), and others arrived in the test 

area, where it was pouring rain. Groves and 

Oppenheimer, standing at the S-10,000 control 

bunker, discussed what to do if the weather did 

not break in time for the scheduled 4:00 a.m. 

test. At 3:30 they pushed the time back to 5:30; 

at 4:00 the rain stopped. Kistiakowsky and his 

team armed the device shortly after 5:00 a.m. 

and retreated to S-10,000. In accordance with his 

policy that each observe from different locations 

in case of an accident, Groves left Oppenheimer 

and joined Bush and Conant at base camp. Those 

in shelters heard the countdown over the public 

address system, while observers at base camp 

picked it up on an FM radio signal.48

The Dawn of the Atomic Age

At precisely 5:30 a.m. on Monday, July 16, 1945, 

the atomic age began. While Manhattan staff 

members watched anxiously, the device exploded 

over the New Mexico desert, vaporizing the tower 

and turning asphalt around the base of the tower 

to green sand. The bomb released approximately 

18.6 kilotons of power, and the New Mexico sky 

was suddenly brighter than many suns. Some 

observers suffered temporary blindness even 

though they looked at the brilliant light through 

smoked glass. Seconds after the explosion came a 

huge blast, sending searing heat across the desert 

and knocking some observers to the ground. A 

steel container weighing over 200 tons, standing 

a half-mile from ground zero, was knocked ajar. 

(Nicknamed Jumbo, the huge container had been 

ordered for the plutonium test and transported to 

the test site but eliminated during final planning.) 
5

Part V

As the orange and yellow fireball stretched up 

and spread, a second column, narrower than the 

first, rose and flattened into a mushroom shape, 

thus providing the atomic age with a visual 

image that has become imprinted on the human 

consciousness as a symbol of power and awesome 

destruction.49

At base camp, Bush, Conant, and Groves shook 

hands. Oppenheimer reported later that the 

experience called to his mind the legend of 

Prometheus, punished by Zeus for giving man 

fire. He also thought fleetingly of Alfred Nobel’s 

vain hope that dynamite would end wars. The 

terrifying destructive power of atomic weapons 

and the uses to which they might be put were to 

haunt many of the Manhattan Project scientists 

for the remainder of their lives.50

The success of the Trinity test meant that a 

second type of atomic bomb could be readied 

for use against Japan. In addition to the uranium 

gun model, which was not tested prior to being 

used in combat, the plutonium implosion device 

detonated at Trinity now figured in American 

Far Eastern strategy. In the end Little Boy, the 

untested uranium bomb, was dropped first 

at Hiroshima on August 6, 1945, while the 

plutonium weapon Fat Man followed three days 

later at Nagasaki on August 9.

Potsdam

The American contingent to the Big Three 

conference, headed by Truman, Byrnes, and 

Stimson, arrived in Berlin on July 15 and spent 

most of the next two days grappling with the 

interrelated issues of Russian participation in 

the Far Eastern conflict and the wording of an 

early surrender offer that might be presented 

to the Japanese. This draft surrender document 

received considerable attention, the sticking point 

being the term “unconditional.” It was clear that 

the Japanese would fight on rather than accept 

terms that would eliminate the Imperial House 

or demean the warrior tradition, but American 

policy makers feared that anything less than 

a more democratic political system and total 

demilitarization might lead to Japanese aggression 

in the future. Much effort went into finding the 

precise formula that would satisfy American war 

aims in the Pacific without requiring a costly 

invasion of the Japanese mainland. In an attempt 

to achieve surrender with honor, the emperor 

had instructed his ministers to open negotiations 

with Russia. The United States intercepted and 

decoded messages between Tokyo and Moscow 

that made it unmistakably clear that the Japanese 

were searching for an alternative to unconditional 

surrender.

Reports on Trinity

Stalin arrived in Berlin a day late, leaving 

Stimson July 16 to mull over questions of postwar 

German administration and the Far Eastern 

situation. After sending Truman and Byrnes a 

memorandum advocating an early warning to 

Japan and setting out a bargaining strategy for 

Russian entry in the Pacific war, Stimson received 

a cable from George L. Harrison, his special 

consultant in Washington, that read:

 Operated on this morning. Diagnosis not yet 

 complete but results seem satisfactory and 

 already exceed expectations. Local press

 release necessary as interest extends great 

 distance. Dr. Groves pleased. He returns 

 tomorrow. I will keep you posted.51 

Stimson immediately informed Truman and 

Byrnes that the Trinity test had been successful. 

The next day Stimson informed Churchill of the 

test. The prime minister expressed great delight 

and argued forcefully against informing the 

Russians, though he later relented. On July 18, 

while debate continued over the wording of the 

surrender message, focusing on whether or not 

to guarantee the place of the emperor, Stimson 

received a second cable from Harrison: 

92
5

The Atomic Bomb and American Strategy

93

 Doctor has just returned most enthusiastic 

 and confident that the little boy is as husky 

 as his big brother. The light in his eyes 

 discernible from here to Highhold and I 

 could have heard his screams from here to 

 my farm.52

Translation: Groves thought the plutonium 

weapon would be as powerful as the uranium 

device and that the Trinity test could be seen as 

far away as 250 miles and the noise heard for 

fifty miles. Initial measurements taken at the 

Alamogordo site suggested a yield in excess of 

5,000 tons of TNT. Truman went back to the 

bargaining table with a new card in his hand.

Further information on the Trinity test 

arrived on July 21 in the form of a long and 

uncharacteristically excited report from Groves’s. 

Los Alamos scientists now agreed that the blast 

had been the equivalent of between 15,000 and 

20,000 tons of TNT, higher than anyone had 

predicted. Groves reported that glass shattered 

125 miles away, that the fireball was brighter than 

several suns at midday, and that the steel tower 

had been vaporized. Though he had previously 

believed it impregnable, Groves stated that he 

did not consider the Pentagon safe from atomic 

attack.53 Stimson informed Marshall and then 

read the entire report to Truman and Byrnes. 

Stimson recorded that Truman was “tremendously 

pepped up” and that the document gave him an 

entirely new feeling of confidence.54 The next 

day Stimson, informed that the uranium bomb 

would be ready in early August, discussed Grove’s 

report at great length with Churchill. The British 

prime minister was elated and said that he now 

understood why Truman had been so forceful 

with Stalin the previous day, especially in his 

opposition to Russian designs on Eastern Europe 

and Germany. Churchill then told Truman that 

the bomb could lead to Japanese surrender 

without an invasion and eliminate the necessity 

for Russian military help. He recommended that 

the President continue to take a hard line with 

Stalin. Truman and his advisors shared Churchill’s 

views. The success of the Trinity test stiffened 

Truman’s resolve, and he refused to accede to 

Stalin’s new demands for concessions in Turkey 

and the Mediterranean.

On July 24, Stimson met with Truman. He 

told the President that Marshall no longer saw 

any need for Russian help, and he briefed the 

President on the latest S-l situation. The uranium 

bomb might be ready as early as August 1 and 

was a certainty by August 10. The plutonium 

weapon would be available by August 6. 

Stimson continued to favor making some sort of 

commitment to the Japanese emperor, though the 

draft already shown to the Chinese was silent on 

this issue.

Truman Informs Stalin

American and British coordination for an 

invasion of Japan continued, with November 1 

standing as the landing date. At a meeting with 

American and British military strategists at 

Potsdam, the Russians reported that their troops 

were moving into the Far East and could enter 

the war in mid-August. They would drive the 

Japanese out of Manchuria and withdraw at the 

end of hostilities. Nothing was said about the 

bomb. This was left for Truman, who, on the 

evening of July 24, approached Stalin without an 

interpreter to inform the Generalissimo that the 

United States had a new and powerful weapon. 

Stalin casually responded that he hoped that it 

would be used against Japan to good effect. The 

reason for Stalin’s composure became clear later 

when it was learned that Russian intelligence had 

been receiving information about the S-l project 

from Klaus Fuchs and other agents since summer 

1942.

The Potsdam Proclamation

A directive, written by Groves and issued by 

Stimson and Marshall on July 25, ordered the 

Army Air Force’s 509th Composite Group to 

attack Hiroshima, Kokura, Niigata, or Nagasaki 
5

Part V

Atomic Bombing of Japan. Reprinted from Vincent C. Jones, Manhattan: The Army and the Atomic Bomb (Washington, 

D.C.: U.S. Government Printing Office, 1985).

94
5

The Atomic Bomb and American Strategy

95

“after about” August 3, or as soon as weather 

permitted.55 The 509th was ready. Tests with 

dummies had been conducted successfully, and 

Operation Bronx, which brought the gun and 

uranium-235 projectile to Tinian aboard the 

U.S.S. Indianapolis and the other components on 

three C-54s, was complete. On July 26, the United 

States learned of Churchill’s electoral defeat and 

Chiang Kai-Shek’s concurrence in the warning 

to Japan. Within hours the warning was issued 

in the name of the President of the United States, 

the president of China, and the prime minister of 

Great Britain (now Clement Attlee). The Russians 

were not informed in advance. This procedure 

was technically correct since the Russians 

were not at war with Japan, but it was another 

indication of the new American attitude that the 

Soviet Union’s aid in the present conflict no longer 

was needed. The message called for the Japanese 

to surrender unconditionally or face “prompt and 

utter destruction.”56 The Potsdam Proclamation 

left the emperor’s status unclear by making no 

reference to the royal house in the section that 

Model of Little Boy Uranium Bomb. Reprinted from Richard G. Hewlett and Oscar E. Anderson, Jr., The New World 1939-1946 Volume 

I of A History of the United States Atomic Energy Commission (University Park: Pennsylvania State University Press, 1962).

promised the Japanese that they could design 

their new government as long as it was peaceful 

and more democratic. While anti-war sentiment 

was growing in Japanese decision-making circles, 

it could not carry the day as long as unconditional 

surrender left the emperor’s position in jeopardy. 

The Japanese rejected the offer on July 29.

Intercepted messages between Tokyo and Moscow 

revealed that the Japanese wanted to surrender 

but felt they could not accept the terms offered 

in the Potsdam Proclamation. American policy 

makers, however, anxious to end the war without 

committing American servicemen to an invasion 

of the Japanese homeland, were not inclined 

to undertake revisions of the unconditional 

surrender formula and cause further delay. 

A Russian declaration of war might convince 

Japan to surrender, but it carried a potentially 

prohibitive price tag as Stalin would expect to 

share in the postwar administration of Japan, a 

situation that would threaten American plans in 

the Far East. A blockade of Japan combined with 
5

Part V

Fat Man Plutonium Bomb Being Readied at Tinian. 

Los Alamos National Laboratory.

conventional bombing was rejected as too timeconsuming and an invasion of the islands as too 

costly. And few believed that a demonstration of 

the atomic bomb would convince the Japanese to 

give up. Primarily upon these grounds, American 

policy makers concluded that the atomic bomb 

must be used. Information that Hiroshima might 

be the only prime target city without American 

prisoners in the vicinity placed it first on the list. 

As the final touches were put on the message 

Truman would issue after the attack, word came 

that the first bomb could be dropped as early 

as August 1. With the end now in sight, poor 

weather led to several days’ delay.

Hiroshima

In the early morning hours of August 6, 1945, a 

B-29 bomber attached to the 509th Composite 

Group took off from Tinian Island and headed 

north by northwest toward the Japanese 

Islands over 1,500 miles away. Its primary 

target was Hiroshima, an important military 

and communications center with a population 

of nearly 300,000 located in the deltas of 

southwestern Honshu Island facing the Inland 

Sea. The Enola Gay, piloted by Colonel Paul 

Tibbets, flew at low altitude on automatic pilot 

before climbing to 31,000 feet as it neared the 

target area. As the observation and photography 

escorts dropped back, the Enola Gay released a 

9,700-pound uranium bomb, nicknamed Little 

Boy, at approximately 8:15 a.m. Hiroshima time. 

Tibbets immediately dove away to avoid the 

anticipated shockwaves of the blast. Forty-three 

seconds later a huge explosion lit the morning 

sky as Little Boy detonated 1900 feet above 

the city, directly over a parade field where the 

Japanese Second Army was doing calisthenics. 

Though already eleven and a half miles away, 

the Enola Gay was rocked by the blast. At first 

Tibbets thought he was taking flak. After a second 

shockwave hit the plane, the crew looked back at 

Hiroshima. “The city was hidden by that awful 

cloud boiling up, mushrooming, terrible and 

incredibly tall,” Tibbets recalled.57 Little Boy killed 

70,000 people (including about twenty American 

airmen being held as POWs) and injured another 

70,000. By the end of 1945, the Hiroshima death 

toll rose to 140,000 as radiation-sickness deaths 

mounted. Five years later the total reached 

200,000. The bomb caused total devastation for 

five square miles, with almost all of the buildings 

in the city either destroyed or damaged.

Within hours of the attack, radio stations began 

reading a prepared statement from President 

Harry Truman informing the American public 

that the United States had dropped an entirely 

new type of bomb on the Japanese city of 

Hiroshima—an atomic bomb with more power 

than 15,000 tons of TNT.58 Truman warned that 

if Japan still refused to surrender unconditionally 

as demanded by the Potsdam Proclamation of 

July 26, the United States would attack additional 

targets with equally devastating results. Two days 

later, on August 8, the Soviet Union declared 

war on Japan and attacked Japanese forces in 

Manchuria, ending American hopes that the war 

would end before Russian entry into the Pacific 

theater.

96
5

The Atomic Bomb and American Strategy

97

Nagasaki

Factional struggles and communications 

problems prevented Japan from meeting Allied 

terms in the immediate aftermath of Hiroshima. 

In the absence of a surrender announcement, 

conventional bombing raids on additional 

Japanese cities continued as scheduled. Then, 

on August 9, a second atomic attack took place. 

Taking off from Tinian at 3:47 a.m., Bock’s Car

(named after its usual pilot) headed for its 

primary target, Kokura Arsenal, located on the 

northern coast of Kyushu Island. Pilot Charles 

Sweeney found unacceptable weather conditions 

and unwelcome flak above Kokura. Sweeney 

made three passes over Kokura, then decided to 

switch to his secondary target even though he had 

only enough fuel remaining for a single bombing 

run. Clouds greeted Bock’s Car as it approached 

Nagasaki, home to the Mitsubishi plant that had 

manufactured the torpedoes used at Pearl Harbor. 

At the last minute, a brief break in the cloud 

cover made possible a visual targeting at 29,000 

feet and Bock’s Car dropped her single payload, 

a plutonium bomb weighing 10,000 pounds and 

nicknamed Fat Man, at 11:01 a.m. The plane 

then veered off and headed to Okinawa for an 

emergency landing. Fat Man exploded 1,650 feet 

above the slopes of the city with a force of 21,000 

tons of TNT.59 Fat Man killed 40,000 people and 

injured 60,000 more. Three square miles of the 

city were destroyed, less than Hiroshima because 

of the steep hills surrounding Nagasaki. By 

January 1946, 70,000 people had died in Nagasaki. 

The total eventually reached 140,000, with a death 

rate similar to that of Hiroshima.60

Surrender

Still the Japanese leadership struggled to come to 

a decision, with military extremists continuing to 

advocate a policy of resistance to the end. Word 

finally reached Washington from Switzerland and 

Sweden early on August 10 that the Japanese, in 

accordance with Hirohito’s wishes, would accept 

the surrender terms, provided the emperor 

retain his position. Truman held up a third 

atomic attack while the United States considered 

a response, finally taking a middle course and 

acknowledging the emperor by stating that his 

authority after the surrender would be exercised 

under the authority of the Supreme Commander 

of the Allied Powers. With British, Chinese, and 

Russian concurrence, the United States answered 

the Japanese on August 11. Japan surrendered on 

August 14, 1945, ending the war that began for 

the United States with the surprise attack at Pearl 

Harbor on December 7, 1941. The United States 

had been celebrating for almost three weeks when 

the formal papers were signed aboard the U.S.S. 

Missouri on September 2.

The Bomb Goes Public

The veil of secrecy that had hidden the atomic 

bomb project was lifted on August 6 when 

President Truman announced the Hiroshima 

raid to the American people. The release of the 

Smyth Report on August 12, which contained 

general technical information calculated to 

satisfy public curiosity without disclosing any 

atomic secrets, brought the Manhattan Project 

into fuller view.61 Americans were astounded to 

learn of the existence of a far-flung, governmentrun, top secret operation with a physical plant, 

payroll, and labor force comparable in size to the 

American automobile industry. Approximately 

130,000 people were employed by the project at 

its peak, among them many of the nation’s leading 

scientists and engineers.

In retrospect, it is remarkable that the atomic 

bomb was built in time to be used in World 

War II. Most of the theoretical breakthroughs in 

nuclear physics dated back less than twenty-five 

years, and with new findings occurring faster 

than they could be absorbed by practitioners in 

the field, many fundamental concepts in nuclear 

physics and chemistry had yet to be confirmed by 

laboratory experimentation. Nor was there any 

conception initially of the design and engineering 

difficulties that would be involved in translating 

what was known theoretically into working 
5

Part V

98

devices capable of releasing the enormous energy 

of the atomic nucleus in a predictable fashion. 

In fact, the Manhattan Project was as much a 

triumph of engineering as of science. Without the 

innovative work of the talented Leslie Groves, as 

well as that of Crawford Greenewalt of DuPont 

and others, the revolutionary breakthroughs in 

nuclear science achieved by Enrico Fermi, Niels 

Bohr, Ernest Lawrence, and their colleagues 

would not have produced the atomic bomb 

during World War II. Despite numerous obstacles, 

the United States was able to combine the forces 

of science, government, military, and industry 

into an organization that took nuclear physics 

from the laboratory and into battle with a weapon 

of awesome destructive capability, making clear 

the importance of basic scientific research to 

national defense.
995

Part VI:

The Manhattan District in 

Peacetime

From the time S-l became public knowledge 

until the Atomic Energy Commission succeeded 

it on January 1, 1947, the Manhattan Engineer 

District controlled the nation’s nuclear program. 

Groves remained in command, intent upon 

protecting America’s lead in nuclear weapons by 

completing and consolidating the organization he 

had presided over for three years in challenging 

wartime conditions. He soon found that 

peacetime held its own challenges.

According to a plan approved by Stimson and 

Marshall in late August 1945, Groves shut down 

the thermal diffusion plant in the K-25 area on 

September 9 and put the Alpha tracks at Y-12 on 

standby during September as well. The improved 

K-25 gaseous diffusion plant now provided 

feed directly to the Beta units. Hanford’s three 

piles continued in operation, but one of the two 

chemical separation areas was closed. Los Alamos 

was assigned the task of producing a stockpile 

of atomic weapons. Actual weapon assembly 

was to be done at Sandia Base in Albuquerque, 

where engineering and technical personnel were 

relocated with the staff previously stationed at 

Wendover Field in western Utah.

Operation Crossroads

In July 1946, during Operation Crossroads, the 

Manhattan Project tested its third and fourth 

plutonium bombs (Trinity and Nagasaki were 

the first two) with a large, invited audience 

of journalists, scientists, military officers, 

congressmen, and foreign observers at Bikini 

Atoll in the Pacific. Shot Able, dropped from a 

B-29 on July 1, sank three ships and performed 

as well as its two predecessors from a technical 

standpoint, though it failed to fulfill its pretest 

publicity buildup. Shot Baker was detonated 

from ninety feet underwater on the morning of 

July 25. Baker produced a spectacular display as 

it wreaked havoc on a seventy-four-vessel fleet 

of empty ships and spewed thousands of tons of 

water into the air. Both Able and Baker yielded 

explosions equivalent to 21,000 tons of TNT, 

though Baker introduced the most subtle hazard 

of the atomic age—radiation fallout.62 Able and 

Baker were the final weapon tests conducted by 

the Manhattan Project and the last American tests 

until the Atomic Energy Commission’s Sandstone 

series began in spring 1948.

Superpower Chill

Between August 1945 and January 1947, while 

Groves fought to maintain the high priority of the 

atomic program in a peacetime environment, the 

euphoria that swept the United States at the end 

of World War II dissipated as Americans found 

themselves embroiled in a new global struggle, 

this time with the Soviet Union. The United States 

held a monopoly on atomic weapons during the 

sixteen months of Groves’s peacetime tenure, 

but less than three years after the Atomic Energy 

Commission succeeded the Manhattan Engineer 

District, the Russians’ secret atomic bomb 

program achieved success with the 1949 test of 

Joe I (which the Americans named after Joseph 

Stalin). During the 1950s, relations between the 

two superpowers remained strained, and both 

added the hydrogen bomb to their arsenals in an 

attempt to achieve military superiority.

Postwar Planning

The beginning of the Cold War in the late 1940s 

was linked to the failure of the World War II allies 

to reach agreements on international controls 

respecting nuclear research and atomic weapons. 

Postwar planning in the United States began in 

earnest in July 1944, when Met Lab scientists in 
5

Chicago issued a “Prospectus on Nucleonics,” 

which included plans for atomic research and 

advocated the creation of an international 

organization to prevent nuclear conflict. In 

August the Military Policy Committee set up a 

Postwar Policy Committee, charged with making 

recommendations on the proper government role 

in postwar atomic research and development. 

The committee, composed of Richard Tolman 

(chairman), Warren Lewis, Henry Smyth, and 

Rear Admiral Earle W. Mills, recommended that 

the best way for the government to maintain 

a vigorous nuclear program was to set up a 

peacetime version of the Office of Scientific 

Research and Development. 

Roosevelt and Churchill included postwar 

planning on their agenda when they met at Hyde 

Park in September 1944. They immediately vetoed 

the idea of an open atomic world (Churchill 

adamantly rejecting Bohr’s recommendation.) 

Bush and Conant, meanwhile, contacted Stimson 

on September 19 and spoke to the necessity 

of releasing selected information on the bomb 

project, reasoning that in a free country the secret 

could not be kept long. When Roosevelt asked 

Bush for a briefing on S-l several days later, Bush 

discovered that Roosevelt had signed an “aidememoire” with Churchill, pledging to continue 

bilateral research with England in certain areas of 

atomic technology.63 Bush feared that Roosevelt 

would institute full interchange with Great Britain 

without consulting his own atomic power experts. 

Bush argued, prophetically, that leaving the 

Russians out of such an arrangement might well 

lead to an arms race among the Allied victors.

The Baruch Plan

Bush and Conant presented their views more fully 

on September 30. They held that the American 

and British lead would last no more than three 

or four years and that security against the bigger 

bombs that surely would result from a worldwide 

arms race could be gained only through 

international agreements aimed at preventing 

secret research and surprise attacks. Bush and 

Conant’s basic philosophy found expression in 

the Acheson-Lilienthal report of March 1946, 

fashioned primarily by Oppenheimer and 

evolving into the formal American proposal for 

the international control of atomic energy known 

as the Baruch Plan.

Bernard Baruch, the elder statesman who had 

served American presidents in various capacities 

since World War I, unveiled the United States 

plan in a speech to the newly-created United 

Nations Atomic Energy Commission on June 14, 

1946. Baruch proposed the establishment of an 

international atomic development authority along 

the lines proposed by the Acheson-Lilienthal 

report, one that would control all activities 

dangerous to world security and possess the 

power to license and inspect all other nuclear 

projects. Once such an authority was established, 

no more bombs should be built and existing 

bombs should be destroyed. Abolishing atomic 

weapons could lay the groundwork for reducing 

and subsequently eliminating all weapons, 

thus outlawing war altogether. The Baruch 

Plan, in Baruch’s words “the last, best hope of 

earth,” deviated from the optimistic tone of the 

Acheson-Lilienthal plan, which had intentionally 

remained silent on enforcement, and set specific 

penalties for violations such as illegally owning 

atomic bombs.64 Baruch argued that the United 

Nations should not allow members to use the 

veto to protect themselves from penalties for 

atomic energy violations; he held that simple 

majority rule should prevail in this area. As on 

enforcement, the Acheson-Lilienthal report had 

studiously avoided comment on the veto issue.65

Not surprisingly, the Soviet Union, a non-nuclear 

power, insisted upon retaining its United Nations 

veto and argued that the abolition of atomic 

weapons should precede the establishment of an 

international authority. Negotiations could not 

100

Part VI
5

The Manhattan District in Peacetime

101

proceed fairly, the Russians maintained, as long as 

the United States could use its atomic monopoly 

to coerce other nations into accepting its plan. 

The Baruch Plan proposed that the United States 

reduce its atomic arsenal by carefully defined 

stages linked to the degree of international 

agreement on control. Only after each stage of 

international control was implemented would 

the United States take the next step in reducing 

its stockpile. The United States position, then, 

was that international agreement must precede 

any American reductions, while the Soviets 

maintained that the bomb must be banned before 

meaningful negotiations could take place.

The debate in the United Nations was a debate 

in name only; neither side budged an inch in the 

six months following Baruch’s United Nations 

speech. In the end, the Soviet Union, unwilling 

to surrender its veto power, abstained from the 

December 31, 1946, vote on Baruch’s proposal on 

the grounds that it did not prohibit the bomb, and 

the American plan became a dead letter by early 

1947—though token debate on the American plan 

continued into 1948. The United States, believing 

that Soviet troops posed a threat to eastern Europe 

and recognizing that American conventional 

forces were rapidly demobilizing, refused to 

surrender its atomic deterrent without adequate 

international controls and continued to develop 

its nuclear arsenal. In an atmosphere of mutual 

suspicion the Cold War set in.

The Debate Over Domestic Legislation

While the international situation grew more 

ominous due to deteriorating relations between 

the United States and the Soviet Union, a 

domestic debate was taking place over the 

permanent management of America’s nuclear 

program. The terms of the debate were framed 

by the Interim Committee in July 1945 when it 

wrote draft legislation proposing a peacetime 

organization with responsibilities very similar 

to those of the Manhattan Project. The draft 

legislation provided for a strong military presence 

on a nine-member board of commissioners and 

strongly advocated the federal government’s 

continued dominance in nuclear research and 

development.

The May-Johnson Bill

The Interim Committee’s draft legislation reached 

President Truman via the State Department 

shortly after the armistice. After affected federal 

agencies approved, Truman advocated speedy 

passage of the congressional version of the 

bill, the May-Johnson bill, on October 3, 1945. 

Groves, Bush, and Conant testified at hearings in 

the House of Representatives that the sweeping 

powers granted the proposed commission were 

necessary and that only government control 

of atomic power could prevent its misuse. 

Although Lawrence, Fermi, and Oppenheimer 

(with some misgivings) regarded the bill as 

acceptable, many of the scientists at the Met 

Lab and at Oak Ridge complained that the bill 

was objectionable because it was designed to 

maintain military control over nuclear research, 

a situation that had been tolerable during the war 

but was unacceptable during peacetime when 

free scientific interchange should be resumed. 

Particularly onerous to the scientific opponents 

were the proposed penalties for security violations 

contained in the May-Johnson bill—ten years in 

prison and a $100,000 fine. Organized scientific 

opposition in Washington slowed the bill’s 

progress, and Arthur H. Vandenberg of Michigan 

held it up in the Senate through a parliamentary 

maneuver.

The McMahon Bill

As support for the May-Johnson bill eroded 

in late 1945, President Truman withdrew his 

support. Vandenberg’s attempt to establish a 

joint House-Senate special committee failed, but 

Brien McMahon of Connecticut successfully 

created and became chair of the Senate’s Special 

Committee on Atomic Energy. Daily hearings 
5

took place until December 20, when McMahon 

introduced a substitute to the May-Johnson 

bill. Hearings on the new McMahon bill began 

in January. Groves opposed McMahon’s bill, 

citing weak security provisions, the low military 

presence, and the stipulation that commission 

members be full-time (Groves thought that 

more eminent commissioners could be obtained 

if work was part-time). Groves also objected 

to the bill’s provision that atomic weapons be 

held in civilian rather than military custody. 

Nevertheless, the Senate approved the McMahon 

bill on June 1, 1946, and the House approved it on 

July 20, with a subsequent conference committee 

eliminating most substantive amendments. 

The sometimes bitter debate between those 

who advocated continued military stewardship 

of America’s atomic arsenal and those who 

saw continued military control as inimical 

to American traditions ended in victory for 

supporters of civilian authority. President Truman 

signed the McMahon Act, known officially as 

the Atomic Energy Act of 1946, on August 1. 

The bill called for the transfer of authority from 

the United States Army to the United States 

Atomic Energy Commission, a five-member 

civilian board serving full-time and assisted by 

a general advisory committee and a military 

liaison committee.66 The Atomic Energy Act 

entrusted the Atomic Energy Commission with 

the government monopoly in the field of atomic 

research and development previously held by its 

wartime predecessor.67

Conclusion

The Manhattan Project, its wartime mission 

completed, gave way to the civilian Atomic 

Energy Commission. How well the Atomic 

Energy Commission would be able to manage the 

nuclear arsenal in a Cold War environment and 

whether it could successfully develop the peaceful 

uses of atomic energy, only time would tell. What 

was clear as the Atomic Energy Commission 

took over at the beginning of 1947 was that the 

success of the Manhattan Project had helped 

cement the bond between basic scientific research 

and national security. Science had gone to war 

and contributed mightily to the outcome. The 

challenge confronting American policy makers 

in the postwar years was to enlist the forces of 

science in the battle to defend the peace.

102

Part VI
1035

TOP POLICY

GROUP

THE PRESIDENT

MILITARY

POLICY COMMITTEE SECRETARY OF WAR INTERIM

COMMITTEE

CHIEF OF

STAFF

GENERAL H.H. ARNOLD

(USAAF)

MAJOR GENERAL

L.R. GROVES

PLANNING FOR

MILITARY OPERATIONS

BRIGADIER GENERAL

T.F. FARRELL, DEPUTY

MANHATTAN DISTRICT

COL. K.D. NICHOLS

OTHER

ESTABLISHMENTS

CLINTON

ENGINEER

WORKS

HANFORD

ENGINEER

WORKS

LOS

ALAMOS

Simplified Chart of Manhattan Project. Adapted from Leslie R. Groves, Now It Can Be Told (New York: Harper 

& Row, 1962).

(Nichols: Administrative matters;

Groves: Other matters)

 Manhattan Project Chart
5

Notes

1. The Einstein letter is reprinted in Vincent C.

 Jones, Manhattan: The Army and the Atomic 

 Bomb (Washington, D.C.: U.S. Government 

 Printing Office, 1985), pp. 609-10.

2. Roosevelt to Einstein, October 19, 1939.

3. The United States had little reliable intelligence 

 on the German bomb effort until late in the 

 war. Thus it was not known until the ALSOS 

 counterintelligence mission that the German 

 program had not proceeded beyond the 

 laboratory stage and had foundered by mid-

 1942. For details of the ALSOS mission see 

 Richard Rhodes, The Making of the Atomic 

 Bomb (New York: Simon & Schuster, 1986), pp. 

 605-10.

4. For details on the German research effort 

 see McGeorge Bundy, Danger and Survival: 

 Choices About the Bomb in the First Fifty Years

 (New York: Random House, 1988), 

 pp. 14-23.

5. Lawrence Badash, “Introduction,” in 

 Reminiscences of Los Alamos, 1943-1945, edited 

 by Lawrence Badash, Joseph O. Hirschfelder, 

 and Herbert P. Broida (Dordrecht, Holland: D. 

 Reidel Publishing Company, 1980), xi.

6. Rhodes, Making of the Atomic Bomb, p. 228.

7. William R. Shea, “Introduction: From 

 Rutherford to Hahn,” in Otto Hahn and the Rise 

 of Nuclear Physics, edited by William R. Shea 

 (Dordrecht, Holland: D. Reidel Publishing 

 Company, 1983), p. 15.

8. Ibid.

9. Jones, Manhattan, p. 10.

10. Richard G. Hewlett and Oscar E. Anderson, 

Jr., The New World, 1939-1946, Volume I, A 

History of the United States Atomic Energy 

Commission (University Park, Pennsylvania: 

Pennsylvania State University Press, 1962), 

 pp. 30-31.

11. Ibid., p. 168.

12. Laura Fermi, Atoms in the Family: My Life 

With Enrico Fermi (Chicago: University of 

Chicago Press, 1954), p. 164.

13. Rhodes, Making of the Atomic Bomb, pp. 336-

38.

14. Hewlett and Anderson, New World, p. 37.

15. Ibid., p. 39.

16. MAUD, while it appears to be an acronym, is 

not. It is simply a codename.

17. Rhodes, Making of the Atomic Bomb, pp. 321-

25 and 369.

18. Bundy, Danger and Survival, pp. 48-49.

19. Hewlett and Anderson, New World, p. 46.

20. Ibid., p. 49.

21. Ibid., p. 48.

22. Ibid., p. 406.

23. Ibid., pp. 74-75.

24. Ibid., pp. 82-83.

25. Henry D. Smyth, A General Account of the 

Development of Methods of Using Atomic 

Energy for Military Purposes Under the 

Auspices of the United States Government, 

1940-1945 (Washington, D.C.: U. S. 

Government Printing Office, 1945), p. 70.

26. Rhodes, Making of the Atomic Bomb, p. 442.

27. Ibid.

28. Hewlett and Anderson, New World, p. 113.

29. Kenneth D. Nichols, The Road to Trinity (New 

York: William Morrow and Company, Inc., 

1987), pp. 121 and 146.

30. Hewlett and Anderson, New World, p. 149.

31. Nichols recounts his adventure in borrowing 

the silver in Road to Trinity, p. 42.

32. Hewlett and Anderson, New World, p. 155.

104
105

33. Ibid.

34. Ibid., p. 165.

35. For more on k see Rhodes, Making of the 

Atomic Bomb, p. 397.

36. Adsorption is a process whereby gases, 

liquids, or dissolved substances are gathered 

on a surface in a condensed layer.

37. Jones, Manhattan, p. 218.

38. Hewlett and Anderson, New World, p. 220.

39. Nichols, Road to Trinity, p. 108.

40. Rhodes, Making of the Atomic Bomb, p. 570.

41. In the Matter of J. Robert Oppenheimer: 

Transcript of Hearing Before Personnel Security 

Board, Washington, D.C., April 12, 1954, 

Through May 6, 1954 (Washington, D.C.: 

Government Printing Office, 1954), 

 pp. 12-13.

42. For details on Fuchs see Robert C. Williams, 

Klaus Fuchs, Atom Spy (Cambridge: Harvard 

University Press, 1987).

43. Rhodes, Making of the Atomic Bomb, 

 pp. 540-41.

44. Ibid., pp. 305-08.

45. Ibid., pp. 644-45.

46. Jones, Manhattan, p. 530.

47. The Franck report, the response of the 

Scientific Panel, and the Interim Committee 

recommendations are discussed in Ibid., 

 pp. 365-72.

48. Rhodes, Making of the Atomic Bomb, 

 pp. 571-72.

49. Ibid., p. 676.

50. See Ibid., pp. 668-78, for more on the Trinity 

test and the responses of those present.

51. Hewlett and Anderson, New World, p. 383.

52. Ibid., p. 386.

53. Leslie R. Groves, Now It Can Be Told, (New 

York: Harper & Row, 1962), p. 434.

54. Herbert Feis, The Atomic Bomb and the End of 

World War II (Princeton: Princeton University 

Press, 1966), p. 85.

55. The directive is reprinted in Rhodes, Making 

of the Atomic Bomb, p. 691.

56. Hewlett and Anderson, New World, p. 395.

57. Paul W. Tibbets, “How to Drop an Atom 

Bomb,” Saturday Evening Post 218 (June 8, 

1946), p. 136.

58. The official yield of Little Boy, as listed in the 

Department of Energy’s “Announced United 

States Nuclear Tests, July 1945 Through 

December 1986,” page 2, January 1987, was 

15,000 tons of TNT (15 kilotons).

59. The official yield of Fat Man listed in Ibid. 

Descriptions of the bombings of Hiroshima 

and Nagasaki derived from Rhodes, Making of 

the Atomic Bomb, pp. 710 and 739-40.

60. Summaries of Hiroshima and Nagasaki 

casualty rates and damage estimates appear in 

Rhodes, Making of the Atomic Bomb, 

 pp. 733-34, 740, 742; Groves, Now It Can 

Be Told, pp. 319, 329-30, 346; and Jones, 

Manhattan, pp. 545-48.

61. The Smyth report is cited in footnote 25.

62. For details of the Baker fallout see Barton C. 

Hacker, The Dragon’s Tail: Radiation Safety in 

the Manhattan Project, 1942-1946 (Berkeley: 

University of California Press, 1987), 

 pp. 135-53.

63. Hewlett and Anderson, New World, 

 pp. 326-27.

64. Ibid., p. 579.

65. Bundy, Danger and Survival, pp. 158-66.

66. The McMahon bill is reprinted in Hewlett and 

Anderson, New World, pp. 714-22.

67. The Manhattan Project ended with the 

transfer of power from the Manhattan 

Engineer District to the Atomic Energy 

Commission, though the Manhattan Engineer 

District itself was not abolished until August 

15, 1947. The Top Policy Group and the 

Military Policy Committee had already 

ceased to exist by the time the Atomic Energy 

Commission took over on January 1, 1947.
5

Select Bibliography

Bundy, McGeorge, Danger and Survival: Choices 

About the Bomb in the First Fifty Years (New York: 

Random House, 1988).

Cantelon, Philip L. and Robert C. Williams, 

editors, The American Atom: A Documentary 

History of Nuclear Policies from the Discovery of 

Fission to the Present, 1939-1984 (Philadelphia: 

University of Pennsylvania Press, 1984).

Groves, Leslie R., Now It Can Be Told (New York: 

Harper & Row, 1962).

Herken, Gregg, Brotherhood of the Bomb: 

The Tangled Lives and Loyalties of Robert 

Oppenheimer, Ernest Lawrence and Edward Teller 

(New York: Henry Holt, 2002).

Hersey, John, Hiroshima (New York: Knopf, 1946).

Hewlett, Richard G. and Oscar Anderson, Jr., The 

New World, 1939-1946, Volume I of A History 

of the United States Atomic Energy Commission

(University Park: Pennsylvania State University 

Press, 1962).

Jones, Vincent C., Manhattan: The Army and 

the Atomic Bomb (Washington, D.C.: U. S. 

Govern ment Printing Office, 1985).

Norris, Robert S., Racing for the Bomb: General 

Leslie R. Groves, the Manhattan Project’s 

Indispensable Man (Hanover, NH: Steerforth 

Press, 2003).

Rhodes, Richard, The Making of the Atomic Bomb

(New York: Simon & Schuster, 1986).

Smyth, Henry D., Atomic Energy for Military 

Pur poses: The Official Report on the Development 

of the Atomic Bomb Under the Auspices of 

the United States Government, 1940-1945

(Washington, D.C.: U. S. Government Printing 

Office, 1945).

United States Atomic Energy Commission, In 

the Matter of J. Robert Oppenheimer: Transcript 

of Hearing Before Personnel Security Board, 

Washington, D.C., April 12, 1954, Through May 

6, 1954 (Washington, D.C.: U. S. Government 

Print ing Office, 1954).

106
1075

Manhattan Project 

Chronology

Date Events

1919 Ernest Rutherford 

discovers the proton by 

artificially transmuting 

an element (nitrogen into 

oxygen).

1930 Ernest O. Lawrence builds 

the first cyclotron in 

Berkeley.

1931 Robert J. Van de Graaff 

develops the electrostatic 

generator.

1932 James Chadwick discovers 

the neutron.

1932 J. D. Cockroft and E. T. S. 

Walton first split the atom.

1932 Lawrence, M. Stanley 

Livingston, and Milton 

White operate the first 

cyclotron.

1934 Enrico Fermi produces 

fission.

December 1938 Otto Hahn and Fritz 

Strassmann discover 

the process of fission in 

uranium.

December 1938 Lise Meitner and Otto 

Frisch con firm the HahnStrassmann discovery and 

communicate their findings 

to Niels Bohr.

January 26, 1939 Bohr reports on the 

Hahn-Strassman results at 

a meeting on theoretical 

physics in Washington, 

D. C.

August 2, 1939 Albert Einstein writes 

President Franklin D. 

Roosevelt, alerting 

the President to the 

importance of research on 

chain reactions and the 

possibility that research 

might lead to developing 

powerful bombs.

September 1, 1939 Germany invades Poland.

October 11-12, 1939 Alexander Sachs discusses 

Eins tein’s letter with 

President Roosevelt. 

Roosevelt decides to act 

and appoints Lyman J. 

Briggs head of the Advisory 

Committee on Uranium.

October 19, 1939 Roosevelt informs Einstein 

 that he has set up a 

 committee to study 

 uranium.

October 21, 1939 The Uranium Committee 

 meets for the first time.

November 1, 1939 The Uranium Committee 

recom mends that the 

government pur chase 

graphite and uranium 

oxide for fission research.
5

March 1940 John R. Dunning and his 

col leagues demonstrate 

that fission is more readily 

produced in the rare 

uranium-235 isotope, 

not the more plentiful 

uranium-238.

Spring- Isotope separation

Summer 1940 methods are investigated.

June 1940 Vannevar Bush is named 

head of the National 

Defense Research 

Committee. The Uranium 

Com mittee becomes a 

scientific sub committee of 

Bush’s organization.

February 24, 1941 Glenn T. Seaborg’s research 

group discovers plutonium.

March 28, 1941 Seaborg’s group 

demonstrates that 

plutonium is fissionable.

May 1941 Seaborg proves plutonium 

is more fissionable than 

uranium-235.

May 17, 1941 A National Academy of 

Sciences report emphasizes 

the necessity of further 

research.

June 22, 1941 Germany invades the 

Soviet Union.

June 28, 1941 Bush is named head of the 

Office of Scientific Research 

and Development. James 

B. Conant replaces Bush 

at the National Defense 

Research Com mittee, 

which becomes an advisory 

body to the Office of 

Scientific Research and 

Development.

July 2, 1941 The British MAUD report 

 concludes that an atomic 

 bomb is feasible.

July 11, 1941 A second National Academy 

of Sciences report confirms 

the findings of the first.

July 14, 1941 Bush and Conant receive 

 the MAUD report.

October 9, 1941 Bush briefs Roosevelt and 

Vice Presi dent Henry A. 

Wallace on the state of 

atomic bomb research. 

Roosevelt instructs Bush to 

find out if a bomb can be 

built and at what cost. Bush 

receives permission to 

explore con struction needs 

with the Army.

November 9, 1941 A third National Academy 

of Sciences report agrees 

with the MAUD report that 

an atomic bomb is feasible.

November 27, 1941 Bush forwards the third 

 National Academy of 

 Sciences report to 

 the President.

December 7, 1941 The Japanese attack 

Pearl Harbor.

December 10, 1941 Germany and Italy declare 

 war on the United States.

December 16, 1941 The Top Policy Committee 

becomes primarily 

responsible for making 

108
1095

broad policy decisions 

relating to uranium 

research.

December 18, 1941 The S-l Executive 

Committee (which 

replaced the Uranium 

Committee in the Office 

of Scientific and Research 

Development) gives 

Lawrence $400,000 to 

continue electromagnetic 

research.

January 19, 1942 Roosevelt responds to 

Bush’s November 27 report 

and approves production of 

the atomic bomb.

March 9, 1942 Bush gives Roosevelt an 

optimistic report on the 

possibility of producing a 

bomb.

May 23, 1942 The S-l Executive 

Committee recom mends 

that the project move to the 

pilot plant stage and build 

one or two piles (reactors) 

to produce plutonium 

and electromagnetic, 

cen trifuge, and gaseous 

diffusion plants to produce 

uranium-235.

June 1942 Production pile designs 

are developed at the 

Metallurgical Laboratory in 

Chicago.

June 17, 1942 President Roosevelt 

approves the S-l 

Executive Committee 

recommenda tion to 

proceed to the pilot plant 

stage and instructs that 

plant con struction be the 

responsibility of the Army. 

The Office of Scientific 

Research and Development 

continues to direct nuclear 

research, while the Army 

delegates the task of plant 

construction to the Corps 

of Engineers.

July 1942 Kenneth Cole establishes 

the health division at the 

Metallurgical Laboratory.

August 7, 1942 The American islandhopping cam paign in the 

Pacific begins with the 

landing at Guadalcanal.

August 13, 1942 The Manhattan Engineer 

District is established in 

New York City, Col onel 

James C. Marshall 

command ing.

August 1942 Seaborg produces a 

microscopic sam ple of pure 

plutonium.

September 13, 1942 The S-l Executive 

Committee visits 

Lawrence’s Berkeley 

laboratory and 

recommends building an 

elec tromagnetic pilot plant 

and a section of a full-scale 

plant in Tennessee.

September 17, 1942 Colonel Leslie R. Groves 

is ap pointed head of the 

Manhattan Engineer District. 

He is promoted to Brigadier 

General six days later.
5

September 19, 1942 Groves selects the Oak 

Ridge, Ten nessee, site for 

the pilot plant.

September 23, 1942 Secretary of War Henry 

Stimson creates a Military 

Policy Committee to help 

make decisions for the 

Manhattan Project.

October 3, 1942 E. I. du Pont de Nemours 

and Com pany agrees 

to build the chemical 

separation plant at Oak 

Ridge.

October 5, 1942 Compton recommends 

an intermediate pile at 

Argonne.

Fall 1942 J. Robert Oppenheimer 

and the luminaries report 

from Berkeley that more 

fissionable material may 

be needed than previously 

thought.

October 19, 1942 Groves decides to establish 

a separate scientific 

laboratory to design an 

atomic bomb.

October 26, 1942 Conant recommends 

dropping the centrifuge 

method.

November 12, 1942 On the recommendation 

of Groves and Conant, 

the Military Policy 

Committee decides 

to skip the pilot plant 

stage on the plutonium, 

elec tromagnetic, and 

gaseous diffusion projects 

and go directly from the 

research stage to industrialscale production. The 

Committee also decides 

not to build a centrifuge 

plant.

November 14, 1942 The S-l Executive 

Committee en dorses the 

recommendations of the 

Military Policy Committee.

November 1942 The Allies invade 

North Africa.

November 25, 1942 Groves selects Los Alamos, 

New Mexico, as the bomb 

laboratory (codenamed 

Project Y). Oppen heimer is 

chosen laboratory director.

December 2, 1942 Scientists led by Enrico 

Fermi achieve the first 

self-sustained nuclear chain 

reaction in Chicago.

December 10, 1942 The Lewis committee 

compromises on the 

electromagnetic method. 

The Military Policy 

Committee decides to build 

the plutonium production 

facilities at a site other than 

Oak Ridge.

December 28, 1942 Roosevelt approves 

detailed plans for building 

production facilities and 

producing atomic weapons.

January 13-14, 1943 Plans for the Y-12 

electromagnetic plant are 

discussed. Groves insists 

that Y-12’s first racetrack be 

finished by July 1.

110
1115

January 14-24, 1943 At the Casablanca 

Conference, Roosevelt and 

British Prime Minister 

Churchill agree upon 

unconditional surrender 

for the Axis powers.

January 16, 1943 Groves selects Hanford, 

Washington, as the site for 

the plutonium produc tion 

facilities. Eventually three 

reac tors, called B, D, and F, 

are built at Hanford.

January 1943 Bush encourages Philip 

Abelson’s research on the 

thermal diffusion process.

February 18, 1943 Construction of Y-12 

begins at Oak Ridge.

February 1943 Groundbreaking for the 

X-10 plutonium pilot plant 

takes place at Oak Ridge.

March 1943 Researchers begin arriving 

at Los Alamos.

April 1943 Bomb design work begins 

at Los Alamos.

June 1943 Site preparation for the K25 gaseous diffusion plant 

commences at Oak Ridge.

Summer 1943 The Manhattan Engineer 

District moves its 

headquarters to Oak Ridge.

July 1943 Oppenheimer reports 

that three times as much 

fissionable material may 

be necessary than thought 

nine months earlier.

August 27, 1943 Groundbreaking for 

the 100-B plutonium 

production pile at Han ford 

takes place.

September 8, 1943 Italy surrenders to Allied 

forces.

September 9, 1943 Groves decides to double 

the size of Y-12.

September 27, 1943 Construction begins on 

K-25 at Oak Ridge.

November 4, 1943 The X-10 pile goes critical 

and pro duces plutonium by 

the end of the month.

Late 1943 John von Neumann 

visits Los Alamos to aid 

implosion research.

December 15, 1943 The first Alpha racetrack 

is shut down due to 

maintenance problems.

January 1944 The second Alpha racetrack 

is started and demonstrates 

maintenance pro blems 

similar to those that 

disabled the first.

January 1944 Construction begins on 

Abelson’s thermal diffusion 

plant at the Philadelphia 

Naval Yard.

February 1944 Y-12 sends 200 grams 

of uranium-235 to Los 

Alamos.

March 1944 The Beta building at Y-12 is 

completed.
5

March 1944 Bomb models are tested at 

Los Alamos.

April 1944 Oppenheimer informs 

Groves about Abelson’s 

thermal diffusion research 

in Philadelphia.

June 6, 1944 Allied forces launch the 

Normandy invasion.

June 21, 1944 Groves orders the 

construction of the S-50 

thermal diffusion plant at 

Oak Ridge.

July 4, 1944 The decision is made to 

work on a calutron with a 

30-beam source for use in 

Y-12.

July 17, 1944 The plutonium gun bomb 

(codenamed Thin Man) is 

abandoned.

July 1944 A major reorganization 

to maximize implosion 

research occurs at Los 

Alamos.

July 1944 Scientists at the 

Metallurgical Laboratory 

issue the “Prospectus on 

Nucleonics,” concerning 

the interna tional control of 

atomic energy.

August 7, 1944 Bush briefs General George 

C. Mar shall, informing 

him that small im plosion 

bombs might be ready 

by mid-1945 and that a 

uranium bomb will almost 

certainly be ready by 

August 1, 1945.

September 1944 Colonel Paul Tibbets’s 

393rd Bom bardment 

Squadron begins test drops 

with dummy bombs called 

pumpkins.

September 13, 1944 The first slug is placed in 

pile 100-B at Hanford.

September 1944 Roosevelt and Churchill 

meet in Hyde Park and 

sign an “aide-memoire” 

pledging to continue 

bilateral research on atomic 

technology.

Summer 1944- The Manhattan Project’s 

Spring 1945 chances for success 

advance from doubtful 

to probable as Oak Ridge 

and Hanford produce 

increasing amounts of 

fis sionable material, and 

Los Alamos makes progress 

in chemistry, metallurgy, 

and weapon design.

September 27, 1944 The 100-B reactor goes 

critical and begins 

operation.

September 30, 1944 Bush and Conant advocate 

interna tional agreements 

on atomic research to 

prevent an arms race.

December 1944 The chemical separation 

plants (Queen Marys) are 

finished at Hanford.

February 2, 1945 Los Alamos receives its first 

plutonium.

112
1135

February 4-11, 1945 Roosevelt, Churchill, and 

Soviet Premier Joseph 

Stalin meet at Yalta.

March 1945 S-50 begins operation at 

Oak Ridge.

March 1945 Tokyo is firebombed, 

resulting in 100,000 

casualties.

March 12, 1945 K-25 begins production at 

Oak Ridge.

April 12, 1945 President Roosevelt dies.

April 25, 1945 Stimson and Groves brief 

President Truman on the 

Manhattan Project.

May 1945 Stalin tells Harry Hopkins 

that he is willing to meet 

with Truman and proposes 

Berlin as the location.

May 7, 1945 The German armed forces 

in Europe surrender to the 

Allies.

May 23, 1945 Tokyo is firebombed again, 

this time resulting in 

83,000 deaths.

May 31-June 1, 1945 The Interim Committee 

meets to make 

recommendations on 

wartime use of atomic 

weapons, international 

regulation of atomic 

information, and legislation 

regarding domestic control 

of the atomic enterprise 

(the Committee’s draft 

legislation becomes the 

basis for the May-Johnson 

bill).

June 6, 1945 Stimson informs President 

Truman that the Interim 

Committee recom mends 

keeping the atomic bomb 

a secret and using it as 

soon as possible without 

warning.

June 1945 Scientists at the 

Metallurgical Laboratory 

issue the Franck Report, 

advocating international 

control of atomic 

research and proposing 

a demonstration of the 

atomic bomb prior to its 

combat use.

June 14, 1945 Groves submits the 

target selection group’s 

recommendation to 

Marshall.

June 21, 1945 The Interim Committee, 

supporting its Scientific 

Panel, rejects the Franck 

Report recommendation 

that the bomb be 

demonstrated prior to 

combat.

July 2-3, 1945 Stimson briefs Truman on 

the In terim Committee’s 

deliberations and outlines 

the peace terms for Japan.

July 16, 1945 Los Alamos scientists 

successfully test a 
5

plutonium implosion 

bomb in the Trinity shot at 

Alamogordo, New Mexico.

July 17- Truman, Churchill, and 

August 2, 1945 Stalin meet in Potsdam.

July 21, 1945 Groves sends Stimson a 

report on the Trinity test.

July 24, 1945 Stimson again briefs 

Truman on the Manhattan 

Project and peace terms 

for Japan. In an evening 

session, Truman informs 

Stalin that the United States 

has tested a powerful new 

weapon.

July 25, 1945 The 509th Composite 

Group is ordered to attack 

Japan with an atomic bomb 

“after about” August 3.

July 26, 1945 Truman, Chinese President 

Chiang Kai-Shek, and new 

British Prime Minister 

Clement Atlee issue the 

Potsdam Proclamation, 

calling for Japan to 

surrender unconditionally.

July 29, 1945 The Japanese reject the 

Potsdam Proclamation.

August 6, 1945 The gun model uranium 

bomb, call ed Little Boy, is 

dropped on Hiroshima. 

Truman announces the raid 

to the American public.

August 8, 1945 Russia declares war 

on Japan and in vades 

Manchuria.

August 9, 1945 The implosion model 

plutonium bomb, called 

Fat Man, is dropped on 

Nagasaki.

August 12, 1945 The Smyth Report, 

containing unclassified 

technical information 

on the bomb project, is 

released.

August 14, 1945 Japan surrenders.

September 2, 1945 The Japanese sign articles 

of sur render aboard the 

U.S.S. Missouri.

September 9, 1945 S-50 shuts down.

September 1945 Y-12 shutdown begins.

October 3, 1945 Truman advocates passage 

of the May-Johnson bill.

December 20, 1945 Senator Brien McMahon 

introduces a substitute 

to the May-Johnson 

bill, which had been 

losing support, in cluding 

Truman’s.

January 1946 Hearings on the McMahon 

bill begin.

June 14, 1946 Bernard Baruch presents 

the American plan for 

international con trol of 

atomic research.

July 1, 1946 Operation Crossroads 

begins with Shot Able, a 

plutonium bomb drop ped 

from a B-29, at Bikini Atoll.

114
115

July 25, 1946 Operation Crossroads 

continues with Shot 

Baker, a plutonium bomb 

detonated underwater, at 

Bikini Atoll.

August 1, 1946 President Truman signs the 

Atomic Energy Act of 1946, 

a slightly amended version 

of the McMahon bill.

December 1946- The Soviet Union opposes

January 1947 the Baruch Plan, rendering 

it useless.

January 1, 1947 In accordance with the 

Atomic Energy Act of 

1946, all atomic energy 

activities are transferred 

from the Manhattan 

Engineer District to the 

newly created United 

States Atomic Energy 

Commission. The Top 

Policy Group and the 

Military Policy Committee 

had already disbanded.

August 15, 1947 The Manhattan Engineer 

District is abolished.

December 31, 1947 The National Defense 

Research Committee and 

the Office of Scien tific 

Research and Development 

are abolished. Their 

functions are transferred to 

the Department of Defense.
5
5

National Security History Series
5 DOE/MA-0002 Revised The Manhattan Project: Making the Atomic Bomb January 2010