THE ATOMIC BOMBINGS OF HIROSHIMA AND NAGASAKI

by The Manhattan Engineer District, June 29, 1946.

Index

FOREWORD

INTRODUCTION

THE MANHATTAN PROJECT INVESTIGATING GROUP

PROPAGANDA

SUMMARY OF DAMAGES AND INJURIES

MAIN CONCLUSIONS

THE SELECTION OF THE TARGET

DESCRIPTION OF THE CITIES BEFORE THE BOMBINGS

Hiroshima

Nagasaki

THE ATTACKS

Hiroshima

Nagasaki

GENERAL COMPARISON OF HIROSHIMA AND NAGASAKI

GENERAL DESCRIPTION OF DAMAGE CAUSED BY THE ATOMIC EXPLOSIONS

TOTAL CASUALTIES

THE NATURE OF AN ATOMIC EXPLOSION

CHARACTERISTICS OF THE DAMAGE CAUSED BY THE ATOMIC BOMBS

CALCULATIONS OF THE PEAK PRESSURE OF THE BLAST WAVE

LONG RANGE BLAST DAMAGE

GROUND SHOCK

SHIELDING, OR SCREENING, FROM THE BLAST

FLASH BURN

CHARACTERISTICS OF INJURIES TO PERSONS

BURNS

MECHANICAL INJURIES

BLAST INJURIES

RADIATION INJURIES

SHIELDING FROM RADIATION

EFFECTS OF THE ATOMIC BOMBINGS ON THE INHABITANTS OF THE CITIES

APPENDIX: Father Siemes' eyewitness account

FOREWORD

This report describes the effects of the atomic bombs which were dropped on

the Japanese cities of Hiroshima and Nagasaki on August 6 and 9, 1945,

respectively. It summarizes all the authentic information that is

available on damage to structures, injuries to personnel, morale effect,

etc., which can be released at this time without prejudicing the security

of the United States.

This report has been compiled by the Manhattan Engineer District of the

United States Army under the direction of Major General Leslie R. Groves.

Special acknowledgement to those whose work contributed largely to this

report is made to:

The Special Manhattan Engineer District Investigating Group,

The United States Strategic Bombing Survey,

The British Mission to Japan, and

The Joint Atomic Bomb Investigating Group (Medical). and particularly to

the following individuals:

Col. Stafford L. Warren, Medical Corps, United States Army, for his

evaluation of medical data,

Capt. Henry L. Barnett, Medical Corps, United States Army, for his

evaluation of medical data,

Dr. R. Serber, for his comments on flash burn,

Dr. Hans Bethe, Cornell University, for his information of the nature of

atomic explosions,

Majors Noland Varley and Walter C. Youngs, Corps of Engineers, United

States Army, for their evaluation of physical damage to structures,

J. 0. Hirschfelder, J. L. Magee, M. Hull, and S. T. Cohen, of the Los

Alamos Laboratory, for their data on nuclear explosions,

Lieut. Col. David B. Parker, Corps of Engineers, United States Army, for

editing this report.

INTRODUCTION

Statement by the President of the United States: "Sixteen hours ago an

American airplane dropped one bomb on Hiroshima, Japan, and destroyed its

usefulness to the enemy. That bomb had more power than 20,000 tons of

T.N.T. It had more than two thousand times the blast power of the British

Grand Slam, which is the largest bomb ever yet used in the history of

warfare".

These fateful words of the President on August 6th, 1945, marked the first

public announcement of the greatest scientific achievement in history. The

atomic bomb, first tested in New Mexico on July 16, 1945, had just been

used against a military target.

On August 6th, 1945, at 8:15 A.M., Japanese time, a B-29 heavy bomber

flying at high altitude dropped the first atomic bomb on Hiroshima. More

than 4 square miles of the city were instantly and completely devastated.

66,000 people were killed, and 69,000 injured.

On August 9th, three days later, at 11:02 A.M., another B-29 dropped the

second bomb on the industrial section of the city of Nagasaki, totally

destroying 1 1/2 square miles of the city, killing 39,000 persons, and

injuring 25,000 more.

On August 10, the day after the atomic bombing of Nagasaki, the Japanese

government requested that it be permitted to surrender under the terms of

the Potsdam declaration of July 26th which it had previously ignored.

THE MANHATTAN PROJECT ATOMIC BOMB INVESTIGATING GROUP

On August 11th, 1945, two days after the bombing of Nagasaki, a message was

dispatched from Major General Leslie R. Groves to Brigadier General Thomas

F. Farrell, who was his deputy in atomic bomb work and was representing him

in operations in the Pacific, directing him to organize a special Manhattan

Project Atomic Bomb Investigating Group.

This Group was to secure scientific, technical and medical intelligence in

the atomic bomb field from within Japan as soon as possible after the

cessation of hostilities. The mission was to consist of three groups:

1. Group for Hiroshima.

2. Group for Nagasaki.

3. Group to secure information concerning general Japanese activities in

the field of atomic bombs.

The first two groups were organized to accompany the first American troops

into Hiroshima and Nagasaki.

The primary purposes of the mission were as follows, in order of

importance:

1. To make certain that no unusual hazards were present in the bombed

cities.

2. To secure all possible information concerning the effects of the bombs,

both usual and unusual, and particularly with regard to radioactive

effects, if any, on the targets or elsewhere.

General Groves further stated that all available specialist personnel and

instruments would be sent from the United States, and that the Supreme

Allied Commander in the Pacific would be informed about the organization of

the mission.

On the same day, 11 August, the special personnel who formed the part of

the investigating group to be sent from the United States were selected and

ordered to California with instructions to proceed overseas at once to

accomplish the purposes set forth in the message to General Farrell. The

main party departed from Hamilton Field, California on the morning of 13

August and arrived in the Marianas on 15 August.

On 12 August the Chief of Staff sent the Theater Commander the following

message:

"FOR MACARTHUR, SIGNED MARSHALL:

"GROVES HAS ORDERED FARRELL AT TINIAN TO ORGANIZE A SCIENTIFIC GROUP OF

THREE SECTIONS FOR POTENTIAL USE IN JAPAN IF SUCH USE SHOULD BE DESIRED.

THE FIRST GROUP IS FOR HIROSHIMA, THE SECOND FOR NAGASAKI, AND THE THIRD

FOR THE PURPOSE OF SECURING INFORMATION CONCERNING GENERAL JAPANESE

ACTIVITIES IN THE FIELD OF ATOMIC WEAPONS. THE GROUPS FOR HIROSHIMA AND

NAGASAKI SHOULD ENTER THOSE CITIES WITH THE FIRST AMERICAN TROOPS IN ORDER

THAT THESE TROOPS SHALL NOT BE SUBJECTED TO ANY POSSIBLE TOXIC EFFECTS

ALTHOUGH WE HAVE NO REASON TO BELIEVE THAT ANY SUCH EFFECTS ACTUALLY EXIST.

FARRELL AND HIS ORGANIZATION HAVE ALL AVAILABLE INFORMATION ON THIS

SUBJECT."

General Farrell arrived in Yokohama on 30 August, with the Commanding

General of the 8th Army; Colonel Warren, who was Chief of the Radiological

Division of the District, arrived on 7 September. The main body of the

investigating group followed later. Preliminary inspections of Hiroshima

and Nagasaki were made on 8-9 and 13-14 September, respectively. Members

of the press had been enabled to precede General Farrell to Hiroshima.

The special groups spent 16 days in Nagasaki and 4 days in Hiroshima,

during which time they collected as much information as was possible under

their directives which called for a prompt report. After General Farrell

returned to the U.S. to make his preliminary report, the groups were headed

by Brigadier General J. B. Newman, Jr. More extensive surveys have been

made since that time by other agencies who had more time and personnel

available for the purpose, and much of their additional data has thrown

further light on the effects of the bombings. This data has been duly

considered in the making of this report.

PROPAGANDA

On the day after the Hiroshima strike, General Farrell received

instructions from the War Department to engage in a propaganda campaign

against the Japanese Empire in connection with the new weapon and its use

against Hiroshima. The campaign was to include leaflets and any other

propaganda considered appropriate. With the fullest cooperation from

CINCPAC of the Navy and the United States Strategic Air Forces, he

initiated promptly a campaign which included the preparation and

distribution of leaflets, broadcasting via short wave every 15 minutes over

radio Saipan and the printing at Saipan and distribution over the Empire of

a Japanese language newspaper which included the description and

photographs of the Hiroshima strike.

The campaign proposed:

1. Dropping 16,000,000 leaflets in a period of 9 days on 47 Japanese cities

with population of over 100,000. These cities represented more than 40% of

the total population.

2. Broadcast of propaganda at regular intervals over radio Saipan.

3. Distribution of 500,000 Japanese language newspapers containing

stories and pictures of the atomic bomb attacks.

The campaign continued until the Japanese began their surrender

negotiations. At that time some 6,000,000 leaflets and a large number of

newspapers had been dropped. The radio broadcasts in Japanese had been

carried out at regular 15 minute intervals.

SUMMARY OF DAMAGES AND INJURIES

Both the Hiroshima and the Nagasaki atomic bombs exhibited similar effects.

The damages to man-made structures and other inanimate objects was the

result in both cities of the following effects of the explosions:

A. Blast, or pressure wave, similar to that of normal explosions.

B. Primary fires, i.e., those fires started instantaneously by the heat

radiated from the atomic explosion.

C. Secondary fires, i.e., those fires resulting from the collapse of

buildings, damage to electrical systems, overturning of stoves, and other

primary effects of the blast.

D. Spread of the original fires (B and C) to other structures.

The casualties sustained by the inhabitants of both cities were due to:

A. "Flash" burns, caused directly by the almost instantaneous radiation of

heat and light at the moment of the explosion.

B. Burns resulting from the fires caused by the explosion.

C. Mechanical injuries caused by collapse of buildings, flying debris,

and forceable hurling - about of persons struck by the blast pressure

waves.

D. Radiation injuries caused by the instantaneous penetrating radiation

(in many respects similar to excessive X-ray exposure) from the nuclear

explosion; all of these effective radiations occurred during the first

minute after initiation of the explosion, and nearly all occurred during

the first second of the explosion.

No casualties were suffered as a result of any persistent radioactivity of

fission products of the bomb, or any induced radioactivity of objects near

the explosion. The gamma radiations emitted by the nuclear explosion did

not, of course, inflict any damage on structures.

The number of casualties which resulted from the pure blast effect alone

(i.e., because of simple pressure) was probably negligible in comparison to

that caused by other effects.

The central portions of the cities underneath the explosions suffered

almost complete destruction. The only surviving objects were the frames of

a small number of strong reinforced concrete buildings which were not

collapsed by the blast; most of these buildings suffered extensive damage

from interior fires, had their windows, doors, and partitions knocked out,

and all other fixtures which were not integral parts of the reinforced

concrete frames burned or blown away; the casualties in such buildings near

the center of explosion were almost 100%. In Hiroshima fires sprang up

simultaneously all over the wide flat central area of the city; these fires

soon combined in an immense "fire storm" (high winds blowing inwards toward

the center of a large conflagration) similar to those caused by ordinary

mass incendiary raids; the resulting terrific conflagration burned out

almost everything which had not already been destroyed by the blast in a

roughly circular area of 4.4 square miles around the point directly under

the explosion (this point will hereafter in this report be referred to as

X). Similar fires broke out in Nagasaki, but no devastating fire storm

resulted as in Hiroshima because of the irregular shape of the city.

In both cities the blast totally destroyed everything within a radius of 1

mile from the center of explosion, except for certain reinforced concrete

frames as noted above. The atomic explosion almost completely destroyed

Hiroshima's identity as a city. Over a fourth of the population was killed

in one stroke and an additional fourth seriously injured, so that even if

there had been no damage to structures and installations the normal city

life would still have been completely shattered. Nearly everything was

heavily damaged up to a radius of 3 miles from the blast, and beyond this

distance damage, although comparatively light, extended for several more

miles. Glass was broken up to 12 miles.

In Nagasaki, a smaller area of the city was actually destroyed than in

Hiroshima, because the hills which enclosed the target area restricted the

spread of the great blast; but careful examination of the effects of the

explosion gave evidence of even greater blast effects than in Hiroshima.

Total destruction spread over an area of about 3 square miles. Over a

third of the 50,000 buildings in the target area of Nagasaki were destroyed

or seriously damaged. The complete destruction of the huge steel works and

the torpedo plant was especially impressive. The steel frames of all

buildings within a mile of the explosion were pushed away, as by a giant

hand, from the point of detonation. The badly burned area extended for 3

miles in length. The hillsides up to a radius of 8,000 feet were scorched,

giving them an autumnal appearance.

MAIN CONCLUSIONS

The following are the main conclusions which were reached after thorough

examination of the effects of the bombs dropped on Hiroshima and Nagasaki:

1. No harmful amounts of persistent radioactivity were present after the

explosions as determined by:

A. Measurements of the intensity of radioactivity at the time of the

investigation; and

B. Failure to find any clinical evidence of persons harmed by persistent

radioactivity.

The effects of the atomic bombs on human beings were of three main types:

A. Burns, remarkable for (1) the great ground area over which they were

inflicted and (2) the prevalence of "flash" burns caused by the

instantaneous heat radiation.

B. Mechanical injuries, also remarkable for the wide area in which

suffered.

C. Effects resulting from penetrating gamma radiation. The effects from

radiation were due to instantaneous discharge of radiation at the moment of

explosion and not to persistent radioactivity (of either fission products

or other substances whose radioactivity might have been induced by

proximity to the explosions).

The effects of the atomic bombs on structures and installations were of two

types:

A. Destruction caused by the great pressure from the blast; and

B. Destruction caused by the fires, either started directly by the great

heat radiation, or indirectly through the collapse of buildings, wiring,

etc.

4. The actual tonnage of T.N.T. which would have caused the same blast

damage was approximately of the order of 20,000 tons.

5. In respect to their height of burst, the bombs performed exactly

according to design.

6. The bombs were placed in such positions that they could not have done

more damage from any alternative bursting point in either city.

7. The heights of burst were correctly chosen having regard to the type of

destruction it was desired to cause.

8. The information collected would enable a reasonably accurate prediction

to be made of the blast damage likely to be caused in any city where an

atomic explosion could be effected.

THE SELECTION OF THE TARGET

Some of the most frequent queries concerning the atomic bombs are those

dealing with the selection of the targets and the decision as to when the

bombs would be used.

The approximate date for the first use of the bomb was set in the fall of

1942 after the Army had taken over the direction of and responsibility for

the atomic bomb project. At that time, under the scientific assumptions

which turned out to be correct, the summer of 1945 was named as the most

likely date when sufficient production would have been achieved to make it

possible actually to construct and utilize an atomic bomb. It was

essential before this time to develop the technique of constructing and

detonating the bomb and to make an almost infinite number of scientific and

engineering developments and tests. Between the fall of 1942 and June

1945, the estimated probabilities of success had risen from about 60% to

above 90%; however, not until July 16, 1945, when the first full-scale test

took place in New Mexico, was it conclusively proven that the theories,

calculations, and engineering were correct and that the bomb would be

successful.

The test in New Mexico was held 6 days after sufficient material had become

available for the first bomb. The Hiroshima bomb was ready awaiting

suitable weather on July 31st, and the Nagasaki bomb was used as soon after

the Hiroshima bomb as it was practicable to operate the second mission.

The work on the actual selection of targets for the atomic bomb was begun

in the spring of 1945. This was done in close cooperation with the

Commanding General, Army Air Forces, and his Headquarters. A number of

experts in various fields assisted in the study. These included

mathematicians, theoretical physicists, experts on the blast effects of

bombs, weather consultants, and various other specialists. Some of the

important considerations were:

A. The range of the aircraft which would carry the bomb.

B. The desirability of visual bombing in order to insure the most

effective use of the bomb.

C. Probable weather conditions in the target areas.

D. Importance of having one primary and two secondary targets for each

mission, so that if weather conditions prohibited bombing the target there

would be at least two alternates.

E. Selection of targets to produce the greatest military effect on the

Japanese people and thereby most effectively shorten the war.

F. The morale effect upon the enemy.

These led in turn to the following:

A. Since the atomic bomb was expected to produce its greatest amount of

damage by primary blast effect, and next greatest by fires, the targets

should contain a large percentage of closely-built frame buildings and

other construction that would be most susceptible to damage by blast and

fire.

B. The maximum blast effect of the bomb was calculated to extend over an

area of approximately 1 mile in radius; therefore the selected targets

should contain a densely built-up area of at least this size.

C. The selected targets should have a high military strategic value.

D. The first target should be relatively untouched by previous bombing, in

order that the effect of a single atomic bomb could be determined.

The weather records showed that for five years there had never been two

successive good visual bombing days over Tokyo, indicating what might be

expected over other targets in the home islands. The worst month of the

year for visual bombing was believed to be June, after which the weather

should improve slightly during July and August and then become worse again

during September. Since good bombing conditions would occur rarely, the

most intense plans and preparations were necessary in order to secure

accurate weather forecasts and to arrange for full utilization of whatever

good weather might occur. It was also very desirable to start the raids

before September.

DESCRIPTION OF THE CITIES BEFORE THE BOMBINGS

Hiroshima

The city of Hiroshima is located on the broad, flat delta of the Ota River,

which has 7 channel outlets dividing the city into six islands which

project into Hiroshima Bay. The city is almost entirely flat and only

slightly above sea level; to the northwest and northeast of the city some

hills rise to 700 feet. A single hill in the eastern part of the city

proper about 1/2 mile long and 221 feet in height interrupted to some

extent the spreading of the blast damage; otherwise the city was fully

exposed to the bomb. Of a city area of over 26 square miles, only 7

square miles were completely built-up. There was no marked separation of

commercial, industrial, and residential zones. 75% of the population was

concentrated in the densely built-up area in the center of the city.

Hiroshima was a city of considerable military importance. It contained the

2nd Army Headquarters, which commanded the defense of all of southern

Japan. The city was a communications center, a storage point, and an

assembly area for troops. To quote a Japanese report, "Probably more than

a thousand times since the beginning of the war did the Hiroshima citizens

see off with cries of 'Banzai' the troops leaving from the harbor."

The center of the city contained a number of reinforced concrete buildings

as well as lighter structures. Outside the center, the area was congested

by a dense collection of small wooden workshops set among Japanese houses;

a few larger industrial plants lay near the outskirts of the city. The

houses were of wooden construction with tile roofs. Many of the industrial

buildings also were of wood frame construction. The city as a whole was

highly susceptible to fire damage.

Some of the reinforced concrete buildings were of a far stronger

construction than is required by normal standards in America, because of

the earthquake danger in Japan. This exceptionally strong construction

undoubtedly accounted for the fact that the framework of some of the

buildings which were fairly close to the center of damage in the city did

not collapse.

The population of Hiroshima had reached a peak of over 380,000 earlier in

the war but prior to the atomic bombing the population had steadily

decreased because of a systematic evacuation ordered by the Japanese

government. At the time of the attack the population was approximately

255,000. This figure is based on the registered population, used by the

Japanese in computing ration quantities, and the estimates of additional

workers and troops who were brought into the city may not be highly

accurate. Hiroshima thus had approximately the same number of people as

the city of Providence, R.I., or Dallas, Tex.

Nagasaki

Nagasaki lies at the head of a long bay which forms the best natural harbor

on the southern Japanese home island of Kyushu. The main commercial and

residential area of the city lies on a small plain near the end of the bay.

Two rivers divided by a mountain spur form the two main valleys in which

the city lies. This mountain spur and the irregular lay-out of the city

tremendously reduced the area of destruction, so that at first glance

Nagasaki appeared to have been less devastated than Hiroshima.

The heavily build-up area of the city is confined by the terrain to less

than 4 square miles out of a total of about 35 square miles in the city as

a whole.

The city of Nagasaki had been one of the largest sea ports in southern

Japan and was of great war-time importance because of its many and varied

industries, including the production of ordnance, ships, military

equipment, and other war materials. The narrow long strip attacked was of

particular importance because of its industries.

In contrast to many modern aspects of Nagasaki, the residences almost

without exception were of flimsy, typical Japanese construction, consisting

of wood or wood-frame buildings, with wood walls with or without plaster,

and tile roofs. Many of the smaller industries and business establishments

were also housed in wooden buildings or flimsily built masonry buildings.

Nagasaki had been permitted to grow for many years without conforming to

any definite city zoning plan and therefore residences were constructed

adjacent to factory buildings and to each other almost as close as it was

possible to build them throughout the entire industrial valley.

THE ATTACKS

Hiroshima

Hiroshima was the primary target of the first atomic bomb mission. The

mission went smoothly in every respect. The weather was good, and the

crew and equipment functioned perfectly. In every detail, the attack was

carried out exactly as planned, and the bomb performed exactly as expected.