The Atomic Bombings of Hiroshima and Nagasaki
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The Manhattan Engineer District >> The Atomic Bombings of Hiroshima and Nagasaki
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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.
The bomb exploded over Hiroshima at 8:15 on the morning of August 6, 1945.
About an hour previously, the Japanese early warning radar net had detected
the approach of some American aircraft headed for the southern part of
Japan. The alert had been given and radio broadcasting stopped in many
cities, among them Hiroshima. The planes approached the coast at a very
high altitude. At nearly 8:00 A.M., the radar operator in Hiroshima
determined that the number of planes coming in was very small - probably
not more than three - and the air raid alert was lifted. The normal radio
broadcast warning was given to the people that it might be advisable to go
to shelter if B-29's were actually sighted, but no raid was expected beyond
some sort of reconnaissance. At 8:15 A.M., the bomb exploded with a
blinding flash in the sky, and a great rush of air and a loud rumble of
noise extended for many miles around the city; the first blast was soon
followed by the sounds of falling buildings and of growing fires, and a
great cloud of dust and smoke began to cast a pall of darkness over the
city.
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