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THE ATOMIC BOMBINGS OF HIROSHIMA AND NAGASAKI
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 EXPLO
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FOREWORD
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 Majo
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INTRODUCTION
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 achieveme
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THE MANHATTAN PROJECT ATOMIC BOMB INVESTIGATING GROUP
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 o
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PROPAGANDA
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 lea
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SUMMARY OF DAMAGES AND INJURIES
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, ove
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MAIN CONCLUSIONS
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:
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THE SELECTION OF THE TARGET
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
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DESCRIPTION OF THE CITIES BEFORE THE BOMBINGS
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
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THE ATTACKS
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
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GENERAL COMPARISON OF HIROSHIMA AND NAGASAKI
GENERAL COMPARISON OF HIROSHIMA AND NAGASAKI
It was not at first apparent to even trained observers visiting the two Japanese cities which of the two bombs had been the most effective. In some respects, Hiroshima looked worse than Nagasaki. The fire damage in Hiroshima was much more complete; the center of the city was hit and everything but the reinforced concrete buildings had virtually disappeared. A desert of clear-swept, charred remains, with only a few strong building frames left standing was a terrifying sight. At Nagasaki there wer
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GENERAL DESCRIPTION OF DAMAGE CAUSED BY THE ATOMIC EXPLOSIONS
GENERAL DESCRIPTION OF DAMAGE CAUSED BY THE ATOMIC EXPLOSIONS
In considering the devastation in the two cities, it should be remembered that the cities' differences in shape and topography resulted in great differences in the damages. Hiroshima was all on low, flat ground, and was roughly circular in shape; Nagasaki was much cut up by hills and mountain spurs, with no regularity to its shape. In Hiroshima almost everything up to about one mile from X was completely destroyed, except for a small number (about 50) of heavily reinforced concrete buildings, mo
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TOTAL CASUALTIES
TOTAL CASUALTIES
There has been great difficulty in estimating the total casualties in the Japanese cities as a result of the atomic bombing. The extensive destruction of civil installations (hospitals, fire and police department, and government agencies) the state of utter confusion immediately following the explosion, as well as the uncertainty regarding the actual population before the bombing, contribute to the difficulty of making estimates of casualties. The Japanese periodic censuses are not complete. Fin
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THE NATURE OF AN ATOMIC EXPLOSION
THE NATURE OF AN ATOMIC EXPLOSION
The most striking difference between the explosion of an atomic bomb and that of an ordinary T.N.T. bomb is of course in magnitude; as the President announced after the Hiroshima attack, the explosive energy of each of the atomic bombs was equivalent to about 20,000 tons of T.N.T. But in addition to its vastly greater power, an atomic explosion has several other very special characteristics. Ordinary explosion is a chemical reaction in which energy is released by the rearrangement of the atoms o
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CHARACTERISTICS OF THE DAMAGE CAUSED BY THE ATOMIC BOMBS
CHARACTERISTICS OF THE DAMAGE CAUSED BY THE ATOMIC BOMBS
The damage to man-made structures caused by the bombs was due to two distinct causes: first the blast, or pressure wave, emanating from the center of the explosion, and, second, the fires which were caused either by the heat of the explosion itself or by the collapse of buildings containing stoves, electrical fixtures, or any other equipment which might produce what is known as a secondary fire, and subsequent spread of these fires. The blast produced by the atomic bomb has already been stated t
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CALCULATIONS OF THE PEAK PRESSURE OF THE BLAST WAVE
CALCULATIONS OF THE PEAK PRESSURE OF THE BLAST WAVE
Several ingenious methods were used by the various investigators to determine, upon visiting the wrecked cities, what had actually been the peak pressures exerted by the atomic blasts. These pressures were computed for various distances from X, and curves were then plotted which were checked against the theoretical predictions of what the pressures would be. A further check was afforded from the readings obtained by the measuring instruments which were dropped by parachute at each atomic attack.
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LONG RANGE BLAST DAMAGE
LONG RANGE BLAST DAMAGE
There was no consistency in the long range blast damage. Observers often thought that they had found the limit, and then 2,000 feet farther away would find further evidence of damage. The most impressive long range damage was the collapse of some of the barracks sheds at Kamigo, 23,000 feet south of X in Nagasaki. It was remarkable to see some of the buildings intact to the last details, including the roof and even the windows, and yet next to them a similar building collapsed to ground level. T
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GROUND SHOCK
GROUND SHOCK
The ground shock in most cities was very light. Water pipes still carried water and where leaks were visible they were mainly above ground. Virtually all of the damage to underground utilities was caused by the collapse of buildings rather than by any direct exertion of the blast pressure. This fact of course resulted from the bombs' having been exploded high in the air....
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SHIELDING, OR SCREENING FROM BLAST
SHIELDING, OR SCREENING FROM BLAST
In any explosion, a certain amount of protection from blast may be gained by having any large and substantial object between the protected object and the center of the explosion. This shielding effect was noticeable in the atomic explosions, just as in ordinary cases, although the magnitude of the explosions and the fact that they occurred at a considerable height in the air caused marked differences from the shielding which would have characterized ordinary bomb explosions. The outstanding exam
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FLASH BURN
FLASH BURN
As already stated, a characteristic feature of the atomic bomb, which is quite foreign to ordinary explosives, is that a very appreciable fraction of the energy liberated goes into radiant heat and light. For a sufficiently large explosion, the flash burn produced by this radiated energy will become the dominant cause of damage, since the area of burn damage will increase in proportion to the energy released, whereas the area of blast damage increases only with the two-thirds power of the energy
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CHARACTERISTICS OF THE INJURIES TO PERSONS
CHARACTERISTICS OF THE INJURIES TO PERSONS
Injuries to persons resulting from the atomic explosions were of the following types: A. Burns, from 1. Flash radiation of heat 2. Fires started by the explosions. B. Mechanical injuries from collapse of buildings, flying debris, etc. C. Direct effects of the high blast pressure, i.e., straight compression. D. Radiation injuries, from the instantaneous emission of gamma rays and neutrons. It is impossible to assign exact percentages of casualties to each of the types
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BURNS
BURNS
Two types of burns were observed. These are generally differentiated as flame or fire burn and so-called flash burn. The early appearance of the flame burn as reported by the Japanese, and the later appearance as observed, was not unusual. The flash burn presented several distinctive features. Marked redness of the affected skin areas appeared almost immediately, according to the Japanese, with progressive changes in the skin taking place over a period of a few hours. When seen after 50 days, th
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MECHANICAL INJURIES
MECHANICAL INJURIES
The mechanical injuries included fractures, lacerations, contusions, abrasions, and other effects to be expected from falling roofs, crumbling walls, flying debris and glass, and other indirect blast effects. The appearance of these various types of mechanical injuries was not remarkable to the medical authorities who studied them. It was estimated that patients with lacerations at Hiroshima were less than 10,600 feet from X, whereas at Nagasaki they extended as far as 12,200 feet. The tremendou
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BLAST INJURIES
BLAST INJURIES
No estimate of the number of deaths or early symptoms due to blast pressure can be made. The pressures developed on the ground under the explosions were not sufficient to kill more than those people very near the center of damage (within a few hundred feet at most). Very few cases of ruptured ear drums were noted, and it is the general feeling of the medical authorities that the direct blast effects were not great. Many of the Japanese reports, which are believed to be false, describe immediate
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RADIATION INJURIES
RADIATION INJURIES
As pointed out in another section of this report the radiations from the nuclear explosions which caused injuries to persons were primarily those experienced within the first second after the explosion; a few may have occurred later, but all occurred in the first minute. The other two general types of radiation, viz., radiation from scattered fission products and induced radioactivity from objects near the center of explosion, were definitely proved not to have caused any casualties. The proper
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SHIELDING FROM RADIATION
SHIELDING FROM RADIATION
Exact figures on the thicknesses of various substances to provide complete or partial protection from the effects of radiation in relation to the distance from the center of explosion, cannot be released at this time. Studies of collected data are still under way. It can be stated, however, that at a reasonable distance, say about 1/2 mile from the center of explosion, protection to persons from radiation injury can be afforded by a layer of concrete or other material whose thickness does not pr
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EFFECTS OF THE ATOMIC BOMBINGS ON THE INHABITANTS OF THE BOMBED CITIES
EFFECTS OF THE ATOMIC BOMBINGS ON THE INHABITANTS OF THE BOMBED CITIES
In both Hiroshima and Nagasaki the tremendous scale of the disaster largely destroyed the cities as entities. Even the worst of all other previous bombing attacks on Germany and Japan, such as the incendiary raids on Hamburg in 1943 and on Tokyo in 1945, were not comparable to the paralyzing effect of the atomic bombs. In addition to the huge number of persons who were killed or injuried so that their services in rehabilitation were not available, a panic flight of the population took place from
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