Chapter III
How the Atomic Bomb Works

Out of the stories of Hiroshima and Nagasaki can be built up, detail by detail, the picture of how the atomic bomb works--the different forms of energy given off, the velocity and intensity of each, the sort of effects each has on animate and inanimate objects. In these factors is the real story of what happened at Hiroshima and Nagasaki, for in them chance circumstances are ruled out.

Spectators' accounts, whether of the New Mexico, the Hiroshima, or the Nagasaki explosion, describe similar pictures. At Nagasaki, for example, the bomb exploded at 1102 with a tremendous flash of blue-white light, like a giant magnesium flare. The flash was accompanied by a rush of heat and was followed by a huge pressure wave and the rumbling sound of the explosion. Curiously enough, this sound was not distinctly noted by those who survived near the center of the explosion, although it was heard as far as 15 miles away. People on the hillsides in the country at a considerable distance from Nagasaki told of seeing the blue-white and then multicolored flash over the city, followed some seconds later by a tremendous clap, like thunder very close overhead. A huge snow-white cloud shot rapidly into the sky and the scene on the ground was obscured first by a bluish haze and then by a purple-brown cloud of dust and smoke.

The survivors were not aware at the time that a radically new bomb had been used. They were conscious of an explosion of tremendous power, but even the Government had no conception, until President Truman's announcement was broadcast, of the new principle of operation. If we strip our minds of any lingering prejudice that the atomic bomb is supernatural or incomprehensible in its operation, we shall see why its uniqueness was not at first recognized.

A. The Nature of the Explosion

The atomic bomb works by explosion. An explosion is, in the words of the Smyth report, simply a "sudden and violent release of a large amount of energy in a small region." As do ordinary high explosives, atomic bombs release energy, though on an unprecedented scale. The energy takes three forms (one of which is new), and all the effects of the bomb can be referred directly to these three kinds of energy. They are:
  1. Heat (which is present in other explosions, as the familiar injuries known as "flash burns" on warships illustrate, but ordinarily not at high enough diffused temperatures to burn a man or set fire to combustible objects at any considerable distance from the explosion).

  2. Radiation (similar to X-rays or to that from radium).

  3. Blast or pressure (as from a demolition bomb).

The whole discussion of the effects of the atomic bomb will be phrased in terms of these three kinds of energy. No other more mysterious or immeasurable forces acted; these were all.

These were enough. The energy released in atomic explosion is of such magnitude and from so concentrated a source that it sets entirely new problems in its use or in protection against it. Ordinary burning or explosion is a chemical reaction in which energy is released during the rearrangement of the atoms of the explosive material. In an atomic reaction, however, the identity of the atoms, not simply their arrangement, is changed. The change is more fundamental: in it, matter is transformed into energy. The energy released when a pound of nitroglycerine explodes would, when converted into heat, raise the temperature of 150 pounds of water by 18° F. The explosion of a pound of uranium would produce an equal temperature rise in 2 billion pounds of water! Clearly, only a small part of the mass in the bomb's active core need be transformed to give an explosion of tremendous power.

At the time of the explosion, then, energy was given off in the forms of light, heat, gamma radiation, and pressure. The whole range of radiations, indeed, seems to have been present. There were heat radiations in the low frequency band


below infrared, visible waves of all colors (as the eyewitness accounts show), and penetrating radiations of very high frequency generally grouped as "gamma rays." Light and radiant heat ("flash heat") sped out in all directions at a rate of 186,000 miles per second, and the gamma rays at the same rate (though their effect was not immediately obvious). The shock waves travelled much more slowly. It may be inferred from tests with high explosives that the rate at a relative short distance from the point of explosion was about 2 miles per second, and dropped rapidly to the speed of sound, or about one-fifth of a mile per second. Thus the light, heat, and gamma radiation reached the target first, followed by shock and sound and the high winds of the blast.

B. Heat

The center of the explosions--several hundred feet above ground--was a ball of fire. Because the radiant heat given off at the explosion easily charred combustible objects while ceasing so quickly that surfaces not in the direct line of radiation were unaffected, there are clearly marked "shadows" visible where objects were shielded against the heat. By projecting back the sharply defined outlines of these shadows, Japanese and Allied scientists have determined the height and diameter of the fireball. The two fireballs were apparently several hundred feet in diameter. The temperature at their core was virtually inconceivable--millions of degrees centigrade. Even at its edge, the temperature was several thousand degrees; reasoning from the heat effects observed on human beings, bubbled roof tile, and combustible materials, Japanese and Allied scientists have placed the figure variously between 3,000° and 9,000° C. Energy given off in heat alone was estimated by Japanese physicists at the astronomical figure of 1013 calories.

The flash heat was intense enough to cause fires, despite the distance of the fireball from the ground. Clothing ignited, though it could be quickly beaten out, telephone poles charred, thatched roofs of houses caught fire. In Hiroshima, the explosion started hundreds of fires almost simultaneously, the most distant of which was found 13,700 feet from ground zero; this, however, probably started when a building with a thatched roof collapsed onto a hot charcoal fire. Fires were started directly by flash heat in such easily ignitible substances as dark cloth, paper, or dry-rotted wood, within about 3,500 feet of ground zero; white-painted, concrete-faced or cement-stuccoed structures reflected the heat and did not ignite. A cedar bark roof and the top of a dry-rotted wooden platform 5,200 feet west of ground zero, were reported to have been ignited by the bomb flash. The majority of initial fires in buildings, however, were started by secondary sources ( kitchen charcoal fires, electric short-circuits, industrial process fires, etc.) In Nagasaki, both Japanese and American fire experts agreed that more fires were caused directly that indirectly, in a ratio of 60 to 40. The range of primary fire there is reported to have exceeded 10,000 feet.

Charred telephone poles were discernible for 10,000 feet south and 13,000 feet north of ground zero at Hiroshima, and for 13,000 feet or more at Nagasaki. Bubbling of roof tile occurred at Hiroshima from ground zero out to 4,000 feet, though with only scattered frequency after 2,000 feet. The same phenomenon was reported at Nagasaki, accompanied again by scarring and peeling of granitic rocks, almost a mile from ground zero. A similar bubbled surface was obtained at the National Bureau of Standards by heating a sample of the tile to 1,800° C. for a period of 4 seconds. The effect so produced extended deeper into the tile than did the bubbling caused by the atomic bomb, which indicates that the explosion of the bomb subjected the tile to a temperature of more than 1,800° C. for less than 4 seconds.

Persons reported feeling heat on their skin as far away as 24,000 feet. Burns of unprotected skin certainly occurred up to 12,000 to 13,000 feet, and reportedly up to 15,000 feet--nearly 3 miles. Serious or third-degree burns were suffered by those directly exposed within 4,500 feet, and occasionally as remote as 7,200 feet. In the immediate area of ground zero, the heat charred corpses beyond recognition.

Clothing as well as buildings afforded considerable protection against the flash. Even a clump of grass or tree leaf was, on occasion, adequate.

The implication clearly is that the duration of the flash was less than the time required for the grass or leaf to shrivel. While an accurate estimate is not possible, the duration could hardly have exceeded a fraction of a second.

C. Radiation

From the chain reaction which produced the mass release of energy in the explosion, a wide


NAGASAKI--Blistered tile found at ground zero.

"Shadow" of band valve wheel on paint of a gas holder at Hiroshima. Radiant heat instantly burned paint where the heat rays were not obstructed. 6,300 feet from ground zero (Japanese photo).


NEW SHOOTS are appearing on this limb of a chestnut tree, about 2,100 feet south of ground zero at Nagasaki, 2 months after that attack, even though the leaves were burned and withered at the time of the explosion (Japanese photo).

TREES SPLINTERED BY BLAST on a Nagasaki hillside, 2,700 feet southwest of ground zero (Japanese photo).


range of radiations were released. The light and heat are familiar elements of explosions, but the free neutrons and high-frequency radiations such as gamma rays are a new phenomenon. These radiations are highly penetrating and lethal.

The damaging penetration of radiation would be possible from three sources:

  1. From the high-frequency radiations, whether neutrons, gamma rays, or other unspecified rays, released in the chain reaction of the bomb.

  2. From lingering radioactivity from deposits of primary fission products scattered in the explosion.

  3. From induced radioactivity in the bombed area, caused by interaction of neutrons with matter penetrated.

Only the first cause seems to have had important effects, though there are detectable pockets of radioactivity in both cities. At Takasu, 10,000 feet from ground zero at Hiroshima, and at Nishiyama, 6,500 feet from ground zero in Nagasaki, scientific measurements weeks after the explosion showed radioactivity. Presumably this was from deposits of primary fission products rather than induced radioactivity. In tests of the ground and bones of victims of radiation disease, certain substances--phosphorus, barium, strontium, rare earths--have shown radioactivity. Though evidence of lingering radioactivity is slight, it is strong enough to leave open the ominous possibility of a different situation had the bomb exploded at ground level.

The radiation apparently had no lasting effects on the soil or vegetation: Seeds later planted within a few hundred feet of ground zero grew normally. Examination of subsurface soil in t he immediate area showed presence of earthworms and other life only a few inches below the surface. The effect on human procreation is as yet undetermined, but pregnant women within a mile of ground zero showed an increased number of miscarriages, and there was in most cases a low sperm count among men in the same area. Stories of harmful effects on people who came into the area after the explosion have been disproved by investigation.

The rays proved lethal for an average radius of 3,000 feet from ground zero. They caused loss of hair up to 7,500 feet and occasionally beyond, and other mild effects up to almost 2 miles.

D. Blast

The pressure or shock wave travelled out in all directions from the explosion. The blast effects produced were uniform, and essentially those of conventional large high-explosive weapons though on a much larger scale. Thus, instead of localized effects such as the collapse of a roof truss or wall panel, entire buildings were crushed or distorted as units.

The blast pressure, as with high explosives, rose almost instantaneously to a peak, declined more slowly, and then fell below atmospheric pressure for a period about three times the period during which it was above atmospheric pressure. The positive period--that during which the pressure was greater than atmospheric--was of much greater peak pressure than the succeeding, or negative phase. Short though the positive phase was--probably only slightly longer than a second--it lasted longer than the positive phase of ordinary bombs. Thus the effect of the atomic bomb on buildings was usually that of a powerful push which shoved buildings over or left them leaning, whereas high explosive bombs strike sharply and much more briefly and tend to punch holes in walls. The duration was also long enough so that almost all building failures came during the positive phase. Comparatively few evidences were found of failures of members during the longer but less intense negative phase; window shutters blown outwards toward the explosion were very rare.

Experiments with high explosives have shown that the face-on peak pressures are approximately two to five times as intense as side-on peak pressures; thus greater damage was inflicted on walls or roofs facing the blast than on similar surfaces parallel to the blast. Near ground zero, the blast struck almost vertically downward. Buildings were crushed if weak, or the roofs were crushed in with little or no damage to the walls. Trunks of trees remained standing, but stripped of their branches; telephone poles, pushed farther out, also remained erect near the center. Many small buildings were virtually engulfed in the pressure wave and simultaneously crushed from different directions. At somewhat greater distances, both horizontal and vertical components of the blast were appreciable, and buildings suffered damage both to roofs and to walls facing the explosion. At considerable distances, where the blast was travelling


BLAST STRUCK DOWNWARD against the roof of the Chinzei School, 1,500 feet from ground zero at Nagasaki, which had been taken over in part for munitions work. The fourth story collapsed completely, but the heavy earthquake-resistant structure protected some machine tools in the first floor from serious damage. Electric transformers and a switchboard did not escape; a combination of blast, fire, and debris destroyed them.


in an almost horizontal direction, damage was predominantly inflicted on walls during the blast. In such cases, the buildings were often completely racked by the inability of roof truss members to transmit the pressure to the far walls.

Shielding was more important at Nagasaki than at Hiroshima, because of the hills that divided the city. Building restrictions in Japan after the 1923 earthquake limited building heights to 100 feet; thus there was little shielding by buildings from these airburst bombs.

Reflection and diffraction effects were observed. Had the blast travelled in completely straight lines, more buildings would have survived in Nagasaki than actually did. Reflection effects were most clearly observed in the destruction of parapet walls of roofs on the side away from the bomb, where reflection of the blast wave from the roof reinforced the blast impinging on the wall directly. They were also visible in the displacing and cracking of concrete decks of bridges within 1,000 feet of ground zero, where reflection of the blast wave from the water struck the bridges where their resistance was least.

The resistance of buildings depended very largely on their construction, as two examples show.

  1. In the area between 2,000 and 3,000 feet from ground zero at Nagasaki, only 9.5 percent of the floor area of reinforced concrete buildings was destroyed or structurally damaged. Yet in the ring between 4,000 and 5,000 feet from ground zero, 56 percent of such buildings was destroyed or structurally damaged. Careful examination showed that the difference lay solely in design, construction detail, and materials: The bomb detonated over a section containing the most carefully and strongly built buildings in the city, the majority multistory earthquake resistant structures. This strength more than compensated for the greater intensity of blast. A rapidly diminishing blast was capable of serious damage to weaker buildings further away, mostly high, single-story industrial buildings, with thin, shell-type, arch roofs.

  2. At both cities, steel-framed buildings with corrugated asbestos walls and roofs suffered less structural damage than those with corrugated iron or sheet-metal walls and roofs. The corrugated asbestos crumbled easily, permitting the blast pressure to equalize itself rapidly around the main framing members, but the steel siding transferred the pressure to the structural members, causing distortion or general collapse.

The limits of blast effects extended 8 miles out, where some glass reportedly shattered in Hiroshima; at the same city, some roof stripping and disturbance of tiles was inflicted at the Japan Steel Co., 4.1 miles from ground zero.

In analyzing the extent of the destruction wrought by the bombs, it is necessary to discriminate between the two cities and between different types of buildings. Equivalent effects are found at Nagasaki over greater areas. Structural damage to reinforced concrete buildings, both earthquake resistant and non earthquake resistant, occurred within an area of 0.05 square mile at Hiroshima, but at Nagasaki similar severe damage was inflicted in an area of 0.43 square mile.

Severe damage to one-story light steel frame buildings was equally extensive at the two cities; the area was 3.3 square miles at Nagasaki and 3.4 square miles at Hiroshima. Heavy steel frame buildings would be studied only at Nagasaki, where they suffered structural damage over an area of 1.8 square miles.

One-story brick buildings with load bearing walls were severely damaged within an area of 8.1 square miles at Nagasaki, and within an area of 6 square miles at Hiroshima. Multistory brick buildings, which were studied only at Hiroshima, were severely damaged within an area of 3.6 square miles.

Wood domestic buildings were severely damaged within an area of 7.5 square miles at Nagasaki, and within an area of 6 square miles at Hiroshima. Wood frame industrial and commercial buildings, which were of inferior construction, were severely damaged within 9.9 square miles at Nagasaki, and 8.5 square miles at Hiroshima.

Maximum blast pressures fall off very rapidly as the distance from the detonation increases. In the two bombed cities, thus, reinforced concrete buildings of good construction were structurally damaged only when within a few hundred feet of ground zero. Indeed, ground zero itself was too distant from air zero for the earthquake-resistant buildings to be collapsed. It is the opinion of the Survey's engineers that at Hiroshima more thorough destruction near ground zero, without significant loss in the scope of destruction, could have been achieved had the bomb been detonated at a lower altitude.


FIRE FRINGE. 8,200 feet from ground zero at Nagasaki, the old police station was completely gutted by fire. Hills protected houses on the right from blast, and fire did not spread to them (Japanese photo).

BLAST BUCKLED THE COLUMNS of this wood frame building, beyond the fire fringe at Hiroshima (7,600 feet from ground zero).


WRECKAGE IN NAGASAKI STREETCAR TERMINAL 1,500 feet north of ground zero. Streetcar in center was blown about 6 feet by the blast (Japanese photo). <

THE HIROSHIMA FIRE DEPARTMENT lost its only aerial ladder truck when the west side main fire station was destroyed by blast and fire, 4,000 feet from ground zero (Japanese photo).


E. The Atomic Bomb compared to other Weapons

In comparing the atomic bomb with other weapons, it is well to remember the importance of the height at which it exploded. Because of this distance from the targets, the atomic bombs did not exert at any point in Hiroshima or Nagasaki the high instantaneous peak pressures of even small high explosive bombs. For example, a single 100-pound bomb exploding at ground level exerts a higher blast pressure over an area of 1,000 square feet (for about 18 feet around its point of detonation) than did the atomic bomb at any point in either city.

That fact will place comparisons of the radii of effectiveness in the proper perspective. Even at the heights from which the atomic bomb was exploded in Japan, its blast effects were on a new scale because the duration of the blast was long compared to that of high explosive bombs. To take only one example: At Nagasaki, brick buildings suffered structural damage within a radius averaging 6,000 feet from ground zero. Comparable damage would be done by a 500-pound high explosive bomb burst at ground level for a radius of 55 feet; by a 1,000-pound bomb for 80 feet; by a 1-ton bomb for 110 feet; and by a 2-ton bomb for 200 feet. A hypothetical 10-ton blockbuster (only 10-ton penetrating bombs have actually been used) could be expected to achieve equivalent damage over a radius of 400 feet. The area of effectiveness of the air-burst atomic bomb against brick buildings thus ranged from 15,000 times as great as that for a 500-pound bomb to 225 times as great as that for the imaginary 10-ton blockbuster.

A simple table shows most strikingly the comparison between the striking forces needed for atomic and for conventional raids. Against the two atomic attacks can be set the data for the most effective single urban attack, that on Tokyo on 9 March 1945, and the average effort and results from the Twentieth Air Force's campaign against Japanese cities:

  Effort and results
Hiroshima Nagasaki Tokyo Average
of 93
Planes 1 1 279 173
Bomb load 11 11 21,667 21,129
Population density per square mile 35,000 65,000 130,000 (3)
Square miles destroyed 4.7 1.8 15.8 1.8
Killed and missing 70/80,000 35/40,000 83,600 1,850
Injured 70,000 40,000 102,000 1,830
Mortality rate per square mile destroyed 15,000 20,000 5,300 1,000
Casualty rate per square mile 32,000 43,000 11,800 2,000

1Atomic. 2Tons. 3Unknown.

What stands out from this compilation, even more than the extent of the destruction from a single concentrated source, is the unprecedented casualty rate from the combination of heat, blast, and gamma rays from the chain reaction.

On the basis of the known destructiveness of various bombs computed from the war in Europe and the Pacific and from tests, the Survey has estimated the striking force that would have been necessary to achieve the same destruction at Hiroshima and Nagasaki. The cause physical damage equivalent to that caused by the atomic bombs, approximately 1,300 tons of bombs (one-fourth high explosives and three-fourth incendiary) would have been required at Nagasaki--in the target area. To place that many bombs in the target area, assuming daylight attacks under essentially the same conditions of weather and enemy opposition that prevailed when the atomic bombs were dropped, it is estimated that 1,600 tons of bombs would have had to be dropped at Hiroshima and 900 tons at Nagasaki. To these bomb loads would have to be added a number of tons of antipersonnel fragmentation bombs to inflict comparable casualties. These would add about 500 tons at Hiroshima and 300 tons at Nagasaki. The total bomb loads would thus be 2,100 tons at Hiroshima (400 HE, 1,200 IB) and 1,200 tons (675 HE, 225 IB) at Nagasaki. With each plane carrying 10 tons, the attacking force required would have been 210 B-29s at Hiroshima and 120 B-29s at Nagasaki.

It should be kept in mind, however, that the area of damage at Nagasaki does not represent the full potential effectiveness of the atomic bomb used there. The damage was limited by the small size of the rather isolated section of the city over which the bomb exploded. Had the target been sufficiently large, with no sections protected by intervening hills, the area of damage would have been about five times as large. An equivalent bomb load which would correspond to the destructive power of the Nagasaki bomb rather than the imperfect results achieved would approximate 2,200 tons of high explosives and incendiaries for physical damage plus 500 tons of fragmentation bombs for casualties, a total of 270 B-29 loads of 10 tons each.


DESTRUCTION OF BUILDINGS WITH BRICK LOAD BEARING WALLS. Note how brick debris lies inside wall facing blast, at remains of a barracks at the Japanese Army Divisional Grounds, 4,200 feet from ground zero at Hiroshima. The Koa Fire Insurance Co., 1,300 feet from ground zero, is completely destroyed except for the heavy walls of the vault.


REINFORCED CONCRETE BUILDINGS STILL STAND--but note how the interiors, as in the operating room of the Nagasaki University Hospital (2,200 feet from ground zero), are burnt out. Fire has consumed the floor, the balcony, and all seats, and distorted the metal railings and pipe.


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