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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"Outside a radius of 4 kilometers and within a radius of 8 kilometers
living creatures were injured by materials blown about by the blast; the
majority were only superficially wounded. Houses were only half or
partially damaged."
The British Mission to Japan interpreted their observations of the
destruction of buildings to apply to similar construction of their own as
follows:
A similar bomb exploding in a similar fashion would produce the following
effects on normal British houses:
Up to 1,000 yards from X it would cause complete collapse.
Up to 1 mile from X it would damage the houses beyond repair.
Up to 1.5 miles from X it would render them uninhabitable without extensive
repair, particularly to roof timbers.
Up to 2.5 miles from X it would render them uninhabitable until first-aid
repairs had been carried out.
The fire damage in both cities was tremendous, but was more complete in
Hiroshima than in Nagasaki. The effect of the fires was to change
profoundly the appearance of the city and to leave the central part bare,
except for some reinforced concrete and steel frames and objects such as
safes, chimney stacks, and pieces of twisted sheet metal. The fire damage
resulted more from the properties of the cities themselves than from those
of the bombs.
The conflagration in Hiroshima caused high winds to spring up as air was
drawn in toward the center of the burning area, creating a "fire storm".
The wind velocity in the city had been less than 5 miles per hour before
the bombing, but the fire-wind attained a velocity of 30-40 miles per hour.
These great winds restricted the perimeter of the fire but greatly added to
the damage of the conflagration within the perimeter and caused the deaths
of many persons who might otherwise have escaped. In Nagasaki, very severe
damage was caused by fires, but no extensive "fire storm" engulfed the
city. In both cities, some of the fires close to X were no doubt started
by the ignition of highly combustible material such as paper, straw, and
dry cloth, upon the instantaneous radiation of heat from the nuclear
explosion. The presence of large amounts of unburnt combustible materials
near X, however, indicated that even though the heat of the blast was very
intense, its duration was insufficient to raise the temperature of many
materials to the kindling point except in cases where conditions were
ideal. The majority of the fires were of secondary origin starting from
the usual electrical short-circuits, broken gas lines, overturned stoves,
open fires, charcoal braziers, lamps, etc., following collapse or serious
damage from the direct blast.
Fire fighting and rescue units were stripped of men and equipment. Almost
30 hours elapsed before any rescue parties were observable. In Hiroshima
only a handful of fire engines were available for fighting the ensuing
fires, and none of these were of first class type. In any case, however,
it is not likely that any fire fighting equipment or personnel or
organization could have effected any significant reduction in the amount of
damage caused by the tremendous conflagration.
A study of numerous aerial photographs made prior to the atomic bombings
indicates that between 10 June and 9 August 1945 the Japanese constructed
fire breaks in certain areas of the cities in order to control large scale
fires. In general these fire breaks were not effective because fires were
started at so many locations simultaneously. They appear, however, to have
helped prevent fires from spreading farther east into the main business and
residential section of Nagasaki.
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. Finally, the great fires that raged in each city totally
consumed many bodies.
The number of total casualties has been estimated at various times since
the bombings with wide discrepancies. The Manhattan Engineer District's
best available figures are:
TABLE A
Estimates of Casualties
Hiroshima Nagasaki
Pre-raid population 255,000 195,000
Dead 66,000 39,000
Injured 69,000 25,000
Total Casualties 135,000 64,000
The relation of total casualties to distance from X, the center of damage
and point directly under the air-burst explosion of the bomb, is of great
importance in evaluating the casualty-producing effect of the bombs. This
relationship for the total population of Nagasaki is shown in the table
below, based on the first-obtained casualty figures of the District:
TABLE B
Relation of Total Casualties to Distance from X
Distance Total Killed per
from X, feet Killed Injured Missing Casualties square mile
0 - 1,640 7,505 960 1,127 9,592 24,7OO
1,640 - 3,300 3,688 1,478 1,799 6,965 4,040
3,300 - 4,900 8,678 17,137 3,597 29,412 5,710
4,900 - 6,550 221 11,958 28 12,207 125
6,550 - 9,850 112 9,460 17 9,589 20
No figure for total pre-raid population at these different distances were
available. Such figures would be necessary in order to compute per cent
mortality. A calculation made by the British Mission to Japan and based on
a preliminary analysis of the study of the Joint Medical-Atomic Bomb
Investigating Commission gives the following calculated values for per cent
mortality at increasing distances from X:
TABLE C
Per-Cent Mortality at Various Distances
Distance from X, Per-cent Mortality
in feet
0 - 1000 93.0%
1000 - 2000 92.0
2000 - 3000 86.0
3000 - 4000 69.0
4000 - 5000 49.0
5000 - 6000 31.5
6000 - 7000 12.5
7000 - 8000 1.3
8000 - 9000 0.5
9000 - 10,000 0.0
It seems almost certain from the various reports that the greatest total
number of deaths were those occurring immediately after the bombing. The
causes of many of the deaths can only be surmised, and of course many
persons near the center of explosion suffered fatal injuries from more than
one of the bomb effects. The proper order of importance for possible
causes of death is: burns, mechanical injury, and gamma radiation. Early
estimates by the Japanese are shown in D below:
TABLE D
Cause of Immediate Deaths
City Cause of Death Per-cent of Total
Hiroshima Burns 60%
Falling debris 30
Other 10
Nagasaki Burns 95%
Falling debris 9
Flying glass 7
Other 7
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 of the explosive material. In an atomic explosion the identity of
the atoms, not simply their arrangement, is changed. A considerable
fraction of the mass of the explosive charge, which may be uranium 235 or
plutonium, is transformed into energy. Einstein's equation, E = mc^2,
shows that matter that is transformed into energy may yield a total energy
equivalent to the mass multiplied by the square of the velocity of light.
The significance of the equation is easily seen when one recalls that the
velocity of light is 186,000 miles per second. The energy released when a
pound of T.N.T. explodes would, if converted entirely into heat, raise the
temperature of 36 lbs. of water from freezing temperature (32 deg F) to
boiling temperature (212 deg F). The nuclear fission of a pound of uranium
would produce an equal temperature rise in over 200 million pounds of
water.
The explosive effect of an ordinary material such as T.N.T. is derived from
the rapid conversion of solid T.N.T. to gas, which occupies initially the
same volume as the solid; it exerts intense pressures on the surrounding
air and expands rapidly to a volume many times larger than the initial
volume. A wave of high pressure thus rapidly moves outward from the center
of the explosion and is the major cause of damage from ordinary high
explosives. An atomic bomb also generates a wave of high pressure which is
in fact of, much higher pressure than that from ordinary explosions; and
this wave is again the major cause of damage to buildings and other
structures. It differs from the pressure wave of a block buster in the
size of the area over which high pressures are generated. It also differs
in the duration of the pressure pulse at any given point: the pressure from
a blockbuster lasts for a few milliseconds (a millisecond is one thousandth
of a second) only, that from the atomic bomb for nearly a second, and was
felt by observers both in Japan and in New Mexico as a very strong wind
going by.
The next greatest difference between the atomic bomb and the T.N.T.
explosion is the fact that the atomic bomb gives off greater amounts of
radiation. Most of this radiation is "light" of some wave-length ranging
from the so-called heat radiations of very long wave length to the
so-called gamma rays which have wave-lengths even shorter than the X-rays
used in medicine. All of these radiations travel at the same speed; this,
the speed of light, is 186,000 miles per second. The radiations are
intense enough to kill people within an appreciable distance from the
explosion, and are in fact the major cause of deaths and injuries apart
from mechanical injuries. The greatest number of radiation injuries was
probably due to the ultra-violet rays which have a wave length slightly
shorter than visible light and which caused flash burn comparable to severe
sunburn. After these, the gamma rays of ultra short wave length are most
important; these cause injuries similar to those from over-doses of X-rays.
The origin of the gamma rays is different from that of the bulk of the
radiation: the latter is caused by the extremely high temperatures in the
bomb, in the same way as light is emitted from the hot surface of the sun
or from the wires in an incandescent lamp. The gamma rays on the other
hand are emitted by the atomic nuclei themselves when they are transformed
in the fission process. The gamma rays are therefore specific to the
atomic bomb and are completely absent in T.N.T. explosions. The light of
longer wave length (visible and ultra-violet) is also emitted by a T.N.T.
explosion, but with much smaller intensity than by an atomic bomb, which
makes it insignificant as far as damage is concerned.
A large fraction of the gamma rays is emitted in the first few microseconds
(millionths of a second) of the atomic explosion, together with neutrons
which are also produced in the nuclear fission. The neutrons have much
less damage effect than the gamma rays because they have a smaller
intensity and also because they are strongly absorbed in air and therefore
can penetrate only to relatively small distances from the explosion: at a
thousand yards the neutron intensity is negligible. After the nuclear
emission, strong gamma radiation continues to come from the exploded bomb.
This generates from the fission products and continues for about one minute
until all of the explosion products have risen to such a height that the
intensity received on the ground is negligible. A large number of beta
rays are also emitted during this time, but they are unimportant because
their range is not very great, only a few feet. The range of alpha
particles from the unused active material and fissionable material of the
bomb is even smaller.
Apart from the gamma radiation ordinary light is emitted, some of which is
visible and some of which is the ultra violet rays mainly responsible for
flash burns. The emission of light starts a few milliseconds after the
nuclear explosion when the energy from the explosion reaches the air
surrounding the bomb. The observer sees then a ball of fire which rapidly
grows in size. During most of the early time, the ball of fire extends as
far as the wave of high pressure. As the ball of fire grows its
temperature and brightness decrease. Several milliseconds after the
initiation of the explosion, the brightness of the ball of fire goes
through a minimum, then it gets somewhat brighter and remains at the order
of a few times the brightness of the sun for a period of 10 to 15 seconds
for an observer at six miles distance. Most of the radiation is given off
after this point of maximum brightness. Also after this maximum, the
pressure waves run ahead of the ball of fire.
The ball of fire rapidly expands from the size of the bomb to a radius of
several hundred feet at one second after the explosion. After this the
most striking feature is the rise of the ball of fire at the rate of about
30 yards per second. Meanwhile it also continues to expand by mixing with
the cooler air surrounding it. At the end of the first minute the ball has
expanded to a radius of several hundred yards and risen to a height of
about one mile. The shock wave has by now reached a radius of 15 miles and
its pressure dropped to less than 1/10 of a pound per square inch. The
ball now loses its brilliance and appears as a great cloud of smoke: the
pulverized material of the bomb. This cloud continues to rise vertically
and finally mushrooms out at an altitude of about 25,000 feet depending
upon meteorological conditions. The cloud reaches a maximum height of
between 50,000 and 70,000 feet in a time of over 30 minutes.
It is of interest to note that Dr. Hans Bethe, then a member of the
Manhattan Engineer District on loan from Cornell University, predicted the
existence and characteristics of this ball of fire months before the first
test was carried out.
To summarize, radiation comes in two bursts - an extremely intense one
lasting only about 3 milliseconds and a less intense one of much longer
duration lasting several seconds. The second burst contains by far the
larger fraction of the total light energy, more than 90%. But the first
flash is especially large in ultra-violet radiation which is biologically
more effective. Moreover, because the heat in this flash comes in such a
short time, there is no time for any cooling to take place, and the
temperature of a person's skin can be raised 50 degrees centigrade by the
flash of visible and ultra-violet rays in the first millisecond at a
distance of 4,000 yards. People may be injured by flash burns at even
larger distances. Gamma radiation danger does not extend nearly so far and
neutron radiation danger is still more limited.
The high skin temperatures result from the first flash of high intensity
radiation and are probably as significant for injuries as the total dosages
which come mainly from the second more sustained burst of radiation. The
combination of skin temperature increase plus large ultra-violet flux
inside 4,000 yards is injurious in all cases to exposed personnel. Beyond
this point there may be cases of injury, depending upon the individual
sensitivity. The infra-red dosage is probably less important because of its
smaller intensity.
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 to be
approximately equivalent to that of 20,000 tons of T.N.T. Given this
figure, one may calculate the expected peak pressures in the air, at
various distances from the center of the explosion, which occurred
following detonation of the bomb. The peak pressures which were calculated
before the bombs were dropped agreed very closely with those which were
actually experienced in the cities during the attack as computed by Allied
experts in a number of ingenious ways after the occupation of Japan.
The blast of pressure from the atomic bombs differed from that of ordinary
high explosive bombs in three main ways:
A. Downward thrust. Because the explosions were well up in the air, much
of the damage resulted from a downward pressure. This pressure of course
most largely effected flat roofs. Some telegraph and other poles
immediately below the explosion remained upright while those at greater
distances from the center of damage, being more largely exposed to a
horizontal thrust from the blast pressure waves, were overturned or tilted.
Trees underneath the explosion remained upright but had their branches
broken downward.
B. Mass distortion of buildings. An ordinary bomb can damage only a part
of a large building, which may then collapse further under the action of
gravity. But the blast wave from an atomic bomb is so large that it can
engulf whole buildings, no matter how great their size, pushing them over
as though a giant hand had given them a shove.
C. Long duration of the positive pressure pulse and consequent small
effect of the negative pressure, or suction, phase. In any explosion, the
positive pressure exerted by the blast lasts for a definite period of time
(usually a small fraction of a second) and is then followed by a somewhat
longer period of negative pressure, or suction. The negative pressure is
always much weaker than the positive, but in ordinary explosions the short
duration of the positive pulse results in many structures not having time
to fail in that phase, while they are able to fail under the more extended,
though weaker, negative pressure. But the duration of the positive pulse
is approximately proportional to the 1/3 power of the size of the explosive
charge. Thus, if the relation held true throughout the range in question,
a 10-ton T.N.T. explosion would have a positive pulse only about 1/14th as
long as that of a 20,000-ton explosion. Consequently, the atomic
explosions had positive pulses so much longer then those of ordinary
explosives that nearly all failures probably occurred during this phase,
and very little damage could be attributed to the suction which followed.
One other interesting feature was the combination of flash ignition and
comparative slow pressure wave. Some objects, such as thin, dry wooden
slats, were ignited by the radiated flash heat, and then their fires were
blown out some time later (depending on their distance from X) by the
pressure blast which followed the flash radiation.
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. The
peak pressure figures gave a direct clue to the equivalent T.N.T. tonnage
of the atomic bombs, since the pressures developed by any given amount of
T.N.T. can be calculated easily.
One of the simplest methods of estimating the peak pressure is from
crushing of oil drums, gasoline cans, or any other empty thin metal vessel
with a small opening. The assumption made is that the blast wave pressure
comes on instantaneously, the resulting pressure on the can is more than
the case can withstand, and the walls collapse inward. The air inside is
compressed adiabatically to such a point that the pressure inside is less
by a certain amount than the pressure outside, this amount being the
pressure difference outside and in that the walls can stand in their
crumpled condition. The uncertainties involved are, first, that some air
rushes in through any opening that the can may have, and thus helps to
build up the pressure inside; and, second, that as the pressure outside
falls, the air inside cannot escape sufficiently fast to avoid the walls of
the can being blown out again to some extent. These uncertainties are such
that estimates of pressure based on this method are on the low side, i.e.,
they are underestimated.
Another method of calculating the peak-pressure is through the bending of
steel flagpoles, or lightning conductors, away from the explosion. It is
possible to calculate the drag on a pole or rod in an airstream of a
certain density and velocity; by connecting this drag with the strength of
the pole in question, a determination of the pressure wave may be obtained.
Still another method of estimating the peak pressure is through the
overturning of memorial stones, of which there are a great quantity in
Japan. The dimensions of the stones can be used along with known data on
the pressure exerted by wind against flat surfaces, to calculate the
desired figure.
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.
The limiting radius for severe displacement of roof tiles in Nagasaki was
about 10,000 feet although isolated cases were found up to 16,000 feet.
In Hiroshima the general limiting radius was about 8,000 feet; however,
even at a distance of 26,000 feet from X in Hiroshima, some tiles were
displaced.
At Mogi, 7 miles from X in Nagasaki, over steep hills over 600 feet high,
about 10% of the glass came out. In nearer, sequestered localities only 4
miles from X, no damage of any kind was caused. An interesting effect was
noted at Mogi; eyewitnesses said that they thought a raid was being made on
the place; one big flash was seen, then a loud roar, followed at several
second intervals by half a dozen other loud reports, from all directions.
These successive reports were obviously reflections from the hills
surrounding Mogi.
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.
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 example of shielding was that afforded by the hills in the
city of Nagasaki; it was the shielding of these hills which resulted in the
smaller area of devastation in Nagasaki despite the fact that the bomb used
there was not less powerful. The hills gave effective shielding only at
such distances from the center of explosion that the blast pressure was
becoming critical - that is, was only barely sufficient to cause collapse -
for the structure. Houses built in ravines in Nagasaki pointing well away
from the center of the explosion survived without damage, but others at
similar distances in ravines pointing toward the center of explosion were
greatly damaged. In the north of Nagasaki there was a small hamlet about
8,000 feet from the center of explosion; one could see a distinctive
variation in the intensity of damage across the hamlet, corresponding with
the shadows thrown by a sharp hill.
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