at
away from windows, of course, or
behind a tree, or even protection of
one part of the body by another
Nuclear Blast Triple Threat
The explosion of an atomic bomb
Some High Temperatures
produces
three
major
effects,
It has been estimated that in
which make it three major weap- | the atomic explosions in Japan,
ons in one.
It devastates by blast, by heat
and by radioactivity.
which took place some 2,000 feet
above the ground, the temperature
It has been
at ground zero, from thermal ra-
estimated that the blast wave, in
liation, was probably between 3,000
and 4,000 degrees Centigrade, 5,400
to 7,200 degrees Fahrenheit. It is
an air burst, is responsible for 50
to 60 per cent of the deaths; the
heat-flash for 20 to 30 per cent,
and radioactivity for 15 to 20 per
true that the temperature fell off
rapidly with increasing distance
cent.
from the burst, but the effects were
An important difference between
definitely noticeable as far as two
an atomic and a conventional ex-
miles away or more.
ated per unit mass is much freater
in the atomic blast. As a consequence, the temperature attained
is much higher, with the result
that a larger proportion of the
energy is emitted as thermal radi-
tion with the thermal radiation
from an atomic bomb is not only
plosion is that the energy liber-
ation
(heat)
explosion.
An atomic
An important point in connec-
the amount of energyin this radia-
tion, but also the fact that nearly
the whole of it is emitted in an
extremely short time, about three
seconds from the initiation of the
explosion. In other words, the intensity o1 the radiation, which is
at the time of the
bomb,
for
example,
a4 measure of the rate at which it
reaches ga particular surface, is
very high.
releases roughly one third of its
total energy in the form of this
radiation. For the nominal atomic
bomb, equal to 20,000 tons of TNT,
the energy emitted in this manner
would be about 6.7 trillion calories,
which is equivalent to about 8,000,600 kilowatt hours.
Because of this high intensity,
the heat accompanying the absorp-
tion of thermal radiation is produced rapidly, most of it on the
surface of the body upon which it
falls.
It is evident that such an enor-
val, high surface temperatures are
attained.
A set of data have led to the
conclusion that exposure to thermal
radiation from a nominal atomic
bomb, on a fairly clear day, would
lead to more or less serious skin
When the radiation falls on mat-
ter, part may be reflected, part
will be absorbed, and the remainder, if any, wil! pass through, ulti-
mately to fall on other portions of
matter. It is the radiation absorbed that is important for the
present purpose.
The extent of this absorption depends on the nature of matter and
|
also upon its color. A black mate-
rial will absorb a much larger pro-
portion of the thermal radiation
falling upon it than wil] the same
material when colored white. Most
of the absorbed thermal radiation
is converted directly into heat.
Since only a small propor-
tion of the heat is dissipated by
conduction during the short inter-
mous amount of radiant energy
would produce considerable damage to living organisms and to
combustible materials,
|
26
burns within a radins of 10,000
feet from ground zero. This ‘s in
general agreement with the experiences in Japan.
However, in spite of its great
range, protection from thermal
radiation is easily achieved. The
rays travel in straight lines, and
so only direct exposure, in the open
or through windows, would lead to
harmful consequences. Shelter behind almost any object, such as
anywhere in the interior of a house,
so as to avoid direct exposure to
the atomic Bali of Fire, would be
effective.
Only fairly close to ground zero
would the thermal radiation be
Where burns did occur througn
clothing, these tended to involve
regions where the clothes were
tightly drawn over the skin, at the
elbows or shoulders, for example,
while areas where the clothing
fitted loosely were unharmed.
Case of the Kimono
expected to penetrate clothing, and
so parts of the body covered in
this way are generally safe from
thermal] radiation burns.
The “Flash-Burns”
One of the striking facts con-
nected with the atomic bombing of
Japan was the large number of
casualties attributed to what have
been called ‘“‘flash-burns," caused
by the instantaneous thermal radiation. It has been estimated that
20 to 30 per cent of fatal casualties at Hiroshima and Nagasaki
were from such burns, as distinct
from those who suffered the more
familiar flame burns,
Thermal radiation burns were
recorded at a distance of 7,500 feet
from ground zero at Hiroshima
and as far as 13,000 feet at Nagasaki. The incidence of these burns,
as might have been expected, was
inversely related to the distance
from the explosion.
A very distinctive feature of the
thermal radiation burns was their
sharp limitation to exposed areas
of the skin facing the center of
Because white or light colors reflected the thermal radiations,
they generally afforded better pro-
tection than dark clothing. Thus
it was not unusual to find burns
through black clothing, but not
through white material worn by
the same individual.
This was strikingly shown in the
case of a woman clad in a kimono
at the time of the explosion. Her
back and arms were badly burned
in a pattern corresponding to the
dark portions of the kimono, while
the skin under the light portions
was unaffected,
Studies on the part played
by
the ultra-violet range of the thermal radiations have led to the con-
clusion that the ultra-violet radiations from an atomic bomb do not
make the major contribution to
skin injuries. This means, therefore, that the infra-red radiation is
the main factor in causing the
flash-burns.
This is a subject of more than
mere scientific interest. If it is the
the explosion. They were consequently sometimes referred to as
“profile burns.’”’ This is because of
the fact mentioned above that ra-
infra-red that is the most important, then there is the possibility
and so only regions directly
posed to it will be affected.
to take cover. or other appropriate
evasive action, thus reducing the
diation
travels in straight lines.
ex-
A striking illustration of this be-
havior was that of a man writing
_before a window. His hands were
seriously burned, but his face and
neck, which were not covered. suffered only slight burns because the
angle of entry of the radiation was
such as to place them in partial
shadow.
Although thermal radiation burns
were largely confined to exposed
parts of the body, there were a
few cases in which such burns occurred through one, and very occasionally more, lavers of clothing. Instances of this kind were
observed only near the center of
the explosion.
that 4 person caught in the open
by
the
explosion
of
an
atomic
bomb might have sufficient time
therma] radiation damage.
This would be possible because
most of the infra-red radiation is
emitted by the Bali of Fire in its
later stages, following the second
temperature maximum, 7,000 degrees Centigrade; that is to say,
from about 0.3 to three seconds
after the explosion.
Thus,
if
protection
could
be
found within one second of the explosion, the exposure to infra-red
radiation would be very roughly
one-third of the total amount re-
ceived
at
that
distance.
Under
many circumstances this difference would be very significant.