3
})
))
Blast Kills Fire
At distances close enough to the
explosion to cause actual ignition
of wood, etc., the blast wind, com-
ing within a few seconds, generally would be strong enough to
blow out the flame.
For this reason it would appear
that relatively few of the numerous fires, which developed almost
instantaneously after the atomic
bombings of Japan out to distances
of 4,000 to 5,000 feet from ground
zero, that is, almost to the limit of
severe blast damage, were directly
caused by thermal radiations from
the bombs.
It is probable that most of the
fires origimated from secondary
causes, such as upsetting of charcoal or wood stoves, which were
common in Japanese homes; electrical short circuits: broken fas
lines, and so on, which were a direct effect of the blast wave. In
several cases, fires in industrial
plants were started by the overturning of furnaces and boilers,
and by the collapse of buildings
upon them,
It is true that fire-fighting seryices and equipment were poor by
American standards,
but it is
doubtful if much could have been
achieved, under the circumstances,
by more efficient fire departments.
At Hiroshima, for example, 70
per cent of the fire-fighting equipment was crushed in the collapse
of firehouses, and 80 per cent of
the personnel were unable to respond. Even if men and machines
had survived the blast, many fires
would have been inaccessible be-
cause of the streets being blocked
with debris.
3
This consisted of a wind that blew
toward the burning area of the
city from all directions, reaching
a maximum velocity of 30 to 40
miles per hour, two to three hours
after the explosion, decreasing to
light or moderate and variable in
direction about six hours later.
It should be noted, however, that
the fire storm was by no means a
special characteristic of the atomic
bomb.
been
Similar fire storms have
reported
as
accompanying
large conflagrations in the United
States, and especially afte: incen-
diary bomb attacks in both Germany and Japan during World
WarII.
The nigh winds are produced
largely by the updraft of the
heated air over an extensive burning area,
They are thus the
equivalent, on a very large scale,
of the draft that sucks air up a
chimney under which a fire is
burning.
In addition to the flash-burns,
many of the casualties from the
atomic
bomb explosions
were
caused by flame burns. In build-
ings collapsed by the blast, many
persons who might otherwise have
survived
their
injuries
trapped and burned. The
were
burns
suffered were of the kind that
might accompany any fire and
were not especially characteristic
of an atomic explosion.
Burns of both types, flash and
flame, were believed to be responsible for more than half of the
fatal casualties and probably at
least three quarters of all the cas-
ualties
saki.
at
Hiroshima and
Naga-
The magnitude of the problem,
therefore, points to the necessity
for making adequate preparations
for dealing with large numbers of
Another contributory factor to
the destruction by fire was the
failure of the water supply in both
Hiroshima and Nagasaki.
The
burned patients in the event of an
emergency. This means the train-
pumping stations were not largely
ing of great numbers in giving the
affected, but serious damage was
sustained by distributing pipes and
mains.
Most of the lines above
most rudimentary first aid for
burns, because a sufficient number
ground were broken by collapsing
buildings and by heat from the
of doctors and nurses could not be
The Fire Storm
sion of nuclear radiations, consisting of gamma rays, similar in nature to X-rays, neutrons, beta particles (electrons! and a small proportion of alpha particles (nuclei
provided.
The explosion
fires that melted the pipes.
of
an
atomic
bomb is accompanied by the emis-
About twenty minutes after the
detonation of the atomic bomb at
Hiroshima there
developed the
phenomenon known as fire storm.
28
of
helium
atoms).
Radiations
emitted within a munute of the
detonation are referred to as initial
nuclear radiations.
Those
emitted after more than a minute
are known as residual.
The initial radiations of importance to us are the gamma rays
and the neutrons
Both have considerable penetrating power, so
that they can reach the earth even
when liberated at appreciable dis-
tances away.
Both can produce
harmtul effects on living organisms.
The energy of the gamma rays
present in the instantaneous, or
prompt nuclear radiation 1s about
3 per cent of the total energy lib-
erated by the bomb, but only a
small proportion of this, perhaps 1
per cent, succeeds in penetrating
any great distance from the bomb.
A somewhat similar amount is
present in the gamma rays emitted by the fission products in the
first minute after an atomic explosion.
Nevertheless,
in
spite
of
the
On
the
other
about
from radiation sickness. If part
of the body were protected by a
suitable shield, it is probable that
a larger dose than 400 r would not
prove
fatal.
Ordinary clothing
can in no sense be regarded as
protective.
At less than 2,100 feet from the
explosion,
physical
and
thermal
destruction are so serious in un-
protected regions that radiological
injury does not need consideration,
At distances greater than 9,000
feet, the dosage is, in generai, too
small to be of serious consequences,
unless it is repeated at short intervals.
At
the
minimum
distance
of
2,100 feet from the expiosion, the
dosage of gamma rays in an unprotected location would be 10,000
To reduce this to below the
mches of lead.
A layer of some
ly effective. Underground shelters
could thus provide adequate pro-
rection agianst the radiation hazard,
Outside Shelters
An outside shelter of the type
used in World War II as a protection against blast bombs, covered
with about twenty inches of packed
nuclear radiation would probably
prove fatal to 50 per cent of human beings, even if protected by
twelve inches of concrete,
However, beyond about 7,000 feet
soil, would decrease the radiation
dosage below the median lethal
value at distances greater than
about 3,000 feet from the explo-
would be
sion. For a height of burst of 2.000
feet, this would represent 2,250
virtually harmless, without protective shielding, whereas exposure to
thermal radiation at this distance
feet or more from ground zero. The
thickness of concrete that would
produce the same effect is roughly
twelve inches, that of iron four
inches, and that of lead about two
inches,
The statement that an unprotected person within 4,200 feet of
an atomic explosion would receive
a median tethal dose of 400 r is
based on the supposition that the
exposure lasts for the whole min-
could produce serious skin burns.
Radiation Dosage
Is
is
thirty inches of soi] would be equal-
hand,
Shielding from gamma rays or
neutrons is not the simple matter
of shielding against thermal radiation.
For example, at a distance
of 3,000 feet from the explosion of
a nominal atomic bomb, the initial
dosage
bomb
require something like twenty inches of concrete or about three
incendiary effect.
Radiation
atomic
Thus a large proportion of human beings exposed to the initial
gamma rays Within 4,200 feet of
an atomic explosion would die
median lethal dose of 400 r would
nuclear radiations do not have any
the nuclear radiations
nominal
4,200 feet.
r.
energy being considerably smaller
than that appearing in the form of
thermal] radiation, the gamma radiation can cauSe an appreciabie
proportion of the atomic bomb
casualties.
beings. The median lethal range
of the gamma radiation from 4
measured
in terms of a unit called the roentgen, or the r. It is usually accepted
that a dose of 400 r of radiation
over the whole body in the course
of a few mintites represents the
median lethal dose that would be
fatal to about 50 per cent of human
ute of the period of initial radia23