Qe a
amount of matter was lost, so
small, in fact, that it was not pos-
sible to weigh it by any method
known to us. We have to burn
3,000 to 7,000 tons of coal to con-
vert the matter of one gram into
energy, a ratio of three to, seven
billion to one.
When energy is obtained by the
burning of coal, the atoms of the
coal, mostly carbon and hydrogen,
remain unchanged, the loss of
matter being due to a rearrange-
ment of the electrons on the outside surface of the atom.
In what is known as atomic en-
ergy, the energy is obtained by a
break up of the atoms used as fuel.
When this happens, an amount of
matter three to seven million times
as preat as in the burning of coal
is converted into energy. As compared with the amount of matter
converted into energy in the explosion of TNT the ratio is 20,000,000
to 1. In other words, the exploSion of one kilogram of fissionable
material, such as Uranium 235 or
releases
an
energy
plutonium,
equivalent to 20,000 tons of TNT.
Uranium 235 (U-235) and plutonium are at present the only two
substances that can be used for the
release of atomic energy, either as
an explosive or for power. U-235
is the only one found in nature in
sizable quantities. Plutonium is
a Man-made element, produced
from the more abundant form of
uranium, known as U-2388, in our
gigantic plants, known as nuclear
reactors, at Hanford, Wash.
U-235 Is the Key
A
third
fissionable
element,
known as Uranium 233, can be
produced artificially out of the
non-fissionable element, thorium,
but to do so it is necessary to use
either U-235 or plutonium. Ura-
nium 235 is thus the key substance
without which no atomic bombs or
atomic power could be obtained.
Uranium 235 is found in nature
mixed With Uranium 238, each ton
of purified uranium metal consist-
ing of 1,986 pounds of U-238 and
only fourteen pounds of U-235. To
separate the latter required the
construction of a billion-dollar
plant at Oak Ridge, Tenn.
Plutonium is produced by an-
a self-multiplying chain reaction,
in which neutrons (fundamental
atomic particles without an elec-
tric charge)
are released.
These
neutrons enter the Uranium 238
in the mixture and convert it into
plutonium.
Just as an ordinary fire needs
oxygen to burn,
an atomic fire
needs neutrons.
These neutrons
come from the nuclei (cores) of
atoms of U-235 or plutenium, each
atom split releasing an average of
two neutrons, these in turn splitting two atoms which release four
neutrons, thus starting a chain reaction. In an atomic bomb these
neutrons are released at such an
incredible rate that as many as
two billion trillion atoms are split
in less than one-millionth of a
second.
The explosion of an atomic bomb
is analogous to spontaneous com-
bustion, the explosion taking place
as soon as a minimum amount of
fissionable material (either U-235
or plutonium) is assembled in one
unit. This minimum amount
known as the critical mass,
is
Amount a Top Secret
The actual amount is a top se-
cret, but for purposes of ilNustra-
tion let us assume that it is ten
kilograms. This would mean that
as soon as ten kilograms of either
U-235 or plutonium were assembled in ome unit, the explosion
would take place automatically,
the reaction starting with a stray
neutron from a cosmic ray coming
from outer space. Hence to explode
an atomic bomb, the ten kilograms
would have to be divided into two
parts that were brought together
by a timing device after it had
been dropped.
The atomic bombs dropped over
Japan and tested at Alamogordo,
N. M., and at Bikini had a power
of 20,000 tons of TNT, which cor-
responds to the splitting of all the
atoms
in
a
kilogram
U-235 or plutonium.
This does not mean,
of
either
however,
that these bombs contained only
one Kilogram of the fissionable
material, because that would mean
an efficiency of 100 per cent, and
while the actual efficiency of the
bomb is a top secret, the handbook
makes it clear that this is less
than 100 per cent.
metal is made to split by means of :
This means that a certain perother method, in which the U-235
in
the
natural
mixed
uranium
3
7h
centage of the atoms remain un-
split after the explosion, going off
as part of the great cloud of radioactive vapor that characterized
the explosion.
The explosion of an atomic bomb
produces several effects, which
vary greatly with the manner in
which it is exploded.
It is first of all a tremendous
blast weapon, concentrating within
itself (that is, the ‘nominal atomic
bomb” used over Japan) the blasting power of 2,000 wartime tenblockbusters. Its temperature after
the explosion reaches more than
1,000,000 degrees centigrade, and
is thus a tremendous incendiary
Weapon, setting great fires in
buildings and causing severe flash
burns in human beings.
Range of Effects
However, and this is very important from the point of view of
Planning an effective civilian de-
fense, the blast effect and the in-
cendiary effect have an effective
range within a rather Hmited radius from the center of the explo-
sion. So while little can be done for
those unfortunates caught in the
open within that central area,
much can be done to take measures
for reducing to a minimum the effects of blast and fire in the region
outside this area.
in
Furthermore,
Japan.
as
was
the
case
many secondary fires
were started in outlying areas by
fiving
debris,
from
overturned
stoves, escaping gas, etc., that
efficient planning could eliminate
altogether or reduce to a minimum.
Many of those who died from
burns that became infected because of lack of first aid can be
saved by the training of personnel
‘
to give first aid for flash burns.
Many others could be saved from
burns by training to duck for shelter,
In addition to the blast and incendiary effects the explosion of
an atomic bomb gives off large
amounts of radiations, The most
serious of these are the instan-
taneous radiations that come off
in the form of gamma rays, similar in nature to very powerful
X-rays. These last but a very short
time, no longer than a flash of
lightning, and like lightning kill
those they strike. However, these,
too, have a rather short effective
range, and those who survive it,
depending on the dosage they received, may be saved by proper
measures,
;
The second principal form of
radiation released in the atomic
bomb explosion is that from the
fission products, some 200 split
fragments of the exploded U-235
or plutonium atoms. If the bomb
is exploded some 2,000 feet above
the ground, as it was in Japan,
and in the air burst (Test ‘“Able”)
over Bikini, these fragments go up
in the great mushroom cloud to
some
50,000 feet and
are there
widely dispersed so that they can
cause little harm.
Qn the other hand, if the bomb
is exploded near the ground, as it
was in Alamogordo,
or under
water, as in Test ‘Baker’ at BiKini, these fission fragments may
constitute a great hazard for some
time,
However, here too we have
learned a great deal from the Bi-
kini test as to the nature of the
danger and
how
to
avoid
proper counter-measures.
it by