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"Within about 1ms after the explosion, some 70-80% of the explosion energy...
is emitted as primary thermal radiation, most of which consists of soft
X-rays."
Glasstone, The Effects of Nuclear Weaponns
FISSION PRINCIPLES
The binding energy per nucleon versus atomic mass has a turning point around
Fe-56. Iron is the most stable element. Elements with atomic masses less
than iron tend to combine, and those with masses greater than iron tend to
split. Radioactivity is an indication of this instability. The problem is that
protons in the nucleus tend to repel each other. There comes a stage where
the nuclear binding energy cannot compete with this repelling force, even if
you add more and more neutrons to the nucleus. Take as an example, the highest
Z naturally occuring element - uranium.
U has many radioactive isotopes. These include U-234, U-235 and U-238. They are
among the longest-living elements in a table of radioactive isotopes.
The U-235 isotope is used in weapons since it has the highest fission cross
section of all the U isotopes, for thermal neutrons.
If you bombard U-238 with thermal neutrons, you might just cause
a transuranic beta decay to Pu-239. Pu does not occur naturally, and is of
use in weapons. If you bombard the radioactive isotopes with slow neutrons
there is a chance that you will split the nuclei in half. In the process, you
release some binding energy, and some more neutrons. For an explosion, you
need a self-sustaining chain reaction which keeps on generating more and more
neutrons. In effect, you need a critical mass of fissionable material to
offset any loss of neutrons. (Instead of hitting other isotopic nuclei, the
neutrons might just wander off.) A sphere of material is used to provide the
least surface area for neutron loss. If the sphere is large enough, neutron
loss will be balanced by neutron generation, resulting in a self-sustaining
reaction. You have an energy release in fission since the mass of the original
atom doesn't equal the mass of the two reaction atoms. The lost energy is
converted to radiation and kinetic energy of the atoms via mass-energy
equivalence. The fission products are around equal size, and are highly
radioactive. Products include Sr, which is absorbed into human bones and
stays there, since it is chemically similar to calcium. Other harmful
products include cesium, similar to potassium. Cesium is distributed
uniformly throughout the body.
The number of fissioning nuclei increases as a geometric progression, with
each generation. Most of the energy in a bomb is released during around the
80th generation.
It is estimated in 10^-6 secs, about 2x10-24 U-235 nuclei split, releasing
HUGE amounts of energy. A single split gives you about 170MeV on average,
whereas a chemical reaction only gives you a few eV.
An example of a fission reaction is:
U-235 + n -> Kr-92 + Xe-142 + 2n + 207 MeV.
The released energy is many orders of magnitude greater than that released
by a chemical reaction using the same amount of matter.
A solid Pu sphere of 6.2kg mass is about 3.3" in diameter. It would be as
big as a tennis ball, but as massive as a bowling ball. The sphere would be
bigger if there was a Po-Be core inside.
Uranium & Plutonium
-------------------
Uranium-235 is very difficult to extract. In fact, for every 25,000 tons
of Uranium ore that is mined from the earth, only 50 tons of Uranium metal can
be refined from that, and 99.3% of that metal is U-238 which is too stable to
be used as an active agent in an atomic detonation. To make matters even more
complicated, no ordinary chemical extraction can separate the two isotopes
since both U-235 and U-238 possess precisely identical chemical
characteristics. The only methods that can effectively separate U-235 from
U-238 are mechanical methods.
U-235 is slightly, but only slightly, lighter than its counterpart,
U-238. A system of gaseous diffusion is used to begin the separating process
between the two isotopes. In this system, Uranium is combined with fluorine
to form Uranium Hexafluoride gas. This mixture is then propelled by low-
pressure pumps through a series of extremely fine porous barriers. Because
the U-235 atoms are lighter and thus propelled faster than the U-238 atoms,
they could penetrate the barriers more rapidly. As a result, the
U-235's concentration became successively greater as it passed through each
barrier. After passing through several thousand barriers, the Uranium
Hexafluoride contains a relatively high concentration of U-235 -- 2% pure
Uranium in the case of reactor fuel, and if pushed further could
(theoretically) yield up to 95% pure Uranium for use in an atomic bomb.
Once the process of gaseous diffusion is finished, the Uranium must be
refined once again. Magnetic separation of the extract from the previous
enriching process is then implemented to further refine the Uranium. This
involves electrically charging Uranium Tetrachloride gas and directing it past
a weak electromagnet. Since the lighter U-235 particles in the gas stream are
less affected by the magnetic pull, they can be gradually separated from the
flow.
Following the first two procedures, a third enrichment process is then
applied to the extract from the second process. In this procedure, a gas
centrifuge is brought into action to further separate the lighter U-235 from
its heavier counter-isotope. Centrifugal force separates the two isotopes of
Uranium by their mass. Once all of these procedures have been completed, all
that need be done is to place the properly molded components of Uranium-235
inside a warhead that will facilitate an atomic detonation.
Supercritical mass for Uranium-235 is defined as 110 lbs (50 kgs) of
pure Uranium.
Depending on the refining process(es) used when purifying the U-235 for
use, along with the design of the warhead mechanism and the altitude at which
it detonates, the explosive force of the A-bomb can range anywhere from 1
kiloton (which equals 1,000 tons of TNT) to 20 megatons (which equals 20
million tons of TNT -- which, by the way, is the smallest strategic nuclear
warhead we possess today. {Point in fact -- One Trident Nuclear Submarine
carries as much destructive power as 25 World War II's}).
While Uranium is an ideally fissionable material, it is not the only one.
Plutonium can be used in an atomic bomb as well. By leaving U-238 inside an
atomic reactor for an extended period of time, the U-238 picks up extra
particles (neutrons especially) and gradually is transformed into the element
Plutonium.
Plutonium is fissionable, but not as easily fissionable as Uranium.
While Uranium can be detonated by a simple 2-part gun-type device, Plutonium
must be detonated by a more complex 32-part implosion chamber along with a
stronger conventional explosive, a greater striking velocity and a
simultaneous triggering mechanism for the conventional explosive packs. Along
with all of these requirements comes the additional task of introducing a fine
mixture of Beryllium and Polonium to this metal while all of these actions are
occurring.
Supercritical mass for Plutonium is defined as 35.2 lbs (16 kgs). This
amount needed for a supercritical mass can be reduced to a smaller quantity of
22 lbs (10 kgs) by surrounding the Plutonium with a U-238 casing.
============================================================================
- Diagram of a Chain Reaction -
-------------------------------
|
|
|
|
[1]------------------------------> o
. o o .
. o_0_o . <-----------------------[2]
. o 0 o .
. o o .
|
\|/
~
. o o. .o o .
[3]-----------------------> . o_0_o"o_0_o .
. o 0 o~o 0 o .
. o o.".o o .
|
/ | \
|/_ | _\|
~~ | ~~
|
o o | o o
[4]-----------------> o_0_o | o_0_o <---------------[5]
o~0~o | o~0~o
o o ) | ( o o
/ o \
/ [1] \
/ \
/ \
/ \
o [1] [1] o
. o o . . o o . . o o .
. o_0_o . . o_0_o . . o_0_o .
. o 0 o . <-[2]-> . o 0 o . <-[2]-> . o 0 o .
. o o . . o o . . o o .
/ | \
|/_ \|/ _\|
~~ ~ ~~
. o o. .o o . . o o. .o o . . o o. .o o .
. o_0_o"o_0_o . . o_0_o"o_0_o . . o_0_o"o_0_o .
. o 0 o~o 0 o . <--[3]--> . o 0 o~o 0 o . <--[3]--> . o 0 o~o 0 o .
. o o.".o o . . o o.".o o . . o o.".o o .
. | . . | . . | .
/ | \ / | \ / | \
: | : : | : : | :
: | : : | : : | :
\:/ | \:/ \:/ | \:/ \:/ | \:/
~ | ~ ~ | ~ ~ | ~
[4] o o | o o [5] [4] o o | o o [5] [4] o o | o o [5]
o_0_o | o_0_o o_0_o | o_0_o o_0_o | o_0_o
o~0~o | o~0~o o~0~o | o~0~o o~0~o | o~0~o
o o ) | ( o o o o ) | ( o o o o ) | ( o o
/ | \ / | \ / | \
/ | \ / | \ / | \
/ | \ / | \ / | \
/ | \ / | \ / | \
/ o \ / o \ / o \
/ [1] \ / [1] \ / [1] \
o o o o o o
[1] [1] [1] [1] [1] [1]
============================================================================
- Diagram Outline -
---------------------
[1] - Incoming Neutron
[2] - Uranium-235
[3] - Uranium-236
[4] - Barium Atom
[5] - Krypton Atom
===========================================================================
I. The History of the Atomic Bomb
------------------------------
On August 2nd 1939, just before the beginning of World War II, Albert
Einstein wrote to then President Franklin D. Roosevelt. Einstein and several
other scientists told Roosevelt of efforts in Nazi Germany to purify U-235
with which might in turn be used to build an atomic bomb. It was shortly
thereafter that the United States Government began the serious undertaking
known only then as the Manhattan Project. Simply put, the Manhattan Project
was committed to expedient research and production that would produce a viable
atomic bomb.
The most complicated issue to be addressed was the production of ample
amounts of `enriched' uranium to sustain a chain reaction. At the time,
Uranium-235 was very hard to extract. In fact, the ratio of conversion from
Uranium ore to Uranium metal is 500:1. An additional drawback is that the 1
part of Uranium that is finally refined from the ore consists of over 99%
Uranium-238, which is practically useless for an atomic bomb. To make it even
more difficult, U-235 and U-238 are precisely similar in their chemical
makeup. This proved to be as much of a challenge as separating a solution of
sucrose from a solution of glucose. No ordinary chemical extraction could
separate the two isotopes. Only mechanical methods could effectively separate
U-235 from U-238. Several scientists at Columbia University managed to solve
this dilemma.
A massive enrichment laboratory/plant was constructed at Oak Ridge,
Tennessee. H.C. Urey, along with his associates and colleagues at Columbia
University, devised a system that worked on the principle of gaseous
diffusion. Following this process, Ernest O. Lawrence (inventor of the
Cyclotron) at the University of California in Berkeley implemented a process
involving magnetic separation of the two isotopes.
Following the first two processes, a gas centrifuge was used to further
separate the lighter U-235 from the heavier non-fissionable U-238 by their
mass. Once all of these procedures had been completed, all that needed to be
done was to put to the test the entire concept behind atomic fission.
Over the course of six years, ranging from 1939 to 1945, more than 2
billion dollars were spent on the Manhattan Project. The formulas for
refining Uranium and putting together a working bomb were created and seen to
their logical ends by some of the greatest minds of our time. Among these
people who unleashed the power of the atomic bomb was J. Robert Oppenheimer.
Oppenheimer was the major force behind the Manhattan Project. He
literally ran the show and saw to it that all of the great minds working on
this project made their brainstorms work. He oversaw the entire project from
its conception to its completion.
Finally the day came when all at Los Alamos would find out whether or not
The Gadget (code-named as such during its development) was either going to be
the colossal dud of the century or perhaps end the war. It all came down to
a fateful morning of midsummer, 1945.
At 5:29:45 (Mountain War Time) on July 16th, 1945, in a white blaze that
stretched from the basin of the Jemez Mountains in northern New Mexico to the
still-dark skies, The Gadget ushered in the Atomic Age. The light of the
explosion then turned orange as the atomic fireball began shooting upwards at
360 feet per second, reddening and pulsing as it cooled. The characteristic
mushroom cloud of radioactive vapor materialized at 30,000 feet. Beneath the
cloud, all that remained of the soil at the blast site were fragments of jade
green radioactive glass. ...All of this caused by the heat of the reaction.
The brilliant light from the detonation pierced the early morning skies
with such intensity that residents from a faraway neighboring community would
swear that the sun came up twice that day. Even more astonishing is that a
blind girl saw the flash 120 miles away.
Upon witnessing the explosion, reactions among the people who created
it were mixed. Isidor Rabi felt that the equilibrium in nature had been
upset -- as if humankind had become a threat to the world it inhabited.
J. Robert Oppenheimer, though ecstatic about the success of the project,
quoted a remembered fragment from Bhagavad Gita. "I am become Death," he
said, "the destroyer of worlds." Ken Bainbridge, the test director, told
Oppenheimer, "Now we're all sons of bitches."
Several participants, shortly after viewing the results, signed petitions
against loosing the monster they had created, but their protests fell on deaf
ears. As it later turned out, the Jornada del Muerto of New Mexico was not
the last site on planet Earth to experience an atomic explosion.
As many know, atomic bombs have been used only twice in warfare. The
first and foremost blast site of the atomic bomb is Hiroshima. A Uranium
bomb (which weighed in at over 4 & 1/2 tons) nicknamed "Little Boy" was
dropped on Hiroshima August 6th, 1945. The Aioi Bridge, one of 81 bridges
connecting the seven-branched delta of the Ota River, was the aiming point of
the bomb. Ground Zero was set at 1,980 feet. At 0815 hours, the bomb was
dropped from the Enola Gay. It missed by only 800 feet. At 0816 hours, in
the flash of an instant, 66,000 people were killed and 69,000 people were
injured by a 10 kiloton atomic explosion.
The point of total vaporization from the blast measured one half of a
mile in diameter. Total destruction ranged at one mile in diameter. Severe
blast damage carried as far as two miles in diameter. At two and a half
miles, everything flammable in the area burned. The remaining area of the
blast zone was riddled with serious blazes that stretched out to the final
edge at a little over three miles in diameter.
On August 9th 1945, Nagasaki fell to the same treatment as Hiroshima.
Only this time, a Plutonium bomb nicknamed "Fat Man" was dropped on the city.
Even though the "Fat Man" missed by over a mile and a half, it still leveled
nearly half the city. Nagasaki's population dropped in one split-second from
422,000 to 383,000. 39,000 were killed, over 25,000 were injured. That
blast was less than 10 kilotons as well. Estimates from physicists who have
studied each atomic explosion state that the bombs that were used had utilized
only 1/10th of 1 percent of their respective explosive capabilities.
While the mere explosion from an atomic bomb is deadly enough, its
destructive ability doesn't stop there. Atomic fallout creates another hazard
as well. The rain that follows any atomic detonation is laden with
radioactive particles. Many survivors of the Hiroshima and Nagasaki blasts
succumbed to radiation poisoning due to this occurance.
The atomic detonation also has the hidden lethal surprise of affecting
the future generations of those who live through it. Leukemia is among the
greatest of afflictions that are passed on to the offspring of survivors.
While the main purpose behind the atomic bomb is obvious, there are many
by-products that have been brought into consideration in the use of all
weapons atomic. With one small atomic bomb, a massive area's communications,
travel and machinery will grind to a dead halt due to the EMP (Electro-
Magnetic Pulse) that is radiated from a high-altitude atomic detonation.
These high-level detonations are hardly lethal, yet they deliver a serious
enough EMP to scramble any and all things electronic ranging from copper wires
all the way up to a computer's CPU within a 50 mile radius.
At one time, during the early days of The Atomic Age, it was a popular
notion that one day atomic bombs would one day be used in mining operations
and perhaps aid in the construction of another Panama Canal. Needless to say,
it never came about. Instead, the military applications of atomic destruction
increased. Atomic tests off of the Bikini Atoll and several other sites were
common up until the Nuclear Test Ban Treaty was introduced. Photos of nuclear
test sites here in the United States can be obtained through the Freedom of
Information Act.
[See Smyth Report for fuller details. Goin's book in References has photos
of nuke sites.]
============================================================================
1994