With a few parts from a hardware store and
some know-how, it is possible to build a weapon of mass destruction.
Well, as long as you can find a few pounds of plutonium on Ebay
to fuel it.
In 1905 Albert Einstein
wrote a number of revolutionary physics papers including his
Special Theory of Relativity. One of the formulas that came
out of this, almost as an afterthought, was E=mc˛. That
is: energy is equal to mass times the speed of light squared.
What Einstein was saying is that matter - everything around
us we can touch and see - is actually the same thing as energy,
just in a different form. The upshot of this is that it should
be possible to convert energy to matter or, visa versa, convert
matter to energy.
A 10 megaton H-bomb test.
An A-bomb (
or atomic bomb) is generally considered to be one
based on the fission principal - that is the splitting
of atoms. An H-bomb (or hydrogen bomb) is based
on the principal of fusion, that is the fusing of atoms
together. H-bombs are generally much more powerful than
A-bombs. The largest A-bomb tops out at the equivalent
of 0.7 megatons of TNT, while the largest H-bomb ever
produced was 50 megatons. The heat and pressures needed
in order to get an H-bomb's fusion reaction going, however,
can only be produced on earth at the heart of a fission
bomb, so in effect every H-bomb has an A-bomb as a part
of its mechanism
The energy we release every day when driving our
car or cooking on a stove comes from chemical reactions.
Two or more chemicals react through the motion of electrons
and the forming and breaking of chemical bonds. One familiar
form of chemical reaction is combustion. For example, the oxygen
in the air reacts with the substances in a candle to release
heat and light. In chemical reactions the amount of measurable
matter involves never changes, if you were able to capture all
the soot, smoke and carbon dioxide released by the candle you
would find they weigh exactly the same amount as the original
candle and oxygen that reacted with it. The material changed
form and released energy but did not disappear.
Einstein's formula suggested that it was possible
to get energy by what we now call a nuclear reaction. This is
the conversion of matter to energy. What's more, the amount
of energy available in even a small amount of matter is, according
to the formula, tremendous. Matter is just sort of a condensed
version of energy, but it isn't a one-to-one relationship. The
conversion factor is the speed of light (already a huge number)
squared (which makes it a really big number). We can picture
this relationship by thinking about water and steam. You can
cool steam (think of this as the energy) down and it becomes
water (think of this as matter) or heat water up to make steam.
It takes a lot of steam to create a few drops of water though,
but only few a ounces of water to create a whole room full of
steam. The same is true of energy and matter. In the atomic
bomb that destroyed Hiroshima only 600 milligrams of uranium
(less than the weight of a dime) was converted to energy, but
it released the same amount of power as at least 13,000 tons
of the conventional chemical explosive TNT.
Converting matter to energy is no easy trick,
however. The sun does it naturally by a process called fusion.
The sun, a gigantic ball of mostly hydrogen gas, has intense
pressures and heat created at its core by its gravity. It is
under this heat and pressure that the hydrogen atoms fuse to
create helium and release energy. Re-creating the intense conditions
required to generate fusion on earth isn't easy, however, so
atomic bombs uses another process called fission.
neutron strikes a uranium atom 2)The uranium atom is split
into a krypton atom and a barium atom releasing some binding
energy along with more neutrons. 3)The neutrons strike
other uranium atoms starting the process all over again.
(Copyright Lee Krystek, 2007)
A fission reaction is just the opposite of fusion.
Instead of atoms being put together, they are split into pieces.
When a neutron (a subatomic particle) with enough energy hits
an atom of radioactive material like uranium, the uranium atom
will split into two smaller atoms and some of the energy that
held the original atom together is released. If the right type
of uranium is used, the split will also release additional neutrons
capable of splitting other atoms. If this process continues
with each new split releasing neutrons which in turn split other
atoms it is called a chain reaction. Because of the speed
involved in a nuclear reaction, billions of atoms can be split
in a tiny fraction of a second. If the reaction proceeds at
a sedate level the fission produces energy in a controllable
manner. This is what is going on in the heart of a nuclear power
plant. The energy released is used to heat water to the point
of steam and the steam spins turbines connected to generators
to make electricity. If the reaction proceeds at an uncontrolled
level, however, a nuclear explosion can result.
This might seem to make nuclear power plants potential
atomic bombs, but the uranium used in the plants is not the
type that could sustain a reaction at a rate high enough to
cause an explosion by itself (nuclear power plants are subject
to explosions caused by steam pressure and other non-nuclear
forces, however). In fact, engineering a device that does not
tear itself apart before the explosion really gets underway
is one of the main design problems of building a bomb.
Uranium or plutonium can be used as fuel for atomic
bombs. Both are highly radioactive. This means they are constantly
shedding subatomic particles including neutrons. Only certain
isotopes of these materials - like uranium 235 and plutonium
239 - consistently give off neutrons of such high energy that
they will split atoms. When enough of the material is put together,
a chain reaction starts and the mass is said to be critical.
The term used for a mass of radioactive material with a growing
chain reaction, splitting more and more atoms with each moment,
is supercritical. While putting enough uranium 235 together
in a single mass will make it supercritical (and create a surge
of radiation that will kill you if you are standing near it
unprotected - see "A Supercritical Accident" below)
it is not enough to create a bomb. The material must be held
in a compressed state long enough for the reaction to take place
while resisting the initial energy of the explosion that will
try and tear it apart. There are two well-known approaches to
doing this. The first is known as the "gun" method.
conventional explosive drives the uranium "bullet"
into the "spike;" bringing the mass to supercritical
and causing the detonation. (Copyright
Lee Krystek, 2007)
The "gun" is the simplest way to build a nuclear
weapon. The atomic bomb used on Hiroshima during World War II
used this approach. The weapon consists of a tube (much like
the barrel of a gun) with half the nuclear charge fixed at one
end and the other half (the moving half) at the opposite end.
A conventional explosive charge was placed behind the moving
portion which can be thought of as the "bullet." When the conventional
charge is detonated, the bullet races down the tube and slams
into the fixed charge at the other end (referred to as the "spike").
Once the two halves of the nuclear fuel are brought together
and held together long enough, the chain reaction starts, the
fuel goes supercritical and the explosion takes place.
While the gun method is easy to engineer, it has
some drawbacks. The biggest one is the need to make sure the
two parts of nuclear fuel come together rapidly enough. As the
two sections get about an inch apart, they will start exchanging
neutrons that might start a chain reaction. If the two parts
go supercritical before they get close enough, the force of
the energy released will blow them apart before the main explosion
gets underway. This type of failure is known as a "fizzle."
Another problem is that this method is less efficient,
requiring between 20 and 25 kilograms (around 44 to 55 pounds)
of uranium. Other approaches can use as little as 15 kilograms
(about 33 pounds). Given that weapon's grade uranium and plutonium
are very hard to get, this is a real disadvantage.
Also, the gun method only works if the uranium
is being used as the fuel. The process of creating plutonium
generally causes it to be contaminated with other materials
which increase the chance of it going supercritical before the
two sections are close enough together. This, in turn, increases
the chances of a fizzle instead of a blast. To make the gun
method work reliably with plutonium, you would have to increase
the speed with which the "bullet" approached the "spike" significantly.
To do this would mean making the tube impracticality long.
explosives press on the "tamper/pusher," compressing
the plutonium until it reaches a supercritical mass. The
initiator floods the area with neutrons to help get the
chain reaction going. (Copyright Lee
For this reason, if you use plutonium to fuel
a bomb you need to use the more sophisticated "implosion" method.
With this approach the nuclear fuel is shaped into a sphere
(called the "pit"). Conventional explosives are put around it.
When these are detonated the force of the explosion squeezes
the pit into a supercritical mass long enough for the explosion
to take place. While the principle sounds easy, it is difficult
to actually make it work. The pit cannot simply be surrounded
by high explosives. The shock wave that compresses it must be
precisely spherical, otherwise the pit material will escape
out through a weak point. To create the necessary explosive
force in a perfect sphere, shaped explosive charges (sometimes
called explosive lens) are used. The "fatman" bomb the leveled
Nagasaki in World War II used 32 charges arranged around the
pit like the faces of a soccer ball. In order to create the
spherical shock wave it isn't only necessary to get the charges
in the right position with the right shape, but they must be
detonated at exactly the right time. A charge that detonates
late will create a hole in the shock wave through which the
pit can escape.
Implosion designs also require a neutron trigger
or "initiator" to flood the pit with neutrons during detonation.
In "fatman" this was done with a small sphere with layers of
beryllium and polonium separated by thin gold foil placed in
the center of the pit. An implosion design may also include
other layers between the explosives and the pit to create a
more powerful explosion. These include a "pusher" (designed
to increase the explosive shock wave hitting the pit), a "tamper"
(to help the pit from blowing apart too quickly once the explosion
starts), and a "reflector" composed of a material that will
reflect neutrons back in the pit increasing the amount of fission.
In some bomb designs these functions are integrated into a single
layer of material.
The plutonium sphere resting
in the fatal testbed.
In 1945 an
atomic bomb worker, Harry K. Daghlian Jr, was killed while
experimenting with plutonium. The test was designed to
see just how much of a neutron reflector was needed to
push the sphere of plutonium to the edge of going supercritical
with the experimenter gauging how close he was getting
by listening to a Geiger counter. As he moved the final
"brick" of reflective material close to the sphere he
realized he should not place it in position, but then
it slipped from his hand. Daghlian knocked the brick away,
but it was too late. The sphere went supercritical with
a flash of blue light. He was exposed to 510 REMs of radiation
and after an agonizing illness, died 28 days later.
The implosion design is generally considered to
be superior in almost every way to the gun design and it is
the choice for any organization with the resources to design
and construct it. One of the major advantages of this approach
is that it is easy to make the implosion design more efficient
by increasing the effectiveness of the conventional explosives.
For example, if the pit is squeezed so that the density is doubled
during detonation it may yield a 10-kiloton explosion. If that
same pit can be compressed to three times its original density,
a 40-kiloton explosion can be generated with no additional nuclear
fuel. The longer the fission material is allowed to react, the
bigger the explosion.
You Build a Bomb?
Building a basic nuclear weapon is not easy, but
not all that hard either. In 1964 the U.S. Army decided to see
just how difficult it was. They hired two professors that had
Ph.Ds in physics, but no experience with nuclear weapons or
access to nuclear secrets. The two were given the task of designing
an atomic bomb using only information available to the general
public. It took them roughly two years, but in the end they
designed an implosion style weapon that could have been made
in a local machine shop which could have produced an explosion
similar to the Hiroshima bomb.
The only thing that they found extremely difficult
to do was to get the proper material to fuel the bomb: uranium
235 or plutonium 239. Only a tiny fraction of natural uranium
that is mined from the ground is isotope 235 and separating
it from the other isotopes is a major chore requiring huge factory
complexes working years to isolate just a few pounds. In fact,
most weapons programs get around this by utilizing plutonium,
which is very rarely in found nature at all, but can be created
by exposing more common types of uranium to radiation in a nuclear
"breeder" reactor. Plutonium is extremely difficult to handle,
however. It is one of the most toxic materials known to man,
especially if inhaled.
It is the difficulty of getting and handling these
fissionable materials that protects us from people building
nuclear bombs in their basements. It is for this reason nonproliferation
of nuclear material is a major concern of most governments and
there is great apprehension about countries who want to build
nuclear reactors capable of "breeding" plutonium fuel. Knowledge
of how to build a bomb is hard to control. Fortunately, so far,
the materials needed have been much easier to keep track of.
Tsar Bomba the largest H-Bomb ever tested.
But for how long?
Note all the information
in this article has been assembled from unclassified public
materials and fall short of the details necessary to build an
actual weapon. This article is for information purposes only,
and in no way are unauthorized persons encouraged to construct
weapons of any sort.