The Difference Between Nuclear Fission & Nuclear Fusion
There are two types of atomic explosions that can be facilitated by
Uranium-235: fission and fusion. Fission, simply put, is a nuclear
reaction in which an atomic nucleus splits into fragments, usually two
fragments of comparable mass, emitting 100 million to several hundred
million volts of energy. This energy is expelled explosively and
violently in the atomic bomb. A fusion reaction is usually started with a
fission reaction, but unlike the fission (atomic) bomb, the fusion
(hydrogen) bomb derives its power from the fusing of nuclei of various
hydrogen isotopes into helium nuclei. This article discusses the A-bomb
or atomic bomb.
The massive power behind the reaction in an atomic bomb arises from the
forces that hold the atom together. These forces are akin to, but not
quite the same as, magnetism.
Atoms are comprised of various numbers and combinations of the three
sub-atomic particles: protons, neutrons and electrons. Protons and
neutrons cluster together to form the nucleus (central mass) of the atom
while the electrons orbit the nucleus much like planets around a sun.
It is the balance and arrangement of these particles that determine the
stability of the atom.
Most elements have very stable atoms which are impossible to split
except by bombardment in particle accelerators. For all practical
purposes, the only natural element whose atoms can be split easily is
uranium, a heavy metal with the largest atom of all natural elements and
an unusually high neutron-to-proton ratio. This higher ratio does not
enhance its "splitability," but it does have an important bearing on its
ability to facilitate an explosion, making uranium-235 an exceptional
candidate for nuclear fission.
There are two naturally-occurring isotopes of uranium. Natural uranium
consists mostly of isotope U-238, with 92 protons and 146 neutrons
(92+146=238) per atom. Mixed with this is a 0.6% accumulation of U-235,
with only 143 neutrons per atom. The atoms of this lighter isotope can
be split, thus it is "fissionable" and useful in making atomic bombs.
Neutron-heavy U-238 has a role to play in the atomic bomb as well since
its neutron-heavy atoms can deflect stray neutrons, preventing an
accidental chain reaction in a uranium bomb and keeping neutrons
contained in a plutonium bomb. U-238 can also be "saturated" to produce
plutonium (Pu-239), a man-made, radioactive element also used in atomic
Both isotopes of uranium are naturally radioactive; their bulky atoms
disintegrating over time. Given enough time (hundreds of thousands of
years) uranium will eventually lose so many particles that it will turn
into lead. This process of decay can be greatly accelerated in what is
known as a chain reaction. Instead of disintegrating naturally and
slowly, the atoms are forcibly split by bombardment with neutrons.
A blow from a single neutron is enough to split the less-stable U-235
atom, creating atoms of smaller elements (often barium and krypton) and
releasing heat and gamma radiation (the most powerful and lethal form of
radioactivity). The chain reaction occurs when "spare" neutrons from
this atom fly out with sufficient force to split other U-235 atoms they
come in contact with. In theory, it is necessary to split only one U-235
atom, which will release neutrons which will split other atoms, which
will release neutrons ... and so on. This progression is not arithmetic;
it is geometric and takes place within a millionth of a second.
The minimum amount to start a chain reaction as described above is known
as super critical mass. For pure U-235, it is 110 pounds (50
kilograms). No uranium is ever quite pure, however, so in reality more
will be needed. U-235, U-238 and Plutonium.
Uranium is not the only material used for making atomic bombs. Another
material is the Pu-239 isotope of the man-made element plutonium.
Plutonium is only found naturally in minute traces, so useable amounts
must be produced from uranium. In a nuclear reactor, uranium's heavier
U-238 isotope can be forced to acquire extra particles, eventually
becoming the plutonium.
Plutonium will not start a fast chain reaction by itself, but this
difficulty is overcome by having a neutron source, a highly radioactive
material that gives off neutrons faster than the Plutonium itself. In
certain types of bombs, a mixture of the elements Beryllium and Polonium
is used to bring about this reaction. Only a small piece is needed
(super critical mass is about 32 pounds, though as little as 22 can be
used). The material is not fissionable in and of itself, but merely acts
as a catalyst to the greater reaction.