Nuclear Fission refers to either a nuclear reaction or a radioactive decay process in which the nucleus of an atom splits into smaller parts (lighter nuclei), often producing free neutrons and photons (in the form of gamma rays), and releasing a very large amount of energy, even by the energetic standards of radioactive decay. The two nuclei produced are most often of comparable but slightly different sizes, typically with a mass ratio of products of about 3 to 2, for common fissile isotopes.Most fissions are binary fissions (producing two charged fragments), but occasionally (2 to 4 times per 1000 events), three positively charged fragments are produced, in a ternary fission. The smallest of these fragments in ternary processes ranges in size from a proton to an argon nucleus.
Fission of heavy elements is an exothermic reaction which can release large amounts of energy both as electromagnetic radiation and as kinetic energy of the fragments (heating the bulk material where fission takes place). In order for fission to produce energy, the total binding energy of the resulting elements must be greater than that of the starting element. Fission is a form of nuclear transmutation because the resulting fragments are not the same element as the original atom.
Splitting the Uranium Atom:
Uranium is the principle element used in nuclear reactors and in certain types of atomic bombs. The specific isotope used is U-235. When a stray neutron strikes a U-235 nucleus, it is at first absorbed into it. This creates U-236. U-236 is unstable and this causes the atom to fission. The fissioning of U-236 can produce over twenty different products. However, the products' masses always add up to 236. The following two equations are examples of the different products that can be produced when U-235 fissions:
U-235 + 1 neutron2 neutrons + 92Kr + 142Ba + ENERGY
U-235 + 1 neutron
In each of the above reactions, 1 neutron splits the atom. When the atom is split, 1 additional neutron is released. This is how a chain reaction works. If more U-235 is present, those 2 neutrons can cause 2 more atoms to split. Each of those atoms releases 1 more neutron bringing the total neutrons to 4. Those 4 neutrons can strike 4 more U-235 atoms, releasing even more neutrons. The chain reaction will continue until all the U-235 fuel is spent. This is roughly what happens in an atomic bomb. It is called a runaway nuclear reaction.
In this animation, one can see how the fissioning of each U-235 atom (red) releases more neutrons (green) that go on to fission more U-235 atoms, thus producing a chain reaction.
Where Does the Energy Come From?
In the section above we described what happens when an U-235 atom fissions. We gave the following equation as an example:
U-235 + 1 neutron
2 neutrons + 92Kr + 142Ba + ENERGYYou might have been wondering, "Where does the energy come from?". The mass seems to be the same on both sides of the reaction:
235 + 1 = 2 + 92 + 142 = 236 = U-236
reference : http://en.wikipedia.org/wiki/Nuclear_fission
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