Fission is the splitting of an atom nucleus. When fission happens, the nucleus splits into several smaller parts. In the process of splitting apart, some of the mass is lost. This lost mass becomes energy. Nuclear fission reactors are examples of controlled fission reaction. Nuclear weapons are examples of an uncontrolled nuclear fission reaction.

Chain reactionCredit: Fastfission

Fission Of Uranium U235

Uranium 235 is the best element to use for a fission reaction because it is a large atom and contains more neutrons than protons. This makes it ideal for a nuclear fission reaction. The amount of U235 for a chain reaction should be around four percent and it is approximately 0.7 percent in nature. Enriching uranium increases the amount of U235 to a higher percentage and makes it useable for a fission reaction. A fission reaction is possible in nature because there is evidence that a partial fission reaction occurred in Gambia sometime in the pasthistory. A chain reaction doesn’t normally occur in nature because the amount of U235 is too low to make it fissionable.


A neutron is a particle with a neutral charge. It has neither a positive nor a negative charge. Because a neutron is neutral it is the particle used to fire at U235 to start a fission reaction. A neutron isn’t deflected by positive or negative charges, and will go to the center of the uranium atom and penetrate deeper. This causes the fission reaction.

Fission Reaction

The neutron splits the uranium atom apart. This contact releases more neutrons that penetrate more uranium atoms. This continues until a neutron has penetrated all the uranium atoms. This is a chain reaction. The process sustains itself as long as neutrons penetrate uranium atoms. When it continues, it is at critical mass. After a uranium atom splits, it has less mass. High amounts of heat and radioactivity replace the lost mass.

Controlled and Uncontrolled Reactions

Cadmium and boron control rods absorb neutrons, and used in nuclear reactors. Raising and lowering the rods increase or decrease the neutron activity by increasing or decreasing rod area to absorb neutrons. This keeps the reaction continuing, but not enough to explode. It is neutron neutral where neutron production and destruction equal each other. When the uranium depletes to the point the reaction is failing it is time to add new fuel. Old fuel rods are extremely radioactive. Nuclear weapons use material processed from spent reactor rods. An atomic bomb is an uncontrolled fission reaction. There is no attempt to control neutron spread. It uses at least 20 percent uranium U235. The result produces high amounts of heat and radioactive material to cause its destruction.

The use of nuclear fission power has been the goal of scientists and researchers since the discovery of nuclear physics.  This invention is being used for both military and peaceful purposes.