The Fission Process


Fission involves the process of a large nuclei splitting into smaller nuclei, either through a nuclear reaction of radioactive decay. Binding energy is related to how stable certain nuclei are, with the largest being seemingly the most unstable, the following graph shows which nucleons are best suited for fission or fusion, with the heaviest nucleons being the best for fission.


(http://www.bbc.co.uk/bitesize/higher/physics/radiation/nuclear_reactions/revision/2/)

The heavier nucleons are inherently unstable, and as a result naturally want to split into two smaller, more stable atoms through the process of fission. However, this process takes millions of years in nature, so something must be done to induce these reactions artificially. This leads to the use of neutron bombardment to induce the same reaction that would have occurred naturally. When a large nuclei absorbs an additional neutron it may become unstable and split into two smaller nuclei and additional free neutrons. An example of a reaction like this would be Uranium-235 absorbing a neutron and then splitting into one Barium-139 atom, one Krypton-94 atom and 3 free neutrons. Barium and Krypton are only one of the hundreds or possible products from the neutron induced Uranium-235 reaction. The following graph shows the distribution of fission fragments based on atomic mass, where the green area represents the fission of uranium-233, blue of plutonium-239, red of uranium-235, and black a mixture of uranium and plutonium.


(http://en.wikipedia.org/wiki/Nuclear_fission_product#/media/File:ThermalFissionYield.svg)

Barium and Krypton are only one of the hundreds or possible products from the neutron induced Uranium-235 reaction. These reactions also release an enormous amount of energy in the form of kinetic energy from the fission products, gamma ray emmision, and fast-neutrons. “The induced fission of this isotope releases an average of 200 MeV per atom, or 80 million kilojoules per gram of Uranium-235. The attraction of nuclear fissionas a source of power can be understood by comparing this value with the 50kJ/g released when natural gas is burned”[2]. Nuclear reactors use the additional free neutrons that are released to induce additional reactions in what is known as a nuclear chain reaction. These chain reactions are controlled within a nuclear reactor, which then harness the energy to produce power. In contrast, in nuclear weapons the chain reaction is left uncontrolled.



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