HKNIC - Material for Fission Material for Fission
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Material for Fission

The number of elements that can be used for fission is very small.

Uranium has two nuclides that occur in nature and may undergo fission. Natural uranium consists of 0.711% of uranium 235 (U-235), which may be split by neutrons of any energy level, although neutrons at low energy will be most effective. The other constituent, uranium 238 makes up of 99.275%, but can only be split by neutrons with much greater energy. Most of the nuclear power generated today comes from the splitting of U-235.

Thorium 232 is another nuclide that occurs in nature and is several times more plentiful than uranium. However, it needs slightly more energy from the incoming neutron to split than U-238, and so is not very commonly used.

There are three other nuclides, uranium 233, plutonium 239 and plutonium 241, which do not occur in nature but can be produced artificially from thorium 232, uranium 238 and plutonium 240 respectively, but which can be split effectively even by neutrons at low energy.

Four Stages in the Fission Progress for U-235

A neutron hitting a U 235 nucleus is absorbed to form a U 236 nucleus. The U 236 nucleus is not stable and it splits to form two lighter nuclei. In the process, energy and two or sometimes three neutrons are released.

The nature of the two lighter nuclei and the number of neutrons released vary from one fission to another. Of the two lighter nuclei, about 0.01% will be formed to have equal mass. However, most will have one heavy and one light nucleus.

The neutrons produced in a nuclear fission may continue to hit other U 235 nuclei to yield more fission and maintain the chain reaction. However, the process will stop if too many neutrons escape from the mass of uranium material, leaving not enough there to maintain fission. If more neutrons are produced than are necessary to maintain the chain reaction and make up for those escaping, the number of fissions will increase and the material will get hotter and will usually melt. The process is self-sustaining if a proper neutron balance can be kept. This situation is a “controlled” chain reaction.

How the Mass of the Split Nuclei of U-235 vary
A Chain Reaction