Uranium-235
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| Uranium-235 | |
|---|---|
| Image:Nuclear fuel pellets.jpeg | |
| General | |
| Name, symbol | Uranium-235, 235U |
| Neutrons | 143 |
| Protons | 92 |
| Nuclide Data | |
| Natural abundance | 0.72% |
| Half-life | 7.038·108 years |
| Decay products | Thorium-231 |
| Isotope mass | 235.0439299(20) u |
| Spin | 7/2- |
| Excess energy | 40914.062 ± 1.970 keV |
| Binding energy | 1783870.285 ± 1.996 keV |
| Decay mode | Decay energy |
| SF | n/a MeV |
| Alpha decay | Template:Qif |
| Template:Qif | |
| Template:Qif | |
Uranium-235 is an isotope of uranium that differs from the element's other common isotope, uranium-238, by its ability to cause a rapidly expanding fission chain reaction, i.e., it is fissile. In fact, U-235 is the only fissile isotope found in nature. It was discovered in 1935 by Arthur Jeffrey Dempster. A uranium nucleus that absorbs a neutron splits into two lighter nuclei; this is called nuclear fission. It releases either two or three neutrons which continue the reaction. In nuclear reactors, the reaction is slowed down by the addition of control rods which are made of elements such as boron, cadmium, and hafnium which can absorb a large number of neutrons. In nuclear bombs, the reaction is uncontrolled and the large amount of energy released creates a nuclear explosion.
The fission of one atom of U-235 generates 200 MeV = Template:Sn J, i.e. 18 TJ/mol = 77 TJ/kg.
Only around 0.72% of all natural uranium is uranium-235, the rest being mostly uranium-238. This concentration is insufficient for a self sustaining reaction in a mass of pure uranium or a light water reactor; enrichment, which just means separating out the uranium-238, must take place to get a usable concentration of uranium-235. Pressurised Heavy Water Reactors, other heavy water reactors, and some graphite moderated reactors are known for using unenriched uranium. For an explosion, approximately 90% purity is required. Uranium which has been processed to boost its uranium-235 propotion is known as enriched uranium, and there are many different levels of enrichment for different purposes.
Uranium-235 has a half-life of 700 million years.