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Uranium-235 (235 U or U-235) is an isotope of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope uranium-238, it is fissile, i.e., it can sustain a nuclear chain reaction. It is the only fissile isotope that exists in nature as a primordial nuclide. Uranium-235 has a half-life of 703.8 million years.
Since nuclear chain reactions may only require natural materials (such as water and uranium, if the uranium has sufficient amounts of 235 U), it was possible to have these chain reactions occur in the distant past when uranium-235 concentrations were higher than today, and where there was the right combination of materials within the Earth's crust.
Fission product yields by mass for thermal neutron fission of U-235 and Pu-239 (the two typical of current nuclear power reactors) and U-233 (used in the thorium cycle). This page discusses each of the main elements in the mixture of fission products produced by nuclear fission of the common nuclear fuels uranium and plutonium.
When a uranium nucleus fissions into two daughter nuclei fragments, about 0.1 percent of the mass of the uranium nucleus [15] appears as the fission energy of ~200 MeV. For uranium-235 (total mean fission energy 202.79 MeV [16]), typically ~169 MeV appears as the kinetic energy of the daughter nuclei, which fly apart at about 3% of the speed of ...
Uranium-235 fissions with low-energy thermal neutrons because the binding energy resulting from the absorption of a neutron is greater than the critical energy required for fission; therefore uranium-235 is fissile. By contrast, the binding energy released by uranium-238 absorbing a thermal neutron is less than the critical energy, so the ...
The symbols are defined as: [3], and are the average number of neutrons produced per fission in the medium (2.43 for uranium-235). and are the microscopic fission and absorption thermal cross sections for fuel, respectively.
The less-moderated neutrons are relatively more likely to be captured by uranium-238 and less likely to fission uranium-235, so light-water reactors require enriched uranium to operate. There are also proposals to use the compound formed by the chemical reaction of metallic uranium and hydrogen ( uranium hydride —UH 3 ) as a combination fuel ...
Change of content of Uranium-235 in natural uranium; the content was 3.65% 2 billion years ago. A key factor that made the reaction possible was that, at the time the reactor went critical 1.7 billion years ago, the fissile isotope 235 U made up about 3.1% of the natural uranium, which is comparable to the amount used in some of today's reactors.