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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.
Natural uranium is made weapons-grade through isotopic enrichment. Initially only about 0.7% of it is fissile U-235, with the rest being almost entirely uranium-238 (U-238). They are separated by their differing masses. Highly enriched uranium is considered weapons-grade when it has been enriched to about 90% U-235. [citation needed]
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 ...
Since the nuclei that can readily undergo fission are particularly neutron-rich (e.g. 61% of the nucleons in uranium-235 are neutrons), the initial fission products are often more neutron-rich than stable nuclei of the same mass as the fission product (e.g. stable zirconium-90 is 56% neutrons compared to unstable strontium-90 at 58%).
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.
Krypton-85, with a half-life 10.76 years, is formed by the fission process with a fission yield of about 0.3%. Only 20% of the fission products of mass 85 become 85 Kr itself; the rest passes through a short-lived nuclear isomer and then to stable 85 Rb. If irradiated reactor fuel is reprocessed, this radioactive krypton may be released into ...
The "special nuclear materials" are also plutonium-239, uranium-233, and enriched uranium (U-235). Note that the 1980 Convention on the Physical Protection of Nuclear Material definition of nuclear material does not include thorium. [4] The NRC has a regulatory process for nuclear materials with five main components. [5]
When a free neutron hits the nucleus of a fissile atom like uranium-235 (235 U), the uranium nucleus splits into two smaller nuclei called fission fragments, plus more neutrons (for 235 U three about as often as two; an average of just under 2.5 per fission). The fission chain reaction in a supercritical mass of fuel can be self-sustaining ...