Uranium-235 Uranium-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 fission chain reaction . It is the only fissile isotope that is primordial and found in relatively significant quantities in nature. Uranium-235 has a half-life of 703.8 million years. It was discovered in 1935 by Arthur Jeffrey Dempster . Its fission cross section for slow thermal neutrons is about 584.3±1 barns. For fast neutrons it is on the order of 1 barn . Most but not all neutron absorptions result in fission; a minority result in neutron capture forming uranium-236 .Fission The fission of one atom of uranium-235 releases inside the reactor. That corresponds to 19.54 TJ/mol, or 83.14 TJ/kg. Another 8.8 MeV escapes the reactor as anti-neutrinos. When nuclides are bombarded with neutrons, one of the many fission reactions that it can undergo is the following :n + → + + 3 n Heavy water reactors, and some graphite moderated reactors , can use natural uranium, but light water reactors must use low enriched uranium because of the higher neutron absorption of light water. Uranium enrichment removes some of the uranium-238 and increases the proportion of uranium-235. Highly enriched uranium , which contains an even greater proportion of uranium-235, is sometimes used in the reactors of nuclear submarines , research reactors and nuclear weapons . If at least one neutron from uranium-235 fission strikes another nucleus and causes it to fission, then the chain reaction will continue. If the reaction will sustain itself, it is said to be critical , and the mass of 235 U required to produce the critical condition is said to be a critical mass . A critical chain reaction can be achieved at low concentrations of U-235 if the neutrons from fission are moderated to lower their speed, since the probability for fission with slow neutrons is greater. A fission chain reaction produces intermediate mass fragments which are highly radioactive and produce further energy by their radioactive decay . Some of them produce neutrons, called delayed neutrons , which contribute to the fission chain reaction. The power output of nuclear reactors is adjusted by the location of control rods containing elements that strongly absorb neutrons, e.g., boron , cadmium , or hafnium , in the reactor core . In nuclear bombs , the reaction is uncontrolled and the large amount of energy released creates a nuclear explosion .Nuclear weapons The Little Boy gun type atomic bomb dropped on Hiroshima on August 6, 1945 was made of highly enriched uranium with a large tamper . The nominal spherical critical mass for an untampered 235 U nuclear weapon is, a sphere in diameter. The material must be 85% or more of 235 U and is known as weapons grade uranium , though for a crude, inefficient weapon, 20% is sufficient. Even lower enrichment can be used, but then the required critical mass rapidly increases. Use of a large tamper, implosion geometries, trigger tubes, polonium triggers, tritium enhancement, and neutron reflectors can enable a more compact, economical weapon using one-fourth or less of the nominal critical mass, though this would likely only be possible in a country that already had extensive experience in engineering nuclear weapons. Most modern nuclear weapon designs use plutonium-239 as the fissile component of the primary stage; however, HEU is often used in the secondary stage as an ignitor for the fusion fuel.Uses Uranium-235 has many uses such as fuel for nuclear power plants , and nuclear weapons such as nuclear bombs. Some artificial satellites, such as the SNAP-10A and the RORSATs were powered by nuclear reactors fueled with U-235.
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