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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 fission chain reaction. It is the only fissile isotope that is a primordial nuclide or found in significant quantity in nature.

Uranium-235 has a half-life of 700 million years and was discovered in 1935 by Arthur Jeffrey Dempster.Its nuclear cross section for slow thermal neutrons is about 1000 barn. 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.The fission of one atom of U-235 generates 202.5 MeV = 3.244 × 10−11 J, i.e. 19.54 TJ/mol= 83.14 TJ/kg.Heavy water reactors, and some graphite moderated reactors can use unenriched uranium, but light water reactors must use low enriched uranium because of light water's neutron absorption. Uranium enrichment removes some of the uranium-238 and increases the proportion of uranium-235. In nuclear weapon design, highly enriched uranium containing 40% or greater U-235 is sometimes used in the secondary stage in place of natural or depleted uranium. Primary stages today most commonly use plutonium but when uranium is used, it is even more highly enriched in U-235.

If at least one neutron from U-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 U-235 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. In nuclear reactors, the reaction is slowed down by the addition of control rods which are made of element 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 fissile uranium in nuclear weapons, containing 85% or more of 235U is known as weapons grade, though for a crude, inefficient weapon 20% is sufficient (called weapon(s)-usable); even less is sufficient, but then the critical mass required rapidly increases. However, use of implosion and neutron reflectors can enable construction of a weapon from a quantity of uranium below the usual critical mass for its level of enrichment, though this would likely only be possible in a country which already had extensive experience in developing nuclear weapons. The Little Boymarker atomic bomb was fueled by highly enriched uranium. Most modern nuclear weapon designs use plutonium as the fissile component of the primary stage however HEU is often used in the secondary stage.

Source Average energy released [MeV]
Instantaneously released energy
Kinetic energy of fission fragments 169.1
Kinetic energy of prompt neutrons     4.8
Energy carried by prompt γ-rays     7.0
Energy from decaying fission products
Energy of β−-particles     6.5
Energy of anti-neutrinos     8.8
Energy of delayed γ-rays     6.3
Sum 202.5
Energy released when those prompt neutrons which don't (re)produce fission are captured     8.8
Energy converted into heat in an operating thermal nuclear reactor 202.5

See also


  1. Some Physics of Uranium
  2. Nuclear fission and fusion, and neutron interactions, National Physical Laboratory.
  • Table of Nuclides.
  • A piece of U-235 (uranium-235, a rare form of uranium) the size of a grain of rice can produce energy equal to that contained in three tons of coal or fourteen barrels of oil. (Contemporary's Science)

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