Uranium-235 Explained

Mass Number:235
Symbol:U
Num Neutrons:143
Num Protons:92
Abundance:0.72%
Decay Product:Thorium-231
Decay Mass:231
Decay Symbol:Th
Parent:Protactinium-235
Parent Mass:235
Parent Symbol:Pa
Parent2:Neptunium-235
Parent2 Mass:235
Parent2 Symbol:Np
Parent3:Plutonium-239
Parent3 Mass:239
Parent3 Symbol:Pu
Mass:235.0439299
Spin:7/2−
Decay Mode1:Alpha
Decay Energy1:4.679

Uranium-235 (235U 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. It was discovered in 1935 by Arthur Jeffrey Dempster. Its fission cross section for slow thermal neutrons is about barns.[1] For fast neutrons it is on the order of 1 barn.[2] Most neutron absorptions induce fission, though a minority (about 15%) result in the formation of uranium-236.[3] [4]

Fission properties

The fission of one atom of uranium-235 releases inside the reactor. That corresponds to 19.54 TJ/mol, or 83.14 TJ/kg.[5] Another 8.8 MeV escapes the reactor as anti-neutrinos. When nuclei are bombarded with neutrons, one of the many fission reactions that it can undergo is the following (shown in the adjacent image):

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 (HEU), 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 continues to sustain itself, it is said to be critical, and the mass of 235U required to produce the critical condition is said to be a critical mass. A critical chain reaction can be achieved at low concentrations of 235U 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 235U nuclear weapon is,[6] which would form a sphere 17.32cm (06.82inches) in diameter. The material must be 85% or more of 235U and is known as weapons grade uranium, though for a crude and inefficient weapon 20% enrichment is sufficient (called weapon(s)-usable). Even lower enrichment can be used, but this results in the required critical mass rapidly increasing. 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;[7] [8] however, HEU (highly enriched uranium, in this case uranium that is 20% or more 235U) is frequently used in the secondary stage as an ignitor for the fusion fuel.

SourceAverage energy
released [MeV]
Instantaneously released energy
Kinetic energy of fission fragments169.1
Kinetic energy of prompt neutrons4.8
Energy carried by prompt γ-rays7.0
Energy from decaying fission products
Energy of β− particles6.5
Energy of delayed γ-rays6.3
Energy released when those prompt neutrons which do not (re)produce fission are captured8.8
Total energy converted into heat in an operating thermal nuclear reactor202.5
Energy of anti-neutrinos8.8
Sum211.3

Natural decay chain

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Uses

Uranium-235 has many uses such as fuel for nuclear power plants and in nuclear weapons such as nuclear bombs. Some artificial satellites, such as the SNAP-10A and the RORSATs were powered by nuclear reactors fueled with uranium-235.[9] [10]

External links

Notes and References

  1. Web site: #Standard Reaction: 235U(n,f) . www-nds.iaea.org . IAEA . 4 May 2020.
  2. Web site: "Some Physics of Uranium", UIC.com.au . 2009-01-18 . usurped . https://web.archive.org/web/20070717070625/http://www.uic.com.au/uicphys.htm . July 17, 2007 .
  3. Web site: Capture-to-fission Ratio . nuclear-power.com . June 26, 2024.
  4. The ratio of neutron capture to fission for uranium-235 . M. J. . Cabell . L. J. . Slee . 1962 . Journal of Inorganic and Nuclear Chemistry . 24 . 12 . 1493–1500 . 10.1016/0022-1902(62)80002-5.
  5. https://web.archive.org/web/20190505175631/http://www.kayelaby.npl.co.uk/atomic_and_nuclear_physics/4_7/4_7_1.html Nuclear fission and fusion, and neutron interactions
  6. Web site: FAS Nuclear Weapons Design FAQ . https://web.archive.org/web/19990507100044/https://fas.org/nuke/intro/nuke/design.htm . 2010-09-02 . 1999-05-07 .
  7. Book: Nuclear Weapon Design . Federation of American Scientists . 2016-06-04 . https://web.archive.org/web/20081226091803/http://www.fas.org/nuke/intro/nuke/design.htm . 2008-12-26 . dead.
  8. Book: Miner , William N. . The Encyclopedia of the Chemical Elements. https://archive.org/details/encyclopediaofch00hamp. registration. Reinhold Book Corporation. New York (NY). 1968. Clifford A. Hampel. Schonfeld, Fred W.. Plutonium. 541. 68029938.
  9. Web site: Schmidt. Glen. SNAP Overview – radium-219 – general background . American Nuclear Society. 27 August 2012. February 2011.
  10. Web site: RORSAT (Radar Ocean Reconnaissance Satellite). daviddarling.info.