Uranium-235

Uranium-235 (²³⁵U) is a minor 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 nature.

"U-235" redirects here. For the World War II submarine, see German submarine U-235.

Uranium-235, ²³⁵U

Uranium metal highly enriched in uranium-235
General
Symbol	^235U
Names	uranium-235, U-235
Protons	92
Neutrons	143
Nuclide data
Natural abundance	0.72%
Half-life	703,800,000 years
Parent isotopes	^235Pa ^235Np ^239Pu
Decay products	^231Th
Isotope mass	235.0439299 u
Spin	7/2−
Excess energy	40914.062 ± 1.970 keV
Binding energy	1783870.285 ± 1.996 keV
Decay modes
Decay mode	Decay energy (MeV)
Alpha	4.679
Isotopes of uranium Complete table of nuclides

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.[1] For fast neutrons it is on the order of 1 barn.[2] Most but not all neutron absorptions result in fission; a minority result in neutron capture forming uranium-236.

Natural decay chain

${\displaystyle {\begin{array}{r}{\ce {^{235}_{92}U->[\alpha ][7.038\times 10^{8}\ {\ce {y}}]{^{231}_{90}Th}->[\beta ^{-}][25.52\ {\ce {h}}]{^{231}_{91}Pa}->[\alpha ][3.276\times 10^{4}\ {\ce {y}}]{^{227}_{89}Ac}}}{\begin{Bmatrix}{\ce {->[98.62\%\beta ^{-}][21.773\ {\ce {y}}]{^{227}_{90}Th}->[\alpha ][18.718\ {\ce {d}}]}}\\{\ce {->[1.38\%\alpha ][21.773\ {\ce {y}}]{^{223}_{87}Fr}->[\beta ^{-}][21.8\ {\ce {min}}]}}\end{Bmatrix}}{\ce {^{223}_{88}Ra->[\alpha ][11.434\ {\ce {d}}]{^{219}_{86}Rn}}}\\{\ce {^{219}_{86}Rn->[\alpha ][3.96\ {\ce {s}}]{^{215}_{84}Po}->[\alpha ][1.778\ {\ce {ms}}]{^{211}_{82}Pb}->[\beta ^{-}][36.1\ {\ce {min}}]{^{211}_{83}Bi}}}{\begin{Bmatrix}{\ce {->[99.73\%\alpha ][2.13\ {\ce {min}}]{^{207}_{81}Tl}->[\beta ^{-}][4.77\ {\ce {min}}]}}\\{\ce {->[0.27\%\beta ^{-}][2.13\ {\ce {min}}]{^{211}_{84}Po}->[\alpha ][0.516\ {\ce {s}}]}}\end{Bmatrix}}{\ce {^{207}_{82}Pb_{(stable)}}}\end{array}}}$

Fission

Nuclear fission seen with a uranium-235 nucleus

The fission of one atom of uranium-235 releases 202.5 MeV (3.24×10⁻¹¹ J) inside the reactor. That corresponds to 19.54 TJ/mol, or 83.14 TJ/kg.[3] Another 8.8 MeV escapes the reactor as anti-neutrinos. When ²³⁵
₉₂U nuclides are bombarded with neutrons, one of the many fission reactions that it can undergo is the following (shown in the adjacent image):

¹
₀n + ²³⁵
₉₂U → ¹⁴¹
₅₆Ba + ⁹²
₃₆Kr + 3 ¹
₀n

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 will 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 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 ²³⁵U nuclear weapon is 56 kilograms (123 lb),[4] a sphere 17.32 centimetres (6.82 in) in diameter. The material must be 85% or more of ²³⁵U and is known as weapons grade uranium, though for a crude, inefficient weapon, 20% is sufficient (called weapon(s)-usable). 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;[5][6] however, HEU (highly enriched uranium, uranium that is 20% or more ²³⁵U) is often used in the secondary stage as an ignitor for the fusion fuel.

Source	Average energy released [MeV][3]
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 delayed γ-rays	6.3
Energy released when those prompt neutrons which don't (re)produce fission are captured	8.8
Total energy converted into heat in an operating thermal nuclear reactor	202.5
Energy of anti-neutrinos	8.8
Sum	211.3

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.[7][8]

References

1. "#Standard Reaction: 235U(n,f)". www-nds.iaea.org. IAEA. Retrieved 4 May 2020.
2. ""Some Physics of Uranium", UIC.com.au". Archived from the original on July 17, 2007. Retrieved 2009-01-18.CS1 maint: BOT: original-url status unknown (link)
3. Nuclear fission and fusion, and neutron interactions, National Physical Laboratory.
4. "FAS Nuclear Weapons Design FAQ". Archived from the original on 1999-05-07. Retrieved 2010-09-02.
5. FAS contributors (ed.). Nuclear Weapon Design. Federation of American Scientists.
6. Miner, William N.; Schonfeld, Fred W. (1968). "Plutonium". In Clifford A. Hampel (ed.). The Encyclopedia of the Chemical Elements. New York (NY): Reinhold Book Corporation. p. 541. LCCN 68029938.
7. Schmidt, Glen (February 2011). "SNAP Overview - radium-219 - general background" (PDF). American Nuclear Society. Retrieved 27 August 2012.
8. "RORSAT (Radar Ocean Reconnaissance Satellite)". daviddarling.info.

External links

• Table of Nuclides.
• DOE Fundamentals handbook: Nuclear Physics and Reactor theory Vol. 1, Vol. 2.
• Uranium | Radiation Protection Program | US EPA
• NLM Hazardous Substances Databank - Uranium, Radioactive
• "The Miracle of U-235", Popular Mechanics, January 1941—one of the earliest articles on U-235 for the general public

Lighter: uranium-234	Uranium-235 is an isotope of uranium	Heavier: uranium-236
Decay product of: protactinium-235 neptunium-235 plutonium-239	Decay chain of uranium-235	Decays to: thorium-231