Uranium-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| 7.04×108 years [1]
Parent isotopes| 235Pa (β−)
235Np (β+)
239Pu (α)
Decay products| 231Th
Isotope mass| 235.043928[2] 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[3]
Isotopes of Chemistry:uranium
Complete table of nuclides

Uranium-235 (The element Chemistry:Uranium does not exist. 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 and was discovered in 1935 by Arthur Jeffrey Dempster.

The release of energy from the fission of Uranium-235 powers most nuclear reactors and nuclear weapons. Uranium enrichment is the process of taking natural uranium and increasing the proportion of uranium-235. Some reactor types can use natural uranium directly, while nuclear weapons and some others reactor types require uranium enriched in U-235.

Uranium-235 has a half-life of 704 million years. Its fission cross section for slow thermal neutrons is about 584.3±1 barns.[4] For fast neutrons it is on the order of 1 barn.[5] Most neutron absorptions induce fission, though a minority (about 15%) result in the formation of uranium-236.[6][7]

## Contents

* 1 Fission properties
* 1.1 Nuclear weapons
* 2 Decay
* 3 Astrophysical dating
* 4 References
* 5 External links

## Fission properties

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.[8] Another 8.8 MeV escapes the reactor as anti-neutrinos. When 23592U nuclei are bombarded with neutrons, one of the many fission reactions that it can undergo is the following (shown in the adjacent image):

10n \+ 23592U → 14456Ba \+ 8936Kr \+ 3 10n

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 56 kilograms (123 lb),[9] which would form 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 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;[10][11] 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.

Source  | Average energy
released [MeV][8]

|

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 do not (re)produce fission are captured  |  8.8
Total energy converted into heat in an operating thermal nuclear reactor  | 202.5 MeV
Energy of anti-neutrinos  |  8.8
Sum  | 211.3 MeV

## Decay

Uranium-235 is an alpha emitter,[12] producing thorium-231. Uranium-235 is the main progenitor of the actinium series, one of the principal actinide decay chains, as it is the longest-lived and sole primordial nuclide (aside from the final end product, lead-207). Beginning with naturally occurring uranium-235, this series includes isotopes of astatine, bismuth, francium, lead, polonium, protactinium, radium, radon, thallium, and thorium, all of which are present in natural uranium sources. The decay proceeds as (only main decay branches shown):

A92235A2922235U→7.04×10A8 yα23190Th→25.52 hβA−23191Pa→3.27×10A4 yα22789Ac{→21.772 y98.62%βA−22790Th→18.693 dα→21.772 y1.38%α22387Fr→22.00 minβA−}A88223A2882223Ra→11.435 dα21986RnA86219A2862219Rn→3.96 sα21584Po→1.781 msα21182Pb→36.16 minβA−21183Bi{→2.14 min99.724%α20781Tl→4.77 minβA−→2.14 min0.276%βA−21184Po→0.516 sα}A82207A2822207Pb

Or in tabular form, including minor branches:

Nuclide  | Decay mode  | Half-life
(a = years)  | Energy released
MeV  | Decay
product

|
|
|
|

235U | α  | 7.04×108 a  | 4.678  | 231Th
231Th | β− | 25.52 h  | 0.391  | 231Pa
231Pa | α  | 3.27×104 a  | 5.150  | 227Ac
227Ac | β− 98.62%
α 1.38%  | 21.772 a  | 0.045
5.042  | 227Th
223Fr
227Th | α  | 18.693 d  | 6.147  | 223Ra
223Fr | β− 99.994%
α 0.006%  | 22.00 min  | 1.149
5.561  | 223Ra
219At
223Ra | α  | 11.435 d  | 5.979  | 219Rn
219At | α 93.6%
β− 6.4%  | 56 s  | 6.342
1.567  | 215Bi
219Rn
219Rn | α  | 3.96 s  | 6.946  | 215Po
215Bi | β− | 7.6 min  | 2.171  | 215Po
215Po | α
β− 2.3×10−4%  | 1.781 ms  | 7.526
0.715  | 211Pb
215At
215At | α  | 37 μs  | 8.177  | 211Bi
211Pb | β− | 36.16 min  | 1.366  | 211Bi
211Bi | α 99.724%
β− 0.276%  | 2.14 min  | 6.750
0.573  | 207Tl
211Po
211Po | α  | 516 ms  | 7.595  | 207Pb
207Tl | β− | 4.77 min  | 1.418  | 207Pb
207Pb | stable  |  |  |

## Astrophysical dating

Knowledge of current and theoretical production ratios of uranium-235 to uranium-238 allows radiometric dating, the time since modern uranium nuclei were formed in stellar nucleosynthesis.

The 1957 B2FH landmark paper in astrophysics explained the r-process by which both nuclei form. The authors predicted their relative abundances, and those of their rapidly alpha-chain decaying parent nuclides. Thus they predicted 1.64 as the 235U/238U ratio contributed to the interstellar medium by r-process events (supernovae and subsequently discovered kilonovae). This takes billions of years to diminish to their present value of 0.0072 (see natural uranium). They investigate scenarios for historical contribution to the solar nebula, before contribution is cut off at the Sun's formation 4.5 billion years ago. The scenarios are: a single supernova, a finite continuous uniform series of supernovae representing the lifetime of the Milky Way, and an infinite series representing the steady-state universe. From the second scenario, they estimated an age of the Milky Way at around 10 billion years, compared to a modern value of 13.61 billion years. Significantly, at this point the oldest known objects were stellar clusters at 6.5 billion years old.[13]

## References

1. ↑ Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S.; Audi, G. (2021). "The NUBASE2020 evaluation of nuclear properties". Chinese Physics C 45 (3). doi:10.1088/1674-1137/abddae. https://www-nds.iaea.org/amdc/ame2020/NUBASE2020.pdf.
2. ↑ Template:AME2020 II
3. ↑ National Nuclear Data Center. "NuDat 2.x database". Brookhaven National Laboratory. http://www.nndc.bnl.gov/nudat2/.
4. ↑ "#Standard Reaction: 235U(n,f)". IAEA. https://www-nds.iaea.org/standards/Data/standards-235U_xs-data.txt.
5. ↑ ""Some Physics of Uranium", UIC.com.au". http://www.uic.com.au/uicphys.htm.
6. ↑ "Capture-to-fission Ratio". https://www.nuclear-power.com/nuclear-power/fission/capture-to-fission-ratio/.
7. ↑ Cabell, M. J.; Slee, L. J. (1962). "The ratio of neutron capture to fission for uranium-235". Journal of Inorganic and Nuclear Chemistry 24 (12): 1493–1500. doi:10.1016/0022-1902(62)80002-5.
8. ↑ 8.0 8.1 Nuclear fission and fusion, and neutron interactions, National Physical Laboratory Archive.
9. ↑ "FAS Nuclear Weapons Design FAQ". https://fas.org/nuke/intro/nuke/design.htm.
10. ↑ Nuclear Weapon Design. Federation of American Scientists. https://fas.org/nuke/intro/nuke/design.htm. Retrieved 2016-06-04.
11. ↑ Miner, William N.; Schonfeld, Fred W. (1968). "Plutonium". in Clifford A. Hampel. The Encyclopedia of the Chemical Elements. New York (NY): Reinhold Book Corporation. p. 541. https://archive.org/details/encyclopediaofch00hamp.
12. ↑ Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S.; Audi, G. (2021). "The NUBASE2020 evaluation of nuclear properties". Chinese Physics C 45 (3). doi:10.1088/1674-1137/abddae. https://www-nds.iaea.org/amdc/ame2020/NUBASE2020.pdf.
13. ↑ Burbidge, E. Margaret; Burbidge, G. R.; Fowler, William A.; Hoyle, F. (1957-10-01). "Synthesis of the Elements in Stars". Reviews of Modern Physics 29 (4): 547–650. doi:10.1103/RevModPhys.29.547. ISSN 0034-6861. Bibcode: 1957RvMP...29..547B.

## External links

* Table of Nuclides.
* DOE Fundamentals Handbook: Nuclear Physics and Reactor Theory Vol. 1 (), Vol. 2 ().
* Radionuclide Basics: Uranium—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

* v
* t
* e

Main isotopes of uranium

* 232U
* 233U
* 234U
* 235U
* 236U
* 238U

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