Uranium-235

Uranium-235
Uranium metal highly enriched in uranium-235 Full table
General
Name, symbol	Uranium-235,^235U
Neutrons	143
Protons	92
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 mode	Decay energy
Alpha	4.679 MeV

Uranium-235 (²³⁵U) 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 a primordial nuclide or found in significant quantity in nature.

Uranium-235 has a half-life of 703.8 million years. It was discovered in 1935 by Arthur Jeffrey Dempster. Its (fission) nuclear cross section for slow thermal neutrons is about 584.994 barns. For fast neutrons it is on the order of 1 barn.^[1] Most but not all neutron absorptions result in fission; a minority result in neutron capture forming uranium-236.

Natural decay chain

${\displaystyle {\begin{array}{l}{\ce {^{235}_{92}U->[\alpha ][7.038\times 10^{8}\ {\ce {y}}]_{90}^{231}Th->[\beta ^{-}][25.52\ {\ce {h}}]_{91}^{231}Pa->[\alpha ][3.276\times 10^{4}\ {\ce {y}}]_{89}^{227}Ac}}{\begin{Bmatrix}{\ce {->[98.62\%\beta ^{-}][21.773\ {\ce {y}}]_{90}^{227}Th->[\alpha ][18.718\ {\ce {d}}]}}\\{\ce {->[1.38\%\alpha ][21.773\ y]_{87}^{223}Fr->[\beta ^{-}][21.8\ {\ce {min}}]}}\end{Bmatrix}}{\ce {^{223}_{88}Ra->[\alpha ][11,434\ {\ce {d}}]_{86}^{219}Rn}}\\{\ce {^{219}_{86}Rn->[\alpha ][3.96\ {\ce {s}}]_{84}^{215}Po->[\alpha ][1.778\ {\ce {ms}}]_{82}^{211}Pb->[\beta ^{-}][36.1\ {\ce {min}}]_{83}^{211}Bi}}{\begin{Bmatrix}{\ce {->[99.73\%\alpha ][2,13\ {\ce {min}}]_{81}^{207}Tl->[\beta ^{-}][4,77\ {\ce {min}}]}}\\{\ce {->[0,27\%\beta ^{-}][2,13\ {\ce {min}}]_{84}^{211}Po->[\alpha ][0,516\ {\ce {s}}]}}\end{Bmatrix}}{\ce {^{207}_{82}Pb_{(stable)}}}\end{array}}}$

Fission

Nuclear fission seen with a uranium-235 nucleus

The fission of one atom of U-235 generates 202.5 MeV = 3.24 × 10⁻¹¹ J, which translates to 19.54 TJ/mol, or 83.14 TJ/kg.^[2] This is around 2.5 million times more than the energy released from burning coal.^[3] When 235
92U nuclides are bombarded with neutrons, one of the many fission reactions that it can undergo is the following (shown visually 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 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. Highly enriched uranium (HEU), which contains an even greater proportion of U-235, is sometimes used in nuclear weapon design.

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 elements 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.

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 cm (6.8") 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 as the fissile component of the primary stage,^[5][6] however HEU is often used in the secondary stage.

Source	Average energy released [MeV][2]
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

References

1. "Some Physics of Uranium", UIC.com.au at the Wayback Machine (archived July 17, 2007)
2. Nuclear fission and fusion, and neutron interactions, National Physical Laboratory.
3. Energy Content in Common Energy Sources, EngineeringToolBox.com.
4. FAS Nuclear Weapons Design FAQ, accessed 2010-9-2. There are numerous other references on the net, and with modern computers, this is fairly easy to calculate, so secrecy cannot aid security.
5. FAS contributors (ed.). Nuclear Weapon Design. Federation of American Scientists.
6. Miner. 1968. p. 541.

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