Isotopes of uranium

Uranium (92U) is a naturally occurring radioactive element that has no stable isotope. It has two primordial isotopes, uranium-238 and uranium-235, that have long half-lives and are found in appreciable quantity in the Earth's crust. The decay product uranium-234 is also found. Other isotopes such as uranium-233 have been produced in breeder reactors. In addition to isotopes found in nature or nuclear reactors, many isotopes with far shorter half-lives have been produced, ranging from 214U to 242U (with the exception of 220U and 241U). The standard atomic weight of natural uranium is 238.02891(3).

Main isotopes of uranium (92U)
Iso­tope Decay
abun­dance half-life (t1/2) mode pro­duct
232U syn 68.9 y SF –
α 228Th
233U trace 1.592×105 y SF –
α 229Th
234U 0.005% 2.455×105 y SF –
α 230Th
235U 0.720% 7.04×108 y SF –
α 231Th
236U trace 2.342×107 y SF –
α 232Th
238U 99.274% 4.468×109 y α 234Th
SF –
β−β− 238Pu
Standard atomic weight Ar, standard(U)
  • 238.02891(3)[1]

Naturally occurring uranium is composed of three major isotopes, uranium-238 (99.2739–99.2752% natural abundance), uranium-235 (0.7198–0.7202%), and uranium-234 (0.0050–0.0059%).[2] All three isotopes are radioactive (i.e., they are radioisotopes), and the most abundant and stable is uranium-238, with a half-life of 4.4683×109 years (close to the age of the Earth).

Uranium-238 is an alpha emitter, decaying through the 18-member uranium series into lead-206. The decay series of uranium-235 (historically called actino-uranium) has 15 members and ends in lead-207. The constant rates of decay in these series makes comparison of the ratios of parent-to-daughter elements useful in radiometric dating. Uranium-233 is made from thorium-232 by neutron bombardment.

Uranium-235 is important for both nuclear reactors and nuclear weapons because it is the only isotope existing in nature to any appreciable extent that is fissile in response to thermal neutrons. Uranium-238 is also important because it is fertile: it absorbs neutrons to produce a radioactive isotope that subsequently decays to the isotope plutonium-239, which also is fissile.

List of isotopes
Nuclide[3]
[n 1]		 Historic
name		 Z N Isotopic mass (Da)[4]
[n 2][n 3]		 Half-life
 Decay
mode
[n 4]		 Daughter
isotope
[n 5][n 6]		 Spin and
parity
[n 7][n 8]		 Natural abundance (mole fraction)
Excitation energy[n 8] Normal proportion Range of variation
214U[5] 92 122 0.52(+0.95−0.21) ms α 210Th 0+
215U[6] 92 123 215.026760(90) 2.24 ms α 211Th 5/2−#
216U[6][7] 92 124 216.024760(30) 2.25(+0.63−0.40) ms[5] α 212Th 0+
216mU[8] 1.31 ms 8+
217U 92 125 217.02437(9) 26(14) ms
[16(+21−6) ms]		 α 213Th 1/2−#
218U 92 126 218.02354(3) 0.65(+0.08−0.07) ms[5] α 214Th 0+
219U 92 127 219.02492(6) 55(25) µs
[42(+34−13) µs]		 α 215Th 9/2+#
221U[9] 92 129 221.02640(11)# 0.66(14) µs α 217Th (9/2+)
222U 92 130 222.02609(11)# 1.4(7) µs
[1.0(+10−4) µs]		 α 218Th 0+
β+ (10−6%) 222Pa
223U 92 131 223.02774(8) 21(8) µs
[18(+10−5) µs]		 α 219Th 7/2+#
224U 92 132 224.027605(27) 940(270) µs α 220Th 0+
225U 92 133 225.02939# 61(4) ms α 221Th (5/2+)#
226U 92 134 226.029339(14) 269(6) ms α 222Th 0+
227U 92 135 227.031156(18) 1.1(1) min α 223Th (3/2+)
β+ (.001%) 227Pa
228U 92 136 228.031374(16) 9.1(2) min α (95%) 224Th 0+
EC (5%) 228Pa
229U 92 137 229.033506(6) 58(3) min β+ (80%) 229Pa (3/2+)
α (20%) 225Th
230U 92 138 230.033940(5) 20.8 d α 226Th 0+
SF (1.4×10−10%) (various)
β+β+ (rare) 230Th
231U 92 139 231.036294(3) 4.2(1) d EC 231Pa (5/2)(+#)
α (.004%) 227Th
232U 92 140 232.0371562(24) 68.9(4) y α 228Th 0+
CD (8.9×10−10%) 208Pb
24Ne
CD (5×10−12%) 204Hg
28Mg
SF (10−12%) (various)
233U 92 141 233.0396352(29) 1.592(2)×105 y α 229Th 5/2+ Trace[n 9]
SF (6×10−9%) (various)
CD (7.2×10−11%) 209Pb
24Ne
CD (1.3×10−13%) 205Hg
28Mg
234U[n 10][n 11] Uranium II 92 142 234.0409521(20) 2.455(6)×105 y α 230Th 0+ [0.000054(5)][n 12] 0.000050–
0.000059
SF (1.73×10−9%) (various)
CD (1.4×10−11%) 206Hg
28Mg
CD (9×10−12%) 184Hf
26Ne
24Ne
234mU 1421.32(10) keV 33.5(20) ms 6−
235U[n 13][n 14][n 15] Actin Uranium
Actino-Uranium		 92 143 235.0439299(20) 7.038(1)×108 y α 231Th 7/2− [0.007204(6)] 0.007198–
0.007207
SF (7×10−9%) (various)
CD (8×10−10%) 186Hf
25Ne
24Ne
235mU 0.0765(4) keV ~26 min IT 235U 1/2+
236U Thoruranium[10] 92 144 236.045568(2) 2.342(3)×107 y α 232Th 0+ Trace[n 16]
SF (9.6×10−8%) (various)
236m1U 1052.89(19) keV 100(4) ns (4)−
236m2U 2750(10) keV 120(2) ns (0+)
237U 92 145 237.0487302(20) 6.75(1) d β− 237Np 1/2+ Trace[n 17]
238U[n 11][n 13][n 14] Uranium I 92 146 238.0507882(20) 4.468(3)×109 y α 234Th 0+ [0.992742(10)] 0.992739–
0.992752
SF (5.45×10−5%) (various)
β−β− (2.19×10−10%) 238Pu
238mU 2557.9(5) keV 280(6) ns 0+
239U 92 147 239.0542933(21) 23.45(2) min β− 239Np 5/2+
239m1U 20(20)# keV >250 ns (5/2+)
239m2U 133.7990(10) keV 780(40) ns 1/2+
240U 92 148 240.056592(6) 14.1(1) h β− 240Np 0+ Trace[n 18]
α (10−10%) 236Th
242U 92 150 242.06293(22)# 16.8(5) min β− 242Np 0+
  1. mU – Excited nuclear isomer.
  2. ( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
  3. # – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).
  4. Modes of decay:
    CD:Cluster decay
    EC:Electron capture
    SF:Spontaneous fission
  5. Bold italics symbol as daughter – Daughter product is nearly stable.
  6. Bold symbol as daughter – Daughter product is stable.
  7. ( ) spin value – Indicates spin with weak assignment arguments.
  8. # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
  9. Intermediate decay product of 237Np
  10. Used in uranium–thorium dating
  11. Used in uranium–uranium dating
  12. Intermediate decay product of 238U
  13. Primordial radionuclide
  14. Used in Uranium–lead dating
  15. Important in nuclear reactors
  16. Intermediate decay product of 244Pu, also produced by neutron capture of 235U
  17. Neutron capture product, parent of trace quantities of 237Np
  18. Intermediate decay product of 244Pu

Actinides vs fission products
Actinides and fission products by half-life
Actinides[11] by decay chain Half-life
range (a)		 Fission products of 235U by yield[12]
4n 4n+1 4n+2 4n+3
4.5–7% 0.04–1.25% <0.001%
228Ra№ 4–6 a † 155Euþ
244Cmƒ 241Puƒ 250Cf 227Ac№ 10–29 a 90Sr 85Kr 113mCdþ
232Uƒ 238Puƒ 243Cmƒ 29–97 a 137Cs 151Smþ 121mSn
248Bk[13] 249Cfƒ 242mAmƒ 141–351 a

No fission products
have a half-life
in the range of
100–210 ka ...

241Amƒ 251Cfƒ[14] 430–900 a
226Ra№ 247Bk 1.3–1.6 ka
240Pu 229Th 246Cmƒ 243Amƒ 4.7–7.4 ka
245Cmƒ 250Cm 8.3–8.5 ka
239Puƒ 24.1 ka
230Th№ 231Pa№ 32–76 ka
236Npƒ 233Uƒ 234U№ 150–250 ka ‡ 99Tc₡ 126Sn
248Cm 242Pu 327–375 ka 79Se₡
1.53 Ma 93Zr
237Npƒ 2.1–6.5 Ma 135Cs₡ 107Pd
236U 247Cmƒ 15–24 Ma 129I₡
244Pu 80 Ma

... nor beyond 15.7 Ma[15]

232Th№ 238U№ 235Uƒ№ 0.7–14.1 Ga

Legend for superscript symbols
₡  has thermal neutron capture cross section in the range of 8–50 barns
ƒ  fissile
m  metastable isomer
№  primarily a naturally occurring radioactive material (NORM)
þ  neutron poison (thermal neutron capture cross section greater than 3k barns)
†  range 4–97 a: Medium-lived fission product
‡  over 200 ka: Long-lived fission product

Uranium-232
Main article: Uranium-232

Uranium-232 has a half-life of 68.9 years and is a side product in the thorium cycle. It has been cited as an obstacle to nuclear proliferation using 233U as the fissile material, because the intense gamma radiation emitted by 208Tl (a daughter of 232U, produced relatively quickly) makes the 233U contaminated with it more difficult to handle. Uranium-232 is a rare example of an even-even isotope that is fissile with both thermal and fast neutrons.[16][17]

Uranium-233
Main article: Uranium-233

Uranium-233 is a fissile isotope of uranium that is bred from thorium-232 as part of the thorium fuel cycle. Uranium-233 was investigated for use in nuclear weapons and as a reactor fuel; however, it was never deployed in nuclear weapons or used commercially as a nuclear fuel.[18] It has been used successfully in experimental nuclear reactors and has been proposed for much wider use as a nuclear fuel. It has a half-life of 1.592×105 years.

Uranium-233 is produced by the neutron irradiation of thorium-232. When thorium-232 absorbs a neutron, it becomes thorium-233, which has a half-life of only 22 minutes. Thorium-233 decays into protactinium-233 through beta decay. Protactinium-233 has a half-life of 27 days and beta decays into uranium-233; some proposed molten salt reactor designs attempt to physically isolate the protactinium from further neutron capture before beta decay can occur.

Uranium-233 usually fissions on neutron absorption but sometimes retains the neutron, becoming uranium-234. The capture-to-fission ratio is smaller than the other two major fissile fuels uranium-235 and plutonium-239; it is also lower than that of short-lived plutonium-241, but bested by very difficult-to-produce neptunium-236.

Uranium-234
Main article: Uranium-234

Uranium-234 is an isotope of uranium. In natural uranium and in uranium ore, 234U occurs as an indirect decay product of uranium-238, but it makes up only 0.0055% (55 parts per million) of the raw uranium because its half-life of just 245,500 years is only about 1/18,000 as long as that of 238U. The path of production of 234U via nuclear decay is as follows: 238U nuclei emit an alpha particle to become thorium-234. Next, with a short half-life, a 234Th nucleus emits a beta particle to become protactinium-234. Finally, 234Pa nuclei each emit another beta particle to become 234U nuclei.[19][20]

234U nuclei usually last for hundreds of thousands of years, but then they decay by alpha emission to thorium-230, except for the small percentage of nuclei that undergo spontaneous fission.

Extraction of rather small amounts of 234U from natural uranium would be feasible using isotope separation, similar to that used for regular uranium-enrichment. However, there is no real demand in chemistry, physics, or engineering for isolating 234U. Very small pure samples of 234U can be extracted via the chemical ion-exchange process—from samples of plutonium-238 that have been aged somewhat to allow some decay to 234U via alpha emission.

Enriched uranium contains more 234U than natural uranium as a byproduct of the uranium enrichment process aimed at obtaining uranium-235, which concentrates lighter isotopes even more strongly than it does 235U. The increased percentage of 234U in enriched natural uranium is acceptable in current nuclear reactors, but (re-enriched) reprocessed uranium might contain even higher fractions of 234U, which is undesirable.[21] This is because 234U is not fissile, and tends to absorb slow neutrons in a nuclear reactor—becoming 235U.[20][21]

234U has a neutron capture cross section of about 100 barns for thermal neutrons, and about 700 barns for its resonance integral—the average over neutrons having various intermediate energies. In a nuclear reactor, non-fissile isotopes capture a neutron breeding fissile isotopes. 234U is converted to 235U more easily and therefore at a greater rate than uranium-238 is to plutonium-239 (via neptunium-239), because 238U has a much smaller neutron-capture cross section of just 2.7 barns.

Uranium-235
Main article: Uranium-235

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 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 neutron is about 504.81 barns. For fast neutrons it is on the order of 1 barn. At thermal energy levels, about 5 of 6 neutron absorptions result in fission and 1 of 6 result in neutron capture forming uranium-236.[22] The fission-to-capture ratio improves for faster neutrons.

Uranium-236
Main article: Uranium-236

Uranium-236 is an isotope of uranium that is neither fissile with thermal neutrons, nor very good fertile material, but is generally considered a nuisance and long-lived radioactive waste. It is found in spent nuclear fuel and in the reprocessed uranium made from spent nuclear fuel.

Uranium-237

Uranium-237 is an isotope of uranium. It has a half life of about 6.75(1) days. It decays into neptunium-237 by beta decay.

Uranium-238
Main article: Uranium-238

Uranium-238 (238U or U-238) is the most common isotope of uranium found in nature. It is not fissile, but is a fertile material: it can capture a slow neutron and after two beta decays become fissile plutonium-239. Uranium-238 is fissionable by fast neutrons, but cannot support a chain reaction because inelastic scattering reduces neutron energy below the range where fast fission of one or more next-generation nuclei is probable. Doppler broadening of 238U's neutron absorption resonances, increasing absorption as fuel temperature increases, is also an essential negative feedback mechanism for reactor control.

Around 99.284% of natural uranium is uranium-238, which has a half-life of 1.41×1017 seconds (4.468×109 years, or 4.468 billion years). Depleted uranium has an even higher concentration of the 238U isotope, and even low-enriched uranium (LEU), while having a higher proportion of the uranium-235 isotope (in comparison to depleted uranium), is still mostly 238U. Reprocessed uranium is also mainly 238U, with about as much uranium-235 as natural uranium, a comparable proportion of uranium-236, and much smaller amounts of other isotopes of uranium such as uranium-234, uranium-233, and uranium-232.

Uranium-239

Uranium-239 is an isotope of uranium. It is usually produced by exposing 238U to neutron radiation in a nuclear reactor. 239U has a half-life of about 23.45 minutes and decays into neptunium-239 through beta decay, with a total decay energy of about 1.29 MeV.[23] The most common gamma decay at 74.660 keV accounts for the difference in the two major channels of beta emission energy, at 1.28 and 1.21 MeV.[24]

239Np further decays to plutonium-239 also through beta decay (239Np has a half-life of about 2.356 days), in a second important step that ultimately produces fissile 239Pu (used in weapons and for nuclear power), from 238U in reactors.

References
  1. Meija, Juris; et al. (2016). "Atomic weights of the elements 2013 (IUPAC Technical Report)". Pure and Applied Chemistry. 88 (3): 265–91. doi:10.1515/pac-2015-0305.
  2. "Uranium Isotopes". GlobalSecurity.org. Retrieved 14 March 2012.
  3. Half-life, decay mode, nuclear spin, and isotopic composition is sourced in:
    Audi, G.; Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S. (2017). "The NUBASE2016 evaluation of nuclear properties" (PDF). Chinese Physics C. 41 (3): 030001. Bibcode:2017ChPhC..41c0001A. doi:10.1088/1674-1137/41/3/030001.
  4. Wang, M.; Audi, G.; Kondev, F. G.; Huang, W. J.; Naimi, S.; Xu, X. (2017). "The AME2016 atomic mass evaluation (II). Tables, graphs, and references" (PDF). Chinese Physics C. 41 (3): 030003-1–030003-442. doi:10.1088/1674-1137/41/3/030003.
  5. Zhang, Z. Y.; Yang, H. B.; Huang, M. H.; et al. (2021). "New α-Emitting Isotope 214U and Abnormal Enhancement of α-Particle Clustering in Lightest Uranium Isotopes" (PDF). Retrieved 24 January 2021. Cite journal requires |journal= (help)
  6. Y. Wakabayashi; K. Morimoto; D. Kaji; et al. (2014). "New Isotope Candidates, 215U and 216U" (PDF). RIKEN Accel. Prog. Rep. 47: xxii.
  7. H. M. Devaraja; S. Heinz; O. Beliuskina; et al. (2015). "Observation of new neutron-deficient isotopes with Z ≥ 92 in multinucleon transfer reactions" (PDF). Physics Letters B. 748: 199–203. Bibcode:2015PhLB..748..199D. doi:10.1016/j.physletb.2015.07.006.
  9. Khuyagbaatar, J.; et al. (2015). "New Short-Lived Isotope 221U and the Mass Surface Near N = 126" (PDF). Physical Review Letters. 115 (24): 242502. Bibcode:2015PhRvL.115x2502K. doi:10.1103/PhysRevLett.115.242502. PMID 26705628.
  10. Trenn, Thaddeus J. (1978). "Thoruranium (U-236) as the extinct natural parent of thorium: The premature falsification of an essentially correct theory". Annals of Science. 35 (6): 581–97. doi:10.1080/00033797800200441.
  11. Plus radium (element 88). While actually a sub-actinide, it immediately precedes actinium (89) and follows a three-element gap of instability after polonium (84) where no nuclides have half-lives of at least four years (the longest-lived nuclide in the gap is radon-222 with a half life of less than four days). Radium's longest lived isotope, at 1,600 years, thus merits the element's inclusion here.
  12. Specifically from thermal neutron fission of U-235, e.g. in a typical nuclear reactor.
  13. Milsted, J.; Friedman, A. M.; Stevens, C. M. (1965). "The alpha half-life of berkelium-247; a new long-lived isomer of berkelium-248". Nuclear Physics. 71 (2): 299. Bibcode:1965NucPh..71..299M. doi:10.1016/0029-5582(65)90719-4.
    "The isotopic analyses disclosed a species of mass 248 in constant abundance in three samples analysed over a period of about 10 months. This was ascribed to an isomer of Bk248 with a half-life greater than 9 [years]. No growth of Cf248 was detected, and a lower limit for the β− half-life can be set at about 104 [years]. No alpha activity attributable to the new isomer has been detected; the alpha half-life is probably greater than 300 [years]."
  14. This is the heaviest nuclide with a half-life of at least four years before the "Sea of Instability".
  15. Excluding those "classically stable" nuclides with half-lives significantly in excess of 232Th; e.g., while 113mCd has a half-life of only fourteen years, that of 113Cd is nearly eight quadrillion years.
  16. "Uranium 232". Nuclear Power. Archived from the original on 26 February 2019. Retrieved 3 June 2019.
  17. http://atom.kaeri.re.kr/nuchart/getEvaf.jsp?mat=9219&lib=endfb7.1
  18. C. W. Forsburg; L. C. Lewis (1999-09-24). "Uses For Uranium-233: What Should Be Kept for Future Needs?" (PDF). Ornl-6952. Oak Ridge National Laboratory.
  19. Audi, G.; Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S. (2017). "The NUBASE2016 evaluation of nuclear properties" (PDF). Chinese Physics C. 41 (3): 030001. Bibcode:2017ChPhC..41c0001A. doi:10.1088/1674-1137/41/3/030001.
  20. Ronen, Y., ed. (1990). High converting water reactors. CRC Press. p. 212. ISBN 0-8493-6081-1. LCCN 89-25332.
  21. Use of Reprocessed Uranium (PDF). Technical Document. Vienna: International Atomic Energy Agency. 2009. ISBN 978-92-0-157109-0. ISSN 1684-2073.
  22. B. C. Diven; J. Terrell; A. Hemmendinger (1 January 1958). "Capture-to-Fission Ratios for Fast Neutrons in U235". Physical Review Letters. 109 (1): 144–150. Bibcode:1958PhRv..109..144D. doi:10.1103/PhysRev.109.144.
  23. CRC Handbook of Chemistry and Physics, 57th Ed. p. B-345
  24. CRC Handbook of Chemistry and Physics, 57th Ed. p. B-423
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