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 1.592×10^5 y SF – α ^229Th ^234U 0.005% 2.455×10^5 y SF – α ^230Th ^235U 0.720% 7.04×10^8 y SF – α ^231Th ^236U trace 2.342×10^7 y SF – α ^232Th ^238U 99.274% 4.468×10^9 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)×10^5 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)×10^5 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)×10^8 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)×10^7 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)×10^9 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	†		^155Eu^þ
^244Cm^ƒ	^241Pu^ƒ	^250Cf	^227Ac^№	10–29 a		^90Sr	^85Kr	^113mCd^þ
^232U^ƒ		^238Pu^ƒ	^243Cm^ƒ	29–97 a		^137Cs	^151Sm^þ	^121mSn
^248Bk[13]	^249Cf^ƒ	^242mAm^ƒ		141–351 a	No fission products have a half-life in the range of 100–210 ka ...
	^241Am^ƒ		^251Cf^ƒ[14]	430–900 a
		^226Ra^№	^247Bk	1.3–1.6 ka
^240Pu	^229Th	^246Cm^ƒ	^243Am^ƒ	4.7–7.4 ka
	^245Cm^ƒ	^250Cm		8.3–8.5 ka
			^239Pu^ƒ	24.1 ka
		^230Th^№	^231Pa^№	32–76 ka
^236Np^ƒ	^233U^ƒ	^234U^№		150–250 ka	‡	^99Tc^₡	^126Sn
^248Cm		^242Pu		327–375 ka			^79Se^₡
				1.53 Ma		^93Zr
	^237Np^ƒ			2.1–6.5 Ma		^135Cs^₡	^107Pd
^236U			^247Cm^ƒ	15–24 Ma			^129I^₡
^244Pu				80 Ma	... nor beyond 15.7 Ma[15]
^232Th^№		^238U^№	^235U^ƒ№	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

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

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

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

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

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

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.
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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