Isotopes of uranium

Actinides and fission products by half-life
Actinides[1] by decay chain				Half-life range (y)	Fission products of ^235U by yield[2]
4n	4n+1	4n+2	4n+3
						4.5–7%	0.04–1.25%	<0.001%
^228Ra^№				4–6	†		^155Eu^þ
^244Cm	^241Pu^ƒ	^250Cf	^227Ac^№	10–29		^90Sr	^85Kr	^113mCd^þ
^232U^ƒ		^238Pu^№	^243Cm^ƒ	29–97		^137Cs	^151Sm^þ	^121mSn
^248Bk[3]	^249Cf^ƒ	^242mAm^ƒ		141–351	No fission products have a half-life in the range of 100–210 k years ...
	^241Am^ƒ		^251Cf^ƒ[4]	430–900
		^226Ra^№	^247Bk	1.3 k – 1.6 k
^240Pu	^229Th^№	^246Cm	^243Am^ƒ	4.7 k – 7.4 k
	^245Cm^ƒ	^250Cm		8.3 k – 8.5 k
			^239Pu^ƒ№	24.1 k
		^230Th^№	^231Pa^№	32 k – 76 k
^236Np^ƒ	^233U^ƒ№	^234U^№		150 k – 250 k	‡	^99Tc^₡	^126Sn
^248Cm		^242Pu		327 k – 375 k			^79Se^₡
				1.53 M		^93Zr
	^237Np^№			2.1 M – 6.5 M		^135Cs^₡	^107Pd
^236U^№			^247Cm^ƒ	15 M – 24 M			^129I^₡
^244Pu^№				80 M	... nor beyond 15.7 M years[5]
^232Th^№		^238U^№	^235U^ƒ№	0.7 G – 14.1 G
Legend for superscript symbols ₡  has thermal neutron capture cross section in the range of 8–50 barns ƒ  fissile m  metastable isomer №  naturally occurring radioactive material (NORM) þ  neutron poison (thermal neutron capture cross section greater than 3k barns) †  range 4–97 y: Medium-lived fission product ‡  over 200,000 y: Long-lived fission product

Uranium (U) is a naturally occurring radioactive element that has no stable isotopes but two primordial isotopes (uranium-238 and uranium-235) that have long half-life and are found in appreciable quantity in the Earth's crust, along with the decay product uranium-234. The relative atomic mass of natural uranium is 238.02891(3). Other isotopes such as uranium-232 have been produced in breeder reactors.

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%).^[6] All three isotopes are radioactive, creating radioisotopes, with the most abundant and stable being uranium-238 with a half-life of 4.4683×10⁹ years (close to the age of the Earth).

Uranium-238 is an α emitter, decaying through the 18-member uranium series into lead-206. The decay series of uranium-235 (historically called actino-uranium) has 15 members that 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.

The isotope 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, that is, can be broken apart by thermal neutrons. The isotope uranium-238 is also important because it absorbs neutrons to produce a radioactive isotope that subsequently decays to the isotope plutonium-239, which also is fissile.

Uranium-232

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.^[7] 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 159,200 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, U-234 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 U-238. The path of production of U-234 via nuclear decay is as follows: U-238 nuclei emit an alpha particle to become thorium-234 (Th-234). Next, with a short half-life, Th-234 nuclei emit a beta particle to become protactinium-234 (Pa-234). Finally, Pa-234 nuclei emit another beta particle to become U-234 nuclei.

U-234 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 U-234 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 U-234. Very small pure samples of U-234 can be extracted via the chemical ion-exchange process—from samples of plutonium-238 that have been aged somewhat to allow some decay to U-234 via alpha emission.

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

U-234 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. U-234 is converted to U-235 more easily and therefore at a greater rate than U-238 is to Pu-239 (via neptunium-239) because U-238 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.^[8] 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 U-238'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 U-238 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 U-238, 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

Isotopes of uranium Full table
General
Name, symbol	U-239,^239U
Neutrons	147
Protons	92
Nuclide data
Half-life	23.45 mins
Decay products	^239Np
Decay mode	Decay energy
Beta decay 20%	1.28 MeV
Beta decay 80%	1.21 MeV

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.^[9] 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.^[10]

²³⁹Np further decays to plutonium-239, in a second important step that ultimately produces fissile ²³⁹Pu (used in weapons and for nuclear power), from ²³⁸U in reactors.

Table

nuclide symbol	historic name	Z(p)	N(n)	isotopic mass (u)	half-life	decay mode(s)[11] [n 1]	daughter isotope(s)[n 2]	nuclear spin	representative isotopic composition (mole fraction)	range of natural variation (mole fraction)
		excitation energy
^215U[12]		92	123		2.24 ms	α	^211Th	5/2−#
^216U[13][14]		92	124		4.3 ms	α	^212Th	0+
^217U		92	125	217.02437(9)	26(14) ms [16(+21−6) ms]	α	^213Th	1/2−#
^218U		92	126	218.02354(3)	6(5) ms	α	^214Th	0+
^219U		92	127	219.02492(6)	55(25) µs [42(+34−13) µs]	α	^215Th	9/2+#
^220U		92	128	220.02472(22)#	60# ns	α	^216Th	0+
						β^+ (rare)	^220Pa
^221U		92	129	221.02640(11)#	700# ns	α	^217Th	9/2+#
						β^+ (rare)	^221Pa
^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+
						SF (6×10^−9%)	(various)
						CD (7.2×10^−11%)	^209Pb ^24Ne
						CD (1.3×10^−13%)	^205Hg ^28Mg
^234U[n 3][n 4]	Uranium II	92	142	234.0409521(20)	2.455(6)×10^5 y	α	^230Th	0+	[0.000054(5)][n 5]	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 6][n 7][n 8]	Actin Uranium Actino-Uranium	92	143	235.0439299(20)	7.04(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		92	144	236.045568(2)	2.342(3)×10^7 y	α	^232Th	0+
						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+
^238U[n 4][n 6][n 7]	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+
						α (10^−10%)	^236Th
^241U		92	149	241.06033(32)#	5# min	β^−	^241Np	7/2+#
^242U		92	150	242.06293(22)#	16.8(5) min	β^−	^242Np	0+

1. Abbreviations:
   CD: Cluster decay
   EC: Electron capture
   IT: Isomeric transition
   SF: Spontaneous fission
2. Bold for stable isotopes, bold italics for nearly-stable isotopes (half-life longer than the age of the universe)
3. Used in uranium-thorium dating
4. Used in uranium-uranium dating
5. Intermediate decay product of ²³⁸U
6. Primordial radionuclide
7. Used in Uranium-lead dating
8. Important in nuclear reactors

Notes

• Evaluated isotopic composition is for most but not all commercial samples.
• The precision of the isotope abundances and atomic mass is limited through variations. The given ranges should be applicable to any normal terrestrial material.
• Geologically exceptional samples are known in which the isotopic composition lies outside the reported range. The uncertainty in the atomic mass may exceed the stated value for such specimens.
• Commercially available materials may have been subjected to an undisclosed or inadvertent isotopic fractionation. Substantial deviations from the given mass and composition can occur.
• Values marked # are not purely derived from experimental data, but at least partly from systematic trends. Spins with weak assignment arguments are enclosed in parentheses.
• Uncertainties are given in concise form in parentheses after the corresponding last digits. Uncertainty values denote one standard deviation, except isotopic composition and standard atomic mass from IUPAC, which use expanded uncertainties.

References

1. 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 isotopes have half-lives of at least four years (the longest-lived isotope 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.
2. Specifically from thermal neutron fission of U-235, e.g. in a typical nuclear reactor.
3. 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. 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 y. No growth of Cf²⁴⁸ was detected, and a lower limit for the β⁻ half-life can be set at about 10⁴ y. No alpha activity attributable to the new isomer has been detected; the alpha half-life is probably greater than 300 y."
4. This is the heaviest isotope with a half-life of at least four years before the "Sea of Instability".
5. Excluding those "classically stable" isotopes 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.
6. Uranium Isotopes, retrieved 14 March 2012
7. C. W. Forsburg and L. C. Lewis (1999-09-24). "Uses For Uranium-233: What Should Be Kept for Future Needs?" (PDF). ORNL-6952 (Oak Ridge National Laboratory).
8. B. C. Diven, J. Terrell, and A. Hemmendinger (1 January 1958). "Capture-to-Fission Ratios for Fast Neutrons in U235". Physical Review Letters 109: 144–150. doi:10.1103/PhysRev.109.144.
9. CRC Handbook of Chemistry and Physics, 57th Ed. p. B-345
10. CRC Handbook of Chemistry and Physics, 57th Ed. p. B-423
11. "Universal Nuclide Chart". nucleonica. (registration required (help)).
12. "New Isotope Candidates, 215U and 216U" http://www.nishina.riken.jp/researcher/APR/APR047/pdf/xxii.pdf
13. "New Isotope Candidates, 215U and 216U" http://www.nishina.riken.jp/researcher/APR/APR047/pdf/xxii.pdf
14. "Observation of new neutron-deficient isotopes with Z ≥ 92 in multinucleon transfer reactions" http://inspirehep.net/record/1383747/files/scoap3-fulltext.pdf

• Isotope masses from:
  • G. Audi; A. H. Wapstra; C. Thibault; J. Blachot; O. Bersillon (2003). "The NUBASE evaluation of nuclear and decay properties" (PDF). Nuclear Physics A 729: 3–128. Bibcode:2003NuPhA.729....3A. doi:10.1016/j.nuclphysa.2003.11.001.
• Isotopic compositions and standard atomic masses from:
  • J. R. de Laeter; J. K. Böhlke; P. De Bièvre; H. Hidaka; H. S. Peiser; K. J. R. Rosman; P. D. P. Taylor (2003). "Atomic weights of the elements. Review 2000 (IUPAC Technical Report)". Pure and Applied Chemistry 75 (6): 683–800. doi:10.1351/pac200375060683.
  • M. E. Wieser (2006). "Atomic weights of the elements 2005 (IUPAC Technical Report)". Pure and Applied Chemistry 78 (11): 2051–2066. doi:10.1351/pac200678112051. Lay summary.
• Half-life, spin, and isomer data selected from the following sources. See editing notes on this article's talk page.
  • G. Audi; A. H. Wapstra; C. Thibault; J. Blachot; O. Bersillon (1999). "The NUBASE evaluation of nuclear and decay properties" (PDF). Nuclear Physics A 729: 3–128. Bibcode:2003NuPhA.729....3A. doi:10.1016/j.nuclphysa.2003.11.001.
  • National Nuclear Data Center. "NuDat 2.1 database". Brookhaven National Laboratory. Retrieved September 2005.
  • N. E. Holden (2004). "Table of the Isotopes". In D. R. Lide. CRC Handbook of Chemistry and Physics (85th ed.). CRC Press. Section 11. ISBN 978-0-8493-0485-9.