Isotopes of uranium

Uranium (₉₂U) is a naturally occurring radioactive element (radioelement) with no stable isotopes. It has two primordial isotopes, uranium-238 and uranium-235, that have long half-lives and are found in appreciable quantity in 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 (except for ²²⁰U). The standard atomic weight of natural uranium is 238.02891(3).

Isotopes of uranium (₉₂U)

Main isotopes[1] Decay abun­dance half-life (t1/2) mode pro­duct 232U synth 68.9 y α 228Th SF – 233U trace 1.592×105 y[2] α 229Th SF – 234U 0.005% 2.455×105 y α 230Th SF – 235U 0.720% 7.038×108 y α 231Th SF – 236U trace 2.342×107 y α 232Th SF – 238U 99.3% 4.468×109 y α 234Th SF – β−β− 238Pu
Standard atomic weight Ar°(U)
	238.02891±0.00003[3] 238.03±0.01 (abridged)[4]
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Natural uranium consists of three main isotopes, ²³⁸U (99.2739–99.2752% natural abundance), ²³⁵U (0.7198–0.7202%), and ²³⁴U (0.0050–0.0059%).^[5] 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 (about 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 (energy production) and nuclear weapons because it is the only isotope existing in nature to any appreciable extent that is fissile in response to thermal neutrons, i.e., thermal neutron capture has a high probability of inducing fission. A chain reaction can be sustained with a large enough (critical) mass of uranium-235. Uranium-238 is also important because it is fertile: it absorbs neutrons to produce a radioactive isotope that decays into plutonium-239, which also is fissile.

List of isotopes

Nuclide [n 1]	Historic name	Z	N	Isotopic mass (Da)[6] [n 2][n 3]	Half-life[1]	Decay mode[1] [n 4]	Daughter isotope [n 5][n 6]	Spin and parity[1] [n 7][n 8]	Natural abundance (mole fraction)
		Excitation energy[n 8]							Normal proportion[1]	Range of variation
214U[7]		92	122		0.52+0.95 −0.21 ms	α	210Th	0+
215U		92	123	215.026720(11)	1.4(0.9) ms	α	211Th	5/2−#
						β+?	215Pa
216U[8]		92	124	216.024760(30)	2.25+0.63 −0.40 ms	α	212Th	0+
216mU		2206 keV			0.89+0.24 −0.16 ms	α	212Th	8+
217U[9]		92	125	217.024660(86)#	19.3+13.3 −5.6 ms	α	213Th	(1/2−)
						β+?	217Pa
218U[8]		92	126	218.023505(15)	650+80 −70 μs	α	214Th	0+
218mU		2117 keV			390+60 −50 μs	α	214Th	8+
						IT?	218U
219U		92	127	219.025009(14)	60(7) μs	α	215Th	(9/2+)
						β+?	219Pa
221U		92	129	221.026323(77)	0.66(14) μs	α	217Th	(9/2+)
						β+?	221Pa
222U		92	130	222.026058(56)	4.7(0.7) μs	α	218Th	0+
						β+?	222Pa
223U		92	131	223.027961(63)	65(12) μs	α	219Th	7/2+#
						β+?	223Pa
224U		92	132	224.027636(16)	396(17) μs	α	220Th	0+
						β+?	224Pa
225U		92	133	225.029385(11)	62(4) ms	α	221Th	5/2+#
226U		92	134	226.029339(12)	269(6) ms	α	222Th	0+
227U		92	135	227.0311811(91)	1.1(0.1) min	α	223Th	(3/2+)
						β+?	227Pa
228U		92	136	228.031369(14)	9.1(0.2) min	α (97.5%)	224Th	0+
						EC (2.5%)	228Pa
229U		92	137	229.0335060(64)	57.8(0.5) min	β+ (80%)	229Pa	(3/2+)
						α (20%)	225Th
230U		92	138	230.0339401(48)	20.23(0.02) d	α	226Th	0+
						SF ?	(various)
						CD (4.8×10−12%)	208Pb 22Ne
231U		92	139	231.0362922(29)	4.2(0.1) d	EC	231Pa	5/2+#
						α (.004%)	227Th
232U		92	140	232.0371548(19)	68.9(0.4) y	α	228Th	0+
						CD (8.9×10−10%)	208Pb 24Ne
						SF (10−12%)	(various)
						CD?	204Hg 28Mg
233U		92	141	233.0396343(24)	1.592(2)×105 y	α	229Th	5/2+	Trace[n 9]
						CD (≤7.2×10−11%)	209Pb 24Ne
						SF ?	(various)
						CD ?	205Hg 28Mg
234U[n 10][n 11]	Uranium II	92	142	234.0409503(12)	2.455(6)×105 y	α	230Th	0+	[0.000054(5)][n 12]	0.000050– 0.000059
						SF (1.64×10−9%)	(various)
						CD (1.4×10−11%)	206Hg 28Mg
						CD (≤9×10−12%)	208Pb 26Ne
						CD (≤9×10−12%)	210Pb 24Ne
234mU		1421.257(17) keV			33.5(2.0) ms	IT	234U	6−
235U[n 13][n 14][n 15]	Actin Uranium Actino-Uranium	92	143	235.0439281(12)	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%)	215Pb 20Ne
						CD (8×10−10%)	210Pb 25Ne
						CD (8×10−10%)	207Hg 28Mg
235m1U		0.076737(18) keV			25.7(1) min	IT	235U	1/2+
235m2U		2500(300) keV			3.6(18) ms	SF	(various)
236U	Thoruranium[10]	92	144	236.0455661(12)	2.342(3)×107 y	α	232Th	0+	Trace[n 16]
						SF (9.6×10−8%)	(various)
						CD (≤2.0×10−11%)[11]	208Hg 28Mg
						CD (≤2.0×10−11%)[11]	206Hg 30Mg
236m1U		1052.5(6) keV			100(4) ns	IT	236U	4−
236m2U		2750(3) keV			120(2) ns	IT (87%)	236U	(0+)
						SF (13%)	(various)
237U		92	145	237.0487283(13)	6.752(2) d	β−	237Np	1/2+	Trace[n 17]
237mU		274.0(10) keV			155(6) ns	IT	237U	7/2−
238U[n 11][n 13][n 14]	Uranium I	92	146	238.050787618(15)[12]	4.468(3)×109 y	α	234Th	0+	[0.992742(10)]	0.992739– 0.992752
						SF (5.44×10−5%)	(various)
						β−β− (2.2×10−10%)	238Pu
238mU		2557.9(5) keV			280(6) ns	IT (97.4%)	238U	0+
						SF (2.6%)	(various)
239U		92	147	239.0542920(16)	23.45(0.02) min	β−	239Np	5/2+	Trace[n 18]
239m1U		133.7991(10) keV			780(40) ns	IT	239U	1/2+
239m2U		2500(900)# keV			>250 ns	SF?	(various)	0+
						IT?	239U
240U		92	148	240.0565924(27)	14.1(0.1) h	β−	240Np	0+	Trace[n 19]
						α?	236Th
241U[13]		92	149	241.06031(5)	~40 min[14][15]	β−	241Np	7/2+#
242U		92	150	242.06296(10)[13]	16.8(0.5) min	β−	242Np	0+
This table header & footer: view

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. Neutron capture product; parent of trace quantities of ²³⁹Pu
19. Intermediate decay product of ²⁴⁴Pu

Actinides vs fission products

Actinides and fission products by half-life v t e
Actinides[16] by decay chain				Half-life range (a)	Fission products of 235U by yield[17]
4n	4n + 1	4n + 2	4n + 3		4.5–7%	0.04–1.25%	<0.001%
228Ra№				4–6 a		155Euþ
248Bk[18]				> 9 a
244Cmƒ	241Puƒ	250Cf	227Ac№	10–29 a	90Sr	85Kr	113mCdþ
232Uƒ		238Puƒ	243Cmƒ	29–97 a	137Cs	151Smþ	121mSn
	249Cfƒ	242mAmƒ		141–351 a	No fission products have a half-life in the range of 100 a–210 ka ...
	241Amƒ		251Cfƒ[19]	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₡
				1.33 Ma	135Cs₡
	237Npƒ			1.61–6.5 Ma	93Zr	107Pd
236U			247Cmƒ	15–24 Ma		129I₡
244Pu				80 Ma	... nor beyond 15.7 Ma[20]
232Th№		238U№	235Uƒ№	0.7–14.1 Ga
₡,  has thermal neutron capture cross section in the range of 8–50 barns ƒ,  fissile №,  primarily a naturally occurring radioactive material (NORM) þ,  neutron poison (thermal neutron capture cross section greater than 3k barns)

Uranium-214

Uranium-214 is the lightest known isotope of uranium. It was discovered at the Spectrometer for Heavy Atoms and Nuclear Structure (SHANS) at the Heavy Ion Research Facility in Lanzhou, China in 2021, produced by firing argon-36 at tungsten-182. It alpha-decays with a half-life of 0.5 ms.^{[21][22][23][24]}

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, because the intense gamma radiation from ²⁰⁸Tl (a daughter of ²³²U, produced relatively quickly) makes 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.^[25][26]

Uranium-233

Main article: Uranium-233

Uranium-233 is a fissile isotope that is bred from thorium-232 as part of the thorium fuel cycle. ²³³U was investigated for use in nuclear weapons and as a reactor fuel. It was occasionally tested but never deployed in nuclear weapons and has not been used commercially as a nuclear fuel.^[27] 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 around 160,000 years.

Uranium-233 is produced by 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 beta decays into protactinium-233. 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

²³⁴U occurs in natural uranium as an indirect decay product of uranium-238, but makes up only 55 parts per million of the uranium because its half-life of 245,500 years is only about 1/18,000 that of ²³⁸U. The path of production of ²³⁴U is this: ²³⁸U alpha decays to thorium-234. Next, with a short half-life, ²³⁴Th beta decays to protactinium-234. Finally, ²³⁴Pa beta decays to ²³⁴U.^[28][29]

²³⁴U alpha decays to thorium-230, except for a small percentage of nuclei that undergo spontaneous fission.

Extraction of small amounts of ²³⁴U from natural uranium could be done using isotope separation, similar to normal 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 aged somewhat to allow some alpha decay to ²³⁴U.

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.^[30] This is because ²³⁴U is not fissile, and tends to absorb slow neutrons in a nuclear reactor—becoming ²³⁵U.^[29][30]

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 makes 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.^[31] The fission-to-capture ratio improves for faster neutrons.

Uranium-236

Main article: Uranium-236

Uranium-236 has a half-life of about 23 million years; and 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 has a half-life of about 6.75 days. It decays into neptunium-237 by beta decay. It was discovered by Japanese physicist Yoshio Nishina in 1940, who in a near-miss discovery, inferred the creation of element 93, but was unable to isolate the then-unknown element or measure its decay properties.^[32]

Uranium-238

Main article: Uranium-238

Uranium-238 (²³⁸U or U-238) is the most common isotope of uranium in nature. It is not fissile, but is fertile: 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.

About 99.284% of natural uranium is uranium-238, which has a half-life of 1.41×10¹⁷ seconds (4.468×10⁹ years). Depleted uranium has an even higher concentration of ²³⁸U, and even low-enriched uranium (LEU) 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 usually produced by exposing ²³⁸U to neutron radiation in a nuclear reactor. ²³⁹U has a half-life of about 23.45 minutes and beta decays into neptunium-239, with a total decay energy of about 1.29 MeV.^[33] 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.^[34]

²³⁹Np then, with a half-life of about 2.356 days, beta-decays to plutonium-239.

Uranium-241

In 2023, in a paper published in Physical Review Letters, a group of researchers based in Korea reported that they had found uranium-241 in an experiment involving ²³⁸U+¹⁹⁸Pt multinucleon transfer reactions.^[35][36] Its half-life is about 40 minutes.^[35]

References

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16. 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.
17. Specifically from thermal neutron fission of uranium-235, e.g. in a typical nuclear reactor.
18. 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]."
19. This is the heaviest nuclide with a half-life of at least four years before the "sea of instability".
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