Isotopes of uranium

Actinides and fission products by half-life
Actinides by decay chain				Half-life range (y)	Fission products of 235U by yield
4n	4n+1	4n+2	4n+3
						4.5–7%	0.04–1.25%	<0.001%
228Ra№				4–6	†		155Euþ
244Cm	241Puƒ	250Cf	227Ac№	10–29		90Sr	85Kr	113mCdþ
232Uƒ		238Pu№	243Cmƒ	29–97		137Cs	151Smþ	121mSn
248Bk	249Cfƒ	242mAmƒ		141–351	No fission products have a half-life in the range of 100–210 k years ...
	241Amƒ		251Cfƒ	430–900
		226Ra№	247Bk	1.3 k – 1.6 k
240Pu	229Th№	246Cm	243Amƒ	4.7 k – 7.4 k
	245Cmƒ	250Cm		8.3 k – 8.5 k
			239Puƒ№	24.1 k
		230Th№	231Pa№	32 k – 76 k
236Npƒ	233Uƒ№	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№			247Cmƒ	15 M – 24 M			129I₡
244Pu№				80 M	... nor beyond 15.7 M years
232Th№		238U№	235Uƒ№	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%). 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

Isotopes of uranium Full table
General
Name, symbol	U-232,232U
Neutrons	140
Protons	92
Nuclide data
Half-life	68.9 years
Parent isotopes	236Pu (α) 232Np (β+) 232Pa (β−)
Decay products	228Th

Uranium 232 (232
92U
140, 232U, U-232) is an isotope of uranium. It 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 of ²³²U's decay products makes the ²³³U contaminated with it more difficult to handle.

Production of ²³³U (through the neutron irradiation of ²³²Th) invariably produces small amounts of ²³²U as an impurity, because of parasitic (n,2n) reactions on uranium-233 itself, or on protactinium-233:

²³²Th (n,γ) ²³³Th (β−) ²³³Pa (β−) ²³³U (n,2n) ²³²U

²³²Th (n,γ) ²³³Th (β−) ²³³Pa (n,2n) ²³²Pa (β−) ²³²U

The decay chain of ²³²U quickly yields strong gamma radiation emitters:

²³²U (α, 68.9 years)

²²⁸Th (α, 1.9 year)

²²⁴Ra (α, 3.6 day, 0.24 MeV) (at this point, the decay chain is identical to that of ²³²Th)

²²⁰Rn (α, 55 s, 0.54 MeV)

²¹⁶Po (α, 0.15 s)

²¹²Pb (β−, 10.64 h)

²¹²Bi (α, 61 m, 0.78 MeV)

²⁰⁸Tl (β−, 3 m, 2.6 MeV) (35.94% branching ratio)

²⁰⁸Pb (stable)

This makes manual handling in a glove box with only light shielding (as commonly done with plutonium) too hazardous, (except possibly in a short period immediately following chemical separation of the uranium from thorium-228, radium-224, radon-220, and polonium) and instead requiring remote manipulation for fuel fabrication.

Unusually for an isotope with even mass number, ²³²U has a significant neutron absorption cross section for fission (thermal neutrons 75 barns (b), resonance integral 380 b) as well as for neutron capture (thermal 73 b, resonance integral 280 b).

Lighter: uranium-231	Isotopes of uranium is an isotope of uranium	Heavier: uranium-233
Decay product of: plutonium-236 (α) neptunium-232 (β+) protactinium-232 (β−)	Decay chain of isotopes of uranium	Decays to: thorium-228 (α)

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.[1] 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. 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. 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.

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.

Lighter: uranium-238	Isotopes of uranium is an isotope of uranium	Heavier: uranium-240
Decay product of: protactinium-239 (β-)	Decay chain of isotopes of uranium	Decays to: neptunium-239 (β-)

Table

nuclide symbol	historic name	Z(p)	N(n)	isotopic mass (u)	half-life	decay mode(s)	daughter isotope(s)	nuclear spin	representative isotopic composition (mole fraction)	range of natural variation (mole fraction)
		excitation energy
215U		92	123		2.24 ms	α	211Th	5/2−#
216U		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)×105 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	Uranium II	92	142	234.0409521(20)	2.455(6)×105 y	α	230Th	0+	[0.000054(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	Actin Uranium Actino-Uranium	92	143	235.0439299(20)	7.04(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		92	144	236.045568(2)	2.342(3)×107 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	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+
						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+

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

• Isotope masses from:
• Isotopic compositions and standard atomic masses from:
• Half-life, spin, and isomer data selected from the following sources. See editing notes on this article's talk page.

Isotopes of protactinium	Isotopes of uranium	Isotopes of neptunium
Table of nuclides