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). The standard atomic weight of natural uranium is 238.02891(3).

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 sufficiently large (critical) mass of uranium-235. 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.

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 undergoes alpha decay with a half-life of 0.5 ms.^{[21][22][23][24]}

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 is a fissile isotope of uranium 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 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 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.

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 just 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 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 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 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.^[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 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 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 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 (²³⁸U or U-238) is the most common isotope of uranium found 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 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.

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]