Uranium (₉₂U) is a naturally occurringradioactive element (radioelement) with nostable isotopes. It has twoprimordial isotopes,uranium-238 anduranium-235, that have longhalf-lives and are found in appreciable quantity inEarth's crust. Thedecay producturanium-234 is also found. Other isotopes such asuranium-233 have been produced inbreeder reactors. In addition to isotopes found in nature or nuclear reactors, many isotopes with far shorter half-lives have been produced, ranging from²¹⁴U to²⁴²U (except for²²⁰U). Thestandard atomic weight ofnatural uranium is238.02891(3).

Natural uranium consists of three mainisotopes,²³⁸U (99.2739–99.2752%natural abundance),²³⁵U (0.7198–0.7202%), and²³⁴U (0.0050–0.0059%).^[5] All three isotopes areradioactive (i.e., they areradioisotopes), and the most abundant and stable is uranium-238, with a half-life of4.4683×10⁹ years (about theage of the Earth).

Uranium-238 is analpha emitter, decaying through the 18-memberuranium series intolead-206. Thedecay 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 inradiometric dating. Uranium-233 is made fromthorium-232 byneutron bombardment.

Uranium-235 is important for bothnuclear reactors (energy production) andnuclear weapons because it is the only isotope existing in nature to any appreciable extent that isfissile in response tothermal neutrons, i.e., thermalneutron capture has a high probability of inducing fission. Achain reaction can be sustained with a large enough (critical) mass of uranium-235. Uranium-238 is also important because it isfertile: it absorbs neutrons to produce a radioactive isotope that decays intoplutonium-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 inLanzhou,China in 2021, produced by firing argon-36 at tungsten-182. It alpha-decays with a half-life of0.5 ms.^{[21][22][23][24]}

Uranium-232 has a half-life of 68.9 years and is a side product in thethorium cycle. It has been cited as an obstacle tonuclear proliferation using²³³U, because the intensegamma radiation from²⁰⁸Tl (a daughter of²³²U, produced relatively quickly) makes²³³U contaminated with it more difficult to handle. Uranium-232 is a rare example of aneven-even isotope that isfissile with both thermal and fast neutrons.^[25][26]

Uranium-233 is a fissile isotope that is bred fromthorium-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 aneutron, it becomesthorium-233, which has a half-life of only 22 minutes. Thorium-233beta decays intoprotactinium-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, becominguranium-234. The capture-to-fission ratio is smaller than the other two major fissile fuels,uranium-235 andplutonium-239; it is also lower than that of short-livedplutonium-241, but bested by very difficult-to-produceneptunium-236.

²³⁴U occurs in natural uranium as an indirect decay product of uranium-238, but makes up only 55 parts permillion of the uranium because itshalf-life of 245,500 years is only about 1/18,000 that of²³⁸U. The path of production of²³⁴U is this:²³⁸Ualpha decays tothorium-234. Next, with a shorthalf-life,²³⁴Thbeta decays toprotactinium-234. Finally,²³⁴Pa beta decays to²³⁴U.^[28][29]

²³⁴Ualpha decays tothorium-230, except for a small percentage of nuclei that undergospontaneous fission.

Extraction of small amounts of²³⁴U from natural uranium could be done usingisotope separation, similar to normal uranium-enrichment. However, there is no real demand inchemistry,physics, or engineering for isolating²³⁴U. Very small pure samples of²³⁴U can be extracted via the chemicalion-exchange process, from samples ofplutonium-238 that have aged somewhat to allow some alpha decay to²³⁴U.

Enriched uranium contains more²³⁴U than natural uranium as a byproduct of the uranium enrichment process aimed at obtaininguranium-235, which concentrates lighter isotopes even more strongly than it does²³⁵U. The increased percentage of²³⁴U in enriched natural uranium is acceptable in current nuclear reactors, but (re-enriched)reprocessed uranium might contain even higher fractions of²³⁴U, which is undesirable.^[30] This is because²³⁴U is notfissile, and tends to absorb slowneutrons in anuclear reactor—becoming²³⁵U.^[29][30]

²³⁴U has aneutron capture cross section of about 100barns forthermal neutrons, and about 700 barns for itsresonance 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 to²³⁵U more easily and therefore at a greater rate thanuranium-238 is toplutonium-239 (vianeptunium-239), because²³⁸U has a much smaller neutron-capturecross section of just 2.7 barns.

Uranium-235 makes up about 0.72% of natural uranium. Unlike the predominant isotopeuranium-238, it isfissile, i.e., it can sustain afissionchain reaction. It is the onlyfissile isotope that is aprimordial nuclide or found in significant quantity in nature.

Uranium-235 has ahalf-life of 703.8million years. It was discovered in 1935 byArthur Jeffrey Dempster. Its (fission) nuclearcross section for slowthermal neutron is about 504.81barns. For fastneutrons 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 forminguranium-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-livedradioactive waste. It is found in spentnuclear fuel and in the reprocessed uranium made from spent nuclear fuel.

Uranium-237 has a half-life of about 6.75 days. It decays intoneptunium-237 bybeta decay. It was discovered by Japanese physicistYoshio 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 commonisotope ofuranium in nature. It is notfissile, but isfertile: it can capture a slowneutron and after twobeta decays become fissileplutonium-239. Uranium-238 is fissionable by fast neutrons, but cannot support a chain reaction because inelastic scattering reducesneutron energy below the range where fast fission of one or more next-generation nuclei is probable. Doppler broadening of²³⁸U'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 of²³⁸U, and even low-enriched uranium (LEU) is still mostly²³⁸U. Reprocessed uranium is also mainly²³⁸U, 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 asuranium-234,uranium-233, anduranium-232.

Uranium-239 is usually produced by exposing²³⁸U toneutron radiation in a nuclear reactor.²³⁹U has a half-life of about 23.45 minutes andbeta decays intoneptunium-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 toplutonium-239.

In 2023, in a paper published inPhysical Review Letters, a group of researchers based in Korea reported that they had founduranium-241 in an experiment involving²³⁸U+¹⁹⁸Pt multinucleon transfer reactions.^[35][36]Its half-life is about 40 minutes.^[35]