Uranium
Isotopes

Uranium, like all elements with an atomic number greater than 82, has no stable isotopes. All isotopes of uranium are radioactive because the strong nuclear force does not prevail over electromagnetic repulsion in nuclides containing more than 82 protons. Nevertheless, the two most stable isotopes, U and U, have half-lives long enough to occur in nature as primordial radionuclides, with measurable quantities having survived since the formation of the Earth. These two nuclides, along with thorium-232, are the only confirmed primordial nuclides heavier than nearly-stable bismuth-209.

Natural uranium consists of three major isotopes: uranium-238 (99.28% natural abundance), uranium-235 (0.71%), and uranium-234 (0.0054%). There are also five other trace isotopes: uranium-240, a decay product of plutonium-244; uranium-239, which is formed when U undergoes spontaneous fission, releasing neutrons that are captured by another U atom; uranium-237, which is formed when U captures a neutron but emits two more, which then decays to neptunium-237; uranium-236, which occurs in trace quantities due to neutron capture on U and as a decay product of plutonium-244; and finally, uranium-233, which is formed in the decay chain of neptunium-237. Additionally, uranium-232 would be produced by the double beta decay of natural thorium-232, though this energetically possible process has never been observed.

Uranium-238 is the most stable isotope of uranium, with a half-life of about 4.463 × 10 years, roughly the age of the Earth. Uranium-238 is predominantly an alpha emitter, decaying to thorium-234. It ultimately decays through the uranium series, which has 18 members, into lead-206. Uranium-238 is not fissile, but is a fertile isotope, because after neutron activation it can be converted to plutonium-239, another fissile isotope. Indeed, the U nucleus can absorb one neutron to produce the radioactive isotope uranium-239. U decays by beta emission to neptunium -239, also a beta-emitter, that decays in its turn, within a few days into plutonium-239. Pu was used as fissile material in the first atomic bomb detonated in the " Trinity test " on 16 July 1945 in New Mexico.

Uranium-235 has a half-life of about 7.04 × 10 years; it is the next most stable uranium isotope after U and is also predominantly an alpha emitter, decaying to thorium-231. Uranium-235 is important for both nuclear reactors and nuclear weapons, because it is the only uranium isotope existing in nature on Earth in significant amounts that is fissile. This means that it can be split into two or three fragments (fission products) by thermal neutrons. The decay chain of U, which is called the actinium series, has 15 members and eventually decays into lead-207. The constant rates of decay in these decay series makes the comparison of the ratios of parent to daughter elements useful in radiometric dating.

Uranium-236 has a half-life of 2.342 × 10 years and is not found in significant quantities in nature. The half-life of uranium-236 is too short for it to be primordial, though it has been identified as an extinct progenitor of its alpha decay daughter, thorium-232. Uranium-236 occurs in spent nuclear fuel when neutron capture on U does not induce fission, or as a decay product of plutonium-240. Uranium-236 is not fertile, as three more neutron captures are required to produce fissile Pu, and is not itself fissile; as such, it is considered long-lived radioactive waste.

Uranium-234 is a member of the uranium series and occurs in equilibrium with its progenitor, U; it undergoes alpha decay with a half-life of 245,500 years and decays to lead-206 through a series of relatively short-lived isotopes.

Uranium-233 undergoes alpha decay with a half-life of 160,000 years and, like U, is fissile. It can be bred from thorium-232 via neutron bombardment, usually in a nuclear reactor; this process is known as the thorium fuel cycle. Owing to the fissility of U and the greater natural abundance of thorium (three times that of uranium), U has been investigated for use as nuclear fuel as a possible alternative to U and Pu, though is not in widespread use as of 2022. The decay chain of uranium-233 forms part of the neptunium series and ends at nearly-stable bismuth-209 (half-life 2.01 × 10 years) and stable thallium -205.

Uranium-232 is an alpha emitter with a half-life of 68.9 years. This isotope is produced as a byproduct in production of U and is considered a nuisance, as it is not fissile and decays through short-lived alpha and gamma emitters such as Tl. It is also expected that thorium-232 should be able to undergo double beta decay, which would produce uranium-232, but this has not yet been observed experimentally.

All isotopes from U to U inclusive have minor cluster decay branches (less than 10 %), and all these bar U, in addition to U, have minor spontaneous fission branches; the greatest branching ratio for spontaneous fission is about 5 × 10 % for U, or about one in every two million decays. The shorter-lived trace isotopes U and U exclusively undergo beta decay, with respective half-lives of 6.752 days and 23.45 minutes.

In total, 28 isotopes of uranium have been identified, ranging in mass number from 214 to 242, with the exception of 220. Among the uranium isotopes not found in natural samples or nuclear fuel, the longest-lived is U, an alpha emitter with a half-life of 20.23 days. This isotope has been considered for use in targeted alpha-particle therapy (TAT). All other isotopes have half-lives shorter than one hour, except for U (half-life 4.2 days) and U (half-life 14.1 hours). The shortest-lived known isotope is U, with a half-life of 660 nanoseconds, and it is expected that the hitherto unknown U has an even shorter half-life. The proton-rich isotopes lighter than U primarily undergo alpha decay, except for U and U, which decay to protactinium isotopes via positron emission and electron capture, respectively; the neutron-rich U, U, and U undergo beta decay to form neptunium isotopes.
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