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.
[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).
The constant rates of decay in these series makes comparison of the ratios of parent-to-daughter elements useful in radiometric dating.
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.
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.
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.
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.
Extraction of small amounts of 234U from natural uranium could be done using isotope separation, similar to normal uranium-enrichment.
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.
It is the only fissile isotope that is a primordial nuclide or found in significant quantity in nature.
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.
[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.
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.