Uranium dioxide in the context of Fuel element failure

Cleveite is an impure radioactive variety of uraninite containing uranium, found in Norway. It has the composition UO₂ with about 10% of the uranium substituted by rare-earth elements. It was named after Swedish chemist Per Teodor Cleve.

Cleveite was the first known terrestrial source of helium, which is created over time by alpha decay of the uranium and accumulates trapped (occluded) within the mineral. The first sample of helium was obtained by William Ramsay in 1895 when he treated a sample of the mineral with acid. Cleve and Abraham Langlet succeeded in isolating helium from cleveite at about the same time.

Uraninite, also known as pitchblende, is a radioactive, uranium-rich mineral and ore with a chemical composition that is largely UO₂ but because of oxidation typically contains variable proportions of U₃O₈. Radioactive decay of the uranium causes the mineral to contain oxides of lead and trace amounts of helium. It may also contain thorium and rare-earth elements.

Uranium mining is the process of extraction of uranium ore from the earth. Almost 50,000 tons of uranium were produced in 2022. Kazakhstan, Canada, and Namibia were the top three uranium producers, respectively, and together account for 69% of world production. Other countries producing more than 1,000 tons per year included Australia, Niger, Russia, Uzbekistan and China. Nearly all of the world's mined uranium is used to power nuclear power plants. Historically uranium was also used in applications such as uranium glass or ferrouranium but those applications have declined due to the radioactivity and toxicity of uranium and are nowadays mostly supplied with a plentiful cheap supply of depleted uranium which is also used in uranium ammunition. In addition to being cheaper, depleted uranium is also less radioactive due to a lower content of short-lived
U and
U than natural uranium.

Uranium is mined by in-situ leaching (57% of world production) or by conventional underground or open-pit mining of ores (43% of production). During in-situ mining, a leaching solution is pumped down drill holes into the uranium ore deposit where it dissolves the ore minerals. The uranium-rich fluid is then pumped back to the surface and processed to extract the uranium compounds from solution. In conventional mining, ores are processed by grinding the ore materials to a uniform particle size and then treating the ore to extract the uranium by chemical leaching. The milling process commonly yields dry powder-form material consisting of natural uranium, "yellowcake", which is nowadays commonly sold on the uranium market as U₃O₈. While some nuclear power plants – most notably heavy water reactors like the CANDU – can operate with natural uranium (usually in the form of uranium dioxide), the vast majority of commercial nuclear power plants and many research reactors require uranium enrichment, which raises the content of
U from the natural 0.72% to 3–5% (for use in light water reactors) or even higher, depending on the application. Enrichment requires conversion of the yellowcake into uranium hexafluoride and production of the fuel (again usually uranium dioxide, but sometimes uranium carbide, uranium hydride or uranium nitride) from that feedstock.

Natural uranium (NU or U_nat) is uranium with the same isotopic ratio as found in nature. It contains 0.711% uranium-235, 99.284% uranium-238, and a trace of uranium-234 by weight (0.0055%). Approximately 2.2% of its radioactivity comes from uranium-235, 48.6% from uranium-238, and 49.2% from uranium-234.

Natural uranium can be used to fuel both low- and high-power nuclear reactors. Historically, graphite-moderated reactors and heavy water-moderated reactors have been fueled with natural uranium in the pure metal (U) or uranium dioxide (UO₂) ceramic forms. However, experimental fuelings with uranium trioxide (UO₃) and triuranium octaoxide (U₃O₈) have shown promise.

Triuranium octoxide (U₃O₈) is a compound of uranium. It is present as an olive green to black, odorless solid. It is one of the more popular forms of yellowcake and is shipped between mills and refineries in this form.

U₃O₈ has potential long-term stability in a geologic environment. In the presence of oxygen (O₂), uranium dioxide (UO₂) is oxidized to U₃O₈, whereas uranium trioxide (UO₃) loses oxygen at temperatures above 500 °C and is reduced to U₃O₈. The compound can be produced by the calcination of ammonium diuranate or ammonium uranyl carbonate. Due to its high stability, it can be used for the disposal of depleted uranium. Its particle density is 8.38 g cm.