gamma (body-centered cubic) from 774.8°C to melting point - this is the most malleable and ductile state.
Its two principal isotopes are 235U and 238U (see Uranium-235 and Uranium-238). Naturally-occurring Uranium also contains a small amount of the 234U isotope, which is a decay product of 238U. The isotope 235U is important for both nuclear reactors and nuclear weapons because it is the only isotope existing in nature to any appreciable extent that is fissile, that is, fissionable by thermal neutrons. The isotope 238U is also important because it absorbs neutrons to produce a radioactive isotope that subsequently decays to the isotope 239Pu (plutonium), which also is fissile.
The artificial 233U isotope is also fissile and is made from 232thorium by neutron bombardment.
Uranium was the first element that was found to be fissile, i.e. upon bombardment with slow neutrons, its 235U isotope becomes the very short lived 236U, that immediately divides into two smaller nuclei, liberating energy and more neutrons. If these neutrons are absorbed by other 235U nuclei, a nuclear chain reaction occurs, and if there is nothing to absorb some neutrons and slow the reaction, it is explosive. The first atomic bomb worked with by this principle (nuclear fission). A more accurate name for both this and the hydrogen bomb (nuclear fusion) would be "nuclear weapon", because only the nuclei participate.
The main use of uranium in the civilian sector is to fuel commercial nuclear power plants, where fuel is typically enriched in U-235 to 2-3%. However, the Canadian Candu reactors use natural unenriched uranium as fuel. Depleted uranium is used in helicopters and airplanes as counterweights on certain wing parts. Other uses include;
Ceramic glazes where small amounts of natural uranium (that is, not having gone through the enrichment process) may be added for color.
The long half-life of the isotope uranium-238 (4.51 × 109) make it well-suited for use in estimating the age of the earliest igneous rocks.
U-238 is converted into plutonium in breeder reactors. Plutonium in turn in used in hydrogen bombs.
In the Manhattan Project the names tuballoy and oralloy were used to refer to natural uranium and enriched uranium respectively. These names are still used occasionally to refer to natural or enriched uranium.
The exploration and mining of radioactive ores in the United States began around the turn of the 20th century. Sources for radium (contained in uranium ore) were sought for use as luminous paint for watch dials and other instruments. Uranium became important for defense purposes during World War II. In 1943, the Union Mines Development Corporation operated mills in Colorado to process uranium ore for the Manhattan Project, which applied atomic power to military use. To ensure adequate supplies of uranium for national defense, Congress passed the U.S. Atomic Energy Act of 1946, creating the Atomic Energy Commission which had the power to withdraw prospective uranium mining land from public purchase, and also to manipulate the price of uranium to meet national needs (leading to a uranium "boom" in the early 1950s). Military requirements declined in the 1960s, and the Government completed its uranium procurement program by the end of 1970. Simultaneously, a new market emerged - commercial nuclear power plants.
Uranium hexafluoride (UF6) is a white solid which forms a vapor at temperatures above 56 degrees Celsius. UF6 is the compound of uranium used for the two most common enrichment processes, gaseous diffusion enrichment and centrifuge enrichment. It is simply called "hex" in the industry.
Yellowcake is uranium concentrate. It takes its name from the color and texture of the concentrates produced by early mining operations, despite the fact that modern mills using higher calcining temperatures produce "yellowcake" that is dull green to almost black. Yellowcake typically contains 70 to 90 percent uranium oxide (U3O8) by weight.
Ammonium diuranate is an intermediate product in the production of yellowcake, and is bright yellow in colour. It is sometimes confusingly called "yellowcake" as well, but this is not a standard name.