Uranium is a dense silvery-gray metal well known for being radioactive. Uranium oxide was used to color yellow glass in the first-century Roman Empire. The practice was recommenced using pitchblende (uranium ore) in central Europe in the Middle Ages.
German chemist Martin Heinrich Klaproth is credited with identifying a new metal in pitchblende in 1789. He named it after the recently-discovered planet Uranus. Uranium was first isolated in 1841 by French chemistry professor Eugène-Melchior Péligot. Uranium continued to be used to color glass.
In the early 20th century, after Marie Curie discovered radium in pitchblende, the ore was mined primarily for its radium. Uranium itself was an unimportant byproduct used as a cheap glaze for pottery and tiles.
When Antoine Henri Becquerel discovered radioactivity in 1896, it was from a sample of uranium fogging an unused photographic plate. Uranium was therefore known to give off some kind of invisible ray.
By the beginning of World War II, some of the causes and effects of radioactivity were understood. Itallian physicist Enrico Fermi was just finishing work on the theoretical concept of a fission chain reaction in uranium. Uranium became the central element of two research projects in the United States during the war, a controlled nuclear fission pile and the atomic bomb.
Post-war, these two technologies have evolved into the two main uses for uranium, power generation and nuclear weapons.
Mining and Production
Uranium occurs naturally in small amounts everywhere, but only the concentrated ores such as pitchblende are commercially interesting. Uranium is extracted from both open-pit mines and underground, depending on its occurrence. The major uranium mining countries are Kazakhstan (38%), Canada (16%), and Australia (11%). Total world production in 2014 was about 70,000 metric tonnes of uranium oxide (U3O8).
Being mildly radioactive, uranium decays into other radioactive elements, eventually resulting in lead. Therefore, natural uranium ore will always contain small amounts of radium, lead, and other decay products. Uranium ore after mining is crushed, ground, and treated with a leaching acid or alkali to dissolve the uranium oxide. The leaching solvent is then concentrated and dried to a powder called yellowcake. Yellowcake is about 80% U3O8 and is traded commercially as an intermediate uranium product. (Yellowcake is actually black, but early versions of it were yellow.)
Uranium consists of two main isotopes that differ in their nuclear properties but are chemically almost identical. Most uranium (99.3%) is U238, which is radioactive but does not fission. Yellowcake for use in some nuclear reactors can simply be converted to uranium dioxide (UO2) and fashioned into control rods. U238 can also be converted to plutonium in a breeder reactor.
For weapons purposes and some reactors, the remaining 0.7% of uranium, which is fissionable U235, must be concentrated through costly efforts. This is called enrichment. Enrichment is a large enterprise that cannot easily be disguised and has only one purpose. When a nation begins its own nuclear weapons program, uranium enrichment is often the indicator.
Uranium is enriched by one of two favorite methods. Uranium converted to hexafluoride gas (UF6) is separated by many iterations of centrifuges or gas diffusion. The result is enriched to 3-5% U235; what remains is depleted uranium.
Properties and Uses
Uranium is a silvery white metal with high density (19 times water, similar to gold) and mildly radioactive. It is usually stored and used as an oxide rather than the pure metal. Uranium has the highest atomic weight (238) and atomic number (92) of the naturally occurring elements.
Depleted uranium is only marginally radioactive and is shaped into high-density projectile weapons. It is also a component of tank armor and is used in counterweights and inertial navigation systems.
Fissionable uranium is not commonly used in weapons now. Countries with advanced nuclear weapons capability now typically use plutonium in place of uranium for triggering fusion bombs.
The main civilian use of uranium is in nuclear power plants. Most power plants use enriched uranium; however, a few use normal uranium. The naval vessels of major navies are also powered by nuclear energy. A breeder reactor converts U238 to plutonium (P239), which is itself fissionable.