Uranium U

Discovery

In 1789 the German chemist Martin Heinrich Klaproth analysed pitchblende, a heavy black mineral mined in central Europe, and announced that it contained a new element. What he actually isolated was an oxide. The metal itself was first obtained in 1841 by the French chemist Eugène-Melchior Péligot, who reduced uranium tetrachloride with potassium.

For decades uranium was valued mainly as a colouring agent for glass and glazes. That changed in 1896, when Henri Becquerel found that uranium salts darkened photographic plates even in the dark, the observation that launched the study of radioactivity.

In 1938 Otto Hahn and Fritz Strassmann detected barium among the products of uranium bombarded with neutrons, and Lise Meitner and Otto Frisch explained the result as nuclear fission: the nucleus splitting in two. In December 1942 Enrico Fermi's team in Chicago achieved the first self-sustaining chain reaction in a pile of uranium and graphite.

Origin of the name

Klaproth named the element after Uranus, the planet William Herschel had discovered only eight years earlier, in 1781. The planet itself honours Ouranos, the Greek god of the sky. The symbol U is the initial letter. The next two elements, neptunium and plutonium, continued the planetary sequence.

Main uses

• Nuclear power: Most commercial reactors burn uranium enriched to roughly 3–5 per cent uranium-235. Fully fissioning one kilogram releases heat comparable to burning several thousand tonnes of coal.
• Naval propulsion: Submarines and aircraft carriers run on compact uranium-fuelled reactors.
• Depleted uranium: What remains after enrichment is mostly uranium-238. Its very high density makes it useful for aircraft counterweights, radiation shielding, armour and armour-piercing ammunition.
• Geochronology: Uranium–lead dating is the standard method for finding the ages of the oldest minerals on Earth and of meteorites.
• Historical colourant: In the nineteenth and early twentieth centuries uranium compounds produced yellow-green glass and bright ceramic glazes.

Isotopes

Natural uranium is about 99.3 per cent uranium-238, 0.72 per cent uranium-235 and a trace of uranium-234. Uranium-238 has a half-life of roughly 4.47 billion years, close to the age of the Earth, while uranium-235's is about 704 million years. Only uranium-235 fissions readily with slow neutrons, which is why reactor fuel has to be enriched. About 1.7 billion years ago, when the uranium-235 fraction was higher, deposits at Oklo in present-day Gabon ran as natural fission reactors. When uranium-238 absorbs a neutron it eventually becomes plutonium-239.

In everyday life

Uranium is everywhere in small amounts: typical soils contain a few milligrams per kilogram, granite somewhat more, and seawater a few micrograms per litre. Its decay chain produces radon, a radioactive gas that can build up in poorly ventilated basements and is a recognised indoor-air hazard. For natural uranium itself, chemical toxicity to the kidneys is usually a greater concern than its radioactivity. Antique 'uranium glass' glows a vivid green under ultraviolet light.