Uranium

Martin Heinrich Klaproth, working in Berlin in 1789 on pitchblende from the Saxon mines, isolated a new substance and named it after Uranus, which William Herschel had discovered only eight years earlier in 1781. What Klaproth actually had was the oxide; Eugene-Melchior Peligot isolated the metal in 1841.

The naming set a pattern. When element 93 was made in 1940 it was called neptunium, after the next planet outward, and element 94 was called plutonium after the one beyond that. Uranium, neptunium, plutonium: three elements named in sequence for three planets.

It is worth saying clearly what this is and what it is not. It is a naming convention adopted by chemists, following the astronomical discoveries of their own time. It is not an inherited correspondence, and there is no tradition behind it. Uranus itself was unknown to antiquity; it is not usefully visible to the naked eye and no classical or medieval system includes it. There was no alchemical Uranus and there could not have been one. The same applies to Neptune and Pluto, found in 1846 and 1930.

Modern astrological practice does assign rulerships to Uranus, Neptune and Pluto, and some twentieth-century esoteric writing extends this to the elements that share the names. That is a real modern practice and can be described as such. What it is not is a continuation of alchemy, and presenting it as one requires suppressing the dates. The seven metals were tied to the seven visible planets because those were the planets there were to see. The moment the list of planets grew, the correspondence could not be extended without being invented.

Cerium and palladium, named for Ceres and Pallas in 1803 and 1802, are the same case a little earlier. Naming a new element after a newly discovered body was simply the fashion of the period.

Natural uranium is about 99.27 per cent uranium-238, half-life roughly 4.5 billion years, and 0.72 per cent uranium-235, half-life about 700 million years. Only the second is fissile, meaning that a slow neutron can split it and release further neutrons, which can go on to split further nuclei.

In December 1938 Otto Hahn and Fritz Strassmann in Berlin bombarded uranium with neutrons and found barium among the products, which made no sense: barium is roughly half the mass of uranium. Hahn wrote to Lise Meitner, his collaborator of thirty years, who had fled to Sweden that summer. Meitner and her nephew Otto Frisch worked out over Christmas that the nucleus had split, calculated the energy released from the mass difference using Einstein's relation, and published the explanation early in 1939. Frisch named it fission. Hahn alone received the Nobel Prize for it in 1944.

Everything that followed, the Manhattan Project, the reactor, the bomb and much of the political history of the second half of the twentieth century, comes from that result.

The Earth had done it first. In 1972 French analysts noticed that uranium from the Oklo mine in Gabon was slightly depleted in uranium-235, below the 0.72 per cent that is otherwise invariant throughout the solar system. The explanation, proposed by Francis Perrin, is that about 1.7 billion years ago the ore body sustained natural nuclear chain reactions. At that time, because uranium-235 decays faster than uranium-238, natural uranium was about three per cent uranium-235, close to the enrichment used in modern power reactors, and groundwater in the porous ore served as the moderator. At least fifteen such zones have been identified. They ran intermittently, in pulses governed by water boiling away and returning, over a period of the order of hundreds of thousands of years.