Uranium (U)

Uranium is the heaviest element that Earth still has in quantity, and the only one whose behaviour has been continuously changing since the planet formed. That is not a figure of speech. Natural uranium is a mixture of two long-lived isotopes decaying at very different rates, so the composition of the ore in the ground today is measurably different from the composition of the same ore two billion years ago. Almost everything interesting about the element — how it is used, how it dates rocks, and why a reactor once switched itself on in West Africa — follows from that single drifting ratio.

A planet eight years old, and a name to match

Martin Heinrich Klaproth announced the element in 1789, working on pitchblende from the silver mines at Joachimsthal in Bohemia. William Herschel had found the seventh planet only eight years earlier, and Klaproth argued that a new world deserved commemoration in a new substance. The naming logic he invented — element after planet — held for a century and a half and produced neptunium and plutonium in turn when the table ran past uranium's square.

Klaproth believed he had isolated the metal. He had not; his black powder was an oxide. The metal itself did not exist as a substance anyone could handle until Eugène-Melchior Péligot reduced uranium tetrachloride with potassium in 1841, half a century later. The data card credits Klaproth, which is the conventional shorthand and is defensible — he identified a new element, which is the harder half of the job — but the metal is Péligot's.

Uranium was in use before either man, in a way neither would have recognised. Yellow glass tesserae from a Roman villa near Naples have been found to contain uranium oxide as the colourant, which puts a compound of element 92 in decorative work more than seventeen centuries before it was identified. The trade rediscovered the trick in the nineteenth century, and uranium glass — pale yellow-green, fluorescing sharply under ultraviolet — was mass-produced for tableware right up until wartime controls took the raw material away.

Two clocks running at different speeds

Uranium as it comes out of the ground is 99.27% uranium-238, with 0.72% uranium-235 alongside it and a trace of uranium-234 that is not a leftover from the solar nebula at all but a granddaughter of uranium-238, constantly regenerated in the ore.

The two main isotopes differ in half-life by a factor of more than six: 4.47 billion years against 704 million. Both were present when the solar system condensed, and the lighter one has been disappearing six times faster ever since. Run the arithmetic backwards and natural uranium was about 3% uranium-235 roughly two billion years ago — which is, to within rounding, the enrichment level that fuels a modern power reactor. The reason enrichment plants exist is that we arrived a couple of billion years too late.

That decaying ratio is also what makes uranium the backbone of deep-time geochronology. Two independent chains run in the same crystal, one ending at lead-206 and the other, from the lighter isotope, at lead-207 — which means a single grain of zircon carries two clocks that must agree. Where they disagree, the disagreement itself is informative, and the concordia diagram built from the pair is the most trusted dating method in geology. Clair Patterson used the lead isotopes it produces to date the Earth, in 1956, at roughly 4.55 billion years — a number that has barely moved since.

Gabon, and the reactor nobody built

In 1972 a routine assay at the Pierrelatte enrichment plant in France returned uranium-235 at 0.717% instead of the 0.720% that natural uranium always gives. The shortfall was tiny and absolutely consistent. Traced back, it led to the Oklo mine in Gabon, and to an explanation that sounded absurd until the fission products were found sitting in the rock.

Around 1.7 billion years ago, when the ore was still rich enough in uranium-235 to sustain a chain reaction, groundwater percolating through unusually concentrated seams acted as a neutron moderator. Sixteen separate zones went critical. They ran at an average of roughly a hundred kilowatts, on and off, for a few hundred thousand years — self-regulating, because as the water boiled away the reaction stalled until it cooled and the water returned. Analysis of trapped xenon isotopes has been used to argue the cycle was on the order of half an hour of operation followed by a couple of hours of quiet.

Oklo is the only place on Earth where this is known to have happened, and it has been studied intensively for an unglamorous modern reason: the fission products it made have sat in wet rock for well over a billion years, which makes the site the longest-running natural experiment in whether a geological repository can hold anything in place.

What the world actually does with it

Nearly all mined uranium becomes reactor fuel, and reactors supply roughly a tenth of global electricity. Nothing else uses the element in comparable tonnage.

The exception worth naming is depleted uranium — the uranium-238 left over after enrichment, of which there are hundreds of thousands of tonnes in storage. Its value is not nuclear but simply that it is extremely dense and cheap because it is a by-product. It has been used as ballast in aircraft control surfaces, as shielding for medical radiation sources, and, controversially, in armour and armour-piercing rounds, where its tendency to shed material and sharpen itself on impact is the ballistic attraction and the environmental objection at once.

Supply is concentrated in a way that surprises people who expect it to look like oil. Kazakhstan alone produces something over forty per cent of the world's uranium, with Canada, Namibia and Australia making up much of the rest; Australia holds the largest known reserves without leading production. Uranium's hazard profile is also frequently misread. For natural uranium the radiological risk is modest — the specific activity is low, precisely because the half-lives are so long — and the dominant toxicity is chemical, acting on the kidneys in the way other heavy metals do.