uranium

Uranium is element 92, the heaviest chemical element that occurs on Earth in any real quantity. It is a dense silvery metal, weakly radioactive, and it has been the source of most of what humans know about the Atomic nucleus — not because anyone planned it that way, but because uranium was the thing that kept behaving strangely.

Properties

property	value
atomic number (Z)	92
standard atomic weight	238.028 91(3), since 1999
electron configuration	[Rn] 5f³ 6d¹ 7s²
block / period	f / 7 (actinide)
melting point	1408 K (1135 °C)
boiling point	4404 K (4131 °C)
density	19.1 g cm⁻³
state at 20 °C	solid

Three isotopes, all radioactive

Every uranium isotope is an alpha emitter. Two of them are primordial — old enough to have survived since before the Solar System formed — and that is the only reason uranium is still here.

isotope	atomic mass (Da)	amount fraction	half-life	decay
²³⁴U	234.040 950(8)	0.000 054(5)	2.455 × 10⁵ y	α, 100%
²³⁵U	235.043 928(8)	0.007 204(6)	7.04 × 10⁸ y	α, 100%
²³⁸U	238.050 79(1)	0.992 742(10)	4.468 × 10⁹ y	α 100%; spontaneous fission 0.0000545%

²³⁸U's half-life is close to the age of the Earth, so roughly half of the planet's original ²³⁸U is still around. ²³⁵U decays six times faster, so its share has been quietly falling for 4.5 billion years — natural uranium was much richer in it in the deep past, a fact with consequences (see Oklo, below).

²³⁴U is not primordial at all. It is a decay product of ²³⁸U, sitting at the abundance the two half-lives dictate.

Both long chains end in lead: ²³⁵U → ²⁰⁷Pb, and ²³⁸U → ²⁰⁶Pb. Those two clocks running side by side in the same mineral grain are the backbone of geochronology.

Discovery, and the accident that mattered more

Martin Heinrich Klaproth identified uranium in 1789, eight years after William Herschel found the planet it is named for. CIAAW notes the naming directly:

The name derives from the planet Uranus, which in Roman mythology was "Father Heaven".

Klaproth had an oxide, not the metal; Eugène-Melchior Péligot isolated metallic uranium in 1841.

The important moment came in 1896. Henri Becquerel was investigating the newly discovered X-rays and studying how uranium salts respond to light, when he found by accident that the salts emit penetrating radiation on their own, with no light needed. That was radioactivity, and it won him a share of the 1903 Nobel Prize in Physics "in recognition of the extraordinary services he has rendered by his discovery of spontaneous radioactivity". Uranium did not just illustrate nuclear physics — it started it.

Fissile, and what that costs

²³⁵U is the only naturally occurring nuclide that will sustain a fission chain reaction. It is also 0.7204% of natural uranium. Everything difficult about nuclear fuel follows from those two sentences.

• Most power reactors need uranium enriched from 0.7% to 3–5% ²³⁵U — low-enriched uranium (LEU).
• Some advanced designs want 7%, or close to 20%: high-assay LEU (HALEU).
• Commercial enrichment is done by spinning gaseous uranium hexafluoride in centrifuges. Laser separation is still in development.
• ²³⁸U is not wasted: it absorbs neutrons and beta-decays to plutonium-239, which is itself fissile. That is what a breeder reactor is for.

Where it comes from

Uranium is mined, and mining is concentrated. World production was 60,213 tonnes U in 2024, and about three-quarters of it came from three countries.

country	2023 (t U)	2024 (t U)	2024 share
Kazakhstan	21,109	23,270	39%
Canada	11,001	14,309	24%
Namibia	6,986	7,333	12%
Australia	4,693	4,598	—
Uzbekistan (est.)	4,000	4,000	—
Russia	2,710	2,738	—
world total	54,433	60,213

Over half of that — 52% in 2024 — now comes from in situ leach mining, where the ore is dissolved underground and pumped up rather than dug out.

Oklo: the reactor nobody built

In May 1972, routine mass spectrometry at the Tricastin enrichment plant in France found uranium hexafluoride from the Oklo mine in Gabon running at 0.60% ²³⁵U instead of the usual 0.72%. Some samples went as low as 0.44%. Seventeen percent of the fissile material was simply missing, which is the sort of discrepancy that gets investigated quickly.

The explanation was that the ore body had already been a reactor. About 1.7 billion years ago, when natural uranium was rich enough in ²³⁵U to go critical on its own, groundwater acted as a moderator and sixteen zones at Oklo sustained fission on and off for a few hundred thousand years, probably never exceeding 100 kW of thermal power. The CEA announced the finding on 25 September 1972. Oklo is still the only place this is known to have happened.

Because of Oklo, CIAAW's standard atomic weight for uranium carries the annotation "g" — the value applies to normal terrestrial sources except that one locality.

Related: chemical element · Isotope · fission · half-life · periodic table · hydrogen · Atomic nucleus · Nuclear binding energy