The 99 % that gets left behind
Enrichment concentrates uranium 235 from 0.72 % to a few per cent. What comes out of the other end of the cascade is uranium with less uranium 235 than it started with — depleted uranium, around 0.2 %.
The ratio is unforgiving. Producing a kilogram of reactor fuel leaves roughly seven kilograms of depleted uranium, and the world’s enrichment plants have been running since the 1940s.
Several hundred thousand tonnes are stored, mostly as uranium hexafluoride in steel cylinders on concrete pads. It is not waste in the regulatory sense — it has uses and could be used again — but there is far more of it than anyone needs.
⚠️ It is slightly less radioactive than natural uranium, because the enrichment removed the more active isotope. The hazard it presents is chemical: uranium is a heavy metal and toxic to the kidneys in the same way lead is.
Why armies use it
Depleted uranium is 19.1 grams per cubic centimetre, nearly twice the density of lead and comparable to tungsten. For a kinetic penetrator — a dart fired to punch through armour by momentum alone — density is the whole point.
It has a second property tungsten lacks. On impact, tungsten deforms and blunts; uranium shears at the edges and stays sharp. The technical term is adiabatic shear banding, and it means the penetrator keeps cutting instead of flattening.
It is also pyrophoric, so fragments burn on impact, which is a further effect inside a vehicle.
⚠️ The controversy is real but often misdirected. The radioactivity of depleted uranium is low; the concerns raised by the WHO and others are about inhaled dust after an impact and about heavy-metal toxicity in contaminated ground. Whether it has caused the illnesses attributed to it after the Gulf and Balkan wars remains disputed, and the epidemiology has not settled it.
How it dated the Earth
Uranium 238 decays to lead 206 through fourteen steps, with an overall half-life of 4.468 billion years — close enough to the age of the solar system that a useful fraction is still around.
Uranium 235 decays to lead 207 with a half-life of 704 million years. Two chains, two clocks, running in the same rock.
That redundancy is what makes the method trustworthy. If both give the same age, the sample has not lost or gained lead, and the answer can be believed.
Clair Patterson used it in 1956 on the Canyon Diablo meteorite, taking the view that meteorites formed at the same time as the Earth and had not been disturbed since. He obtained 4,550 million years.
⚠️ Getting there required inventing the modern clean room. Patterson found his samples swamped by environmental lead — and following that contamination to its source is what led him to leaded petrol, and eventually to its ban.
Fertile, not fissile
Uranium 238 will not sustain a chain reaction. But it absorbs neutrons, and what it becomes afterwards will.
U-238 + n → U-239 → Np-239 → Pu-239The two decays take about two days and a further two. Plutonium 239 is fissile, so it burns in the same reactor that made it.
This happens in every uranium reactor, continuously. By the end of a fuel cycle something like a third of the energy produced comes from plutonium bred in place rather than from the uranium 235 that was loaded.
⚠️ A breeder reactor is designed to make more fissile material than it consumes, by surrounding the core with a blanket of uranium 238. In principle it turns the 99 % into fuel. In practice the programmes have been slow, expensive and few, and most of the depleted uranium is still sitting on concrete pads.
The chain to lead
Uranium 238’s decay chain is the longest of the three natural series, and it passes through some familiar names.
U-238 → Th-234 → Pa-234 → U-234 → Th-230
→ Ra-226 → Rn-222 → ... → Pb-206Radium 226 is on it — the isotope Marie Curie isolated, and the one that gave the element its reputation. So is radon 222, the gas that seeps from granite bedrock into basements and is the second leading cause of lung cancer.
Every one of those is present in uranium ore in proportion to its half-life, which is why uranium mining and milling produce tailings that remain hazardous long after the uranium itself has gone.