The discovery nobody expected
In December 1938 Otto Hahn and Fritz Strassmann bombarded uranium with neutrons in Berlin and found barium in the residue. Barium has 56 protons; uranium has 92. Nothing in the physics of the time allowed a nucleus to lose that much.
Hahn wrote to Lise Meitner, his collaborator of thirty years, who had fled to Sweden that summer to escape the Nazi race laws. Over Christmas, walking in the snow at Kungälv with her nephew Otto Frisch, she worked out what had happened: the nucleus had split in two, and the missing mass had become energy.
Meitner and Frisch published the interpretation in January 1939 and gave it a name borrowed from biology — fission.
⚠️ Hahn received the Nobel Prize in Chemistry in 1944, alone. Meitner, who had supplied the explanation, was not included. The omission is now generally regarded as one of the committee’s worst.
The number that decides everything
Natural uranium is 99.27 % uranium 238 and 0.72 % uranium 235. Only the 0.72 % will fission with slow neutrons.
That fraction is too small to sustain a chain reaction in most arrangements, so almost everything built around uranium exists to change it. A light water reactor needs 3 to 5 %. A weapon needs above 90 %.
Concentrating it means separating two atoms that are chemically the same. They form identical compounds, dissolve identically, react identically. The only handle is mass: 235 against 238, a difference of 1.3 %.
The Manhattan Project tried four methods at once because nobody knew which would work: gaseous diffusion, electromagnetic separation in calutrons, thermal diffusion, and centrifuges. The first three all contributed to Little Boy.
⚠️ Centrifuges won in the end and are what every enrichment programme uses now. A modern cascade runs thousands of machines at tens of thousands of revolutions per minute, each one shifting the balance by a fraction of a per cent.
Hiroshima
Little Boy was a gun-type device: two subcritical pieces of highly enriched uranium fired into one another down a barrel. Crude, heavy, and so certain to work that it was never tested before use.
It carried about 64 kilograms of uranium. Roughly one kilogram fissioned; the explosion scattered the rest before it could react.
The plutonium device tested at Trinity and dropped on Nagasaki was more efficient and far harder to build, requiring implosion to compress the core symmetrically. Uranium was the simpler path, and the harder material to obtain.
That trade-off still holds. Producing weapons-grade uranium is an industrial undertaking visible from orbit; turning it into a device is comparatively straightforward.
Why enrichment is a diplomatic problem
The same cascade produces reactor fuel and weapons material. Only the running time differs.
This is the difficulty at the centre of every negotiation over a nuclear programme. A country enriching to 5 % is doing something a hundred other countries do. The machinery to reach 90 % is the machinery it already has.
⚠️ The IAEA treats 20 % as the threshold for highly enriched uranium, not because a weapon at 20 % is practical but because the work from 0.72 % to 20 % is most of the separative effort. Past that point, the remaining step is comparatively short.
Inspection regimes therefore count centrifuges and measure material flow rather than trying to judge intent.
Oklo, and the reactor that ran itself
Uranium 235 decays faster than uranium 238 — 704 million years against 4,468 million. So the proportion was higher in the past.
About 1,700 million years ago it stood near 3 %, the same figure reactors are enriched to now. At Oklo in Gabon, groundwater seeped into an unusually rich deposit and acted as a moderator, and a chain reaction started on its own.
It ran intermittently for hundreds of thousands of years, boiling away its own moderator and restarting as the water returned, in cycles of roughly three hours.
It was found in 1972 because a routine sample assayed at 0.717 % instead of 0.720 %. The shortfall was uranium that had already been burned, two thousand million years before anyone thought of burning it.