Hold up a single glossy pellet of nuclear fuel — a cylinder of uranium about the size of the top joint of your little finger. It weighs only a few grams. Yet the energy locked inside it, if you could unleash it all, matches roughly a tonne of coal. A handful of these pellets can keep a family's lights, heating and hot water running for years.
Where is all that energy hiding? Not in the chemistry — you are not going to burn uranium
like coal or petrol. The energy is buried in the very heart of the atom, in the tightly-packed
Most nuclei are perfectly stable and refuse to break. But a few enormous ones — above all uranium-235 and plutonium-239 — are so big and so crammed with protons and neutrons that they are only just holding themselves together. Give one of them the gentlest nudge and it will fly apart. That nudge is a single passing neutron.
Here is the sequence, step by step:
Almost all of that energy appears as the kinetic energy of the two fragments:
they fly apart at enormous speed, crash into the surrounding material, and their motion becomes
heat. That heat is the prize — it is what we use to make electricity. Like the
radioactive nuclei you met in
Just as with radioactive decay, a fission reaction must balance: the mass numbers (top) and the atomic numbers (bottom) each add up to the same total on both sides of the arrow. A typical fission of uranium-235 is:
Let's check that it balances.
So one uranium-235 nucleus has become a barium nucleus and a krypton nucleus — two entirely different elements — plus 3 spare neutrons ready to cause trouble elsewhere. Uranium fission does not always split the same way (sometimes you get caesium and rubidium, or other pairs), but every route obeys the same two balancing rules, and every route hands back 2 or 3 neutrons.
Add up the mass of everything after the split — the two fragments and the spare neutrons — and weigh it against the uranium and neutron you started with. The products are very slightly lighter. A tiny sliver of mass has simply vanished. It has not been destroyed — it has been converted into energy, according to the most famous equation in physics.
Mass and energy are two faces of the same thing. A mass
The lost mass in a single fission is unimaginably small, yet
Here is the clever part. Each fission is triggered by a neutron — and each fission releases two or three more. Those fresh neutrons can dive into neighbouring uranium nuclei and split them, releasing yet more neutrons, which split still more nuclei… A single event can set off a self-sustaining cascade called a chain reaction.
Everything now depends on how many of the released neutrons go on to cause the next fission. Use the controls below. In Runaway mode, more than one neutron per split succeeds, so the number of fissions doubles every generation — 1, 2, 4, 8, … — an exponential blow-up in a fraction of a second. In Controlled mode, spare neutrons are mopped up so that exactly one neutron per split goes on: the reaction ticks over steadily, generation after generation, without ever exploding.
A branching diagram of a nuclear chain reaction. An incoming neutron splits the first nucleus. In controlled mode a single line of fissions marches steadily across the generations; in runaway mode each fission triggers two more, so the count doubles each generation — 1, 2, 4, 8.
That single switch — steady versus doubling — is the difference between a power station quietly heating water and a nuclear weapon. Both run on exactly the same fission; only the neutron bookkeeping differs.
A nuclear reactor is a machine whose entire job is to hold a chain reaction at the "steady" setting — never dying out, never running away — and to carry the heat off to make electricity. It has four key parts:
The chain runs like this: fission heats the fuel → the coolant carries that heat to a boiler → the heat boils water into steam → the steam drives a turbine → the turbine spins a generator → out comes electricity. The nuclear part only replaces the coal fire; from the boiler onwards, a nuclear power station is just an ordinary steam power station.
Fission looks almost too good — a pellet the size of a fingertip out-punching a tonne of coal, and no carbon dioxide pouring into the sky. But it comes with a serious problem. The daughter nuclei left behind are themselves radioactive, and many have very long half-lives — they will keep emitting dangerous radiation for hundreds or even thousands of years.
This nuclear waste cannot simply be thrown away. It has to be sealed up, shielded, and stored safely for a very long time — an engineering and political headache that every country using nuclear power has to solve. Weighing the enormous, low-carbon energy against the long-lived waste is one of the big real-world debates about how we should make electricity.
Burning coal shuffles the electrons around atoms — a chemical change that squeezes out a
few electronvolts of energy per atom. Fission converts a scrap of mass from the nucleus,
and thanks to that enormous
Do the sums and completely fissioning just one kilogram of uranium-235 releases roughly the same energy as burning two to three thousand tonnes of coal — enough to run a large power station for hours from a lump you could hold in one hand. That is why a single truckload of nuclear fuel can do the work of whole trainloads of coal.
In 1938 the chemists Otto Hahn and Fritz Strassmann fired neutrons at uranium in Berlin and were baffled to find barium — a much lighter element — in the leftovers. It made no sense until the physicist Lise Meitner, forced to flee Nazi Germany, worked out with her nephew Otto Frisch what had really happened over that Christmas: the uranium nucleus had split in two. Frisch borrowed a word from biology, where a living cell divides, and called it fission.
Within a few short years that quiet laboratory puzzle had become both nuclear power and the atomic bomb — one of the fastest, and most fateful, journeys from pure curiosity to world-changing technology in all of science.