Nuclear Fission

Unlocking the energy in the nucleus

The nucleus of an atom is astonishingly small — about 100,000 times smaller than the atom itself — yet it holds a staggering amount of energy. Nuclear fission is the process that unlocks some of it by splitting a heavy nucleus in two. A single kilogram of uranium fuel can release as much energy as burning several thousand tonnes of coal. This is the reaction that powers nuclear submarines, drives roughly a tenth of the world's electricity, and, in its uncontrolled form, made the atomic bomb.

To follow this lesson you'll need a clear picture of the nucleus from introduction to atoms and the idea of unstable nuclei from introduction to radioactivity.

What "fission" means

The word fission simply means splitting. In nuclear physics it refers to a large, heavy nucleus breaking into two smaller nuclei (called daughter nuclei), plus a few spare neutrons and a burst of energy.

Only certain nuclei undergo fission usefully. The star of the show is uranium-235 (a uranium isotope with 92 protons and 143 neutrons). Its nucleus is so large that it is barely held together, balanced on the edge of instability. Give it a small nudge and it will split.

That nudge is a neutron. Here is the sequence for induced fission:

1. A slow-moving neutron strikes a uranium-235 nucleus and is absorbed, briefly forming uranium-236.
2. This new nucleus is highly unstable — it wobbles, stretches, and splits into two smaller nuclei (for example barium and krypton).
3. The split also flings out two or three fast neutrons and releases a large amount of energy, mostly as kinetic energy of the fragments.

The chain reaction

Look again at step 3: each fission produces more neutrons than the one that started it. Those neutrons can go on to split other uranium-235 nuclei, each of which releases yet more neutrons, and so on. This self-sustaining process is a chain reaction.

One fission → 2–3 neutrons → each triggers another fission → 4–9 neutrons → and so on, multiplying rapidly.

If left unchecked, a chain reaction can grow explosively in a fraction of a second. The whole art of nuclear engineering is keeping it just barely self-sustaining — exactly one neutron from each fission going on to cause the next — so the energy is released steadily and safely.

Inside a nuclear reactor

A power-station reactor turns the chain reaction into a controllable heat source. Its key parts each have a clear job:

• Fuel rods — contain the uranium (enriched so a few percent is uranium-235). This is where fission happens.
• Moderator — a material such as water or graphite between the fuel rods. The neutrons released by fission are too fast to be easily absorbed, so the moderator slows them down, making further fissions likely.
• Control rods — rods of a neutron-absorbing material such as boron or cadmium. They are lowered into the core to soak up spare neutrons and slow the reaction, or raised to speed it up. Pushing them fully in shuts the reactor down.
• Coolant — usually water under high pressure, which carries the heat away.

The heat is used to boil water into steam, which spins a turbine connected to a generator — the same final step as any other thermal power station. You can see that final stage in how we generate electricity.

Where the energy comes from: E = mc²

Here is the deep part. If you carefully add up the mass of all the bits after a fission — the two daughter nuclei plus the neutrons — the total is very slightly less than the mass of the original uranium nucleus plus the neutron that hit it. A tiny amount of mass has disappeared.

It hasn't really been destroyed. It has been converted into energy, following Einstein's famous equation:

E = mc² energy = mass lost × (speed of light)²

The speed of light, c, is about 300,000,000 m/s, so c² is an enormous number (9 × 10¹⁶). This means even a microscopic loss of mass releases a colossal amount of energy.

Worked example. In one fission event, the mass lost ("mass defect") is about 3.2 × 10⁻²⁸ kg. How much energy is released?

E = mc² = (3.2 × 10⁻²⁸) × (3 × 10⁸)² = (3.2 × 10⁻²⁸) × (9 × 10¹⁶) = 2.88 × 10⁻¹¹ J per fission.

That sounds tiny — but a single gram of uranium contains over 10²¹ nuclei. Multiply that small energy by trillions of fissions and you get the staggering output of a reactor. For comparison, this is millions of times more energy per atom than any chemical reaction such as burning, because chemical reactions only rearrange electrons, never touching the far deeper energy of the nucleus.

The benefits and the costs

Fission is a remarkable energy source, but it comes with trade-offs:

• Pros: huge energy from a small amount of fuel; produces no carbon dioxide during operation, so it is a low-carbon way to make electricity; reliable, around-the-clock power.
• Cons: the daughter nuclei are radioactive waste, some staying hazardous for thousands of years; accidents (Chernobyl, Fukushima) can release radiation; building reactors is expensive; and the same technology can be misused for weapons.

Try it yourself! 🧪

You cannot split atoms at home, but you can model the heart of fission — the chain reaction — and feel why control is everything.

Demo — the domino (or mousetrap) chain reaction.

1. Stand a large number of dominoes upright, arranged so that when one falls it knocks over two or more others (a branching, tree-like layout rather than a single line).
2. Tip over the very first domino — this represents the first neutron starting a fission.
3. Watch the collapse race outward, each domino toppling several more, faster and faster. This is an uncontrolled chain reaction, like a bomb: once started, it runs away completely.
4. Now reset and place a few heavy books flat on the table as barriers between branches, removing some paths. Tip the first domino again. This time the reaction spreads more slowly and may even stop. The books are acting like control rods, absorbing the "neutrons" and limiting the spread.

The single line, the branching avalanche, and the version tamed by barriers capture the three states of a real reactor: shut down, runaway, and controlled. Keeping a reactor in that delicate middle state — exactly one new fission for each old one — is the whole job of a nuclear power station.