Splitting the atom: nuclear fission
Nuclear fission is the process in which the nucleus of a heavy atom splits into two lighter nuclei. In a reactor the fuel is usually uranium-235. When a slow-moving neutron strikes a uranium-235 nucleus, the nucleus becomes unstable and breaks apart into two smaller nuclei, releasing 2 or 3 more neutrons and a large amount of energy as heat.
The chain reaction
Each fission releases extra neutrons, and these can strike other uranium-235 nuclei, causing them to split too. This self-sustaining process is called a chain reaction. If left uncontrolled it would release energy explosively, so a reactor must control the rate carefully.
Parts of a nuclear reactor
| Part | Job |
| Fuel rods | Contain uranium-235, the source of energy |
| Control rods (boron/cadmium) | Absorb spare neutrons to control the reaction rate |
| Moderator (water/graphite) | Slows down neutrons so fission continues |
| Coolant | Carries heat away to boil water into steam |
Example
To slow the reaction, control rods are pushed further into the reactor core. They soak up neutrons, so fewer nuclei split and less heat is produced. To speed it up, the rods are drawn out a little.
The difference between a reactor and a nuclear bomb is control. In a reactor the chain reaction is kept steady so that, on average, only one neutron from each fission goes on to cause the next fission. If the reaction were allowed to grow without limit, the energy would be released all at once. The control rods, moderator and coolant work together to keep the process safe and steady.
From heat to electricity
The heat released boils water into steam. The steam turns a turbine, which spins a generator to produce electricity — the same final steps as a coal power station, but with fission as the heat source.
Key idea
Fission: neutron + uranium-235 → two lighter nuclei + 2–3 neutrons + energy. The extra neutrons keep a controlled chain reaction going.