AP® Environmental Science · Unit 6 · Topic 6.6
Nuclear Power
Nuclear power splits uranium atoms in a controlled chain reaction, and the heat that reaction releases boils water to spin a turbine, the same way a coal plant does, but with a fuel source that packs far more energy per gram.
A uranium-235 atom splits when it absorbs a neutron, breaking into two smaller atoms, a few free neutrons, and a burst of energy. Each fission event releases roughly a million times more energy than breaking a chemical bond does, because that energy comes from the nucleus itself rather than from electrons being shared or transferred. That's why a fingertip-sized pellet of uranium fuel can match the energy of a ton of coal: you're tapping a far denser energy source, not a stronger force.
The freed neutrons go on to split more uranium atoms, and that chain reaction is what a reactor is built to control. Control rods, made of materials like boron or cadmium that absorb neutrons, slide in and out of the core to speed up, slow down, or fully stop the reaction. Inserting them fully drops the reaction below the point where it sustains itself, shutting fission down within seconds. The heat fission produces boils water into steam, the steam spins a turbine, and the turbine spins a generator, the exact same electricity-making step used in a coal or natural gas plant.
Reactor fuel is only enriched to a few percent U-235, far below the enrichment a weapon needs, so even a runaway reaction stays slow compared to an explosive chain reaction. That's why a meltdown is a thermal and structural disaster, not a nuclear explosion. The real danger after a shutdown is decay heat: the fission products left in the fuel keep radioactively decaying and releasing heat for hours or days, so cooling has to continue long after the chain reaction itself has stopped.
Key terms
- Fission
- The splitting of a heavy nucleus like uranium-235 into smaller atoms, releasing neutrons and energy.
- Chain reaction
- A self-sustaining sequence where neutrons from one fission event trigger further fission events.
- Control rods
- Neutron-absorbing rods inserted into a reactor core to slow or stop the chain reaction.
- Decay heat
- Heat released by the continued radioactive decay of fission products even after the chain reaction stops.
- Enrichment
- The process of increasing the proportion of fissile U-235 in uranium fuel.
Watch it explained
- How it worksWalk me through a fission chain reaction and the control rods that keep it steady.
- How it worksHow does a reactor turn splitting atoms into steam, turbine spin, and electricity?
- Common mistakeWhy doesn't a nuclear reactor explode like a bomb when it fails?
- Going furtherWhat happens inside a core when the cooling water is lost and decay heat keeps building?
Check yourself
End of topic.