How does nuclear power work?
A nuclear power plant splits uranium-235 nuclei in a controlled chain reaction, and the heat from that fission turns water into steam. The steam spins a turbine connected to a generator, just as in a coal or gas plant; only the heat source is different.
Watch it explained
How does nuclear power work?
A neutron splits a uranium-235 nucleus and energy bursts out; the freed neutrons split more nuclei in a chain reaction; control rods lowered between the fuel rods absorb neutrons to keep it steady; the heat boils water into steam, which spins a turbine and a generator that sends electricity to a house.
Scenes, in order:
- One Atom Splits, Energy Bursts
- Fission Starts A Chain
- Control Rods Tame It
- Heat Makes Steam
- Turbine Spins, Electricity Flows
Splitting a nucleus releases energy
When a uranium-235 nucleus absorbs a slow neutron, it becomes unstable and splits into two lighter nuclei, plus two or three free neutrons. One common split is ²³⁵U + n → ¹⁴¹Ba + ⁹²Kr + 3n. The pieces together have slightly less mass than what went in, and that missing mass appears as energy, E = mc².
Each fission releases about 200 million electron volts, mostly as the kinetic energy of the fragments, which becomes heat in the fuel. Burning one carbon atom releases only a few electron volts, which is why a small amount of uranium fuel can run a plant for months.
Keeping the chain reaction steady
The free neutrons from one fission can split more nuclei. If on average exactly one neutron from each fission causes another, the reactor is critical and its power holds steady. Three parts keep it there:
- Fuel: uranium enriched to about 3 to 5% uranium-235, up from about 0.7% in natural uranium.
- Moderator: in most reactors, water. It slows neutrons down, and slow neutrons are far more likely to split uranium-235.
- Control rods: made of neutron absorbers such as boron or cadmium. Lowering them between the fuel rods absorbs neutrons and slows the reaction; raising them speeds it up.
Fuel this dilute cannot explode like a nuclear weapon, which needs highly enriched material and a fast, uncontrolled reaction.
From heat to electricity
In a pressurized water reactor, the most common type, the water around the core is kept under high pressure so it does not boil. It carries heat to a steam generator, where it boils water in a separate loop. That steam spins a turbine, the turbine turns a generator, and a condenser cools the steam back into water. The clouds over cooling towers are water vapor. Fission itself releases no carbon dioxide, but the used fuel stays radioactive for a long time and is stored in pools and then sealed casks.
On exams, keep fission (splitting heavy nuclei, used in power plants) apart from fusion (joining light nuclei, which powers the Sun). Both release energy because the products are more tightly bound than what went in.
The film on this page
The Isotoon video on this page was made from the question "How does nuclear power work?" It follows the chain above: a neutron splits a uranium-235 nucleus, the freed neutrons start a chain reaction, control rods drop in to steady it, and the heat makes steam that spins a turbine and a generator. A follow-up like "Why does splitting an atom release energy?" would continue it. For the background, see what an isotope is, what an atom is and the physics page.
Questions
- Why does splitting uranium release energy?
- The two smaller nuclei are more tightly bound than the uranium nucleus, so the products have slightly less mass than the starting nucleus plus neutron. That mass difference is released as energy according to E = mc².
- What do control rods do in a nuclear reactor?
- They absorb neutrons. Lowering them into the core leaves fewer neutrons to cause new fissions, which slows the chain reaction; pulling them out lets it speed up.
- Is nuclear power fission or fusion?
- Every commercial nuclear power plant today uses fission, the splitting of heavy nuclei such as uranium-235. Fusion power is still experimental.