Inside a nuclear reactor: understanding nuclear fission

A nuclear power plant boils water to spin a turbine, just like a coal plant. The difference is where the heat comes from: splitting uranium nuclei. Follow the energy from a single nucleus all the way to the electricity in your home.

Under 15Everything is made of tiny atoms, and each atom has a centre called the nucleus. Some big, heavy nuclei, like uranium, can be split in two by hitting them with a tiny particle called a neutron. When that happens, a lot of energy comes out as heat, and more neutrons fly out, which can split more nuclei. A nuclear power plant controls this very carefully, uses the heat to make steam, and the steam spins a machine that makes electricity.
20+ · going deeperEach U-235 fission releases about 200 MeV, mostly as fragment kinetic energy, with 2–3 prompt neutrons (ν̄ ≈ 2.4) and about 0.65% delayed neutrons from fragment decays. Thermal reactors moderate neutrons to ~0.025 eV, where the U-235 fission cross-section (~580 barns) is hundreds of times larger than at MeV energies. The effective multiplication factor keff = η f p ε PNL (six-factor formula); delayed neutrons stretch the effective generation time from ~10⁻⁴ s to ~0.1 s, which is what makes control with mechanical rods possible.
1 · Inside the atom2 · Chain reaction3 · Reactor tour4 · Control rods5 · Energy conversion6 · Radiation & safety7 · Compare sources8 · Knowledge check

1Inside the atom: one fission event

A slow neutron is absorbed by a uranium-235 nucleus, a fissile isotope. It becomes uranium-236 in an excited, wobbling state, which splits into two lighter fission fragments plus 2 or 3 new neutrons, releasing about 200 MeV of energy, mostly as the fragments' motion, which becomes heat. Each event is different: press the button again to see other possible fragment pairs. The animation is schematic: real nuclei are about 10⁻¹⁴ m across.

2Chain reaction explorer (conceptual model)

Each fission releases 2–3 neutrons, but not all of them cause another fission: some escape, and some are absorbed without splitting anything. The average number of new fissions caused by one fission is the multiplication factor k. Subcritical (k < 1): the chain dies out on average. Critical (k = 1): it is self-sustaining, steady power. Supercritical (k > 1): it grows. A power reactor is run at exactly critical.

This is a simplified random branching model to show the idea, not a neutron-transport simulation. Each run is different because fission is a random process.

3Tour the plant: click any part

Containment & reactor vessel water moderator fuel rods control rods hot coolantcooler water back steam generator turbinesteam generator ⚡ condenser cooling water
Click a part of the plantThis is a simplified pressurised water reactor (PWR), the most common type. Hot water from the core heats a separate water loop into steam, which turns the turbine.

4Control rods (qualitative demonstration)

Chain reaction is

Control rods contain neutron absorbers such as boron, cadmium or hafnium. Pushing them in soaks up more neutrons, so fewer cause fissions and the reaction slows; pulling them out lets it speed up. Operators keep the reactor just critical, and in an emergency all the rods drop in at once (a scram), shutting down the chain reaction in seconds.

Rod positions and the "reaction rate" scale here are illustrative, not the operating values of any real reactor. Fission heat from the fuel's radioactive products continues after shutdown, which is why cooling must keep running.

5From nucleus to socket: energy conversion

Nuclear energy (binding energy released in fission) → thermal energy (hot fuel and coolant) → mechanical energy (spinning turbine) → electrical energy (generator). Most of the heat cannot become electricity: a steam turbine is a heat engine, and the second law of thermodynamics limits its efficiency. The leftover heat goes to the condenser and its cooling water or cooling towers. Steam power stations of all kinds, coal, gas or nuclear, typically turn roughly a third of their heat into electricity.

The flow widths above are illustrative of a typical steam cycle, not measurements of any particular plant.

6Radiation and safety

α Alpha particles

Helium nuclei (2 protons, 2 neutrons). Heavy and slow, stopped by a sheet of paper or the outer layer of skin, but harmful if swallowed or breathed in.

paper ✋

β Beta particles

Fast electrons or positrons from nuclear decay. Pass through paper but are stopped by a few millimetres of aluminium or plastic.

paperaluminium ✋

γ Gamma rays

High-energy light (photons). Very penetrating: thick lead or concrete reduces them strongly, though never completely to zero.

paperaluminiumlead / concrete ✋

n Neutrons

Uncharged, so they pass through metal easily. Best slowed and absorbed by materials rich in hydrogen (water, concrete) plus absorbers like boron.

leadwater / concrete ✋

Defence in depth. Civilian reactors keep radioactive material behind several barriers: ceramic fuel pellets, sealed metal fuel cladding, the thick steel reactor vessel, and a reinforced-concrete containment building. Shielding protects workers, cooling systems (with backups) remove heat even after shutdown, and engineered safety systems such as automatic scrams and emergency core cooling act without needing an operator. Accidents such as Chernobyl (1986) and Fukushima (2011) showed what happens when these layers fail, and led to stricter designs and regulation worldwide.

7Compare energy sources

Every energy source has some environmental impact, from mining and manufacturing to land use and end-of-life. "Operational emissions" means emissions while generating, not over the whole life cycle.

8Knowledge check

Glossary

Fissile
A nucleus that can be split by slow (thermal) neutrons, such as U-235 or Pu-239.
Moderator
A material (usually water or graphite) that slows fast neutrons down, making them far more likely to cause fission in U-235.
Control rod
A rod of neutron-absorbing material moved in or out of the core to control the chain reaction.
Critical
The state where, on average, each fission causes exactly one more fission (k = 1): a steady chain reaction.
Coolant
The fluid (water in most reactors) that carries heat away from the core.
Decay heat
Heat from the radioactive decay of fission products, which continues after the chain reaction stops.
Containment
The sealed, reinforced building around the reactor that keeps radioactive material in during accidents.