How Nuclear Power Stations Work

A nuclear power station uses controlled nuclear fission to generate electricity.

The fission process

1. A uranium-235 (or plutonium-239) nucleus absorbs a slow-moving neutron.
2. The nucleus becomes unstable and splits into two smaller nuclei (fission fragments).
3. Two or three neutrons are released, along with a large amount of energy.
4. The released neutrons can cause further fission events: a chain reaction.

Components of a nuclear reactor

Fuel rods: Contain enriched uranium (U-235). This is where fission occurs.

Control rods: Made of boron or cadmium (materials that absorb neutrons). Inserting control rods deeper absorbs more neutrons and slows the chain reaction. Withdrawing them allows more neutrons to cause fission, increasing the reaction rate.

Moderator: Usually water or graphite. Slows down the fast neutrons released by fission to the correct speed for absorption by U-235 nuclei. Slow (thermal) neutrons are more likely to be captured and cause fission.

Coolant: Usually water or CO$_2$ gas. Carries thermal energy away from the reactor core to a heat exchanger, where it boils water to produce steam.

Shielding: Thick concrete walls surround the reactor to absorb radiation and protect workers.

From reactor to electricity

Reactor $\rightarrow$ heat exchanger $\rightarrow$ steam $\rightarrow$ turbine $\rightarrow$ generator $\rightarrow$ electricity.

Advantages and disadvantages

• No CO$_2$ emissions during operation (low carbon).
• Reliable and high-power output (good for base load).
• Radioactive waste is produced (requires long-term storage).
• Risk of accidents (very low probability but high consequence).
• Decommissioning is expensive.

Common errors and how to correct them

• Saying control rods slow down neutrons. Control rods ABSORB neutrons. The MODERATOR slows them down.
• Confusing the moderator and the coolant (they can be the same substance, e.g. water, but they have different roles).

How to apply this in an exam

Name and explain the role of each component (fuel rods, control rods, moderator, coolant, shielding). Describe the chain reaction and how it is controlled.