Section 1 of 5Main Components
Fuel rods
- Rods holding enriched uranium or plutonium, in which fission takes place and thermal energy is released.
Moderator
- Water or graphite slows the fast neutrons so they are more likely to be absorbed by uranium-235.
Control rods
- Neutron-absorbing rods, for example of boron, raised or lowered to control the rate of fission.
Coolant
- A fluid pumped through the core carries away thermal energy; in some designs it is also the moderator.
Shielding
- A thick steel vessel and concrete wall stop neutrons and gamma rays reaching workers.
Section 2 of 5From Fuel to Electricity
1
Fission in the fuel rods transfers nuclear energy to the kinetic energy of the fragments, which heats the fuel.
2
The coolant flows past the fuel and carries thermal energy to a heat exchanger.
3
In the heat exchanger water is boiled to produce high-pressure steam.
4
The steam spins the turbine, transferring thermal energy to kinetic energy of the turbine blades.
5
The turbine drives a generator, which transfers kinetic energy to electrical energy that is sent to the National Grid.
Section 3 of 5Controlling the Reaction
Controlling the Reaction
Raising rods
- Pulling control rods out means fewer neutrons are absorbed, so the rate of fission and the power output increase.
Lowering rods
- Pushing control rods in absorbs more neutrons, so the rate of fission and the power output decrease.
Emergency
- In an emergency all rods drop fully into the core to stop the chain reaction quickly.
Cooling after shutdown
- The fuel and fission products keep producing thermal energy after shutdown, so cooling must continue.
| Part | Job |
|---|---|
| Control rods | Absorb neutrons |
| Moderator | Slow neutrons |
| Coolant | Remove thermal energy |
| Turbine and generator | Produce electricity |
Section 4 of 5Safety Features
Containment
- The reactor is housed in a strong building designed to hold radioactive material if there is an accident.
Monitoring
- Sensors measure temperature, pressure and radiation levels continuously so that faults are detected early.
Handling fuel
- Spent fuel is moved by remote control and stored under water to absorb radiation and remove heat.
Regulation
- Operators work under strict safety rules and inspections to keep risks as low as possible.
Section 5 of 5Exam Checklist & Tips
6.4.3 – Key Points for the Exam
- Moderator A material in a reactor that slows down neutrons.
- Control rod A neutron-absorbing rod used to adjust the rate of fission.
- Coolant A fluid that removes thermal energy from the reactor core.
- Heat exchanger A device that transfers thermal energy from the coolant to water, producing steam.
- Reactor core The part of the reactor where the fuel rods are held and fission takes place.
Common Mistakes
- Saying the reactor burns uranium like a fire; the energy comes from fission, not combustion.
- Confusing the moderator with control rods; one slows neutrons and the other absorbs them.
- Leaving out the heat exchanger or turbine when describing the energy transfer chain.
- Thinking the reactor stops producing heat as soon as the chain reaction is stopped.