2.9.2Forces, Space and Radioactivity
Nuclear Fission Reactors
5 min read 5 parts Lesson 39 of 41 Unit quiz
Section 1 of 5Parts of a Reactor
The purpose of a reactor is to collect thermal energy from controlled fission and use it to make electricity.
Fuel rods
- These hold the fissile fuel, such as uranium-235, and become very hot as fission takes place.
Moderator
- Water or graphite slows down fast neutrons so that they are more likely to be absorbed and cause fission.
Control rods
- Rods of boron absorb neutrons, and they are raised or lowered between the fuel rods to control the reaction rate.
Coolant
- A liquid or gas pumped round the core carries thermal energy away to a heat exchanger.
Section 2 of 5Controlling the Reaction
Controlling the Reaction
Lowering the rods
- More neutrons are absorbed, so fewer cause fission and the rate of the reaction falls.
Raising the rods
- Fewer neutrons are absorbed, so more cause fission and the rate of the reaction increases.
Steady operation
- The rods are adjusted so exactly one neutron from each fission causes another fission.
Emergency shut-down
- In an emergency all control rods are fully lowered, which stops the chain reaction.
Section 3 of 5From Fission to Electricity
1
Fission in the fuel rods transfers energy to thermal energy and heats the coolant.
2
The hot coolant passes through a heat exchanger and turns water into high-pressure steam.
3
The steam turns the blades of a turbine, transferring thermal energy to kinetic energy.
4
The turbine drives a generator, which transfers kinetic energy to electrical energy.
5
The steam is then cooled and condensed, and the water returns to the heat exchanger to be used again.
Section 4 of 5Advantages and Disadvantages
Low carbon emissions
- Reactors do not release carbon dioxide while operating, so they do not add to the greenhouse effect.
Reliable output
- A reactor produces electricity continuously and is not affected by weather.
Radioactive waste
- Waste stays dangerous for thousands of years and must be stored safely, which is expensive and difficult.
Risk and cost
- An accident can release radioactive material, and building and decommissioning a power station is very costly.
Section 5 of 5Exam Checklist & Tips
2.9.2 – Key Points for the Exam
- Moderator A material that slows down neutrons so they can cause fission.
- Control rod A rod that absorbs neutrons to control the rate of fission.
- Coolant A fluid that removes thermal energy from the reactor core.
- Radioactive waste Material left over from nuclear power that remains radioactive.
Common Mistakes
- Saying control rods slow down the neutrons; the moderator slows them, while control rods absorb them.
- Thinking that raising the control rods slows the reaction; raising them speeds it up.
- Describing the reactor itself as making electricity; the turbine and generator do this.
- Writing that nuclear power is carbon-free in every respect, forgetting mining and construction.