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7.3.3Radioactivity and Particles

Nuclear Reactors — Edexcel International GCSE Science Double Award 4SD0 - Physics Revision Notes & Topical Questions

5 min read 9 parts Lesson 15 of 17

📚 Edexcel International GCSE Science Double Award 4SD0 - Physics Hub / Topical Revision
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Part 1 of 7The job of a reactor

A nuclear reactor is designed to sustain a controlled chain reaction in which, on average, exactly one neutron from each fission causes another fission.

The job of a reactor
The thermal energy released is used to produce steam, which turns a turbine connected to a generator.
The reactor core is therefore a heat source in the same way a boiler is in a coal-fired power station.

Part 2 of 7Main components

PartMaterial (typical)Function
Fuel rodsPellets of enriched uranium oxide in metal tubesContain the fissile material; fission occurs here and heats the rods
ModeratorGraphite or waterSlows fast neutrons by collisions so that they can cause further fission
Control rodsBoron or cadmiumAbsorb neutrons to control the rate of fission
CoolantWater, carbon dioxide gas or liquid sodiumCarries thermal energy from the core to the heat exchanger
ShieldingThick concrete, steelAbsorbs radiation so workers and the environment are protected

Part 3 of 7How the reaction is controlled

The control rods can be raised or lowered into the core.

Lowered
When the rods are lowered further in, they absorb more neutrons, fewer neutrons are left to cause fission, and the reaction slows.
Raised
When they are raised, more neutrons survive, and the reaction speeds up.
In an emergency, all the control rods are dropped fully into the core, absorbing so many neutrons that the chain reaction stops (a "shutdown" or "scram").

The moderator is necessary because the neutrons released in fission move too fast to be readily absorbed by uranium-235. They lose energy in collisions with the nuclei of the moderator, which should absorb few neutrons themselves, and become slow "thermal" neutrons.

Part 4 of 7From fission to electricity

1
Fission in the fuel rods transfers energy to thermal energy in the fuel and moderator.
2
The coolant is pumped past the rods, takes up the thermal energy and becomes hot.
3
In a heat exchanger the hot coolant transfers its thermal energy to water, turning the water into high-pressure steam.
  • The coolant circuit is separate so that radioactive coolant does not go through the turbine.
4
The steam turns a turbine, which turns a generator; electricity is produced.
5
The steam is condensed in a cooling tower or by river or sea water, and the water returns to the heat exchanger.

Overall efficiency is often around 35 to 40%, with the rest of the energy wasted as thermal energy to the surroundings.

Part 5 of 7Safety, waste and comparison

Reactors are enclosed by a thick steel pressure vessel and a concrete containment building, so that in an accident radioactive material is kept in.

Safety, waste and comparison
The fuel must be changed from time to time, since fission fragments build up and absorb neutrons.
Used fuel is highly radioactive and hot: it is stored in water ponds for years to cool,
  • then reprocessed or sealed and buried.
    • (see the lesson on dangers of radiation)

Compared with fossil fuel stations, reactors release no carbon dioxide in operation and need very little fuel, but they cost more to build and decommission, and the waste must be managed for thousands of years.

Part 6 of 7Following the energy and the neutrons

Think of the reactor as two linked flows.

Neutrons
The first is the flow of neutrons: they are released fast, slowed by the moderator, absorbed by uranium-235 to cause fission,
  • or captured by the control rods.
The number of neutrons available is adjusted using the control rods.
Energy
The second is the flow of energy: fission, thermal store of the fuel, coolant, steam, turbine (kinetic), generator (electrical).
At each stage some energy is wasted as heat, so the overall efficiency is well under 100%.

In exams, learn each component with its purpose and a typical material, and say what would happen without it: no moderator means few neutrons cause fission; no control rods means the reaction could speed up; no coolant means the core would overheat and possibly melt; no shielding means harmful radiation escapes.

Part 7 of 7Worked examples

  1. A reactor is running too fast and its output power is rising. Explain what an operator should do with the control rods and why.

  2. A power station has a reactor core producing thermal power of 3000 MW. The electrical output is 1050 MW. Calculate the efficiency and the power wasted.

  3. Why must the moderator be made of a material that absorbs very few neutrons?

Key terms

TermMeaning
Fuel rodTube containing enriched uranium in which fission takes place.
ModeratorMaterial that slows down fast neutrons in a reactor.
Control rodNeutron-absorbing rod raised or lowered to control the fission rate.
CoolantFluid that carries thermal energy away from the reactor core.
Heat exchangerDevice transferring thermal energy from the coolant to water to make steam.
ShieldingThick concrete and steel absorbing radiation from the reactor.

Avoid these mistakes

1
Confusing the moderator and the control rods. The moderator slows neutrons; the control rods absorb them.
2
Saying control rods are made of uranium. They are made of neutron absorbers such as boron or cadmium.
3
Thinking the coolant is only for safety. It also transfers thermal energy to make steam.
4
Thinking a reactor is like a bomb. In a reactor the reaction is held steady, not allowed to run away.
5
Forgetting the shielding or the sealed containment, which protect people from radiation.

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