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ATOMIC STRUCTURE · CHAPTER 11 OF 12
How does nuclear fission generate electricity?
One nucleus splits and releases neutrons. Those neutrons can cause more nuclei to split. How do we control that chain, remove the energy and turn it into electrical power?
By the end, you’ll be able to…
- describe neutron-induced fission and its products
- explain a chain reaction and how its rate can be controlled
- distinguish the roles of moderator, control rods, coolant and shielding
- trace energy transfers from fuel to generator
- use supplied energy and power data
- evaluate nuclear power using emissions, reliability, waste, cost and safety evidence.
Fission means splitting a nucleus
In nuclear fission, a heavy nucleus splits into two smaller nuclei and releases energy. Suitable uranium or plutonium nuclei can undergo fission. Spontaneous fission can happen, but in a reactor fission is usually induced by a nucleus absorbing a neutron.
For example, uranium-235 absorbs a neutron and briefly forms an excited uranium-236 nucleus. It can split, emitting further neutrons and gamma radiation. Different fissions can produce different fragments; two or three neutrons are commonly released.
Follow one induced fission
Watch a neutron enter a uranium-235 nucleus. The nucleus becomes unstable and splits, releasing two smaller nuclei, three neutrons and energy.
* Pink circles represent whole positively charged nuclei, not individual protons. White circles represent neutrons. The pink wavy line represents gamma radiation. Sizes, distances and speeds are not to scale.
Check the nucleon numbers
One possible fission channel is shown below. You do not need to memorise this particular pair of fragments.
Upper numbers
235 + 1 = 141 + 92 + 3
236 = 236
Lower numbers
92 + 0 = 56 + 36 + 0
92 = 92
* The equation balances nucleon number and charge. Gamma photons and the energy released are omitted from this number-check equation; gamma contributes zero to both rows.
Where does the released energy go?
The fission fragments and emitted neutrons move with kinetic energy. Gamma radiation also carries energy. Interactions with surrounding material transfer energy to the fuel and coolant, increasing their internal energy.
The products have a slightly smaller total rest mass than the starting nucleus and neutron. The difference corresponds to released energy. Balancing nucleon numbers does not mean that exact rest masses in kilograms are unchanged.
Does a chain reaction always grow?
Watch all three outcomes together. Pink circles represent nuclei undergoing fission; moving white circles represent neutrons.
* These are simplified examples. Real fission events release several neutrons, but not all cause further fissions. A nuclear reactor controls the chain reaction to maintain a steady rate of energy release.
Inside a nuclear power station
A controlled chain reaction releases energy in the reactor core. The station must carry that energy away safely and use it to generate electricity. First, explore the whole power station; then look more closely at the reactor.
How does a nuclear power station work?
Watch the whole power station working, or select a component to highlight it and discover its role.
* Simplified pressurised-water reactor design. The three fluid circuits remain separate. Colours indicate relative temperatures, not exact temperatures. Moving dots and steam clouds illustrate fluid flow, not individual molecules.
How do control rods keep a nuclear reactor under control?
The reactor's fission rate is increasing. Watch what happens when you lower the control rods into the core to absorb more neutrons.
Which explanation has mixed up the components?
From the reactor to electrical output
The generator does not receive energy directly from nuclei. Follow the energy transfers from reactor to generator.
The energy-transfer chain
Not all the released energy becomes electrical output. Some is transferred to the surroundings, including through the condenser and cooling system.
Link energy and power:
Compare useful output with input:
Percentage efficiency = efficiency × 100%
Use joules, watts and seconds in E = P × t, or use power values in the same units when calculating efficiency.
Three worked examples
Calculate electrical energy output:
A hypothetical station supplies 800 MW for 60 s. 800 MW = 800,000,000 W.
E = P × t
E = 800,000,000 × 60
E = 4.8 × 1010 J
Calculate percentage efficiency:
A plant receives 3000 MW from the reactor and supplies 1000 MW electrically.
Efficiency = useful power output ÷ input power × 100%
Efficiency = 1000 ÷ 3000 × 100%
Efficiency ≈ 33%
Calculate energy released:
Given 3.2 × 10−11 J per fission, find the release from 5.0 × 1012 fissions.
Total energy = number of fissions × energy per fission
E = (5.0 × 1012) × (3.2 × 10−11)
E = 160 J
* Supplied example values, not specifications of a particular power station. The third energy-per-fission value is given, not something to memorise.
What remains after the fuel has been used?
Fission fragments are usually neutron-rich and radioactive. Used fuel can contain these products, unused fuel and other radioactive nuclei formed in the reactor.
Waste must be contained, shielded and managed for periods appropriate to its activity and half-lives. Some products release substantial energy through radioactive decay after the chain reaction stops, so used fuel and the core can still need cooling.
Decommissioning includes making a retired station safe, dismantling equipment and managing contaminated or radioactive materials. Different wastes require different treatment and storage.
What should a balanced decision consider?
Compare benefits with limitations, then evaluate the evidence behind a claim.
Useful features
- Fission does not involve burning a carbon fuel, so the fission process itself does not emit carbon dioxide.
- A small mass of fuel can release a large amount of energy.
- Nuclear stations can provide substantial output for long periods without depending on sunshine or wind at the time.
Costs and responsibilities
- Building and decommissioning stations require substantial resources and planning.
- Radioactive waste requires secure long-term management; some materials remain hazardous for long periods.
- Accidents or loss of cooling can have serious consequences, so multiple safety systems and trained operation are essential.
- Mining, construction, fuel processing and transport have environmental impacts, including life-cycle emissions.
- Fuel is finite, maintenance can interrupt generation, and public concerns influence decisions.
“No carbon dioxide means no environmental impact.”
This conclusion is too broad. Distinguish direct operating emissions from the whole life cycle, and consider waste, land, water, resource use, costs and safety.
When comparing energy sources, consider environmental impact, reliability, costs and safety. A balanced conclusion should use scientific evidence and recognise both advantages and disadvantages.
Check your understanding
Choose one answer for each question. Feedback appears underneath.
“The moderator absorbs all the neutrons, so the generator gets energy directly from gamma rays. When control rods are inserted, cooling can stop immediately.” Correct the explanation.
Include the moderator, control rods, coolant, turbine, generator and radioactive fission products.
Reveal the answer
The moderator slows neutrons, making further fission in suitable U-235 nuclei more likely. Control rods absorb neutrons to regulate or stop the chain.
Energy from fission products and radiation increases the internal energy of the fuel and coolant. This energy is used to produce steam, which turns a turbine connected to a generator.
The generator transfers energy electrically; it does not directly collect electricity from gamma rays. Radioactive products continue to release decay heat after shutdown, so cooling is still needed.
CHAPTER 11
Chapter summary
- Absorbing a neutron can induce a heavy nucleus to split into two smaller nuclei, releasing energy, neutrons and gamma radiation.
- Emitted neutrons can cause further fissions: a chain reaction.
- Steady operation requires one further fission per fission on average.
- A moderator slows neutrons; control rods absorb them; coolant carries energy away.
- Steam drives a turbine connected to a generator; some energy reaches the surroundings.
- Radioactive fission products and decay heat require management after shutdown.
- Evaluate nuclear power using life-cycle emissions, reliability, waste, resources, cost, safety and evidence.