Nuclear Fission and Energy

This lesson covers nuclear fission, the process by which heavy nuclei split to release energy, how a chain reaction is sustained and controlled in a nuclear power station, and the advantages, disadvantages and challenges of nuclear energy. This is the final topic in the Particle Model and Radioactivity section of the Edexcel GCSE Combined Science specification (1SC0).

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What Is Nuclear Fission?

Nuclear fission is the splitting of a large, unstable nucleus into two smaller nuclei (called daughter nuclei or fission fragments), accompanied by the release of:

• Two or three neutrons
• A large amount of energy (as kinetic energy of the fragments and neutrons, and gamma radiation)

The Process

1. A slow-moving (thermal) neutron is absorbed by a large, unstable nucleus (e.g. uranium-235 or plutonium-239).
2. The nucleus becomes even more unstable.
3. The nucleus splits into two smaller daughter nuclei of roughly equal size.
4. Two or three neutrons are released.
5. A large amount of energy is released.

graph LR
    N["Slow neutron"] --> U["Uranium-235<br/>nucleus"]
    U --> F1["Daughter<br/>nucleus 1"]
    U --> F2["Daughter<br/>nucleus 2"]
    U --> N1["Neutron"]
    U --> N2["Neutron"]
    U --> N3["Neutron"]
    U --> E["Energy<br/>(kinetic + gamma)"]

Example Fission Equation

$^{235}_{92}\text{U} + ^{1}_{0}\text{n} \rightarrow ^{141}_{56}\text{Ba} + ^{92}_{36}\text{Kr} + 3\;^{1}_{0}\text{n} + \text{energy}$92235​U+01​n→56141​Ba+3692​Kr+301​n+energy

Note: The mass numbers and atomic numbers are conserved (they balance on both sides).

Exam Tip: You do not need to remember specific fission equations, but you must be able to complete or balance one if given partial information. Check that the mass numbers add up on both sides and the atomic numbers add up on both sides.

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Chain Reactions

The neutrons released by one fission event can be absorbed by other uranium-235 nuclei, causing them to undergo fission too. Each fission releases more neutrons, which cause more fissions, and so on. This is a chain reaction.

graph TD
    A["1 fission event"] --> B["2-3 neutrons released"]
    B --> C["Each neutron causes<br/>another fission"]
    C --> D["More neutrons released"]
    D --> E["Chain reaction<br/>continues"]

Types of Chain Reaction

Type	Description	Result
Controlled	On average, exactly one neutron from each fission goes on to cause another fission	Steady release of energy (nuclear power station)
Uncontrolled	Every released neutron causes further fissions; the number of fissions increases exponentially	Massive, rapid release of energy (nuclear explosion)

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Nuclear Power Stations

A nuclear power station uses controlled fission to generate electricity.

Key Components

Component	Function
Fuel rods	Contain the fissile material (usually enriched uranium-235 or plutonium-239)
Control rods	Made of a neutron-absorbing material (e.g. boron or cadmium); inserted or withdrawn to control the rate of fission
Moderator	Slows down the fast neutrons produced by fission to thermal (slow) speeds so they are more likely to be absorbed by uranium-235 nuclei. Usually water or graphite.
Coolant	Carries thermal energy away from the reactor core to the heat exchanger. Often water (which can also act as the moderator).
Heat exchanger	Transfers thermal energy from the coolant to water in a separate circuit, producing steam.
Turbine and generator	Steam drives a turbine, which turns a generator to produce electricity.
Thick concrete shielding	Surrounds the reactor to absorb radiation and protect workers.

graph LR
    A["Fuel rods<br/>(fission)"] --> B["Coolant carries<br/>thermal energy"]
    B --> C["Heat exchanger<br/>(produces steam)"]
    C --> D["Turbine<br/>(spins)"]
    D --> E["Generator<br/>(electricity)"]
    F["Control rods"] -->|"absorb neutrons<br/>to control rate"| A
    G["Moderator"] -->|"slows neutrons"| A

How Control Rods Work

• Lowering the control rods further into the reactor absorbs more neutrons → fewer fissions → less energy released → reactor power decreases.
• Raising the control rods absorbs fewer neutrons → more fissions → more energy released → reactor power increases.
• In an emergency, control rods are fully inserted (SCRAM) to shut down the reactor.

Exam Tip: A common question asks you to explain the role of the moderator and control rods. The moderator slows neutrons (so they can be absorbed); the control rods absorb neutrons (to control the rate of fission). Do not confuse the two.

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Why Must Neutrons Be Slowed Down?

Fast neutrons produced during fission are too fast to be easily absorbed by uranium-235 nuclei. The moderator slows them down to thermal speeds through collisions, making capture by U-235 much more likely.

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Advantages and Disadvantages of Nuclear Power

Advantages	Disadvantages
No greenhouse gases produced during operation (no CO₂)	Produces radioactive waste that remains hazardous for thousands of years
Very high energy density — a small amount of fuel produces a huge amount of energy	High cost of building and decommissioning nuclear power stations
Reliable — can generate electricity continuously (not dependent on weather)	Risk of nuclear accidents (e.g. Chernobyl, Fukushima)
Reduces dependence on fossil fuels	Uranium is a non-renewable resource (though supplies will last a long time)
Relatively small land footprint compared to wind or solar farms for the same output	Public concern and political opposition

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Nuclear Waste

Nuclear power stations produce radioactive waste. This waste is categorised by its level of radioactivity.

Category	Activity level	Half-life	Examples	Disposal
Low-level waste (LLW)	Low	Short	Protective clothing, tools, paper	Sealed in containers and buried in shallow landfill sites
Intermediate-level waste (ILW)	Moderate	Moderate	Reactor components, chemical sludges	Encased in concrete and stored in deep underground facilities
High-level waste (HLW)	Very high	Long (thousands of years)	Spent fuel rods	Cooled in water pools, then vitrified (turned into glass) and stored deep underground in secure facilities

The Problem with High-Level Waste

• HLW remains dangerously radioactive for thousands of years.
• It generates a lot of heat and must be cooled before storage.
• There is currently no universally agreed long-term disposal method.
• It must be stored securely to prevent contamination of the environment.

Exam Tip: When discussing nuclear waste, always mention the long half-lives of the waste products and the challenge of storing them safely for extended periods. This shows higher-level thinking.

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Nuclear Fission vs Burning Fossil Fuels

Feature	Nuclear fission	Burning fossil fuels
Fuel	Uranium-235 or plutonium-239	Coal, oil, natural gas
Greenhouse gases?	None during operation	Large amounts of CO₂
Waste	Radioactive (small volume but hazardous)	CO₂, SO₂, particulates (large volume)
Energy per kg of fuel	Very high (~80 000 000 MJ/kg for U-235)	Low (~30 MJ/kg for coal)
Fuel availability	Limited but long-lasting reserves	Finite and depleting faster
Risk	Nuclear accident; waste storage	Climate change; air pollution

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Common Misconceptions