What you'll learn
This revision guide covers nuclear fission and nuclear fusion as specified in the AQA GCSE Physics specification. You'll understand how these nuclear processes release energy, their applications in power generation and stars, and their advantages and disadvantages. This topic links directly to atomic structure and radioactivity.
Key terms and definitions
Nuclear fission — the splitting of a large, unstable atomic nucleus into two smaller nuclei, releasing energy, neutrons and gamma radiation.
Nuclear fusion — the joining of two light atomic nuclei to form a heavier nucleus, releasing energy in the process.
Chain reaction — a self-sustaining series of fission reactions where neutrons released from one fission event trigger further fission events.
Control rods — materials (such as boron) that absorb neutrons in a nuclear reactor to control the rate of fission.
Moderator — a substance (such as water or graphite) that slows down neutrons in a nuclear reactor to increase the probability of further fission.
Radioactive waste — spent nuclear fuel and contaminated materials produced during fission reactions that remain radioactive for thousands of years.
Coolant — a fluid (often water or carbon dioxide) that transfers thermal energy from the reactor core to the steam generators.
Critical mass — the minimum amount of fissile material needed to sustain a chain reaction.
Core concepts
Nuclear fission process
Nuclear fission occurs when a large, unstable nucleus absorbs a neutron and splits into two smaller nuclei of roughly similar size. This process releases:
- Two or three neutrons
- Energy (mostly as kinetic energy of the fission products)
- Gamma radiation
The most commonly used fissile materials are uranium-235 and plutonium-239. When a uranium-235 nucleus absorbs a slow-moving neutron, it becomes uranium-236, which is highly unstable and immediately splits.
A typical fission equation:
U-235 + neutron → Ba-141 + Kr-92 + 3 neutrons + energy
The fission products (barium and krypton in this example) vary, but the process always releases neutrons and energy. The daughter nuclei produced are usually radioactive themselves and contribute to radioactive waste.
The energy released in fission comes from the conversion of mass into energy according to Einstein's equation E = mc². A small amount of mass is "lost" during the reaction and converted into a large amount of energy.
Chain reactions and nuclear reactors
In a chain reaction, the neutrons released from one fission event go on to cause further fission reactions in nearby uranium-235 nuclei. Each fission produces 2-3 neutrons, creating the potential for an exponential increase in reactions.
For a controlled chain reaction in a nuclear power station:
- Moderators slow down fast neutrons to thermal speeds, increasing the probability that they will be absorbed by uranium-235 nuclei
- Control rods absorb excess neutrons to maintain exactly one neutron from each fission causing one further fission event
- The reactor core is surrounded by thick concrete shielding to absorb radiation
- A coolant transfers thermal energy from the core to heat exchangers
The control rods can be raised or lowered:
- Lowered further into the core = more neutrons absorbed = slower reaction rate
- Raised out of the core = fewer neutrons absorbed = faster reaction rate
In an emergency, control rods drop fully into the reactor core to stop the chain reaction completely.
Nuclear power stations
Nuclear power stations generate electricity through controlled fission reactions. The process works as follows:
- Nuclear fission in the reactor core releases thermal energy
- The coolant absorbs this thermal energy and transfers it to water in a heat exchanger
- Water boils to produce steam
- Steam drives turbines connected to generators
- Generators produce electricity
- Steam is condensed back to water and recycled
Advantages of nuclear power:
- No carbon dioxide emissions during operation (unlike fossil fuel power stations)
- Very high energy density — small amounts of fuel produce large amounts of energy
- Reliable baseload power that doesn't depend on weather conditions
- Existing fuel reserves (uranium) are sufficient for many decades
Disadvantages of nuclear power:
- Produces radioactive waste that remains hazardous for thousands of years
- High initial construction costs and long build times
- Risk of catastrophic accidents (though extremely rare)
- Decommissioning old reactors is expensive and time-consuming
- Nuclear fuel is non-renewable
- Potential target for terrorism
- Creates materials that could be used for nuclear weapons
Radioactive waste management
Nuclear fission produces radioactive waste products with varying half-lives:
- Low-level waste (contaminated clothing, tools): stored in secure facilities until activity decreases
- Intermediate-level waste (reactor components): encased in concrete and stored
- High-level waste (spent fuel rods): the most dangerous category, requiring long-term storage solutions
High-level waste management strategies include:
- Cooling in water pools for several years
- Encasing in glass (vitrification) and steel containers
- Deep geological disposal in stable rock formations
- Monitored retrievable storage facilities
The waste remains radioactive for thousands of years, creating ethical questions about passing problems to future generations.
Nuclear fusion process
Nuclear fusion joins light nuclei together to form heavier nuclei, releasing energy in the process. This is the reaction that powers stars, including our Sun.
The most practical fusion reactions for energy generation involve hydrogen isotopes:
- Deuterium (hydrogen-2): one proton, one neutron
- Tritium (hydrogen-3): one proton, two neutrons
A typical fusion equation:
Deuterium + Tritium → Helium-4 + neutron + energy
For fusion to occur, nuclei must overcome the electrostatic repulsion between their positive charges. This requires:
- Extremely high temperatures (around 100 million °C) to give nuclei enough kinetic energy
- High pressure to increase the probability of collisions
At these temperatures, matter exists as plasma (a state where electrons are stripped from atoms).
Fusion in stars
Stars form from clouds of dust and gas (mainly hydrogen) drawn together by gravitational forces. As the material collapses:
- Gravitational potential energy converts to kinetic energy
- Temperature and pressure increase at the core
- When core temperature reaches approximately 10 million °C, fusion begins
- Energy released from fusion creates outward pressure
- This outward pressure balances gravitational collapse
A star remains stable for billions of years while it has sufficient hydrogen fuel. The energy released by fusion reaches Earth as electromagnetic radiation, providing warmth and light essential for life.
Our Sun fuses hydrogen nuclei into helium nuclei in a multi-step process called the proton-proton chain. More massive stars can fuse heavier elements in their cores.
Fusion as an energy source on Earth
Scientists have been attempting to develop controlled fusion reactors for decades. The potential advantages are enormous:
Advantages of fusion power:
- Fuel (hydrogen isotopes) is abundant — deuterium can be extracted from seawater
- No carbon dioxide emissions
- No long-lived radioactive waste (unlike fission)
- No possibility of runaway chain reactions
- Very high energy output per kilogram of fuel
Challenges preventing commercial fusion:
- Achieving and maintaining the extreme temperatures required (hotter than the Sun's core)
- Containing the plasma without it touching container walls (which would cool it instantly)
- Using magnetic fields or lasers to confine the plasma
- Energy input to start and maintain fusion currently exceeds energy output
- Extremely expensive experimental facilities required
Current experimental reactors like JET (Joint European Torus) and ITER (International Thermonuclear Experimental Reactor) aim to achieve sustainable fusion reactions. Despite significant progress, commercial fusion power stations remain decades away from operation.
Worked examples
Example 1: Explaining chain reactions (4 marks)
Question: Describe how a chain reaction is created and maintained in a nuclear reactor. [4 marks]
Answer:
- A neutron is absorbed by a uranium-235 nucleus [1 mark]
- The nucleus undergoes fission, releasing 2 or 3 neutrons [1 mark]
- These neutrons can be absorbed by other uranium-235 nuclei, causing further fission reactions [1 mark]
- Control rods and moderators maintain exactly one neutron from each fission causing one further fission, keeping the reaction steady [1 mark]
Example 2: Comparing fission and fusion (6 marks)
Question: Compare nuclear fission and nuclear fusion in terms of the processes involved and the nuclei used. [6 marks]
Answer: Fission:
- Large nucleus splits into two smaller nuclei [1 mark]
- Uses heavy nuclei such as uranium-235 or plutonium-239 [1 mark]
- Initiated by neutron absorption [1 mark]
Fusion:
- Two light nuclei join to form a heavier nucleus [1 mark]
- Uses light nuclei such as deuterium and tritium (hydrogen isotopes) [1 mark]
- Requires extremely high temperatures and pressures [1 mark]
Example 3: Energy from nuclear reactions (3 marks)
Question: Explain why both nuclear fission and nuclear fusion release energy. [3 marks]
Answer:
- In both processes, a small amount of mass is converted into energy [1 mark]
- This follows Einstein's equation E = mc² [1 mark]
- The products have slightly less mass than the reactants, and this "missing" mass is released as energy [1 mark]
Common mistakes and how to avoid them
Confusing fission with fusion: Remember fission = splitting (both words contain 'si'), fusion = joining (think of fusing two things together). Fission uses heavy nuclei like uranium; fusion uses light nuclei like hydrogen.
Thinking fusion is currently used for power generation: Fusion only occurs in experimental reactors and stars. All current nuclear power stations use fission. If a question asks about nuclear power stations, it refers to fission.
Forgetting that both processes release energy: Both fission and fusion release energy through mass being converted to energy (E = mc²). Don't state that one releases energy while the other absorbs it.
Misunderstanding control rod function: Control rods absorb neutrons to slow down the chain reaction. Lowering them further reduces the reaction rate; raising them increases it. They don't "release neutrons" or "speed up neutrons."
Confusing moderators with control rods: Moderators slow down neutrons to increase the probability of fission; control rods absorb neutrons to control the reaction rate. These are different components with different functions.
Overlooking the role of temperature in fusion: Always mention that fusion requires extremely high temperatures (tens of millions of degrees) to overcome electrostatic repulsion between nuclei. This is why fusion is so difficult to achieve on Earth.
Exam technique for "Nuclear fission and nuclear fusion"
Command words matter: "Describe" requires you to state features or characteristics; "explain" requires reasons or mechanisms. For a 4-mark "explain" question about chain reactions, you must describe the process AND explain how it continues.
Use correct terminology: Don't use vague terms like "atoms break apart." Use precise language: "nucleus undergoes fission" or "nuclei undergo fusion." Examiners reward scientific terminology.
Show both sides for evaluation questions: When asked to evaluate or discuss nuclear power, present both advantages (no CO₂, high energy density) and disadvantages (radioactive waste, high costs). Aim for balanced coverage unless the question specifies otherwise.
Link answers to context: If a question provides data about a nuclear power station's output or asks about a star's lifecycle, refer explicitly to the information given. AQA questions often include contexts requiring you to apply knowledge rather than just recall it.
Quick revision summary
Nuclear fission splits large nuclei (uranium-235) into smaller nuclei, releasing neutrons and energy. Chain reactions in nuclear reactors are controlled by moderators and control rods. Nuclear power produces no CO₂ but creates radioactive waste. Nuclear fusion joins light nuclei (hydrogen isotopes) into heavier nuclei, powering stars. Fusion requires extremely high temperatures and is not yet commercially viable on Earth, but offers advantages including abundant fuel and minimal waste. Both processes convert mass into energy according to E = mc².