What you'll learn
This guide covers nuclear fission and chain reactions as specified in the AQA GCSE Physics curriculum. You'll learn how heavy nuclei split to release energy, understand the conditions needed for chain reactions, and explore how nuclear reactors control these processes. These concepts are essential for questions worth up to 6 marks in your exam.
Key terms and definitions
Nuclear fission — the splitting of a large, unstable atomic nucleus into two smaller nuclei of roughly equal size, releasing energy and neutrons
Chain reaction — a self-sustaining sequence of fission reactions where neutrons released from one fission event cause further fission events
Control rods — rods made of neutron-absorbing material (such as boron) that regulate the rate of fission in a nuclear reactor
Moderator — a material (usually water or graphite) that slows down fast-moving neutrons so they can cause further fission
Critical mass — the minimum mass of fissile material needed to maintain a self-sustaining chain reaction
Uranium-235 — a fissile isotope of uranium that readily undergoes fission when struck by slow neutrons
Nuclear fuel — fissile material (typically uranium-235 or plutonium-239) used in reactors to sustain fission reactions
Daughter nuclei — the two smaller nuclei produced when a larger nucleus undergoes fission
Core concepts
The process of nuclear fission
Nuclear fission occurs when a large unstable nucleus absorbs a neutron and splits into two smaller nuclei. The most commonly used fissile material is uranium-235 (U-235).
When a slow-moving neutron collides with a U-235 nucleus:
- The nucleus becomes unstable and splits into two smaller nuclei (daughter nuclei)
- Two or three fast-moving neutrons are released
- Energy is released in the form of kinetic energy of the products
- Gamma radiation is also emitted
The daughter nuclei produced are radioactive and have roughly equal mass. Common products include barium-141 and krypton-92, though various combinations are possible.
A typical fission equation looks like this:
U-235 + neutron → Ba-141 + Kr-92 + 3 neutrons + energy
The mass of the products is slightly less than the mass of the original nucleus plus the neutron. This "missing" mass has been converted into energy according to Einstein's equation E = mc². Even a tiny mass converted produces enormous amounts of energy because c² (the speed of light squared) is such a large number.
Why fission releases energy
The key to understanding energy release lies in binding energy per nucleon. This measures how tightly nucleons (protons and neutrons) are held together in a nucleus.
Iron-56 has the highest binding energy per nucleon of all elements. Nuclei lighter or heavier than iron have lower binding energy per nucleon.
When a heavy nucleus like uranium-235 splits into medium-sized nuclei:
- The daughter nuclei have higher binding energy per nucleon than uranium
- The nucleons are more tightly bound in the products
- The "extra" binding energy is released as kinetic energy
- This energy appears as movement of the products and gamma radiation
For GCSE purposes, you need to know that fission releases energy because the products are more stable than the original nucleus, not memorise complex binding energy graphs.
Chain reactions: controlled and uncontrolled
A chain reaction begins when neutrons from one fission event cause further fission events. Since each fission of U-235 releases 2-3 neutrons, and each neutron can potentially cause another fission, the number of fission events can increase rapidly.
Uncontrolled chain reactions occur when:
- Each fission event leads to multiple further fissions
- The reaction rate increases exponentially
- Enormous energy is released in a fraction of a second
- This is what happens in a nuclear weapon
Controlled chain reactions occur when:
- On average, only one neutron from each fission causes another fission
- The reaction proceeds at a steady, constant rate
- Energy is released in a controlled, usable way
- This is what happens in a nuclear power station
For a chain reaction to be self-sustaining, the fissile material must exceed the critical mass. Below this mass, too many neutrons escape from the surface without causing fission, and the reaction dies out.
Nuclear reactors and control mechanisms
Nuclear power stations use controlled chain reactions to generate electricity. The reactor core contains several key components:
Nuclear fuel rods contain enriched uranium (typically 3-5% U-235, compared to 0.7% in natural uranium). The fuel is arranged in rods to maximise surface area.
Control rods are made from materials that absorb neutrons, such as boron or cadmium. They serve crucial functions:
- Lowering them into the reactor absorbs more neutrons, slowing the reaction
- Raising them allows more neutrons to cause fission, speeding up the reaction
- Fully inserting them stops the chain reaction (emergency shutdown)
- Their position is continuously adjusted to maintain steady power output
The moderator slows down fast neutrons released by fission. Slow neutrons are much more likely to cause fission in U-235 than fast neutrons. Common moderators include:
- Water (also serves as coolant in many reactors)
- Graphite (used in Advanced Gas-Cooled Reactors)
The coolant transfers thermal energy from the reactor core. In the UK's pressurised water reactors, water acts as both moderator and coolant. The heated coolant transfers energy to water in a heat exchanger, producing steam that drives turbines to generate electricity.
A concrete shield (typically 2-3 metres thick) surrounds the reactor to:
- Absorb radiation, particularly neutrons and gamma rays
- Protect workers and the environment
- Contain radioactive material in case of accidents
Comparison with nuclear fusion
While not the main focus of this topic, you should distinguish fission from nuclear fusion:
Nuclear fission:
- Splits heavy nuclei (uranium, plutonium)
- Produces radioactive waste
- Currently used in power stations
- Works at normal reactor temperatures
Nuclear fusion:
- Joins light nuclei (hydrogen isotopes)
- Produces less problematic waste
- Not yet commercially viable for electricity generation
- Requires temperatures of millions of degrees
Both processes release energy, but through opposite mechanisms. Fission breaks apart heavy unstable nuclei; fusion combines light nuclei to form more stable products.
Environmental and safety considerations
Nuclear fission has significant implications for energy generation:
Advantages:
- No carbon dioxide emissions during operation
- Very high energy density (small fuel mass produces large energy output)
- Reliable baseload electricity generation
- Reduced dependence on fossil fuels
Disadvantages:
- Produces highly radioactive waste requiring secure storage for thousands of years
- Risk of catastrophic accidents (Chernobyl, Fukushima)
- Potential for nuclear material to be used in weapons
- High construction and decommissioning costs
- Public concerns about safety
The UK currently operates several nuclear power stations, including Sizewell B in Suffolk and Hinkley Point C (under construction). These contribute approximately 15-20% of UK electricity generation.
Spent nuclear fuel remains dangerously radioactive. High-level waste is currently stored in cooling ponds and dry casks, with plans for deep geological disposal facilities. Intermediate and low-level waste goes to facilities like the UK's repository near Drigg in Cumbria.
Worked examples
Example 1: Describing nuclear fission (4 marks)
Question: Describe what happens when a uranium-235 nucleus undergoes nuclear fission after absorbing a neutron.
Mark scheme answer:
- The uranium-235 nucleus becomes unstable (1 mark)
- It splits into two smaller nuclei of roughly equal size / two daughter nuclei (1 mark)
- Two or three neutrons are released (1 mark)
- Energy is released / gamma radiation is emitted (1 mark)
Exam tip: This is a "describe" question worth 4 marks, so you need four distinct points. Don't waste time explaining WHY fission releases energy unless asked.
Example 2: Explaining chain reactions (3 marks)
Question: Explain how a chain reaction occurs in nuclear fission.
Mark scheme answer:
- Neutrons released from one fission event (1 mark)
- Strike other uranium-235 nuclei / cause further fission (1 mark)
- Each fission releases more neutrons that can cause additional fissions / reaction becomes self-sustaining (1 mark)
Exam tip: "Explain" requires you to give reasons or mechanisms. Show the cause-and-effect relationship between successive fission events.
Example 3: Control rods function (3 marks)
Question: A nuclear reactor uses control rods. Explain how these control rods are used to control the rate of fission in the reactor.
Mark scheme answer:
- Control rods are made of material that absorbs neutrons / boron or cadmium (1 mark)
- Lowering them into the reactor absorbs more neutrons, reducing the reaction rate (1 mark)
- Raising them allows more neutrons to cause fission, increasing the reaction rate (1 mark)
Alternative acceptable points:
- Position can be adjusted to maintain steady rate
- Fully inserting them stops the chain reaction
Exam tip: When a question asks about function, explain WHAT the component does AND HOW it achieves this effect.
Example 4: Calculation-style question (2 marks)
Question: In one fission event, a uranium-235 nucleus releases 3.2 × 10⁻¹¹ J of energy. Calculate how many fission events are needed to release 1 joule of energy.
Mark scheme answer:
Number of fissions = 1 J ÷ (3.2 × 10⁻¹¹ J) (1 mark for method)
= 3.125 × 10¹⁰ OR 3.1 × 10¹⁰ (1 mark for correct answer)
Exam tip: Show your working clearly. Even if your final answer is wrong, you can gain method marks.
Common mistakes and how to avoid them
Confusing fission with fusion — Remember: fission SPLITS heavy nuclei (uranium), fusion JOINS light nuclei (hydrogen). The "ss" in fission suggests splitting.
Saying energy is "produced" or "created" — Energy cannot be created. It is released or transferred from mass-energy to kinetic energy. Use precise language.
Describing control rods as "slowing neutrons" — Control rods ABSORB neutrons; moderators SLOW neutrons. These are different components with different functions.
Not giving enough detail in "describe" questions — If a question is worth 4 marks, you need 4 distinct points. Generic answers like "the nucleus splits and releases energy" won't access all marks.
Forgetting that daughter nuclei are radioactive — The products of fission are themselves unstable and decay further, which is why nuclear waste remains hazardous for thousands of years.
Incorrectly stating all neutrons cause further fission — In a controlled reaction, on average only ONE neutron per fission causes another fission. Others are absorbed by control rods or escape.
Exam technique for "Nuclear fission and chain reactions"
Command word precision: "Describe" requires you to state what happens (usually 1 mark per distinct feature). "Explain" requires reasons or mechanisms (showing cause and effect for full marks). "Compare" means give similarities AND differences.
Diagrams and annotations: If asked to draw a diagram of a reactor, clearly label fuel rods, control rods, moderator, coolant, and shielding. Add brief annotations explaining function if the question asks for it (usually 1 mark per correctly labelled and explained component).
Extended response questions: Nuclear fission may appear in 6-mark questions about energy resources. Structure your answer: define fission, explain the chain reaction process, describe reactor control, evaluate advantages and disadvantages. Use scientific terminology precisely throughout.
Equation questions: You won't need to balance complex nuclear equations at GCSE, but you should recognise the general form: U-235 + neutron → 2 daughter nuclei + neutrons + energy. Know that mass number is conserved (total before = total after).
Quick revision summary
Nuclear fission occurs when heavy nuclei like uranium-235 split after absorbing a neutron, producing two daughter nuclei, 2-3 neutrons, and energy. Chain reactions happen when released neutrons cause further fission. Nuclear reactors control this process using control rods (absorb neutrons), moderators (slow neutrons), and coolant (remove thermal energy). Controlled reactions maintain steady energy output; uncontrolled reactions increase exponentially. Fission releases energy because daughter nuclei have higher binding energy per nucleon than uranium.