# Fission and fusion

> Physics · Edexcel IGCSE 4PH1
> Source: https://www.owlsprep.com/study/edexcel-igcse-physics-s7-fission-and-fusion/

This guide covers all Edexcel IGCSE Physics 4PH1 content for nuclear fission, fusion, chain reactions, and nuclear reactor components, aligned with specification points 7.17–7.26 for your exam preparation.

**Prerequisites:** [Basic atomic structure and isotopes](https://www.owlsprep.com/study/edexcel-igcse-physics-s7-atomic-structure-radioactivity/); [Balancing nuclear equations](https://www.owlsprep.com/study/edexcel-igcse-physics-s7-nuclear-decay-equations/)

## Learning objectives

- Explain how nuclear fission of U-235 occurs and the products formed
- Describe how a chain reaction is set up and controlled in nuclear reactors
- State the function of moderator, control rods and shielding in fission reactors
- Distinguish between nuclear fission and fusion
- Explain why fusion requires very high temperature and pressure
- Recall that fusion is the energy source for stars

## Nuclear Fission Basics

**Nuclear Fission** — Splitting of a heavy unstable nucleus into two smaller radioactive daughter nuclei, triggered by absorption of a slow neutron, releasing energy as kinetic energy of fission products plus 2-3 fast neutrons.

*Example:* Uranium-235 is the most common fuel used in nuclear fission reactors.

Nuclear reactions including fission, fusion and radioactive decay are all sources of large amounts of energy, far more per unit mass than chemical reactions like burning fossil fuels. When a slow (thermal) neutron collides with a U-235 nucleus, it is absorbed, making the nucleus highly unstable. This unstable nucleus splits almost immediately into two smaller roughly equal mass daughter nuclei, plus 2 or 3 high speed neutrons. All fission products have high kinetic energy, which is converted to thermal energy in the reactor core to generate electricity.

**Worked example:** Write a balanced nuclear equation for the fission of U-235 after absorbing a neutron, producing barium-141, krypton-92 and 3 neutrons.

1. Step 1: Write the left-hand side reactants: U-235 plus a neutron
2. $$^{235}_{92}\text{U} + ^{1}_{0}\text{n}$$
3. Step 2: Write the right-hand side products: Ba-141, Kr-92 and 3 neutrons
4. $$^{141}_{56}\text{Ba} + ^{92}_{36}\text{Kr} + 3^{1}_{0}\text{n}$$
5. Step 3: Check mass number (top) and proton number (bottom) are balanced on both sides
6. Mass number left: 235 + 1 = 236; Right: 141 + 92 + 3 = 236 ✔️
7. Proton number left: 92 + 0 = 92; Right: 56 + 36 + 0 = 92 ✔️
8. Final balanced equation:
9. $$^{235}_{92}\text{U} + ^{1}_{0}\text{n} \rightarrow ^{141}_{56}\text{Ba} + ^{92}_{36}\text{Kr} + 3^{1}_{0}\text{n}$$

> **Exam tip:** Always check both mass number and proton number balance for nuclear fission equations, marks are awarded for both correct notation and balancing.

*Calculator:* allowed

## Chain Reactions & Nuclear Reactor Components

**Chain Reaction** — Self-sustaining reaction where neutrons released from one fission event collide with other U-235 nuclei, triggering further fission events that release more neutrons to continue the reaction.

If at least one neutron from each fission event goes on to trigger another fission, the reaction is self-sustaining. If more than one neutron triggers fission, the reaction rate increases exponentially, which is how atomic bombs work. In nuclear reactors, the reaction is carefully controlled to maintain a steady rate where exactly one neutron per fission goes on to cause another fission.

- **Moderator**: Typically made of graphite or water, slows fast neutrons released from fission to thermal speeds so they are easily absorbed by U-235 nuclei (fast neutrons bounce off U-235 without being absorbed).
- **Control rods**: Made of boron or cadmium, absorb excess neutrons. Lowering the rods further into the reactor core absorbs more neutrons, slowing or stopping the reaction; raising them increases the reaction rate.
- **Shielding**: Thick layers of lead and concrete surrounding the reactor core absorb alpha, beta and gamma radiation released during fission, protecting plant workers and the surrounding public from harmful radiation exposure.

**Worked example:** A nuclear reactor operator notices the fission reaction rate is increasing faster than the safe limit. State what the operator should do, and explain how this action corrects the rate.

1. Step 1: State the action: Lower the control rods deeper into the reactor core.
2. Step 2: Explain the mechanism: Control rods are made of neutron-absorbing material like boron. Lowering them increases the number of excess neutrons they absorb, reducing the number of neutrons available to trigger further fission events, slowing the reaction rate back to the safe steady state.

> **Exam tip:** Don't mix up moderator and control rod functions: moderators slow neutrons, control rods absorb them. You will lose marks if you confuse these two roles.

*Calculator:* allowed

## Nuclear Fusion Basics

**Nuclear Fusion** — Process where two small light nuclei join together to form a single larger heavier nucleus. A small amount of mass is lost during this process, which is converted to a very large amount of energy.

*Example:* Fusion of hydrogen isotopes deuterium and tritium is the energy source for all stars including our Sun.

Fusion releases significantly more energy per unit mass than fission, and produces far less long-lived radioactive waste, so it is being researched as a future clean energy source. However, fusion does not occur at normal temperatures and pressures because all nuclei have positive charge from their protons, so they repel each other due to electrostatic forces.

> **info**
>
> Fusion requires temperatures of over 100 million °C and extremely high pressure to give nuclei enough kinetic energy to overcome electrostatic repulsion and collide close enough for the strong nuclear force to pull them together. These conditions exist naturally in the core of stars, where gravity creates the high pressure and temperature needed for sustained fusion.

**Worked example:** Explain why scientists are researching nuclear fusion as an alternative to fission for electricity generation, despite the technical challenges.

1. Step 1: State the benefits of fusion over fission:
2. - Fusion fuel (hydrogen isotopes) is abundant in seawater, while U-235 for fission is a limited non-renewable resource.
3. - Fusion produces very little long-lived radioactive waste, unlike fission which produces highly radioactive waste that must be stored safely for thousands of years.
4. - There is no risk of a catastrophic meltdown with fusion, as the reaction stops immediately if conditions are not maintained, unlike fission chain reactions which can run out of control if not managed properly.
5. Step 2: Note the challenge: The extreme temperature and pressure required for sustained fusion are very difficult and expensive to replicate on Earth, so commercial fusion reactors are not yet available.

*Calculator:* allowed

## Comparing Fission and Fusion

You will often be asked to state the key differences between fission and fusion in exams, so make sure you can clearly distinguish these two processes.

| Feature | Nuclear Fission | Nuclear Fusion |
| --- | --- | --- |
| Process | Splits large heavy nuclei | Joins small light nuclei |
| Fuel used | Uranium-235 (or plutonium) | Hydrogen isotopes (deuterium, tritium) |
| Natural occurrence | Does not occur naturally on Earth | Occurs naturally in star cores |
| Waste product | Highly radioactive long-lived waste | Very little low-level radioactive waste |
| Reaction conditions | Room temperature/pressure, requires slow neutrons | Extremely high temperature and pressure required |
| Energy released per kg | Very large | ~4x larger than fission |

**Check your understanding**

1. Which process powers the Sun?

   - Fission
   - Fusion
   - Radioactive decay
   - Chemical combustion

   *Why:* Correct! Fusion of hydrogen nuclei into helium in the Sun's core releases the energy that reaches Earth as light and heat.

2. What is the role of a moderator in a nuclear reactor?

   - Absorb excess neutrons
   - Slow down fast neutrons
   - Shield workers from radiation
   - Heat water to make steam

   *Why:* Correct! Moderators like graphite or water slow neutrons so they can be absorbed by U-235 nuclei to trigger fission.

*Calculator:* allowed

## Common pitfalls

- **Wrong:** Stating that moderators absorb neutrons
  - Why it fails: Confusing the role of moderators and control rods. Examiners regularly test this distinction, and mixing the two loses marks.
  - Correct: Remember: Moderators slow neutrons, control rods absorb neutrons. Use the mnemonic 'Slow Mo, Absorb Rod' to recall.
- **Wrong:** Saying fusion only requires high temperature, not pressure
  - Why it fails: Both high temperature and pressure are needed: temperature gives nuclei speed to overcome repulsion, pressure forces nuclei close enough to collide. Missing either loses a mark.
  - Correct: Always state both very high temperature AND pressure are required for fusion to occur.
- **Wrong:** Balancing only mass number, not proton number, in fission equations
  - Why it fails: Marks are awarded for both correct mass number and proton number balance, as well as correct particle notation.
  - Correct: Check both top (mass number) and bottom (proton number) values add up to the same total on both sides of the equation before finalizing your answer.
- **Wrong:** Stating that fusion occurs naturally on Earth
  - Why it fails: There are no natural conditions on Earth with high enough temperature and pressure to sustain fusion, unlike star cores.
  - Correct: Neither occurs naturally on Earth under normal conditions — fusion occurs naturally in stars, and on Earth fission happens only in engineered reactors.
- **Wrong:** Claiming chain reactions occur in fusion reactors
  - Why it fails: Chain reactions are specific to fission, as they rely on neutrons released from fission triggering further reactions. Fusion does not produce neutrons in a way that creates a self-sustaining chain reaction the same way.
  - Correct: Only associate chain reactions with nuclear fission, not fusion.

## Cheatsheet

| Key Concept | Quick Facts |
| --- | --- |
| Fission of U-235 | Trigger: slow neutron; Products: 2 daughter nuclei + 2-3 neutrons + energy |
| Chain reaction | 1+ neutron per fission triggers another fission; controlled in reactors, uncontrolled in bombs |
| Moderator | Graphite/water: slows neutrons for U-235 absorption |
| Control rods | Boron/cadmium: absorb neutrons to control reaction rate |
| Shielding | Thick lead/concrete: absorbs radiation to protect workers |
| Fusion | Joins light nuclei; releases more energy than fission; needs high T + pressure to overcome proton repulsion |
| Fusion in stars | Sun fuses H to He; gravity provides required pressure/temperature |
| Fission vs Fusion | Fission = split heavy nuclei; Fusion = join light nuclei |

## What's next

Now that you have mastered nuclear fission and fusion for Edexcel IGCSE Physics, you can connect this knowledge to other parts of the syllabus to build a complete understanding. This content directly links to energy resources in Unit 4, where you will compare nuclear power to other electricity generation methods, and to the astronomy unit (Unit 8) where you learn about stellar life cycles powered by fusion. You should also practice answering past paper questions on this topic to familiarize yourself with exam phrasing and mark scheme expectations, as questions on reactor components and fusion conditions are very common. Make sure you can confidently balance fission nuclear equations and clearly distinguish between fission and fusion processes before moving on to more advanced topics.

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