Welcome! Nuclear power stations split atoms to make electricity. Stars join atoms together. Scientists are racing to copy the stars here on Earth. In this lesson you’ll learn how fission and fusion work, then advise the government on the power of the future. It takes about one hour. (Separate Physics only.)
Before you start
- You need: your exercise book or paper and a pen.
- Write the title and date: Fission and fusion – Power of the Future.
- The golden rule: write your answer first, then tap Show answer. Correct mistakes in a different colour.
- Look out for the blue 📝 boxes. Copy each one into your book neatly – these are your revision notes.
📝 Copy into your book
By the end of this lesson I will be able to:
1. describe nuclear fission and a chain reaction
2. explain how a reactor controls the chain reaction
3. describe nuclear fusion
4. evaluate fission and fusion as energy sources.
Step 1: Starter (5 minutes)
Fission and fusion both release huge amounts of energy from the nucleus of an atom. The words are clues! “Fission” is like “fissure” (a crack). “Fusion” is like “fuse” (join). Guess what each one does to a nucleus.
Show answer
Check your guesses against the box below.
📝 Copy into your book
FISSION = SPLITTING a large nucleus into two smaller nuclei.
FUSION = JOINING two light nuclei to make a heavier nucleus.
Both release a lot of energy.
Step 2: Read and write (8 minutes)
First, copy these three questions into your book. Then read the text below carefully and answer them in full sentences.
- What usually has to happen to a uranium nucleus before it splits?
- What is released when it splits?
- Where does nuclear fusion happen naturally?
📖 Read: Splitting and joining nuclei
Nuclear fission is the splitting of a large, unstable nucleus, such as uranium-235 or plutonium-239. Fission rarely happens on its own. Usually the nucleus must first absorb a neutron. It then splits into two smaller nuclei of roughly equal size, and releases two or three neutrons, gamma rays and a large amount of energy. The released neutrons can go on to split more nuclei, which release more neutrons – a chain reaction. In a nuclear reactor the chain reaction is controlled; in a nuclear weapon it is not.
Nuclear fusion is the joining of two light nuclei, such as types of hydrogen, to form a heavier nucleus. Some of the mass is converted into energy. Fusion happens naturally in stars, including the Sun. It needs extremely high temperatures and pressures, which is why scientists have not yet built a working fusion power station.
Show answers
(a) It absorbs a neutron.
(b) Two smaller nuclei, two or three neutrons, gamma rays and energy.
(c) In stars, such as the Sun.
📝 Copy and complete
For fission to happen, a uranium nucleus usually has to absorb a __________. It splits into two __________ nuclei and releases two or three __________, gamma rays and __________.
Check your gaps
neutron · smaller · neutrons · energy
Step 3: Key facts (10 minutes)
Your task: draw a simple diagram of a chain reaction – one neutron hitting a uranium nucleus, which splits and releases three neutrons, each hitting another nucleus. Then copy the steps underneath:
📝 Copy into your book
Nuclear fission:
1. A uranium-235 nucleus absorbs a neutron.
2. It splits into two smaller nuclei.
3. Two or three neutrons, gamma rays and energy are released.
4. The neutrons split more nuclei → a CHAIN REACTION.
A reactor must stop the chain reaction getting out of control. Copy how:
📝 Copy into your book
CONTROL RODS absorb some of the neutrons, so the chain reaction happens at a steady rate. In a reactor it is controlled; in a nuclear weapon it is uncontrolled.
Now read about fusion, then complete the box.
In fusion, two light nuclei join together. Some of their mass is converted into energy, which is released as radiation. Fusion powers the Sun and all other stars. On Earth it needs extremely high temperatures and pressures, which is why it is so difficult to use for electricity.
📝 Copy and complete
In fusion, two __________ nuclei join to form a __________ nucleus. Some of the __________ is converted into energy. Fusion powers the __________. It is hard to do on Earth because it needs extremely high __________ and __________.
Check your gaps
light · heavier · mass · stars (the Sun) · temperatures · pressures
Quick check: write one difference and one similarity between fission and fusion.
Show answer
Difference: fission splits a large nucleus; fusion joins two small nuclei.
Similarity: both release a lot of energy from the nucleus.
Step 4: Main task – Power of the Future (15 minutes)
The government needs your advice. It has money to spend on the country’s future electricity. Should it build more fission power stations now, or invest in fusion research?
| Fission power stations | Fusion power stations | |
|---|---|---|
| Available? | Yes – in use today | Not yet – still experimental; working power stations may be decades away |
| Fuel | Uranium or plutonium – non-renewable, mined | Types of hydrogen – some can be extracted from seawater, so there’s a lot of it |
| Carbon dioxide when generating | None | None |
| Waste | Radioactive waste that must be stored for thousands of years | Far less long-lasting radioactive waste |
| Safety | Small risk of a major accident | The reaction stops if conditions aren’t right |
| Reliability | Reliable | Expected to be reliable |
First, copy the writing frame – you’ll use it for Q3 in the exam practice too:
📝 Copy into your book
How to evaluate:
FOR – at least 2 reasons, using data
AGAINST – at least 2 problems
JUDGEMENT – “Overall, … because …”
Your task: write your advice with these headings: For fission · Against fission · For and against fusion · My advice (finish with “Overall, the government should… because…”).
Stuck? Think about now versus the future. Can the country wait decades for electricity?
Show model advice
For fission: it’s available now and reliable, so it can provide electricity at any time. It releases no carbon dioxide when generating, so it helps tackle climate change.
Against fission: uranium is non-renewable and will run out. It produces radioactive waste that must be stored safely for thousands of years, and there is a small risk of a major accident.
Fusion: for – its fuel is plentiful, it releases no CO₂ and produces far less long-lasting waste. Against – it isn’t available yet and may not be for decades, and it’s very difficult because it needs extremely high temperatures and pressures.
My advice: overall, the government should build some fission power stations now to provide reliable, low-carbon electricity, and fund fusion research, because fusion could replace fission in the future without the waste problem. Any clear, justified advice is a good answer!
✅ Check: did you include points for and against, and a clear final judgement?
Step 5: Exam practice (14 minutes)
Look at the marks – they tell you how many points to make. Answer in full sentences, then mark yourself.
Q1 (2 marks) State two differences between nuclear fission and nuclear fusion.
Show model answer
Fission splits a large nucleus, whereas fusion joins two small nuclei (1). Fission is used in power stations today, whereas fusion happens in stars (and isn’t yet used to generate electricity) (1). (Also accept: fission uses heavy nuclei like uranium, fusion uses light nuclei like hydrogen.)
Q2 (4 marks) Describe how a chain reaction happens in a nuclear reactor, and how it is controlled.
Hint: your two 📝 boxes from Step 3 have all four marks in them!
Show model answer
A uranium-235 nucleus absorbs a neutron and splits into two smaller nuclei (1), releasing two or three neutrons (and energy) (1). These neutrons are absorbed by other uranium nuclei, which also split, releasing more neutrons (1). Control rods absorb some of the neutrons to keep the rate of fission steady (1).
Q3 (6 marks) Evaluate the use of nuclear fission to generate electricity.
Hint: use your “How to evaluate” frame – for, against, then a judgement.
Show model answer
For: no carbon dioxide is released when generating, so it doesn’t contribute to climate change. It is reliable and can generate electricity at any time. A small amount of fuel releases a very large amount of energy.
Against: uranium is non-renewable and will run out. It produces radioactive waste that must be stored safely for a very long time. There is a small risk of a major accident. Power stations are very expensive to build and to decommission.
Judgement: e.g. “Overall it is a useful low-carbon, reliable source, but the waste and cost mean it should be used alongside renewable resources.”
✅ Add up your marks out of 12 and write the score in your book.
Step 6: Exit ticket (8 minutes)
Explain it to a friend: in 3 sentences, write how you’d explain the difference between fission and fusion to someone in Year 7. Then finish:
- Control rods are important because…
- I think the power of the future is… because…
- One thing I’m still not sure about is…
🎉 Well done – lesson complete! Your 📝 boxes are your revision notes for this topic. Want more? Try the Hazards, uses, fission and fusion questions.
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