How a nucleus splits in fission, how a reactor keeps a chain reaction under control, and how fusion powers the stars. Separate Physics only.
Objectives
Describe nuclear fission, including the part played by a neutron.
Explain how a chain reaction is sustained, and calculate the neutrons after a number of steps.
Describe the jobs of the control rods, moderator, coolant and shielding in a reactor.
Describe nuclear fusion and explain why it needs a very high temperature and pressure.
Higher tier: compare fission and fusion, including fuel, waste and where each is used.
Nuclear fission and fusion explained
Start here
Have you seen the video where a hundred mousetraps sit in a tray, each with a ping pong ball on top, and one ball is dropped in so that the whole tray erupts in a few seconds? Each trap that snaps throws balls that set off more traps, and the number going off grows faster every second, which is exactly how a chain reaction works.
A nuclear power station does something similar with atoms. A big nucleus splits in two, and it throws out a few small particles and a burst of energy. Those particles can set off more nuclei, so the energy keeps coming as long as there is fuel left in the reactor.
The Sun does something completely different. Instead of splitting big nuclei, it squashes tiny hydrogen nuclei together to make a bigger one. That also releases energy, and it has been lighting and warming the Earth for billions of years.
Splitting is called fission, and joining is called fusion. Both change the nucleus, which is why they release far more energy per kilogram of fuel than burning coal or gas ever could. They also create very different problems, which you will meet in this lesson.
By the end you will be able to do three things. You will describe fission and fusion in your own words, and explain how a reactor keeps a chain reaction under control. You will also work out how many neutrons there are after a number of steps, and compare the two processes in an exam answer.
Nuclear fission
Fission means splitting. The fuel in a nuclear reactor is a large, unstable nucleus such as uranium-235 or plutonium-239. A nucleus like this is a bit like a very full water balloon, which looks calm but only needs a small nudge to burst everywhere.
The nudge is a neutron. The uranium-235 nucleus absorbs a neutron, which means it takes it in, and it becomes heavier and even more unstable. A tiny fraction of a second later it splits into two smaller nuclei, which we call the fission products, and they fly apart at high speed.
Two or three more neutrons fly out when the nucleus splits. Energy is also released, and most of it is the kinetic energy of the fast-moving fragments, which means their movement energy. A little is carried away by gamma radiation, and all of it ends up as heat in the reactor.
This energy comes from the nucleus itself, where a tiny amount of mass is changed into energy. That is why a small amount of fuel can release so much, which is the great attraction of nuclear power. A single kilogram of uranium gives vastly more energy than a kilogram of coal.
The two smaller nuclei are usually radioactive as well. This is one reason why the waste from a reactor needs such careful handling, as you will see later in this lesson. For now, remember the order of events, which is neutron in, nucleus splits, then neutrons and energy out.
A flow with four boxes. A neutron hits a uranium-235 nucleus. The nucleus splits into two smaller nuclei. Two or three neutrons are released. Energy is released.
Chain reactions
The neutrons that are released by one fission can be absorbed by other uranium nuclei nearby. Those nuclei split too, and release more neutrons of their own. This is called a chain reaction, because each fission is a link that leads on to the next.
Think about passing on a rumour. If you tell 2 friends, and each of them tells 2 more, the number who know it doubles at every step. A chain reaction multiplies in just the same way, which is why it can get out of hand so fast if nobody is watching it.
In words: number of neutrons after n steps = neutrons released per fission to the power n. In symbols: N = rⁿ, where r is the number of neutrons that cause a new fission each time. For example, if each fission releases 2 neutrons that both cause another fission, starting from 1 neutron you get 2, then 4, then 8, then 16.
If the chain reaction is left uncontrolled, the number of fissions grows faster and faster, and the energy is released in a fraction of a second. That is what happens in a nuclear bomb. In a power station, the chain reaction is kept steady, so that on average only one neutron from each fission goes on to cause another.
Keeping it steady means the energy comes out at a constant rate that we can use to make electricity. This takes some clever engineering, which is the subject of the next step. The key idea to remember is that the number of neutrons multiplies at every step, it does not just go up by a fixed amount.
A bar chart of the number of neutrons in a chain reaction where each fission releases 2 neutrons that cause more fissions. It starts at 1, then 2, 4, 8 and 16. The bars double each time.
Inside a nuclear reactor
A reactor is built to keep a chain reaction steady and to use the heat it makes. The fuel is packed into fuel rods, and the rods sit in the core of the reactor. A power station is really just a very clever kettle, because its job is to heat water into steam, so the nuclear part is only the heat source.
Control rods are made of a material, such as boron, that absorbs neutrons. They are lowered between the fuel rods to soak up neutrons and slow the reaction down, and they are raised to let the reaction speed up. In an emergency they can be dropped in all the way to shut the reactor down, which stops the chain reaction almost at once.
The neutrons from fission come out very fast, and fast neutrons are not easily absorbed. A moderator, such as water or graphite, slows them down. Slow neutrons are much more likely to be absorbed by uranium-235 and cause another fission, so the moderator helps the chain reaction to carry on.
A coolant, such as water, flows through the core and carries the heat away. The heat is used to make steam, and the steam turns a turbine that turns a generator. The core is surrounded by thick concrete shielding, which absorbs the radiation so that the workers and the people living nearby stay safe.
It is easy to mix up the jobs of the control rods and the moderator, so try this. The control rods absorb neutrons, like a sponge mopping up spilt water. The moderator only slows neutrons down, like a speed bump on a road, and the neutrons are still there.
A flow with five boxes. Fission in the fuel rods. The coolant carries the heat away. The heat turns water into steam. Steam turns a turbine. A generator makes electricity.
Nuclear fusion
Fusion means joining. In fusion, two light nuclei, such as the nuclei of hydrogen isotopes, join to form one heavier nucleus such as helium. Some of the mass is changed into energy, and that energy is released as heat and light, which is what we see from the Sun.
This is what powers the Sun and all the other stars. In the core of the Sun, which is at around 15 million degrees Celsius, hydrogen nuclei are fused into helium all the time, millions of tonnes of them every second. The enormous pull of the Sun’s own gravity squeezes the gas and makes the pressure huge.
The big problem is that nuclei are positive, and positive charges repel each other. Think of trying to push the matching ends of two strong magnets together. You need a lot of force to get them close enough to touch, and nuclei are even harder to push together.
That is why fusion needs very high temperature and very high pressure. The nuclei have to move so fast that they smash into each other despite the repulsion, and that only happens when the gas is extremely hot. On Earth we do not have a star’s gravity to help, so we have to make these conditions in some other way, and that is very hard.
Scientists have built experimental fusion reactors that use strong magnetic fields to hold the extremely hot gas away from the walls. So far nobody has built one that works as a power station. That is why fusion is not yet used to make the electricity that you use at home.
A flow with four boxes. Two hydrogen nuclei are forced together. Very high temperature and pressure beat the repulsion. The nuclei join to make helium. Energy is released.
Comparing fission and fusion
In an exam you are often asked to compare the two processes. Fission splits a large nucleus, and it needs a neutron to start it. Fusion joins two small nuclei, and it needs extreme heat and pressure to get started, because the nuclei repel each other.
Fission is used in power stations today. Fusion powers the stars, but it is not yet a working power source on Earth. This is one of the most common differences that you will be asked for, so learn it well.
Higher tier: you may be asked to compare the fuel and the waste. Fission uses uranium or plutonium, which have to be mined and processed, and the waste stays radioactive for a very long time. Fusion uses hydrogen, which is plentiful, and the main product is helium, so it makes little long-lived waste.
Both processes release energy because some of the mass of the nuclei is changed into energy, and in both the amounts are huge compared with chemical reactions. Neither one produces carbon dioxide while it is running, so neither adds to the greenhouse effect in the same way as burning fossil fuels. That makes them attractive when we want low-carbon electricity.
When you write a comparison, use words such as whereas, but and however, so that both sides are clear. A list of two separate facts does not earn the mark by itself. A good answer might say that fission splits large nuclei whereas fusion joins small nuclei, and that only fission is used in power stations today.
Waste, risks and benefits
The used fuel from a fission reactor is dangerous. It contains radioactive fission products, and some of them stay radioactive for thousands of years. It has to be stored very carefully, first in cooling ponds and then in sealed containers, and the long-term plan is to bury it deep underground.
Reactor accidents are rare, but when one happens it is serious. Radioactive material can escape into the air and the soil, and it can cause illness and spread over a wide area, which is why accidents are remembered for so long. That is why reactors have thick shielding, many safety systems and strict rules.
There are good points too. A nuclear power station makes a lot of electricity from a small amount of fuel, and it runs day and night whatever the weather. It also releases no carbon dioxide while it is running, unlike a coal or gas power station, which helps with climate change.
When you answer an opinion question, give one benefit and one drawback, and say why. For example, you could say that fission gives a steady supply of electricity without carbon dioxide, but the waste stays dangerous for a very long time. Using the word because makes your answer stronger, as it shows that you can explain, not just list.
For fusion, the benefits are plentiful fuel and little long-lived waste. The drawback is the huge difficulty of making and holding the extreme conditions. It is also very expensive to build, and the machines are huge and complicated.
Recap
Fission splits a large nucleus, such as uranium-235, after it absorbs a neutron. The split releases two or three neutrons and a lot of energy. Those neutrons can cause more fissions, which is a chain reaction, and each step multiplies the number of neutrons.
In a reactor the chain reaction is kept steady. Control rods absorb neutrons, the moderator slows neutrons down, the coolant carries heat away to make steam and the concrete shielding absorbs radiation. After n steps the number of neutrons is the number released per fission to the power n.
Fusion joins light nuclei, such as hydrogen, to make helium. It needs very high temperature and pressure to beat the repulsion between positive nuclei. It powers the stars, but it is not yet a working power source on Earth, because the conditions are so hard to make.
In an exam you might be asked to describe what happens in fission, or to state the job of a control rod or moderator. You might be asked why fusion needs a very high temperature, or to give one advantage of fusion over fission. A neutron count question is just repeated multiplying, so write out each step to show your working.
Write short, clear answers that use the key words. For example, say that the moderator slows neutrons so they are more likely to be absorbed. Also say which process you are talking about, because fission and fusion are easy to swap by mistake when you are nervous, and a swapped word costs the mark.
Key points
Fission splits a large nucleus after it absorbs a neutron, releasing neutrons and energy.
A chain reaction multiplies the neutrons at each step.
Control rods absorb neutrons. The moderator slows them. The coolant carries heat away.
Fusion joins light nuclei and needs very high temperature and pressure.
Fission is used in power stations. Fusion powers stars.
Fission waste stays radioactive for a very long time.
Worked examples
Example 1
A chain reaction starts with 1 neutron. Each fission releases 2 neutrons, and each of those causes another fission. How many neutrons are there after 4 steps?
The number of neutrons doubles at each step.
After 4 steps the number is 2 × 2 × 2 × 2.
Work out the answer: 16 neutrons.
Answer: 16 neutrons
Common slips
Slip: The number goes up by 2 at each step. It doubles at each step. It is not adding 2.
Slip: After 4 steps the number is 2 + 4 = 6. You must multiply by 2 four times, not add.
Slip: 8 neutrons. 8 is the number after 3 steps. After 4 steps there are 16.
Example 2
Each fission releases 3 neutrons, and each of those causes another fission. Starting from 1 neutron, how many neutrons are there after 3 steps?
The number of neutrons triples at each step.
After 3 steps the number is 3 × 3 × 3.
Work out the answer: 27 neutrons.
Answer: 27 neutrons
Common slips
Slip: The number goes up by 3 at each step. It triples at each step. It does not add 3.
Slip: After 3 steps the number is 3 × 3 = 9. There are three steps, so you multiply by 3 three times.
Slip: 9 neutrons. 9 is the number after 2 steps. After 3 steps there are 27.
Example 3
Explain why a nuclear reactor needs a moderator.
The neutrons released by fission are moving very fast.
The moderator slows the neutrons down.
Slow neutrons are more likely to be absorbed by uranium-235, so they cause more fission.
Answer: It slows the neutrons down so they are more likely to be absorbed and cause more fission.
Common slips
Slip: The neutrons released by fission are moving too slowly. They come out very fast, and that is the problem.
Slip: The moderator absorbs the neutrons. Control rods absorb neutrons. The moderator slows them down.
Slip: Slow neutrons carry the heat away to make steam. The coolant carries heat away. Slow neutrons are simply easier to absorb.
Questions and answers
Nuclear fission and fusion practice questions for GCSE Combined Science Physics. Try each one, then open the answer. Easy questions come first.
Question 1 Easy
Name the process in which a large nucleus splits into two smaller nuclei.
Show answer
Answer: fission
The nucleus splits.
This is fission.
Question 2 Easy
Name the process that releases energy in the Sun.
Show answer
Answer: fusion
The Sun joins hydrogen nuclei.
This is fusion.
Question 3 Easy
Name one fuel used in a fission reactor.
Show answer
Answer: uranium-235
Reactors use heavy elements.
Uranium-235 is used.
Question 4 Easy
State what a nucleus absorbs to start fission.
Show answer
Answer: a neutron
A particle with no charge is absorbed.
It absorbs a neutron.
Question 5 Medium
Each fission releases 2 neutrons and each of those causes another fission. Starting from 1 neutron, how many neutrons are there after 5 steps?
Show answer
Answer: 32 neutrons
The number doubles each step.
2 × 2 × 2 × 2 × 2 = 32
Question 6 Medium
State the job of the moderator in a reactor.
Show answer
Answer: It slows the neutrons down.
Neutrons from fission are fast.
The moderator slows them down.
Question 7 Medium
State one job of the control rods.
Show answer
Answer: They absorb neutrons to control the rate.
The rate of the reaction has to stay steady.
The rods absorb neutrons.
Question 8 Medium
State one difference between fission and fusion.
Show answer
Answer: Fission splits a large nucleus, but fusion joins light nuclei.
Fission involves a heavy nucleus.
Fission splits a nucleus but fusion joins nuclei.
Exam-style questions
Longer questions in the style of a written paper, with the marking points. These are Yumio practice questions.
Exam-style question 14 marks
A nuclear power station uses fission of uranium-235.
(a) Describe what happens in a fission reaction. [2 marks]
(b) State the job of the control rods. [1 mark]
(c) State the job of the moderator. [1 mark]
Show answer and marking points
Answer: (a) A uranium nucleus absorbs a neutron and splits into two smaller nuclei, releasing neutrons and energy. (b) They absorb neutrons to control the rate. (c) It slows the neutrons down.
(a) a nucleus absorbs a neutron and splits into two smaller nuclei (1)
(a) neutrons and energy are released (1)
(b) absorb neutrons (1)
(c) slow the neutrons (1)
Exam-style question 24 marks
Stars such as the Sun release energy by nuclear fusion.
(a) State what happens in nuclear fusion. [1 mark]
(b) Explain why fusion needs a very high temperature. [2 marks]
(c) State one advantage of fusion over fission. [1 mark]
Show answer and marking points
Answer: (a) Two light nuclei join to form a heavier nucleus. (b) The nuclei are positive, so they repel, and they must move very fast to get close enough to fuse. (c) It makes little long-lived radioactive waste.
(a) light nuclei join to form a heavier nucleus (1)
(b) nuclei are positive, so they repel (1)
(b) they must move very fast to get close enough (1)
(c) little long-lived waste, or plentiful fuel (1)
Common mistakes
Saying fusion splits nuclei
Fusion joins light nuclei. Fission splits heavy ones.
Saying control rods slow the neutrons down
Control rods absorb neutrons. The moderator is the part that slows them.
Saying fusion is used in power stations today
Fission is used in power stations. Fusion powers stars but is not yet a working power source.
Saying a chain reaction adds the same number of neutrons each step
The number multiplies at each step. With 2 neutrons per fission, 1 becomes 2, 4, 8, 16.
Saying the coolant stops the reaction
The coolant carries heat away to make steam. Control rods are what control the reaction.
Questions students ask
Is Nuclear fission and fusion on the Foundation or Higher tier?
Nuclear fission and fusion is on both Foundation and Higher papers for GCSE Combined Science Physics, aimed at around grade 6.
What is a common mistake on Nuclear fission and fusion?