Edexcel IGCSE Physics 7(c) Fission and Fusion — Notes
Written for Pearson Edexcel International GCSE Physics (4PH1), Science Double Award (4SD0) and Science Single Award (4SS0), linear and modular. The Topic 7 units are on the 7(b) Radioactivity page.
Which parts do I need?
Single Award candidates need: nuclear reactions as a source of energy (7.17), how a neutron splits U-235 and what fission produces (7.18, 7.19), the role of reactor shielding (7.22) and that stars are powered by fusion (7.25). Single Award can skip every section tagged Double · Physics: chain reactions (7.20), control rods and the moderator (7.21), comparing fusion with fission (7.23), fusion and mass loss (7.24), and why fusion needs high temperature and pressure (7.26). Fission equations also need 7.8 (balancing nuclear equations), so they are Double · Physics. Nothing on this page is Physics-only.
What this page covers
| Spec | Modular no. | You need to be able to… | Route |
|---|---|---|---|
| 7.17 | 8.17 | know that fission, fusion and radioactive decay all release energy | Single · Double · Physics |
| 7.18 | 8.18 | explain how a neutron colliding with U-235 makes it split, releasing energy as kinetic energy of the products | Single · Double · Physics |
| 7.19 | 8.19 | know that each U-235 fission gives two radioactive daughter nuclei and two or three neutrons | Single · Double · Physics |
| 7.20 | 8.20 | describe how the released neutrons can cause further fissions in a chain reaction | Double · Physics |
| 7.21 | 8.21 | describe what the control rods and the moderator do in a reactor | Double · Physics |
| 7.22 | 8.22 | explain why a reactor is surrounded by shielding | Single · Double · Physics |
| 7.23 | 8.23 | explain how fusion differs from fission | Double · Physics |
| 7.24 | 8.24 | describe fusion as small nuclei joining into a larger one, with a loss of mass and a release of energy | Double · Physics |
| 7.25 | 8.25 | know that stars get their energy from fusion | Single · Double · Physics |
| 7.26 | 8.26 | explain, using the repulsion between protons, why fusion needs very high temperature and pressure | Double · Physics |
Modular candidates: Topic 7 is Topic 8 in the modular specifications. The modular number is the same in 4XPH1 Unit 2 (4WPH2) and 4XSD1 Physics Unit 6 (4WSD6).
In one minute
- Fission, fusion and radioactive decay are all nuclear reactions that release energy.
- Fission: a U-235 nucleus absorbs a neutron, becomes unstable and splits into two radioactive daughter nuclei and two or three neutrons. The energy is released as kinetic energy of these products.
- Thick concrete shielding around a reactor absorbs the radiation from the core.
- Stars are powered by fusion.
- The released neutrons can split more U-235 nuclei: a chain reaction. Control rods absorb neutrons; the moderator slows neutrons down. Double · Physics
- Fusion joins small nuclei into a larger one; the products have less mass and energy is released. It needs very high temperature and pressure to overcome the repulsion between the positive nuclei. Double · Physics
Which nuclear reactions release energy? Single · Double · Physics
A nuclear reaction changes the nucleus of an atom, not just its electrons. Three kinds of nuclear reaction release energy:
- Radioactive decay: an unstable nucleus emits alpha, beta or gamma radiation. The radiation carries energy away; when it is absorbed, the surroundings warm up.
- Fission: a large nucleus splits into two smaller nuclei. This is the energy source in nuclear power stations.
- Fusion: two small nuclei join to make a larger nucleus. This is the energy source of stars, including the Sun.
The energy released by one nuclear reaction is millions of times greater than the energy released when one molecule reacts in a chemical reaction such as burning, which is why a small mass of nuclear fuel releases so much energy.
What happens when a U-235 nucleus undergoes fission? Single · Double · Physics
- A slow-moving neutron collides with a uranium-235 nucleus and is absorbed by it.
- The nucleus becomes uranium-236, which is very unstable.
- It splits into two smaller daughter nuclei (for example barium and krypton) and releases two or three neutrons.
- The energy released is carried away as the kinetic energy of the daughter nuclei and neutrons. Gamma rays are also usually emitted.
- The daughter nuclei are themselves radioactive — they decay, which is one reason spent fuel from a reactor is dangerous waste.
- In a reactor the fast-moving products collide with surrounding atoms, so their kinetic energy becomes thermal energy. This heats the coolant, which is used to make steam to drive turbines.
- The two daughter nuclei are not the same every time: U-235 can split in many different ways.
Describe how energy is released from uranium-235 by fission. Name the products of the fission.
- A neutron collides with and is absorbed by a U-235 nucleus.
- The nucleus becomes unstable and splits into two (radioactive) daughter nuclei and two or three neutrons.
- The energy is released as kinetic energy of the daughter nuclei and neutrons.
Answers that say an electron, a proton or an atom hits the uranium do not earn the first mark — it must be a neutron.
How do you write a fission equation? Double · Physics
Not required for Single Award (4SS0): balancing nuclear equations is statement 7.8.
A fission equation is balanced in the same way as a decay equation: the top numbers (mass numbers) add up to the same total on both sides, and so do the bottom numbers (atomic numbers). Remember to include the neutron that starts the fission on the left.
\[ {}^{1}_{0}\text{n}+{}^{235}_{\ 92}\text{U}\rightarrow{}^{141}_{\ 56}\text{Ba}+{}^{92}_{36}\text{Kr}+3\,{}^{1}_{0}\text{n} \]- Top: 1 + 235 = 236 and 141 + 92 + 3 × 1 = 236.
- Bottom: 0 + 92 = 92 and 56 + 36 + 3 × 0 = 92.
(a) In one fission of U-235, the products are xenon-140 (\({}^{140}_{\ 54}\text{Xe}\)), an isotope of strontium (Sr) and two neutrons. Determine the mass number and atomic number of the strontium nucleus.
(b) In another fission, the products are \({}^{137}_{\ 55}\text{Cs}\), \({}^{96}_{37}\text{Rb}\) and some neutrons. Determine the number of neutrons released.
(a) Top: 1 + 235 = 236, so 236 = 140 + A + 2 × 1, giving A = 94.
Bottom: 92 = 54 + Z, giving Z = 38. The nucleus is \({}^{94}_{38}\text{Sr}\).
(b) Top: 236 = 137 + 96 + k, so k = 3 neutrons. (Bottom check: 55 + 37 = 92.)
Common error in (a): forgetting the incoming neutron and using 235 instead of 236 on the left.
How is a chain reaction set up? Double · Physics
Not required for Single Award (4SS0).
- Each fission releases two or three neutrons.
- If these neutrons are absorbed by other U-235 nuclei, those nuclei also undergo fission and release more neutrons. This is a chain reaction.
- Uncontrolled: if more than one neutron from each fission causes another fission, the number of fissions grows very rapidly and a huge amount of energy is released in a very short time (as in a nuclear bomb).
- Controlled: in a nuclear reactor, on average exactly one neutron from each fission goes on to cause another fission, so energy is released at a steady rate.
A fission of U-235 releases two neutrons.
(a) Assume every neutron released causes a new fission. Calculate the number of fissions in the 5th generation if the first generation is one fission. (b) Explain how many of the two neutrons must go on to cause a fission for a reactor to release energy at a steady rate, and what happens to the other neutron.
(a) Each generation doubles: 1 → 2 → 4 → 8 → 16 fissions in the 5th generation.
(b) One neutron from each fission must cause one further fission, so the number of fissions per second stays constant. The other neutron is absorbed (for example by a control rod) or escapes from the core without causing fission.
What do the control rods and the moderator do? Double · Physics
Not required for Single Award (4SS0).
| Part | Typical material | What it does |
|---|---|---|
| control rods | boron or cadmium | absorb neutrons. Lowering them further into the core absorbs more neutrons, so fewer fissions happen and the rate of fission falls. Raising them increases the rate. Fully lowered, they stop the chain reaction. |
| moderator | graphite or water | slows down the neutrons released by fission. The fast neutrons collide with the moderator's atoms and lose kinetic energy. Slow neutrons are much more likely to be absorbed by U-235 nuclei and cause fission, so the chain reaction can continue. |
| fuel rods | uranium (containing U-235) | where the fission takes place |
| coolant | water or carbon dioxide | carries the thermal energy away from the core to make steam |
A moderator is chosen because it absorbs very few neutrons; it does not stop the reaction — it makes fission more likely. Control rods are the part that reduces the rate.
The thermal energy output of a reactor suddenly starts to increase. Explain what the operators should do to the control rods, and how this reduces the energy output.
- Lower the control rods further into the core.
- The control rods absorb (more) neutrons.
- So fewer neutrons are available to be absorbed by U-235 nuclei, and fewer fissions happen per second.
- Less energy is released each second, so the output falls.
Answers must name neutrons. “The control rods absorb particles / radiation / energy” is not credited.
Why is a reactor surrounded by shielding? Single · Double · Physics
- The reactor core emits large amounts of ionising radiation — gamma rays and neutrons from fission, and radiation from the radioactive daughter nuclei.
- This radiation would damage the cells of workers and people nearby.
- The core is surrounded by a thick steel vessel and very thick concrete shielding, which absorbs the radiation so that very little escapes into the surroundings.
Single Award: you need the shielding but not the control rods or moderator.
What is nuclear fusion, and why does it power the stars? Single · Double · Physics
Stars, including the Sun, release energy by nuclear fusion. In the core of a star, hydrogen nuclei fuse to make helium nuclei, releasing the energy that the star radiates as light and heat. This is the part Single Award candidates need.
Fusion: joining nuclei and losing mass Double · Physics
- Check: top 2 + 3 = 5 = 4 + 1; bottom 1 + 1 = 2 = 2 + 0.
- The mass numbers balance, but the actual mass of the products is very slightly less than the mass of the hydrogen nuclei. That small loss of mass is what is released as energy.
Why does fusion need very high temperatures and pressures? Double · Physics
- Nuclei contain protons, so all nuclei are positively charged. Two nuclei repel each other (electrostatic repulsion), and the repulsion gets stronger as they get closer.
- At low temperatures the nuclei move too slowly: they are pushed apart before they get close enough to fuse.
- At a very high temperature (millions of °C) the nuclei move so fast that they have enough kinetic energy to overcome the repulsion and get close enough to fuse.
- A very high pressure (high density) means nuclei are packed closely together, so collisions happen often enough for fusion to keep going.
- These conditions exist in the core of a star. They are very difficult to create and contain on Earth, which is why there are no fusion power stations yet.
How is fusion different from fission? Double · Physics
Not required for Single Award (4SS0).
| Fission | Fusion | |
|---|---|---|
| What happens | a large nucleus splits into two smaller nuclei | two small nuclei join to form a larger nucleus |
| Typical nuclei | uranium-235 (heavy) | isotopes of hydrogen (light) |
| How it starts | absorbing a neutron | nuclei colliding at very high speed |
| Conditions needed | no high temperature or pressure needed | very high temperature and pressure |
| Products | two radioactive daughter nuclei and 2–3 neutrons | a larger nucleus (such as helium), which is usually not radioactive |
| Where it happens | nuclear power stations | stars, including the Sun |
| Energy released | yes | yes |
(a) Explain two differences between nuclear fusion and nuclear fission. (b) A student says, “Fusion is easy to start, because hydrogen nuclei are small.” Explain why the student is wrong.
(a) Any two, each stated for both processes:
- Fusion joins small nuclei into a larger nucleus; fission splits a large nucleus into smaller nuclei.
- Fusion needs very high temperature and pressure; fission does not.
- Fission produces radioactive daughter nuclei; fusion products are usually not radioactive.
- Fission is started by a nucleus absorbing a neutron; fusion is not.
(b) Hydrogen nuclei are positively charged, so they repel each other. They only get close enough to fuse if they are moving extremely fast, which needs a very high temperature, and they only collide often enough at very high pressure (density). These conditions are very hard to create, so fusion is hard to start.
A difference needs both sides: “fusion needs a high temperature” alone is half an answer — add “but fission does not”.
Exam tips for 7(c)
In fission, control-rod and moderator answers, the particle is always a neutron. Answers that say “particles”, “atoms”, “electrons” or “nuclei” lose the mark even when the rest is right.
Fission does not need a high temperature or pressure, and radioactive daughter nuclei come from fission, not fusion. Learn the comparison table row by row.
Do not just write “control rods control the reaction”. Say they absorb neutrons, and that how far they are lowered decides how many neutrons are left to cause fission, and so the rate of fission.
The moderator slows the neutrons down so they are more likely to be absorbed by U-235 and cause fission. It is chosen because it absorbs very few neutrons, and it does not slow the reaction.
For a full answer on why fusion needs extreme conditions, link each one to a reason: high temperature gives the nuclei enough kinetic energy to overcome the repulsion; high pressure makes collisions frequent.
Common misconceptions
| Misconception | Why it is wrong | Correct idea |
|---|---|---|
| A proton or an electron is fired at U-235 to start fission. | A proton is repelled by the nucleus; electrons do not cause fission. | Fission of U-235 is caused by absorbing a neutron, which has no charge. |
| The moderator slows down the chain reaction. | It slows the neutrons, which makes fission more likely. | Control rods reduce the rate; the moderator keeps fission going. |
| Energy from fission is released as heat directly. | The products are first fast-moving. | Energy is released as kinetic energy of the products, which then heats the surroundings. |
| Fusion produces radioactive waste in the same way as fission. | The radioactive daughter nuclei are a feature of fission. | Fusion of hydrogen produces helium, which is not radioactive. |
| In fusion the mass numbers do not balance because mass is lost. | The nucleon number is conserved; only the actual mass falls very slightly. | Mass numbers balance in every nuclear equation. |
Key facts
| Fact | Route |
|---|---|
| fission, fusion and radioactive decay all release energy | All |
| U-235 + neutron → two radioactive daughter nuclei + 2 or 3 neutrons + kinetic energy | All |
| shielding (thick concrete) absorbs radiation from the reactor core | All |
| stars are powered by fusion | All |
| chain reaction: neutrons from one fission cause further fissions | Double · Physics |
| control rods absorb neutrons; moderator slows neutrons | Double · Physics |
| fusion: small nuclei join, mass is lost, energy is released | Double · Physics |
| fusion needs high temperature and pressure to overcome repulsion between positive nuclei | Double · Physics |
There are no Topic 7 relationships on the Equation List.
Check your understanding
1. Name the particle that must be absorbed by a U-235 nucleus for it to undergo fission. All
A neutron.
2. State the form of the energy released when a U-235 nucleus splits. All
Kinetic energy of the fission products (the daughter nuclei and neutrons).
3. Explain why a nuclear reactor is surrounded by thick concrete. All
The core emits ionising radiation (gamma rays and neutrons); the concrete absorbs it, so workers and the public are protected from cell damage.
4. A fission is described by \({}^{1}_{0}\text{n}+{}^{235}_{\ 92}\text{U}\rightarrow{}^{146}_{\ 57}\text{La}+{}^{87}_{35}\text{Br}+y\,{}^{1}_{0}\text{n}\). Determine y. Double · Physics
236 = 146 + 87 + y, so y = 3. (Bottom: 57 + 35 = 92 ✓.)
5. Show that the fusion equation \({}^{2}_{1}\text{H}+{}^{2}_{1}\text{H}\rightarrow{}^{3}_{2}\text{He}+{}^{1}_{0}\text{n}\) is balanced. Double · Physics
Top: 2 + 2 = 4 = 3 + 1. Bottom: 1 + 1 = 2 = 2 + 0. Both balance.
6. Describe what happens to the rate of fission if the control rods are raised, and explain why. Double · Physics
The rate increases: fewer neutrons are absorbed by the control rods, so more neutrons are absorbed by U-235 nuclei and cause fission.
7. Explain why fusion does not happen in hydrogen gas at room temperature. Double · Physics
Hydrogen nuclei (protons) are positive and repel each other. At room temperature they move too slowly to overcome the repulsion and get close enough to fuse; the density is also far too low for frequent collisions.
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