Edexcel IGCSE Physics Fission and fusion questions

Fission and fusion syllabus

Questions on fission and fusion can test recall, calculations, explanations, diagrams, data handling and practical skills. You should be able to:

• 7.17 know that nuclear reactions, including fission, fusion and radioactive decay, can be a source of energy
• 7.18 understand how a nucleus of U-235 can be split (the process of fission) by collision with a neutron and that this process releases energy as kinetic energy of the fission products
• 7.19 know that the fission of U-235 produces two radioactive daughter nuclei and a small number of neutrons
• 7.20 describe how a chain reaction can be set up if the neutrons produced by one fission strike other U-235 nuclei
• 7.21 describe the role played by the control rods and moderator in the fission process
• 7.22 understand the role of shielding around a nuclear reactor
• 7.23 explain the difference between nuclear fusion and nuclear fission
• 7.24 describe nuclear fusion as the creation of larger nuclei resulting in a loss of mass from smaller nuclei, accompanied by a release of energy
• 7.25 know that fusion is the energy source for stars
• 7.26 explain why nuclear fusion does not happen at low temperatures and pressures, due to electrostatic repulsion of protons

How to answer fission and fusion questions

1. Track mass number and atomic number separately when completing nuclear equations.
2. Compare radiation using both ionising ability and penetrating power.
3. For half-life, repeat the same halving interval and keep the starting quantity clear.
4. Distinguish irradiation from contamination and connect each risk reduction to the exposure route.

Fission and fusion example questions and worked answers

These examples show how information in a diagram, graph or experimental context becomes part of a complete exam answer.

Example 1: Using a Diagram

Question 1

The diagram shows some of the parts of a nuclear power station.
Describe the process of controlled fission of U-235 in the nuclear reactor.

Final answer

A neutron is absorbed by a nucleus of U-235, making it unstable. The nucleus splits into two smaller daughter nuclei, releasing energy and further neutrons. These neutrons can cause more U-235 nuclei to split, producing a chain reaction.

The moderator slows down the neutrons, increasing the likelihood of further fissions. Control rods absorb neutrons; inserting or removing them controls the rate of the chain reaction. The energy released heats the water coolant, and the heat is transferred in the heat exchanger to produce steam.

Mark scheme points

Any six of the following points can earn the six marks:

1. M1 A neutron is captured or absorbed by a U-235 nucleus, producing an unstable state.
2. M2 The nucleus breaks up into smaller daughter nuclei.
3. M3 Energy is released.
4. M4 Neutrons are released.
5. M5 The released neutrons can cause further fissions.
6. M6 This produces a chain reaction.
7. M7 The moderator slows the neutrons, increasing the likelihood or rate of fission.
8. M8 Control rods absorb neutrons and reduce the reaction rate.
9. M9 The control rods can be inserted or removed to control the reaction.
10. M10 The released energy is used to heat water.
11. M11 Heat is transferred to another part of the system, such as through a heat exchanger.

Explanation

For the fission event, identify the particle as a neutron and the material splitting as the U-235 nucleus. State both the products of fission and the released energy. The released neutrons maintain the chain reaction by causing further fissions.

Controlled means that the reaction rate is adjusted: the moderator slows neutrons, while control rods absorb neutrons. Finally, link the nuclear energy to the power station by explaining that it heats water and that the heat is transferred to produce steam.

Common mistakes

• Writing that electrons are absorbed or released. The particles involved in the chain reaction are neutrons.
• Saying that a whole atom, molecule or cell undergoes fission. It is the U-235 nucleus that splits.
• Describing only one fission and omitting that released neutrons cause further fissions in a chain reaction.
• Confusing the roles of the moderator and control rods: the moderator slows neutrons, while control rods absorb them.
• Stopping at “energy is released” without explaining that it heats water and is transferred for steam production.

Example 2: Using a Diagram

Question 2

This is a question about nuclear energy. Nuclear fusion can take place between different isotopes of hydrogen to produce an isotope of helium. Complete the nuclear equation for this process.

Final answer

²₁H + ³₁H → ⁴₂He + ¹₀n

See diagram:

Mark scheme points

1. M1 The mass number of helium is 4.
2. M2 The atomic number of the neutron is 0.

Explanation

The total mass number is conserved. The two hydrogen nuclei have a total mass number of 2 + 3 = 5. If helium has mass number 4, the remaining neutron has mass number 1.

The total atomic number is also conserved: 1 + 1 = 2. Helium has atomic number 2, while a neutron has no protons, so its atomic number is 0.

• Fill the upper blank over helium with 4.
• Fill the lower blank beside the neutron with 0.

Common mistakes

• Do not use 5 as helium’s mass number; one neutron is also produced.
• Do not put 1 as the neutron’s atomic number. A neutron contains no proton, so its atomic number is 0.
• Keep mass number as the upper number and atomic number as the lower number.

Practise Fission and fusion questions

Build a focused practice set from this part of the Edexcel IGCSE Physics syllabus.