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: Explaining a nuclear reaction

June 2011 · Paper 2P · Question 8

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 U-235 nucleus, making it unstable so that it splits into two daughter nuclei. This fission releases energy and further neutrons. Those neutrons can cause more U-235 nuclei to split, forming a chain reaction. The graphite moderator slows neutrons, while control rods absorb neutrons; inserting or withdrawing the rods controls the reaction rate. The released energy heats the coolant, which carries heat to the heat exchanger and transfers it to water to produce steam.

Mark scheme points

Any six valid points from the list are sufficient; you do not need to include every point.

1. M1 A neutron is captured or absorbed by a U-235 nucleus, making it unstable.
2. M2 The nucleus breaks up into daughter nuclei.
3. M3 Energy is released.
4. M4 Neutrons are released.
5. M5 Released neutrons can cause further fissions.
6. M6 The repeated fissions form a chain reaction.
7. M7 The moderator slows down neutrons.
8. M8 Control rods absorb neutrons and reduce the fission rate.
9. M9 Inserting or withdrawing control rods changes how many neutrons are absorbed and controls the reaction.
10. M10 The released energy is used to heat water.
11. M11 A heat-transfer process is described, such as hot coolant circulating to the heat exchanger and transferring heat to water.

Explanation

First, a neutron is absorbed by a U-235 nucleus. The resulting unstable nucleus splits into daughter nuclei. This fission releases energy and more neutrons.

Some of the released neutrons may be absorbed by other U-235 nuclei, causing further fissions. When this process repeats, it is called a chain reaction. The graphite moderator slows the neutrons, helping them cause further fissions.

Control rods absorb neutrons. Inserting them absorbs more neutrons and reduces the reaction rate; withdrawing them absorbs fewer neutrons. This allows the chain reaction to be controlled.

Energy from fission heats the coolant. The circulating coolant carries the heat to the heat exchanger, where it is transferred to water to produce steam.

Common mistakes

• Do not say that electrons are absorbed or released: the particles involved in the chain reaction are neutrons.
• Fission is the splitting of a nucleus, not an atom, molecule or cell.
• Keep the roles distinct: the moderator slows neutrons, while the control rods absorb them.

Example 2: Interpreting a nuclear process

2017 specimen · Paper 1P · Question 9(a)(i)

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

The completed nuclear equation is:

Mark scheme points

1. M1 Write 4 as the mass number of helium.
2. M2 Write 0 as the atomic number of the neutron.

Explanation

The total mass number and total atomic number must each balance across the equation.

• Top numbers: the hydrogen nuclei have a total mass number of 2 + 3 = 5. The neutron has mass number 1, so helium needs 5 − 1 = 4.
• Bottom numbers: the total atomic number is 1 + 1 = 2. Helium already has atomic number 2, so the neutron needs 0.

Common mistakes

• Do not give the neutron atomic number 1. Its mass number is 1, but its atomic number is 0 because it contains no protons.

Practise Fission and fusion questions

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