Nuclear Fission & Fusion

IGCSE Edexcel Physics
7.17–7.26 Nuclear fission, chain reactions, nuclear reactors and fusion in stars
Key Concepts: Fission: a heavy nucleus (e.g. U-235) splits into two smaller nuclei after absorbing a neutron, releasing energy and further neutrons — which can trigger a chain reaction. Fusion: two light nuclei (e.g. hydrogen isotopes) combine at very high temperature to form a heavier nucleus, releasing large amounts of energy. Fusion powers stars.

Section A — Nuclear Fission

1. Define nuclear fission. [2]
2. Describe what happens when a uranium-235 nucleus absorbs a slow neutron. Include the products released. [4]
3. Explain what is meant by a chain reaction. State the condition required for an uncontrolled chain reaction to occur. [3]

Section B — Nuclear Reactors

4. Describe the role of each component in a nuclear reactor: [6]

a) Fuel rods

b) Control rods

c) Moderator

d) Coolant

5. Explain why control rods are pushed further into the reactor core when the power output needs to be reduced. [2]

Section C — Nuclear Fusion

6. Define nuclear fusion. [2]
7. Explain why nuclear fusion requires extremely high temperatures and pressures. [3]
8. State where nuclear fusion occurs naturally in the universe. [1]
9. Compare nuclear fission and fusion in terms of: the nuclei involved, the energy released per reaction, and the waste produced. [3]

Total marks: 26

Mark Scheme

1. Nuclear fission is the splitting of a heavy nucleus [1] into two smaller (daughter) nuclei after absorbing a neutron, releasing energy [1] [2]
2. U-235 absorbs a slow neutron [1]; the unstable nucleus splits into two smaller nuclei [1]; releasing 2 or 3 fast neutrons [1]; and a large amount of energy (as kinetic energy and gamma radiation) [1] [4]
3. The neutrons released by one fission can trigger further fissions [1]; each fission releases more neutrons, leading to an exponentially increasing reaction [1]; condition: the sample must be at or above critical mass [1] [3]
4. a) Fuel rods: contain the fissile material (e.g. uranium-235) [1]; b) Control rods: absorb neutrons to control the rate of fission [1]; c) Moderator (e.g. graphite or water): slows neutrons down so they are more likely to cause fission [1]; d) Coolant: carries heat away from the core to generate steam for turbines [1] [6 — 1.5 marks each component; award if clearly correct]
5. Control rods absorb neutrons [1]; pushing them deeper absorbs more neutrons, reducing the rate of fission and lowering power output [1] [2]
6. Nuclear fusion is the joining of two light nuclei [1] to form a heavier nucleus, releasing a large amount of energy [1] [2]
7. Light nuclei are positively charged and repel each other (electrostatic repulsion) [1]; very high kinetic energy (high temperature) is needed for the nuclei to get close enough for the strong nuclear force to take over [1]; high pressure keeps the nuclei close together to increase the probability of fusion [1] [3]
8. In stars (e.g. the Sun) [1]
9. Fission uses heavy nuclei (e.g. U-235); fusion uses light nuclei (e.g. hydrogen isotopes) [1]; fusion releases more energy per reaction than fission [1]; fission produces radioactive waste (spent fuel rods); fusion produces helium which is not radioactive [1] [3]