Fission and Fusion Basics

IGCSE Edexcel Physics
7.17-7.26 Fission and Fusion
Key Concepts: Fission splits a heavy nucleus; fusion joins light nuclei. Fusion powers stars.

Section A: Fission

1. Describe what happens during fission of U-235. [3]
2. Explain how a chain reaction is controlled in a nuclear reactor. [3]

Section B: Fusion

3. Describe nuclear fusion and state where it happens naturally. [3]
4. Explain why fusion requires very high temperature and pressure. [2]
Total marks: 11

Mark Scheme

1. U-235 absorbs neutron, splits into two nuclei and releases neutrons and energy [3]
2. Control rods absorb neutrons; moderator slows neutrons [3]
3. Fusion joins light nuclei; occurs in stars [3]
4. High temp/pressure needed to overcome repulsion [2]