S4.11.7 For the adult
Fission and fusion
A lump of uranium 235 can give out far more energy than a lump of coal of the same mass. Where does the energy come from?
What this chapter teaches
In fission a large nucleus, such as uranium 235, absorbs a neutron and splits into two smaller nuclei, releasing neutrons and energy. The neutrons can cause more fissions, which is a chain reaction. In a reactor control rods absorb neutrons to keep the chain steady and a moderator slows them. In fusion light nuclei join, which needs a very high temperature. The Sun is powered by fusion of hydrogen into helium.
Three questions to ask
There is no one right wording. Listen for the idea.
- What is a chain reaction, and how does a reactor keep it steady?
- Why does fusion need such a high temperature?
- What is the difference between fission and fusion?
After the chapter, the learner can
- Describe how a chain reaction works in a nuclear reactor
- Explain how fusion powers the Sun
What they copy into their notebook
Fission: a large nucleus splits, releasing neutrons and energy. Each neutron can cause more fission, a chain reaction. Control rods absorb neutrons to keep it steady. Fusion: light nuclei join. It needs great heat. The Sun is powered by fusion.