Nuclear Fission and Fusion
Fission splits a large nucleus after it absorbs a neutron, releasing energy and more neutrons that can keep a chain reaction going, which a reactor controls with rods. Fusion joins small nuclei instead, and needs the temperature and pressure found in stars.
What a learner can do afterwards
- Describes a fission chain reaction and the job of the control rods and moderator
- Explains why fusion needs enormous temperature and pressure to start
- Compares the two processes by what goes in, what comes out and the waste each leaves
1 · Read
Fission starts when a heavy nucleus like uranium 235 swallows a neutron and wobbles apart into two lighter nuclei. The split releases energy plus two or three fresh neutrons, and those neutrons can split further nuclei. Left alone that doubling becomes a runaway chain reaction.
Reactors tame the chain with a moderator and control rods. The moderator slows neutrons so splits stay likely, while the rods soak up spare neutrons to hold the rate steady. Push the rods in to calm the reactor, and pull them out to raise power.
Jon compares the two star stories. Fusion joins light hydrogen into heavier nuclei and releases energy, but both nuclei are positive and fight coming close. Only star grade temperature and pressure give them the speed and squeeze to touch. That is why fusion lights the Sun but not yet our grid.
Compare the two by what goes in and what comes out. Fission feeds on heavy nuclei and leaves split fragments plus long lived waste stored for centuries. Fusion feeds on light hydrogen and leaves helium plus a fast neutron, with far shorter lived waste. Fission wins on practicality for now.
Fission splits heavy atoms in a tamed chain, fusion joins light ones in stars.
2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.