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What is nuclear fission?

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Quick answer

Nuclear fission is the splitting of a heavy atomic nucleus, such as uranium, into smaller nuclei, releasing a large burst of energy and extra neutrons. Those neutrons can split more nuclei in a chain reaction, which is how nuclear power plants and atomic bombs produce their enormous energy.

Key things to understand

  1. A neutron strikes a heavy nucleus such as uranium-235, making it unstable so it splits into two lighter nuclei.
  2. The split releases energy as heat, plus two or three more neutrons.
  3. Those neutrons can split nearby nuclei, creating a self-sustaining chain reaction.
  4. Power plants control the reaction to boil water into steam; weapons let it run away uncontrolled.
  5. Fission splits heavy atoms apart, whereas fusion joins light atoms together.

People also ask

What is the difference between fission and fusion?
Fission splits a heavy nucleus into lighter ones; fusion fuses light nuclei such as hydrogen into a heavier one. Both release energy, and fusion is what powers the Sun.
How do power plants control fission?
Control rods absorb excess neutrons, slowing or speeding the chain reaction so it gives off steady heat instead of running away.
Why does fission release so much energy?
A tiny part of the nucleus's mass converts into energy, and by Einstein's E=mc² even a little mass yields a huge amount of energy.