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Notes and summary: nuclear fission

Short answer

Fission of a nucleus like uranium-235 releases on average 2.5 neutrons, which can split other nuclei, causing a chain reaction and huge energy release. Thermal neutrons are most effective, so a moderator slows them. The minimum uranium mass needed to sustain the reaction is the critical mass.

Read the text given below carefully, make notes and write summary of it: When fission occurs, an average of 2.5 neutrons are emitted from the nucleus. [passage of about 230 words on chain reaction in uranium, thermal neutrons, moderators and critical mass] ... The minimum quantity of uranium required to initiate the chain reaction is called the critical mass.

Notes

Title: Nuclear chain reaction

  1. Fission
    • A nucleus (e.g., uranium-235) splits; on average 2.5 neutrons are emitted
  2. Chain reaction
    • Emitted neutrons strike other nuclei, causing more fission
    • Number of fissions increases rapidly; large energy released
  3. Thermal neutrons
    • Fast neutrons must be slowed to thermal speed to cause fission efficiently in U-235
  4. Moderator
    • Material (graphite, heavy water) slows neutrons by collisions
  5. Critical mass
    • Minimum quantity of uranium needed to start and sustain the chain reaction

Summary

When a uranium nucleus undergoes fission, it emits two or three neutrons, 2.5 on average. These neutrons can split other nuclei, so that the process continues as a chain reaction and releases a great deal of energy. Because slow (thermal) neutrons are most effective, a moderator such as graphite or heavy water is used to slow them down. The minimum quantity of uranium needed to start the chain reaction is the critical mass.

(About 70 words.)

✓ Answer checked by Claude Opus 5.5 (high effort)

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