3.8.1.7
Chain reactions sustain fission — critical mass keeps them controlled
Nuclear Energy & Binding Energy — AQA A-Level Physics
Key Definition
Chain reaction — A self-sustaining series of fission reactions where neutrons released by one fission event go on to cause further fission events.
Key Definition
Critical mass — The minimum mass of fissile material required to sustain a steady chain reaction, where exactly one neutron from each fission event goes on to cause another fission.
- Each fission releases 2 or 3 neutrons. In a controlled reactor, exactly one causes another fission.
- Subcritical mass: reaction dies out. Critical mass: steady reaction. Supercritical mass: runaway reaction (explosion).
- Nuclear reactors maintain critical mass. Nuclear weapons use supercritical mass.