- If neutrons from the fission of uranium-235 continue to split other nuclei causing further fission, a chain reaction has occurred.
- A chain reaction refers to a process in which neutrons released in fission produce an additional fission in at least one further nucleus. This nucleus in turn produces neutrons, and the process repeats.
- The process may be controlled (nuclear power) or uncontrolled (nuclear weapons).
- In order for a chain reaction to take place, a minimum of one neutron from each fission must trigger further fission.
- At the same time, the mass of fission material must exceed a certain minimal mass known as the critical mass.
- A critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon its nuclear properties (e.g. the nuclear fission cross-section), its density, its shape, its enrichment, its purity, its temperature and its surroundings.
- If the material is less than this critical mass, too many neutrons escape without hitting any nuclei thus preventing a chain reaction from happening.
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