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How does critical mass allow a self-sustaining fission chain reaction?

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U.S. hydrogen bomb test in the Marshall Islands — nuclear weapon A test of a U.S. thermonuclear weapon (hydrogen bomb) at Enewetak atoll in the Marshall Islands, November 1, 1952.

Critical mass is the minimum amount of fissile material required to sustain a nuclear chain reaction. A chain reaction occurs when neutrons released by the fission of one nucleus induce fission in other nuclei, creating a self-sustaining sequence.1234

The ability to sustain this reaction depends on the multiplication factor, k, which represents the ratio of fissions in one generation to the preceding one. Critical mass allows for a self-sustaining reaction by ensuring that enough neutrons remain within the material to maintain k at or above unity, rather than being lost to leakage or non-fission capture.43

Several factors influence this process:
* Geometry: A spherical shape minimizes surface area relative to volume, reducing neutron leakage.2
* Reflectors: Surrounding the core with materials that scatter escaping neutrons back into the fissile material can significantly lower the required mass.42
* Density: Compressing the core increases the likelihood that emitted neutrons will strike a nucleus before escaping, thereby lowering the critical mass.2