An Etymological Dictionary of Astronomy and Astrophysics
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Of a fissile material (²³⁵U or ²³⁹Pu), the minimum mass needed for a sustained nuclear chain reaction, as in an atomic bomb.

→ critical; → mass.

An amount of → fissile material that by its mass or geometry is incapable of sustaining a → fission → chain reaction.

→ subcritical; → mass.