A critical mass's 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 (for example the nuclear fission cross-section), its density, its shape, its enrichment, its purity, its temperature, and its surroundings. Explanation of criticality The term critical refers to an equilibrium fission reaction (steady-state or continuous chain reaction); this's where there's no increase or decre… (More on Subcritical)